pubs.acs.org/OrganometallicsPublished on Web 07/12/2010r 2010 American Chemical Society

Organometallics 2010, 29, 3341–3349 3341

DOI: 10.1021/om100274g

Dicationic Palladium(II) Complexes as Active Lewis Acid Catalysts for

Polarized Nazarov Cyclization

Jing Zhang, Tulaza Vaidya, William W. Brennessel, Alison J. Frontier,* andRichard Eisenberg*

Department of Chemistry, University of Rochester, Rochester, New York 14627

Received April 6, 2010

Two new bidentate phosphonamidite ligands- 1,2-bis((3RS)-3,3-diphenyltetrahydro-2-oxa-6R-aza-1-phosphapentaleno)ethane, L1, and 1,2-bis((3RS)-3,3-diphenyltetrahydro-2-oxa-6R-aza-1-phosphapenta-leno)benzene,L2- were prepared from (S)-R,R-diphenylprolinol and either 1,2-bis(dichlorophosphino)-ethane or 1,2-bis(dichlorophosphino)benzene, respectively.Reactions ofL1 andL2with one equivalent ofPd(PhCN)2Cl2 in dichloromethane result in the formation of the neutral complexes Pd(L1-P,P)Cl2, 2, andPd(L2-P,P)Cl2, 3, in good yields (85%and92%, respectively). Complexes 2 and 3were fully characterizedbyNMR spectroscopy, elemental analyses, andX-ray crystallography. Chloride abstraction from 1 and 2using two equivalents of AgSbF6 in the presence of excess diethylisopropylidenemalonate (DIM) leads tothe formationof the respectivedicationicpalladiumcomplexes [Pd(L1-P,P)(DIM)][SbF6]2,4, and [Pd(L2-P,P)(DIM)][SbF6]2, 5. As a result of the cationic nature of 4 and 5 and the labile coordination of DIM,these complexes exhibit high activity as electrophilic catalysts for theNazarov cyclization of electronicallypolarized substrates. For those substrates in which a quaternary carbon center is generated uponNazarovcyclization, spirocycle formation occurs via aWagner-Meerwein rearrangement following cyclization. Inthese cases, addition of NaBAr4

f as a cofactor to the reaction system improves the selectivity to spirocycleformation relative to the simple Nazarov product.

Introduction

Cationic palladium(II) complexes of the type of [Pd(diphos-phine)(solvent)]2þ have been studied extensively over the pastdecade as electrophilic and/or Lewis acidic catalysts for im-portant transformations including hydroarylation1 and hydro-amination2 of olefins, Friedel-Crafts alkylation,3Diels-Aldercycloddition,4 aldol condensation,5 enantioselective glyoxylate-and ketone-ene reactions,6 asymmetric addition of organome-tallic reagents to R,β-unsaturated carbonyl compounds,7 and

copolymerization of carbon monoxide and olefins.8 Thesecomplexes are characterized by defined square-planar coordi-nation geometries, often with adjacent coordination sites occu-pied by labile solvent molecules and poorly coordinatingligands. Additionally, these cationic complexes are often saltsof noncoordinating anions. Through the extraordinary varietyof ligands that have been studied for Pd(II) catalysis, thecomplexes have been shown to be electronically and stericallytunable. In this paper, we report new cationic palladium com-plexes that contain a new chiral chelating ligand and show thatthese complexes are catalytically active forNazarov cyclizationand for a particular set of polarized Nazarov substrates, spiro-cycle formation that occurs following ring closure.Our interest in Nazarov cyclization chemistry of polarized

substrates, i.e., divinyl andaryl vinyl carbonyl substrateshavingone electron-rich and one electron-poor double bond, wasstimulated by (1) the fact that the Nazarov reaction can beused in the synthesis of natural products and bioactive mole-cules containing highly functionalized five-membered carbo-cycles9 and (2) the discovery that the cationic iridium(III) com-plex [Ir(Me)(CO)(dppe)(DIB)][BArf4]2, 1 (where dppe=1,2-bis(diphenylphosphino)ethane, DIB=1,2-diiodobenzene, andBArf4

- =B(3,5-(CF3)2C6H3)4), is an active catalyst for the

*To whom correspondence should be addressed. E-mail: eisenberg@chem.rochester.edu, frontier@chem.rochester.edu.(1) Cucciolito, M. E.; D’Amora, A.; Tuzi, A.; Vitagliano, A. Orga-

nometallics 2007, 26, 5216.(2) Phua, P. H.;White, A. J. P.; de Vries, J. G.; Hii, K. K.Adv. Synth.

Catal. 2006, 348, 587.(3) Doherty, S.; Knight, J. G.; Smyth, C. H.; Harrington, R. W.;

Clegg, W. Organometallics 2007, 26, 5961.(4) Pignat, K.; Vallotto, J.; Pinna, F.; Strukul, G. Organometallics

2000, 19, 5160.(5) (a) Sodeoka,M.; Ohrai, K.; Shibasaki,M. J. Org. Chem. 1995, 60,

2648. (b) Nishikata, T.; Kobayashi, Y.; Kobayshi, K.; Yamamoto, Y.;Miyaura, N. Synlett 2007, 3055. (c) Hamashima, Y.; Sodeoka, M. Chem.Rec. 2004, 4, 231. (d) Umebayashi, N.; Hamashima, Y.; Hashizume, D.;Sodeoka, M. Angew. Chem., Int. Ed. 2008, 47, 4196.(6) (a) Becker, J. J.; Van Orden, L. J.; White, P. S.; Gagne, M. R. Org.

Lett.2002,4, 727. (b)Luo,H.K.;Khim,L.B.; Schumann,H.; Lim,C.; Jie,T.X.;Yang,H.Y.Adv. Synth. Catal. 2007, 349, 1781. (c)Mikami, K.; Kawakami, Y.;Akiyama, K.; Aikawa, K. J.Am.Chem. Soc. 2007, 129, 12950. (d)Aikawa,K.;Hioki, Y.; Mikami, K. J. Am. Chem. Soc. 2009, 131, 13922.(7) (a) Nishikata, T.; Yamamoto, Y.; Gridnev, I. D.; Miyaura, N.

Organometallics 2005, 24, 5025. (b) Nishikata, T.; Yamamoto, Y.; Miyaura,N. Chem. Lett. 2007, 36, 1442. (c) Nishikata, T.; Kiyomura, S.; Yamamoto,Y.; Miyaura, N. Synlett 2008, 2487.

(8) Bajracharya, G. B.; Koranne, P. S.; Tsujihara, T.; Takizawa, S.;Onitsuka, K.; Sasai, H. Synlett 2009, 310.

(9) (a) Janka,M.;He,W.; Frontier,A. J.; Flaschenriem,C.; Eisenberg,R.Tetrahedron 2005, 61, 6193. (b) He,W.; Huang, J.; Sun, X.; Frontier, A. J.J.Am.Chem.Soc.2008, 130, 300. (c)Malona, J. A.; Cariou,K.; Frontier, A. J.J. Am. Chem. Soc. 2009, 131, 7560.

3342 Organometallics, Vol. 29, No. 15, 2010 Zhang et al.

Nazarov reaction, as well as for cationic polymerizations ofelectron-rich olefins.10

Over the past decade, several transition metal complexes havebeen reported for promoting the Nazarov cyclization,11 and withchiral ligandspresent,modest togoodenatioselectivitieshavebeenobtained. The latter reports include catalysts such as Sc(pybox)-(OTf)3,

12 Cu(pybox)(OTf)2,13 and Ni(bis{(R)-1-[(S)-2-(diphe-

nylphosphino)ferrocenyl]ethyl}cyclohexylphosphine)(THF)]-[ClO4]2.

14 For complex 1, however, unselective binding of thesubstrate carbonyl oxygen atoms to the two adjacent but inequi-valent binding sites at the iridium center generates two isomers,Aand B, as observed by low-temperature 1H NMR spectro-scopy.9a,15Thebinding selectivitymight explainwhy the resultingasymmetric induction is poor despite the use of a chiral bis-phosphine ligand such as (R)-(þ)-BINAP (where (R)-(þ)-BI-NAP=(R)-(þ)-2,20-bis(diphenylphosphino)-1,10-binaphthyl).16

Thenotion that square-planarpalladium(II) complexes of thetype [Pd(PP*)(solvent)2]

2þ (PP*=chiral bidentate phosphineligand) would haveC2 symmetry with two adjacent and equiva-lent binding sites at the metal center suggested that such a com-plex could improve the enantioselectivity ofNazarov cyclizationcatalysis, thus stimulating the present study.We describe hereinthe synthesis and characterization of two new dicationic palla-dium(II) complexes containing newbidentate phosphonamiditeligands, [Pd(L1-P,P)(DIM)][SbF6]2 (4) (L1=1,2-bis((3RS)-3,3-diphenyltetrahydro-2-oxa-6R-aza-1-phosphapentaleno)ethane)and [Pd(L2-P,P)(DIM)][SbF6]2 (5) (L2=1,2-bis((3RS)-3,3-di-phenyltetrahydro-2-oxa-6R-aza-1-phosphapentaleno)benzene),and their catalytic properties in theNazarov cyclization reactionand spirocycle formation with polarized substrates.

Results and Discussion

Synthesis and Characterization of New Chiral Bidentate

Ligands 1,2-Bis((3rS)-3,3-diphenyltetrahydro-2-oxa-6r-aza-1-phosphapentaleno)ethane (L1) and 1,2-Bis((3RS)-3,3-diphenylte-trahydro-2-oxa-6a-aza-1-phosphapentaleno)benzene (L2). (S)-R,R-Diphenylprolinol is a commercially available compoundthat can also be prepared easily from L-proline. Its derived

P-chiral monodentate and bidentate ligands have been reportedby Reetz,17 Dorta,18 and Schmalz,19 while rhodium complexescontaining some of these ligands have been investigated forasymmetric hydrogenation of olefins17a and hydroboration ofstyrene.17b,19 Following a procedure similar to that used forthe preparation of 1-methyl-3,3-diphenyltetrahydro-3H-pyrrolo-[1,2-c][1,3,2]oxazaphosphole,17b (S)-R,R-diphenylprolinol wasreacted with half an equivalent of 1,2-bis(dichlorophosphino)-ethane in the presence of excess triethylamine at 0 �C to result inthe formation of 1,2-bis((3RS)-3,3-diphenyltetrahydro-2-oxa-6R-aza-1-phosphapentaleno)ethane (L1) in 55% yield (eq 1).The 31PNMR spectrum ofL1 shows a singlet at δ 167.7, indica-ting that the two phosphorus atoms of L1 are magneticallyequivalent.The resonance is shifteddownfieldby22ppm in com-parison to the corresponding resonance in 1,2-bis(dichlorophos-phino)ethane (δ 189.8). The 1HNMR spectrum ofL1 exhibits agroupof signals containing one doublet (δ 7.47, 2H) and two tri-plets (δ 7.11, 2 H and 7.05, 1 H) for one phenyl group, and an-other doublet (δ 7.28, 2H) and two triplets (δ 6.99, 2Hand 6.92,1H) for the second phenyl group, indicating inequivalence of thetwoprolinol phenyl groups. LigandL1was also characterized by13C{1H} NMR spectroscopy and elemental analysis. Similarly,when (S)-R,R-diphenylprolinol was reacted with half an equiva-lent of 1,2-bis(dichlorophosphino)benzene in the presence ofexcess triethylamine at 0 �C for 6 h, 1,2-bis((3RS)-3,3-diphenylte-trahydro-2-oxa-6R-aza-1-phosphapentaleno)benzene (L2) wasobtained as an analytically pure solid in 35%yield (eq 2). LigandL2hasa relatively rigido-phenylenebackbone insteadof an ethy-lene group as inL1. The 31P{1H} NMR spectrum ofL2 shows asinglet at δ 145.3, which corresponds to an upfield shift of ∼7ppm relative to that of 1,2-bis(dichlorophosphino)benzene andconfirms the equivalence of the two phosphorus atoms in theligand. The identity ofL2was further confirmed by its 1HNMRand 13C{1H} NMR spectra as well as by its elemental analysis.

Synthesis of Complexes Pd(L1-P,P)Cl2 (2) and Pd(L2-P,P)-Cl2 (3). Reaction of Pd(PhCN)2Cl2 with one equivalent of 1,2-bis((3RS)-3,3-diphenyltetrahydro-2-oxa-6R-aza-1-phosphapen-taleno)ethane,L1, indichloromethane results in the formationof

(10) (a) Albietz, P. J., Jr.; Cleary, B. P.; Paw,W.; Eisenberg, R. J. Am.Chem. Soc. 2001, 123, 12091. (b) Albietz, P. J., Jr.; Cleary, B. P.; Paw, W.;Eisenberg, R. Inorg. Chem. 2002, 41, 2095.(11) (a)He,W.;Herrick, I.R.;Atesin,T.A.;Caruana,P.A.;Kellenberger,

C. A.; Frontier, A. J. J. Am. Chem. Soc. 2008, 130, 1003. (b) He, W.; Sun, X.;Frontier,A. J.J.Am.Chem.Soc.2003,125, 14278. (c)Huang, J.; Frontier, A. J. J.Am. Chem. Soc. 2007, 129, 8060. (d) Malona, J. A.; Colbourne, J. M.; Frontier,A. J.Org.Lett. 2006,8, 5661. (e) Fujiwara,M.;Kawatsura,M.;Hayase, S.;Nanjo,M.; Itoh, T. Adv. Synth. Catal. 2009, 351, 123. (f) Kawatsura, M.; Higuchi, Y.;Hayase, S.; Nanjo,M.; Itoh, T.Synlett 2008, 1009. (g)Walz, I.; Bertogg,A.; Togni,A. Eur. J. Org. Chem. 2007, 2650.(12) (a) Liang, G.; Gradl, S. N.; Trauner, D.Org. Lett. 2003, 5, 4931.

(b) Liang, G.; Trauner, D. J. Am. Chem. Soc. 2004, 126, 9544.(13) Aggarwal, V. K.; Belfield, A. J. Org. Lett. 2003, 5, 5075.(14) Walz, I.; Togni, A.Chem. Commun. (Cambridge, U. K.) 2008, 4315.(15) Janka, M.; He, W.; Frontier, A. J.; Eisenberg, R. J. Am. Chem.

Soc. 2004, 126, 6864.(16) (a) Atesin, A. C.; Zhang, J.; Vaidya, T.; Brennessel,W.W.; Frontier,

A. J.; Eisenberg, R. Inorg. Chem. 2010, 49, 4331. (b) Vaidya, T.; Atesin, A. C.;Herrick, I.R.; Frontier,A. J.; Eisenberg,R.Angew.Chem., Int.Ed.2010,49, 3363.

(17) (a) Reetz, M. T.; Mehler, G.; Bondarev, O. Chem. Commun.(Cambridge, U. K.) 2006, 2292. (b) Bondarev, O. G.; Goddard, R. Tetra-hedron Lett. 2006, 47, 9013.

(18) Mariz, R.; Briceno, A.; Dorta, R. Organometallics 2008, 27, 6605.(19) (a) Kranich, R.; Eis, K.; Geis, O.; Muhle, S.; Bats, J. W.;

Schmalz, H. G. Chem.-Eur. J. 2000, 6, 2874. (b) Blume, F.; Zemolka,S.; Fey, T.; Kranich, R.; Schmalz, H. G. Adv. Synth. Catal. 2002, 344, 868.

Article Organometallics, Vol. 29, No. 15, 2010 3343

Pd(L1-P,P)Cl2, 2, in 85% yield (eq 3). The 31P{1H} NMRspectrum of 2 exhibits one singlet at δ 176.2, indicating adownfield shift of 8 ppm relative to the free ligand L1. Colorlesscrystals suitable for X-ray diffraction were obtained by slowdiffusion of hexanes into a concentrated solution of 2 indichloromethane at room temperature. Crystallographic data,intensity data collection, and refinement parameters are shownin Table 1, while complete structural information can be foundfrom the CIF supplied in the Supporting Information. AnORTEP diagram of the structure of 2 is shown in Figure 1, withselected bond distances and angles given in Table 2.

Themolecular structure of 2 showsa square-planar coordina-tiongeometryof thePd(II) centerwith twocisphosphinedonorsofL1 and two chloride ligands. The complex possesses approxi-mate C2 symmetry with the two phosphorus atoms of L1 in Sconfiguration. The bond distances and angles in 2 are close invalue to related Pd(II) dichloride di(phosphine) complexes suchas Pd(dppe)Cl2

20 and Pd((R)-(þ)-BINAP)Cl2 (2.245 A).21

The analogous complex withL2 as the ligand, Pd(L2-P,P)Cl2(3), was obtained by the reaction of L2 with one equivalent ofPd(PhCN)2Cl2 in92%yield (eq4).The 31P{1H}NMRspectrumof 3 exhibits one singlet at δ 162.7, suggesting a downfieldchemical shift of 17 ppm relative to free L2. Colorless crystalssuitable for X-ray diffraction were obtained by slow diffusion ofpentane into a concentrated solution of 3 in dichloromethane atroom temperature. Table 1 contains crystallographic data, inten-sity data collection, and refinement parameters, with completestructural details in the CIF format (see Supporting Informa-tion). An ORTEP drawing of the structure of 3 is shown inFigure2,withselectedbonddistancesandangles listed inTable3.

Themolecular structure of 3 confirms a Pd(II) square-planarcoordination geometry defined by the two phosphorus atoms

of L2 and two chloride atoms. The P1-Pd1-P2 angle of87.41(5)� in 3 is essentially equivalent to that of 2 (86.85(5)�).The ligand L2 possesses an approximateC2 symmetry with thetwo phosphorus donor atoms in S configuration. The bonddistances are similar to those of 2 and related Pd(PP)Cl2 com-plexes noted above.

Table 1. CrystalData,DataCollection, andStructureRefinement

Parameters for Pd(L1-P,P)Cl2, 2, and Pd(L2-P,P)Cl2, 3

empirical formula C40H46Cl10N2O2P2Pd C40.5H39Cl3N2O2P2Pdfw 1109.63 860.43temperature 100.0(1) K 100.0(1) Kwavelength 0.71073 A 0.71073 Acryst syst orthorhombic monoclinicspace group P212121 P21unit cell dimens a = 19.357(3) A a = 10.642(1) A

b = 22.915(3) A b = 24.796(3) Ac = 31.461(3) A c = 15.569(2) A

β = 101.667(2)�volume 13955(3) A3 4023.2(9) A3

Z 12 4density (calcd) 1.584 Mg/m3 1.421 Mg/m3

absorp coeff 1.079 mm-1 0.776 mm-1

F(000) 6744 1756cryst color,morphology

colorless, block colorless, plate

cryst size 0.24 � 0.24� 0.20 mm3

0.22 � 0.16� 0.05 mm3

θ range fordata collection

1.52 to 33.14� 1.57 to 28.28�

index ranges -29 e h e 29,-35 e k e 34,

-47 e l e 47

-14 e h e 14,-33 e k e 33,

-20 e l e 20reflns collected 202 837 56 617indep reflns 52 835 [R(int)

= 0.1012]19 941 [R(int)= 0.0960]

obsd reflns 35 381 13 599completeness to θ 99.4% 99.9%absorp corr multiscan multiscanmax. andmin transmn

0.8131 and 0.7817 0.9622 and 0.8478

data/restraints/params

52 835/0/1540 19 941/1/910

goodness-of-fiton F2

1.009 0.936

final R indices[I > 2σ(I )]

R1 = 0.0697,wR2 = 0.1517

R1 = 0.0562,wR2 = 0.0957

R indices(all data)

R1 = 0.1179,wR2 = 0.1764

R1 = 0.0898,wR2 = 0.1060

absolute structparam

-0.02(2) -0.01(2)

largest diff peakand hole

4.011 and-1.487 e A-3

0.853 and-0.675 e A-3

Figure 1. ORTEPdiagramof 2with thermal ellipsoids drawn atthe 50% level. All hydrogen atoms and cocrystallized solventmolecules (CH2Cl2) have been removed for clarity.

(20) Steffen, W. L.; Palenik, G. J. Inorg. Chem. 1976, 15, 2432.(21) Ozawa,F.;Kubo,A.;Matsumoto,Y.;Hayashi,T.;Nishioka,E.;

Yanagi, K.; Moriguchi, K. Organometallics 1993, 12, 4188.

3344 Organometallics, Vol. 29, No. 15, 2010 Zhang et al.

Synthesis and Characterization of Dicationic Complexes

[Pd(L1-P,P)(DIM)][SbF6]2 (4) and [Pd(L2-P,P)(DIM)][SbF6]2(5) (DIM=diethylisopropylidenemalonate). In order to obtaindicationic palladium(II) complexes having adjacent labile sites,chloride abstraction from Pd(L1-P,P)Cl2, 2, using two equiva-lents of AgSbF6 was carried out in the presence of DIM indichloromethane. The resultant complex [Pd(L1-P,P)(DIM)]-[SbF6]2, 4, was obtained in 71% yield (eq 3). The 31P{1H}NMR spectrum of 4 exhibits a singlet at δ 181.0, which indi-cates a downfield shift of 5 ppm compared to that of 2. ThepresenceofDIMin4was confirmedby its 13C{1H}NMRspec-trum with a singlet at δ 174.0, corresponding to the carbonylcarbons of the ligand and indicating chelation toPd(II) by theirequivalence. The chemical shift of the carbonyl carbon of freeDIM is δ 165.9. Likewise, reaction of Pd(L2-P,P)Cl2, 3, withtwo equivalents of AgSbF6 in the presence of excess DIMresults in the formation of [Pd(L2-P,P)(DIM)][SbF6]2, 5, in76% yield (eq 4). The 31P{1H} NMR spectrum of 5 exhibits asinglet at δ 160.2, which is 2.5 ppm upfield that of 3, and the13C{1H} NMR spectrum shows only one singlet at δ 174.0 forthe two carbonyl carbon atoms, consistent with the proposedC2 symmetric complex.

Catalytic Studies of Nazarov Cyclization and Spirocycle

Formation of Aryl Vinyl β-Ketoesters Using 4 and 5. Initialcatalytic studies were carried out using the aryl vinylβ-ketoester 6a. Using 10 mol % of either 4 or 5 in dichloro-methane at room temperature, 6a was converted to theNazarov product 6b in 90% or 85% yield, respectively, after24 h (eq 5). The catalytic activity of 4 and 5 for this reaction isthus similar to that of Cu(OTf)2 but slower than that ofdicationic Ir(III) dppe complex 1.9a,11a In view of the chiralnature of L1 and L2, the enantioselectivity of 6b in eachreaction was measured and found to be 5% and 3% forcatalysis by 4 or 5, respectively.

Nazarov Cyclization Catalysis Leading to Quaternary

Carbon Generation and Spirocycle Formation. β-Ketoesteralkylidene substrates having a methyl substituent atthe C1 position were next examined (eq 6). It has previouslybeen reported that for substrates having a methyl sub-stituent at C1, spirocycle formation can occur in com-petition with simple Nazarov cyclization.11c In eq 6, sub-strate a reacts to form the Nazarov cyclization product bor one of two spirocycle products, c or d.11c,16a Scheme 1illustrates the branching point in the reaction path-way of the palladium-bound oxyallyl cation intermediateformed upon conrotation. Proton elimination generates theNazarov product exclusively, while two types of spirocyclicproducts can form depending on the nature of the Rgroup. The formation of spirocycle products has been pre-viously analyzed and is thought to occur via a Wagner-Meerwin rearrangement upon ring closure that involves aC-C bond shift and subsequent migration of a R or Hgroup from the C5 position to C1.11c,16a The mechanism ofthis single-pot transformation and migratory aptitude of

Table 3. Selected Bond Lengths and Angles of Pd(L2-P,P)Cl2, 3

Bond Length (A)

Pd(1)-P(1) 2.207(1) Pd(1)-P(2) 2.213(2)Pd(1)-Cl(1) 2.362(2) Pd(1)-Cl(2) 2.350(1)P(1)-O(11) 1.602(4) P(1)-N(11) 1.646(4)P(2)-O(12) 1.608(4) P(2)-N(12) 1.646(5)

Bond Angles (deg)

P(1)-Pd(1)-P(2) 87.41(5) P(1)-Pd(1)-Cl(1) 89.68(5)P(2)-Pd(1)-Cl(2) 89.75(5) Cl(1)-Pd(1)-Cl(2) 93.14(5)Cl(2)-Pd(1)-P(1) 177.13(6) Cl(1)-Pd(1)-P(2) 176.70(5)

Table 2. Selected Bond Lengths and Angles of Pd(L1-P,P)Cl2, 2

Bond Length (A)

Pd(1)-P(1) 2.222(1) Pd(1)-P(2) 2.214(1)Pd(1)-Cl(1) 2.364 (1) Pd(1)-Cl(2) 2.357(1)P(1)-O(11) 1.614(4) P(1)-N(11) 1.676(5)P(2)-O(12) 1.612(4) P(2)-N(12) 1.636(5)

Bond Angles (deg)

P(1)-Pd(1)-P(2) 86.85(5) P(1)-Pd(1)-Cl(1) 89.44(5)P(2)-Pd(1)-Cl(2) 89.66(5) Cl(1)-Pd(1)-Cl(2) 94.55(5)Cl(2)-Pd(1)-P(1) 173.37(5) Cl(1)-Pd(1)-P(2) 173.43(5)

Figure 2. ORTEPdiagramof 3with thermal ellipsoids drawn atthe 50% level. All hydrogen atoms and cocrystallized solventmolecules (CH2Cl2) have been removed for clarity.

Scheme 1

(22) Spek, A. L. PLATON: A multipurpose crystallographic tool;version 30016; Utrecht University: Utrecht, The Netherlands: 2003.

(23) Yakelis, N. A.; Bergman, R. G. Organometallics 2005, 24, 3579.

Article Organometallics, Vol. 29, No. 15, 2010 3345

various R groups have been elucidated in detail usingCu(II)24 and dicationic Ir(III) catalysts.11c,16a

When 7a is treatedwith catalytic amounts of 4 or 5 (10mol%), a single diastereomer from Nazarov cyclization, 7b, isisolated as the final product with the relative configurationsat C1 and C5 corresponding to conrotation of the Z isomerof 7a in high yield (Table 4, entries 1 and 2). This observationis in agreement with E/Z isomerization of 7a prior tocyclization and results obtained using Cu(OTf)2 as thecatalyst.11a This transformation does not proceed withouta catalyst.11a In order to assess the activity of a morecommon chiral di(phosphine) palladium complex, Pd((R)-(þ)-BINAP)Cl2 was prepared and treated with two equiva-lents of AgSbF6 in the presence of excess DIM in dichlor-omethane. After removal of the precipitate (AgCl), theresulting yellow solution was used in situ as the catalyst forthe cyclization of 7a. The Nazarov product 7b was isolatedafter 12 h at room temperature in 90%yield and an ee of 3%.Despite the presence of chiral ligands L1, L2, or (R)-(þ)-BINAP in these palladium complexes, the resultant asym-metric induction was minimal. These observations suggestthat the site of cyclization (C1, C5) that leads to the forma-tion of the carbon stereocentermay be too far from the chiralphosphine ligand to cause significant levels of enantioinduction.

Interestingly, when substrate 8a, with a p-methoxyphe-nyl substituent at the C5 position, is reacted in the presenceof 10 mol % of 4, Nazarov product 8b is obtained in only19% yield, while spirocycle 8d, in which the p-methoxy-phenyl group has migrated from C5 to C1, is formed in57% yield (entry 5). It is worth noting that the formationof 8d from 8a as reported byHuang and Frontier employeda stoichiometric amount of Cu(SbF6)2.

11c Similarly, cata-lyst 5 promotes the cyclization of 8a, affording 8b and 8d

in yields of 29% and 58%, respectively (entry 6). In thesestudies, the Nazarov and spirocycle products are identi-fied by their 1H NMR spectra and ESI mass spectrometricdata, and the relative amounts are determined using theNMR data. All of the products have been fully characte-rized previously.11c,16a

While 9a yields only the Nazarov product upon reaction(entry 7), substrate 10a, with a cinnamyl group at the C5position, yields both Nazarov and spirocycle products withrelative amounts of each product depending on the catalyst.With 4 as the catalyst, 10b is obtained as the major product(71%) accompanied by a smaller amount of spirocycle 10d(24%), in which the cinnamyl group has shifted from C5 toC1 (entry 8). In contrast, the reaction of 10a with 5 underidentical conditions leads predominantly to spirocycle 10d

(47%, entry 9). For this reaction, 10b is only aminor productcomponent (4%). Substrate 11a, with a cycloheptenyl ringinstead of a cyclohexenyl ring, undergoes cyclization morerapidly with catalysts 4 and 5 than does 10a. The productdistribution is, however, dependent on the catalyst. Whencomplex 4 is used, the only product is the Nazarov cycliza-tion product 11b (95%), with no spirocycle products formed(entry 10). On the other hand, when 5 is used, a mixture of11b and 11d in a nearly 1:1 ratio (40%and 45%, respectively)is obtained (entry 11).

In a recent study of Nazarov cyclization and spirocycleformation with substrates 7a-11a catalyzed by cationic Ir(III)complexes,16a it was found that the addition of certain salts caninfluence the relative product distributions significantly. Inparticular, the addition of a stoichiometric equivalent (withrespect to the substrate) of NaBAr4

f to the reaction solutionswas observed to alter the product distribution in favor ofspirocycle formation. The rationale for the change in productdistributionwith addedNaBAr4

f was thatNaþ ions bind to thesubstrate (and product) carbonyl oxygen atoms to inhibit thereversible proton loss and addition needed to complete theNazarov cyclization. Instead, if Hþ loss is inhibited, a C-Cshift occurs that results ultimately in spirocycle formationfollowing R or H migration from C5 to C1.16a

On the basis of the previously reported observations andanalysis, the effect of NaBAr4

f on product distribution wasexamined using 4 and 5 as catalysts. When 7awas treated with10 mol % of 5 and one equivalent of NaBAr4

f at roomtemperature for 5.5 h, spirocycle 7c was formed in good yield(85%),whileNazarovproduct 7bwasobtained in just 7%yield(entry 4). Catalyst 5was better than 4 for spirocycle formationin terms of yield and enantioselectivity (compare entries 3 and4). Similarly, the formation of spirocycle 11d from the cycliza-tion of 11a was improved significantly by the addition of oneequivalent of NaBAr4

f (65%), while the yield of Nazarovproduct 11b was reduced to 11% (entry 12). The influence ofNaBAr4

f on the product distribution using the Pd(II) cationic 5is more pronounced than what was previously reported withthe Ir(III) catalyst [Ir(CO)(Me)(DIM)(R-(þ)-BINAP)]2þ(X)2(whereX-=SbF6

-, BAr4f-)16a andmay therefore expand the

utility of the transformations seen for the synthesis of morecomplex natural products and bioactive molecules.

Conclusion

Two neutral palladium(II) dichloride complexes, 2 and 3,containing new bidentate phosphonamidite ligands L1 andL2 have been synthesized and fully characterized. X-raydiffraction of 2 and 3 indicates the two phosphorus atomsin L1 and L2 are in the S configuration. The correspond-ing dicationic palladium(II) complexes [Pd(L1-P,P)(DIM)]-[SbF6]2 (4) and [Pd(L2-P,P)(DIM)][SbF6]2 (5), with theweaklycoordinating ligand DIM and a hexafluoroantimonate(VI)counteranion, were prepared from 2 and 3, respectively. Both4 and 5 exhibit Lewis acidity and are effective catalysts forthe Nazarov cyclization of polarized substrates. For thosesubstrates in which a quaternary C is generated upon conrota-tion, competing formation of a spirocyclic product is alsoobserved. The product distribution is also found to be influ-enced by the addition of NaBArf, with spirocycle formationbeing favored significantly as the amount of NaBArf is in-creased relative to substrate. The effect is based on the presenceof Naþ and is explained on the basis of proton loss from the(24) APEX2, 3.0 ed.; Bruker AXS: Madison, WI, 2009.

3346 Organometallics, Vol. 29, No. 15, 2010 Zhang et al.

Table 4. Catalysis of Nazarov Cyclization and Spirocycle Formation Using 4 and 5

aReaction conditions: substrate (5mM) and catalyst (0.5mM) in dichloromethane. b Isolated yields. cNaBAr4f (5mM)was used as additive. d 80%of

9a was recovered; TMP = 2,4,6-trimethoxyphenyl; PMP = 4-methoxyphenyl; PNP = 4-nitrophenyl.

Article Organometallics, Vol. 29, No. 15, 2010 3347

Nazarov ring-closed intermediate versus a C-C shift to form aspirocycle.

Experimental Section

General Methods. Unless otherwise stated, all the reactionsandmanipulationswereperformed inoven-driedglasswareunder apurified nitrogen atmosphere using either standard Schlenk tech-niques or an inert-atmosphere glovebox. All solvents (dichloro-methane, THF, toluene, hexanes, and diethyl ether) were purifiedand dried under nitrogen by conventional methods. Dichloro-methane-d2 and benzene-d6 were purchased from CambridgeIsotope Laboratories, degassed, and stored in dry glovebox.(S)-R,R-Diphenylprolinol, 1,2-bis(dichlorophosphino)ethane, and1,2-bis(dichlorophosphino)benzene were purchased from StremChemicals, Inc. NaBAr4

f 23 and substrates 6a-11a11c were pre-

pared according to the reported procedures.Elemental analyses were done at the University of Rochester

or Quantitative Technologies Inc. (QTI). 1H NMR, 31P{1H}NMR, and 13C{1H} NMR spectra were recorded on a BrukerAvance 400 MHz or a Bruker Avance 500 MHz spectrometer.1H and 13C{1H} chemical shifts are reported in parts per millionafter referencing to the appropriate solvent resonances. 31P{1H}chemical shifts are reported after an external calibration toan 85% solution of phosphoric acid. The assignment ofNMR resonances for L1, L2, 2, and 3 were verified using 2DNMR experiments (HSQC). A few broad 13C NMR resonanceswere observed for complexes 4 and 5, while some signalsoverlapped.

Synthesis of 1,2-Bis((3aS)-3,3-diphenyltetrahydro-2-oxa-6a-aza-1-phosphapentaleno)ethane, L1. An oven-dried 50 mLSchlenk flask was charged with (S)-R,R-diphenylprolinol (0.506 g,2mmol) and triethylamine (0.606 g, 6mmol) in THF (10mL). Thecolorless solution was cooled to 0 �C, and 1,2-dichlorophosphi-noethane (0.232 g, 1 mmol) was added dropwise maintaining thetemperature below 5 �C. Triethylammonium chloride precipitatedimmediately as a white solid. After 2 h at 0 �C, the mixture waswarmed to and stirred at room temperature for an additional 6 h.The precipitate was then removed by filtration, and the colorlessfiltrate was concentrated under vacuum. The residue was dissolvedin THF (3 mL), and hexanes (15 mL) were added to precipitate awhite solid. After removing the solid, the filtrate was concentratedto about 9mL and then cooled at-25 �C for 2 days. The solid thatprecipitated was separated by filtration and washed with hexanes(3� 1.5 mL). Ligand L1was isolated as a white solid in 55% yield(0.325 g). 31P{1H} NMR (162MHz, C6D6, δ): 167.7 (s).

1HNMR(400MHz, C6D6, δ): 0.73-0.79 (m, 2H, P-CHH-), 1.18-1.23 (m,2H,H4), 1.44-1.55 (m, 6H,P-CHH-,N3,H4), 1.62-1.67 (m, 2H,H3), 3.00-3.02 (m,2H,H5), 3.15-3.17 (m,2H,H5), 4.32-4.34 (m,2H, H2), 6.92 (t, 2H, JHH= 7.2 Hz, Ph-para-H), 6.99 (t, 4H,JHH=7.2Hz, Ph-meta-H), 7.05 (t, 2H, JHH=7.2Hz, Ph-para-H),7.11 (t, 4H, JHH=7.2Hz, Ph-meta-H), 7.28 (d, 4H, JHH=7.2Hz,Ph-ortho-H), 7.47 (d, 4H, JHH=7.2 Hz, Ph-ortho-H). 13C{1H}NMR (100 MHz, C6D6, δ): 25.7 (s, pyrrol-C4), 29.6 (s, prolinol-C3), 30.3 (dd, JPC=33.1 Hz, JPC=26.5 Hz, P-CH2), 51.8 (t, JPC=14.0 Hz,, prolinol-C5), 71.0 (s, prolinol-C2), 92.7 (t, JPC= 5.7 Hz,C-O-P), 126.9 (s, phenyl-C), 127.2 (s, phenyl-C), 127.3 (s, phenyl-C), 127.8 (s, phenyl-C, overlapped with C6D6), 128.3 (s, phenyl-C,overlapped with C6D6), 144.7 (s, phenyl-C), 147.3 (s, phenyl-C).

Anal. Calcd for C36H38N2O2P2: C, 72.96, H, 6.46, N, 4.73. Found:C, 72.92, H, 6.39, N, 4.61.

Synthesis of 1,2-Bis((3rS)-3,3-diphenyltetrahydro-2-oxa-6a-aza-1-phosphapentaleno)benzene, L2. To an oven-dried 50 mLSchlenk flask were added (S)-R,R-diphenylprolinol (0.506 g, 2mmol), triethylamine (0.606 g, 6 mmol), and THF (10 mL). Thecolorless solution was cooled to 0 �C, and a solution of 1,2-bis(dichlorophosphino)benzene (0.280 g, 1 mmol) in THF (3mL) was added dropwise so that the temperature remainedwithin 0 to 5 �C. Triethylammonium chloride was observedimmediately as a white precipitate. The reaction mixture wasallowed to stir at 0 �C for 2 h and then warmed to roomtemperature. After 4 h of stirring, the precipitate was isolatedafter vacuum filtration and the pale yellow filtrate was taken todryness under vacuum. The residue was dissolved in THF (6mL), and then hexanes (24 mL) were added. The precipitate soobtainedwas isolated by filtration, washedwith hexanes (3� 1.5mL), and dried under vacuum for several hours. Ligand L2 wasobtained in a pure form in 31%yield (0.2 g). 31P{1H}NMR (162MHz, CD2Cl2, δ): 145.3 (s). 1H NMR (400 MHz, CD2Cl2, δ):0.58-0.65 (m, 2H, H4), 1.54-1.61 (m, 2H,, H3,), 1.82-1.87 (m,2H, H3), 2.06-2.15 (m, 2H, H4), 3.32-3.36 (m, 2H, H5),3.49-3.55 (m, 2H, H5), 4.04 (br d, 2H, JHH=6 Hz, H2), 6.67(dd, 2H, JPH=3.2 Hz, JHH=5.6 Hz, phenyl-H)), 7.16-7.27 (m,18H, phenyl-H), 7.39 (d, 4H, JHH=7.6 Hz, phenyl-H). 13C{1H}NMR (100 MHz, CD2Cl2, δ): 25.6 (s, pyrrol-C4), 29.1 (s, pro-linol-C3), 50.9 (t, JPC=14.5 Hz, prolinol-C5), 71.1 (s, prolinol-C2), 93.6 (t, JPC=4.9Hz, C-O-P), 127.16 (s, phenyl-C), 127.23(s, phenyl-C), 127.27 (s, phenyl-C), 127.4 (s, phenyl-C), 128.0 (s,phenyl-C), 128.1 (s, phenyl-C), 129.7 (t, JPC = 6.1 Hz, phenyl-C), 144.8 (s, phenyl-C), 146.1 (s, phenyl-C), 148.6 (t, JPC = 9.4Hz, phenyl-C). Anal. Calcd for C40H38 N2O2P2: C, 74.99, H,5.98, N, 4.37. Found: C, 74.28, H, 5.46, N, 4.33.

Synthesis of [(L1-P,P)PdCl2], 2. Ligand L1 (0.059 g, 0.1mmol), Pd(PhCN)2Cl2 (0.038 g, 0.1 mmol), and CH2Cl2 (5 mL)were added to an oven-dried 20 mL vial. Stirring at roomtemperature for 8 h led to a change in the color of the reactionmixture from orange to pale yellow. Upon removal of the solventunder vacuum, the residue was redissolved in CH2Cl2 (3 mL). Theaddition of hexanes (12 mL) provided a yellow solid, which wasseparated after filtration and washed with hexanes (3 � 1.5 mL).Complex Pd(L1-P,P)Cl2 (2) was obtained in 85% yield (0.062 g).31P{1H} NMR (162 MHz, CD2Cl2, δ): 176.2 (s). 1H NMR (400MHz, CD2Cl2, δ): 0.80-0.91 (m, 4H, (-P-CH2)2-), 1.11-1.17(m, 2H, prolinol-H3), 1.75-1.82 (m, 2H, prolinol-H4), 1.84-1.92(m, 2H, prolinol-H3), 2.09-2.17 (m, 2H, prolinol-C4), 3.24-3.31(m, 2H, prolinol-H5), 4.17-4.24 (m, 2H, JHH=9.6Hz, JHH=10.8Hz, prolinol-H5), 4.49 (dd, 1H, JHH =4.8 Hz, JHH= 9.8 Hz,prolinol-H2), 4.55 (dd, 1H, JHH=4.8 Hz, JHH=9.8 Hz, pro-linol-H2), 7.24 (t, 2H, JHH=7.2Hz, phenyl-H), 7.32 (t, 10H, JHH=7.2 Hz, phenyl-H), 7.45 (d, 4H, JHH = 7.2 Hz, phenyl-H),7.50-7.52 (m, 4H, phenyl-H). 13C{1H} NMR (100 MHz, C6D6,δ): 26.1 (s, pyrrol-C4), 30.7 (s, prolinol-C3), 35.4 (t, JPC=32.0Hz,P-CH2), 51.5 (br s, prolinol-C5), 70.5 (s, prolinol-C2), 93.1 (t,JPC = 2.1 Hz, C-O-P), 126.2 (s, phenyl-C), 127.5 (s, phenyl-C),128.0 (s, phenyl-C), 128.4 (s, phenyl-C), 128.9 (s, phenyl-C), 129.2(s, phenyl-C), 141.9 (t, JPC = 3.3 Hz, phenyl-C), 143.6 (s, phenyl-C). Anal. Calcd for C36H38N2O2P2PdCl2: C, 56.16, H, 4.97, N,3.64. Found: C, 55.83, H, 4.92, N, 3.54.

3348 Organometallics, Vol. 29, No. 15, 2010 Zhang et al.

Synthesis of [(L2-P,P)PdCl2], 3.Anoven-dried 20mLvial waschargedwithL2 (0.064 g, 0.1mmol), Pd(PhCN)2Cl2 (0.038 g, 0.1mmol), and CH2Cl2 (5 mL). The orange solution was stirred atroom temperature for 12 h to provide a yellow color. Removal ofthe solvent under vacuum led to a residue, which was redissolved inCH2Cl2 (2.5 mL). The addition of hexanes (20 mL) furnished ayellow precipitate, which was isolated upon filtration and washedwith hexanes (3� 1.5mL). Pd (L2-P,P) Cl2 (3) was isolated in 92%yield (0.075 g). 31P{1H}NMR (162MHz,CD2Cl2, δ): 162.7 (s).

1HNMR (400 MHz, CD2Cl2, δ): 1.51-1.54 (m, 2H, prolinol-H4),1.95-2.07 (m, 4H, prolinol-H4 and H3), 2.26-2.30 (m, 2H,prolinol-H3), 3.20-3.25 (m, 2H, prolinol-H5), 4.28-4.35 (m, 2H,JHH=9.1 Hz, JHH=17.2 Hz, prolinol-H5), 4.93-5.01 (m, 2H,prolinol-H2), 6.49 (br m, 2H, phenyl-H), 6.88-6.90 (m, 2H,phenyl-H), 7.23 (t, 2H, JHH = 7.2 Hz, Phenyl-H), 7.29-7.31 (m,10H, phenyl-H), 7.51-7.52 (m, 4H, phenyl-H), 7.58 (d, 4H, JHH=7.6Hz, phenyl-H). 13C{1H} (100MHz, CD2Cl2, δ): 27.0 (s, pyrrol-C4), 31.7 (s, prolinol-C3), 48.4 (br s, prolinol-C5), 72.5 (s, prolinol-C2), 94.1 (t, JPC= 3.7 Hz, C-O-P), 126.4 (s, phenyl-C), 126.8 (s,phenyl-C), 127.7 (s, phenyl-C), 128.2 (s, phenyl-C), 128.8 (s, phenyl-C), 129.0 (s, phenyl-C), 129.6 (br d, JPC=32.1Hz, phenyl-C), 129.9(d, JPC=27.5 Hz, phenyl-C), 132.7 (br s, phenyl-C), 142.1 (br s,phenyl-C), 143.2 (s, phenyl-C), 143.8 (s, phenyl-C), 144.4 (s, phenyl-C), 144.9 (s, phenyl-C). Anal. Calcd for C40H38Cl2N2O2P2Pd: C,58.73, H, 4.68, N, 3.42. Found: C, 58.85, H, 4.57, N, 3.41.Synthesis of [(L1-P,P)Pd(DIM)][SbF6]2, 4. To an oven-dried

20 mL vial were added [(L1-P,P)PdCl2] (60 mg, 0.078 mmol),diethylisopropylidene malonate (DIM) (156 mg, 0.78 mmol), andCH2Cl2 (30 mL). After AgSbF6 (55 mg, 0.16 mmol) was added tothe yellow solution, the mixture was rapidly stirred at roomtemperature for 16 h. The precipitate thus formed was removedby vacuum filtration using a pad of Celite. The pale yellow filtratewas concentrated to about 5 mL, and then hexanes (20 mL) wereadded slowly to precipitate a pale yellow solid. This solid wascollected, washed with hexanes (3 � 1.5 mL), and dried undervacuum for several hours to yield 4 in a 71%yield (76mg). 31P{1H}NMR (162 MHz, CD2Cl2, δ): 181.0 (s). 1H NMR (400 MHz,CD2Cl2, δ): 1.24 (t, 6H, JHH = 7.2 Hz), 1.22-1.33 (m, 2H),1.45-1.48 (m, 2H), 1.62-1.65 (m, 2H), 2.00-2.13(m,4H), 2.22 (brs, 6H), 3.48-3.54 (m, 4H), 4.05-4.20 (m, 4H), 4.22-4.30 (m, 2H),4.70-4.78(m, 2H), 7.26-7.40 (m, 12H, JHH = 7.2 Hz), 7.44 (d,4H, JHH = 7.2 Hz), 7.53 (m, 4H, JHH = 6.0 Hz). 13C{1H} NMR(125 MHz, CD2Cl2, δ): 13.7 (s), 25.3 (s), 26.0 (s), 30.9 (s), 34.5 (d,JPC= 36.2 Hz), 34.9 (d, JPC= 33.2 Hz), 50.2 (br s), 67.3 (s), 72.2(s), 95.7 (br s), 125.3 (s), 125.5 (s), 127.0 (s), 128.7 (s), 128.9 (s),129.4 (s), 129.9 (s), 140.5 (br s), 141.8 (s), 168.7 (s), 174.0 (s). Anal.Calcd for C46H54F12N2O6P2PdSb2 3 1/4CH2Cl2: C, 39.91, H, 3.94,N, 2.01. Found: C, 39.88, H, 3.95, N, 2.19. The presence of 1/4molecule of dichloromethane in the sample used for elementalanalysis was confirmed by 1H NMR spectroscopy.Synthesis of [(L2-P,P)Pd(DIM)][SbF6]2, 5. An oven-dried 20

mL vial was charged with [(L2-P,P)PdCl2] (58.4 mg, 0.071mmol) and DIM (143 mg, 0.71 mmol) in CH2Cl2 (30 mL). Theyellow mixture was treated with AgSbF6 (49 mg, 0.14 mmol)followed by rapid stirring at room temperature for 16 h. Theprecipitate formed was removed via filtration through a pad ofCelite, and the yellow filtrate was evaporated down to about4mL.Hexanes (20mL)were added slowly to precipitate a yellowsolid. The solidwas collected,washedwith hexanes (3� 1.5mL),and then dried under vacuum for several hours to yield 5 in 76%yield (78 mg). 31P{1H} NMR (202 MHz, CD2Cl2, δ): 159.5 (s).1H NMR (500 MHz, CD2Cl2, δ): δ 1.16 (t, 6H, JHH = 7.5 Hz),1.58-1.63 (m, 2H), 2.01-2-04 (m, 2H), 2.27-2.33 (m, 4H), 2.28(br s, 6H), 3.45-3.50 (m, 2H), 3.71-3.75 (m, 2H), 3.93-3.97 (m,2H), 4.03-4.09 (m, 2H), 5.09-5.16(m, 2H), 6.69-6.71 (m, 2H),7.26-7.58 (m, 22H). 13C{1H}NMR (125MHz, CD2Cl2, δ): 13.6(s), 25.1 (s), 27.0 (s), 31.6 (s), 47.2 (br s), 67.5 (s), 73.4 (s), 96.2(br s), 125.3 (s), 125.5 (s), 125.7 (s), 126.4 (s), 128.7 (s), 129.1(s), 129.5 (s), 129.9 (s), 129.7 (d, JPC = 1.9 Hz), 130.5 (s), 135.6(br s), 140.9 (s), 143.1 (s), 167.1 (s), 174.0 (s). Anal. Calcd for

C50H54F12N2O6P2PdSb2: C, 42.33, H, 3.84, N, 1.97. Found: C,42.33, H, 4.19, N, 2.04.

General Procedures for Catalytic Reactions with Complexes 4

and 5. (a) In a glovebox, a J-Young tube was charged witheither catalyst 4 or 5 (0.005 mmol) and substrate 6a (0.05 mmol)in CD2Cl2 (0.7 mL) and sealed with a Teflon cap. After sub-jecting the tube to the desired temperature, the reactionprogress was monitored by 1H NMR spectroscopy. Uponcompletion, the product was purified by flash column chroma-tography and confirmed by 1H NMR spectroscopy relativeto that reported previously for Nazarov cyclization and spiro-cycle products.

(b) In a glovebox, an oven-dried 20 mL vial was chargedwith either complex 4 or 5 (0.005 mmol) and the appropriatesubstrate (7a-11a) (0.05 mmol) (Table 2) in CH2Cl2 (10 mL)and sealed with a Teflon cap. For those reactions examiningthe influence of NaBAr4

f on product distribution, 0.05 mmolof NaBAr4

f was added at this point. The solution was stirredat room temperature for several hours, and the ensuingreaction was monitored by thin-layer chromatography. Afterthe starting substrate was consumed, the solution wastaken to dryness and the residue was redissolved and passedthrough a small silica gel column to remove the metal com-plexes. The organic products were collected and their iden-tities were confirmed by comparison of their 1H NMR spectrawith those of previously reported samples.16a The yields ofthe Nazarov and spirocycle products were obtained by compar-ison of their 1H NMR resonances versus mesitylene (10 μL)used as an internal standard. Each experiment was repeated atleast two times.

Crystal Structure Determinations of Complexes 2 and 3. Eachcrystal was placed onto the tip of a glass fiber andmounted on aBruker SMART Platform diffractometer equipped with anAPEX II CCD area detector.24 All data were collected at100.0(1) K using Mo KR radiation (graphite monochromator).For each sample a preliminary set of cell constants and anorientation matrix were determined from reflections harvestedfrom three orthogonal wedges of reciprocal space. Full datacollections were carried out with frame exposure times of60-120 s at a detector distance of 4 cm. Randomly orientedregions of reciprocal space were surveyed for each sample: fourmajor sections of frames were collected with 0.50� steps in ω atfour different j settings and a detector position of -38� in 2θ.The intensity data were corrected for absorption,25 and final cellconstants were calculated from the xyz centroids of approxi-mately 4000 strong reflections from the actual data collectionafter integration.26 Structures were solved using SIR9727 andrefined using SHELXL-97.28 Space groups were determined onthe basis of systematic absences, intensity statistics, and Cam-bridge Structural Database frequencies.29 Direct-method solu-tions were calculated, which provided most non-hydrogenatoms from the difference Fourier map. Full-matrix least-squares (on F2)/difference Fourier cycles were performed, whichlocated the remaining non-hydrogen atoms. Non-hydrogenatoms were refined with anisotropic displacement parameters,and hydrogen atoms were placed in ideal positions and refinedas riding atoms with relative isotropic displacement parameters.Absolute configuration for both structures was determined byanomalous dispersion effects. All refinements were run tomathematical convergence.

(25) Sheldrick, G. M. SADABS, 2008/1 ed.; University of G€ottingen:Germany, 2008.

(26) SAINT, 7.60A ed.; Bruker AXS: Madison, WI, 2008.(27) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.;

Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.;Spagna, R. SIR97: A new program for solving and refining crystalstructures; Istituto di Cristallografia, CNR: Bari, Italy, 1999.

(28) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr.2008, 64, 112.

(29) Allen, F. H.Acta Crystallogr., Sect. B: Struct. Sci. 2002, 58, 380.

Article Organometallics, Vol. 29, No. 15, 2010 3349

The crystal structure of 2 contains three independent palla-dium molecules and four independent cocrystallized dichloro-methane molecules in the asymmetric unit, with all atoms ingeneral positions, while the crystal structure of 3 has twoindependent palladium molecules and one cocrystallized di-chloromethane molecule in the asymmetric unit, with all atomsin general positions. In the crystal structure of 3, there is asolvent channel parallel to the a-axis that contains highlydisordered solvent that could not be modeled satisfactorily.The reflection contributions from this solvent were removedusing the “Squeeze” function of program PLATON, whichdetermined there to be 143 electrons in 434 A3 removed per unitcell.22 Since the exact identity and quantity of this solvent areunknown, it was not included in the molecular formula, and

thus, calculations that derive from the molecular formula(F(000), density, etc.) are known to be incorrect.

Acknowledgment. The authors thank the NationalScience Foundation (Grant CHE 0847851) for suppor-ting this work.

Supporting Information Available: Experimental details and1H NMR and MS data for previously reported organic com-pounds obtained from catalytic experiments using complexes 4and 5 are included. X-ray crystallographic data of 2 and 3 areprovided in CIF format. This material is available free of chargevia the Internet at http://pubs.acs.org.