pubs.acs.org/Organometallics Published on Web 10/09/2009 r 2009 American Chemical Society

6462 Organometallics 2009, 28, 6462–6468

DOI: 10.1021/om900736a

Contacted Ion-Pair Lithium Alkylamidoaluminates: Intramolecular

Alumination (Al-H Exchange) Traps for TMEDA and PMDETA

Ben Conway,† Joaquın Garcıa-�Alvarez,*,‡ Eva Hevia,† Alan R. Kennedy,†

Robert E. Mulvey,*,† and Stuart D. Robertson†

†WestCHEM, Department of Pure and Applied Chemistry, University of Strathclyde,Glasgow G1 1XL, U.K., and ‡Departamento de Quımica Org�anica e Inorg�anica, Instituto Universitario de

Quımica Organomet�alica “Enrique Moles” (Unidad Asociada al CSIC), Facultad de Quımica,Universidad de Oviedo, E-33071 Oviedo, Spain

Received August 24, 2009

Exploring the reactivity of mixed-metal synergic bases, it is found that the lithium TMP aluminateiBu3Al(TMP)Li functions as a dual TMP/alkyl base, exhibiting 2-foldAMMAl (alkali-metal-mediatedalumination) toward TMEDA to yield the heterobimetallic bis-(deprotonated TMEDA) derivative[Li{Me2NCH2CH2N(Me)CH2}2Al(iBu)2] (2). In contrast, the amide enriched aluminate iBu2Al-(TMP)2Li acts as only a single-fold amido base toward TMEDA or PMDETA to afford the amine-deprotonated derivatives [Li{Me2NCH2CH2N(Me)CH2}(TMP)Al(iBu)2] (4) and [Li{Me2NCH2-CH2N(Me)CH2CH2N(Me)CH2}(TMP)Al(iBu)2] (5), respectively. On their own, the aluminum com-pounds iBu3Al or iBu2Al(TMP) are not sufficiently strong bases tometalate TMEDAorPMDETA, soin 2, 4, and 5, the R-deprotonations of TMEDA and PMDETA are synergic in origin, as theintramolecular communication between Li and Al appears to activate the TMP and iBu bases. Thisspecial behavior can be attributed to intramolecular proximity effects between the active basecomponent (TMP or iBu) and the ligating TMEDA or PMDETA molecule. X-ray crystallographystudies reveal2 is a contacted ion-pair ate containing twoR-aluminatedTMEDAligands,which chelatethe lithium cation which is linked to the distorted tetrahedral Al center by two N bridges from themetalated junction of the TMEDAmolecules. In contrast, 4 and 5 have a mixed NCH2-TMP bridgingligand set, completed by two terminal iBu ligands on Al and a chelating metalated TMEDA orPMDETA ligand attached to Li, respectively. In addition, the 1H, 7Li, and 13C{1H} spectra of 2(recorded in C6D6 solutions), 4, and 5 (recorded in cyclohexane solutions-d12) are disclosed.

Introduction

First reported in 2004 by Uchiyama et al., lithium TMP-aluminate “iBu3Al(TMP)Li” (where TMP is the stericallydemanding amide 2,2,6,6-tetramethylpiperidide) is an excel-lent base reagent in THF, realizing highly chemo- andregioselective deprotonative aluminations of functionalizedaromatic and heteroaromatic compounds as well as offunctionalized allylic compounds.1 Theoretical and experi-mental studies have revealed that this TMP-aluminateacts as an amido base, exhibiting only single-step directed

ortho-metalation (DoM) reactivity.2-4 Furthermore, on thebasis of our synthetic, reactivity, NMR spectroscopic, andX-ray crystallographic studies, a reaction scheme involvingpreviously unconsidered solvent-separated ion-pair speciesand a dismutation process was postulated.5 In addition, wewere able to report5 the complex [(THF)3 3Li{O(dC)-N(iPr)2(C6H4)}Al(iBu)3] as the first tangible metallo inter-mediate of a direct alumination reaction (performed in THFsolutions) of N,N-diisopropylbenzamide, or indeed of anyfunctionalized aromatic compound, to be structurally de-fined and fully characterized.6 We also described that carry-ing out the same reaction in a nonpolar solvent (hexane)

*Towhomcorrespondence should be addressed. E-mail: r.e.mulvey@strath.ac.uk (R.E.M.) and garciajoaquin.uo@uniovi.es (J.G.-A.).(1) (a) Uchiyama,M.; Naka, H.; Matsumoto, Y.; Ohwada, T. J. Am.

Chem. Soc. 2004, 126, 10526. (b) Naka, H.; Uchiyama, M.; Matsumoto, Y.;Wheatley, A. E. H.; McPartlin, M.; Morey, J. V.; Kondo, Y. J. Am. Chem.Soc. 2007, 129, 1921.(2) Naka,H.;Morey, J. V.; Haywood, J.; Eisler, D. J.;McPartlin,M.;

Garcia, F.; Kudo, H.; Kondo, Y.; Uchiyama, M.; Wheatley, A. E. H.J. Am. Chem. Soc. 2008, 130, 16193.(3) This is in contrast to most TMP-zincate bases which exhibit two-

stepDoM, see:Nobuto,D.;Uchiyama,M. J.Org. Chem. 2008, 73, 1117,although the second step can be inhibited by coordination saturation at thezinc center, see: Clegg,W.; Conway, B.; Graham, D. V.; Hevia, E.; Kennedy,A. R.; Mulvey, R. E.; Russo, L.; Wright, D. S. Chem.;Eur. J. 2009, 15,7074.

(4) For an authoritative reviewon “complex-inducedproximity effect(CIPE)” inDoMsee:Whisler,M.C.;MacNeil, S.; Snieckus, V.; Beak, P.Angew Chem., Int. Ed. 2004, 43, 2206. For reviews in DoM of aromaticsystems through alkali-metal-mediated alumination (and metalation in gen-eral), see: (a) Mulvey, R. E.; Mongin, F.; Uchiyama, M.; Kondo, Y. Angew.Chem., Int. Ed. 2007, 46, 3802. (b)Mulvey, R. E.Acc.Chem.Res. 2009, 42,743.

(5) Conway, B.; Hevia, E.; Garcia-Alvarez, J.; Graham, D. V.;Kennedy, A. R.; Mulvey, R. E. Chem. Commun. 2007, 5241.

(6) Crystal structures of three different Lewis base-stabilized deriva-tives [L 3Li(μ-TMP)(μ-iBu)(iBu)2] have been previously described by usin: Garcia-Alvarez, J.; Hevia, E.; Kennedy, A. R.; Klett, J.; Mulvey,R. E. Chem. Commun. 2007, 2402.

Article Organometallics, Vol. 28, No. 22, 2009 6463

instead of THF, the aforementioned LiTMP-aluminate canfunction as a dual TMP/alkyl base in the synthesis of thecomplex [{PhC(dO)N(iPr)2} 3Li{2-[1-C(dO)N(iPr)2]C6H4}-{Me2NCH2CH2N(Me)CH2}Al(iBu)2] (1) (see Figure 1) thatcombines ortho-alumination of N,N-diisopropylbenzamidewith methyl-alumination of TMEDA (N,N,N0,N0-tetra-methylethylenediamine, Me2NCH2CH2NMe2).

7 As pre-viously noted by Clayden,8 “direct deprotonation R tonitrogen is usually impossible to achieve (except for superba-sic reagents) unless the lone pair of the nitrogen atom isinvolved in conjugation with a carbonyl group or delocalizedaround an aromatic ring.” This general process, direct R-metalation of tertiary amines, is a desirable synthetic route topolar heteroatom-containing organometallics but is ham-pered due, as previously mentioned, to the destabilization ofthe developing carbanions by the repulsionwith the lone pairelectron density of the adjacent nitrogen atom.9 Usually,multistep procedures such as BF3-activation of amines,10

transmetalation (in particular, tin/lithium exchange),11 orC-S/C-Te bond breakages12 are necessary to access theseuseful synthetic reagents, but generally these provide onlymoderate yields. Only a few examples exist for which directdeprotonation of otherwise nonfunctionalized amines hasbeen demonstrated. Although TMEDA’s primary role inorganometallic chemistry13 is as a bidentate N donor ligand,it does, on prolonged exposure to certain organoalkalireagents, undergo different direct R-metalation reactions,

the regioselectivity of which depends on the identity ofthe deprotonating agent. Thus, by means of tBuLi14a ornBuLi,14b,c lithiation of TMEDA in the terminal methylgroup is achieved, while its methylenemetalation is observedwhen the superbase LICKOR is used,14a followed by elim-ination of potassium dimethylamide. Only a select fewmethods have been proven to be effective in direct deproto-nation of other nonfunctionalized amines. To date,these include deprotonation of: (i) monodentate amines asN-methylpiperidine (metalated at a CH3 position by theLochmann-Schlosser base),15 (ii) bidentate amines as1,3-dimethyl-1,3-diazacyclohexane (deprotonated at thedisfavored position between the two nitrogen atoms withtBuLi)16a,b and N,N,N0,N0-tetramethylcyclohexane-1,2-dia-mine ((R,R)-TMCDA, lithiated at a terminal CH3 group bytBuLi),17 (iii) tridentate amines as N,N0,N00-trimethyl-1,4,7-triazacyclononane18 (lithiated at a CH3 position by nBuLi),MeN[(CH2)2NMe2]2 (N,N,N0,N00,N00-pentamethyldiethyle-netriamine, PMDETA, lithiated at a terminal or centralCH3 group depending on the concentration of the alkyllithium reagent),19 and 1,3,5-trimethyl-1,3,5-triazacyclohex-ane (TMTAC, deprotonated at a methylenic (CH2) groupbetween the two nitrogen atoms with tBuLi),16a,b and (iv)tetradentate amines as bis(3-methyl-1,3-diazacyclohex-1-yl)methane and its five-ring analogue (regioselectively de-protonated at both endocyclic NCH2N units by tBuLi).16c

In this work, we present the regioselective direct lithium-mediated methyl-alumination of TMEDA by two differentLiTMP-aluminates. Thus, iBu3Al(TMP)Li is capable ofmetalating two molecules of TMEDA in a one-pot reaction,establishing, as previously described in a communication byus,7 that this TMP-aluminate can function as a dual TMP/alkyl base, while moving to the dialkyl-diamido aluminateiBu2Al(TMP)2Li only one molecule of TMEDA could bemetalated. Significantly, this dialkyl-diamido aluminateiBu2Al(TMP)2Li can also undergo lithium-mediated meth-yl-alumination of a terminal CH3-group of PMDETA.

Results and Discussion

As we have previously described, mixing LiTMP, iBu3Al,TMEDA, andN,N-diisopropylbenzamide in bulk hydrocar-bon solvent generates the heterobimetallic trianioniccomplex [{PhC(dO)N(iPr)2} 3Li{2-[1-C(dO)N(iPr)2]C6H4}-{Me2NCH2CH2N(Me)CH2}Al(iBu)2] (1) (see Figure 1). At-tempting to shed more light on this unexpected R-alumina-tion of TMEDA, the same reaction methodology wasemployed but avoiding the addition of the aromatic amide.

Figure 1. Structure of [{PhC(dO)N(iPr)2} 3Li{2-[1-C(dO)N-(iPr)2]C6H4}{Me2NCH2CH2N(Me)CH2}Al(iBu)2] (1).

(7) Garcia-Alvarez, J.; Graham, D. V.; Kennedy, A. R.; Mulvey, R.E.; Weatherstone, S. Chem. Commun. 2006, 3208.(8) Clayden, J. In Organolithiums: Selectivity for Synthesis, Tetrahe-

dron Organic Chemistry Series, 2002, 23, p 14.(9) (a) Boche,G.; Lohrenz, J. C.W.;Opel,A.; Sapse,A.-M.; Schleyer,

P. v. R. inLithiumChemistry; JohnWiley & Sons: NewYork, 1995, p. 195.(b) Bordwell, N. R.; Vanler, R.; Zhang, X. J. Am. Chem. Soc. 1991, 113,9856. (c) Schlosser, M., Ed.Organometallics in Synthesis;AManual 2nded.; Wiley: New York, 2002.(10) Kessar, S. V.; Singh, P. Chem. Rev. 1997, 97, 721.(11) (a) Seyferth, D.; Weiner, M. A. J. Org. Chem. 1959, 24, 1395.

(b) Peterson, D. J. J. Organomet. Chem. 1970, 21, 63. (c) Peterson, D. J. J.Am.Chem.Soc. 1971, 93, 4027. (d) Tian,X.;Woski,M.; Lustig, C.; Pape, T.;Fr€ohlich, R.; Le Van, D.; Bergander, K.; Mitzel, N. W. Organometallics2005, 24, 82. (e) Tian, X.; Fr€ohlich, R.; Pape, T.; Mitzel, N. W. Organome-tallics 2005, 24, 5294. (f) Tian, X.; Fr€ohlich, R.;Mitzel, N.W.Dalton Trans.2005, 380.(12) (a) Gawley, R. E.; Zhang, Q. J. Org. Chem. 1995, 60, 5763.

(b) Quintard, J.-P.; Elissondo, B.; Jousseaume, B. Synthesis 1984, 495. (c)Trost, B. M., Fleming, I., Eds. in Comprehensive Organic Synthesis;Pergamon Press: New York, 1991. (d) Strohmann, C.; Abele, B. C. Angew.Chem., Int. Ed. Engl. 1996, 35, 2378.(13) For an enlightening discussion on the role of TMEDA in group 1

chemistry, see: (a) Collum, D. B. Acc. Chem. Rev. 1992, 25, 448. See also:(b) Langer, A.W.Polyamine-ChelateAlkaliMetal Compounds; Advances inChemistry Series; American Chemical Society: Washington, DC, 1974; p 130.(14) (a) K€ohler, F. H.; Hertkorn,N.; Bl€umel, J.Chem. Ber. 1987, 120,

2081. (b) Harder, S.; Lutz, M. Organometallics 1994, 13, 5173.(c) Hildebrand, A.; Lonnecke, P.; Silaghi-Dumitrescu, L.; Silaghi-Dumitrescu,I.; Hey-Hawkins, E.Dalton Trans. 2006, 967. (d) Gessner, V. H.; Strohmann,C. J. Am. Chem. Soc. 2008, 130, 14412.

(15) (a) Schlosser, M.; Hartmann, J. Angew. Chem. 1973, 85, 544.Angew. Chem., Int. Ed. 1973, 12, 508. (b) Bauer, W.; Lochmann, L. J. Am.Chem. Soc. 1992, 114, 7482.

(16) (a) Bojer, D.; Kamps, I.; Tian, X.; Hepp, A.; Pape, T.; Fr€ohlich,R.;Mitzel, N.W.Angew. Chem. 2007, 119, 4254. Angew. Chem., Int. Ed.2007, 46, 4176. (b) See also: K€ohn, R. D.; Seifert, G.; Kociok-K€ohn, G.Chem. Ber. 1996, 129, 1327. (c) Kamps, I.; Mix, A.; Berger, R. J. F.;Neumann, B.; Stammler, H.-G.; Mitzel, N. W. Chem. Commun. 2009, DOI:10.1039/b909561f.

(17) (a) Strohmann, C.;Gessner, V.H. Angew.Chem. 2007, 119, 8429(b) Angew. Chem., Int. Ed. 2007, 46, 8281.

(18) Arnold, J.; Knapp, V.; Schmidt, J. A. R.; Shafir, A. J. Chem.Soc., Dalton Trans. 2002, 3273.

(19) (a) Schakel, M.; Aarnts,M. P.; Klumpp, G.W.Recl. Trav. Chim.Pays-Bas 1990, 109, 305. (b) Klumpp, G. W.; Luitjes, H.; Schakel, M.; deKanter, E. J. J.; Schmitz,R. F.; vanEikemaHommes,N. J.R.C.Angew.Chem.1992, 104, 624. Angew. Chem., Int. Ed. Engl. 1992, 31, 633.(c) Luitjes, H.; Schakel; Aarnts, M. P.; Schmitz, R. F.; de Kanter, E. J. J.;Klumpp,G.W.Tetrahedron 1997, 53, 9977. (d)Gessner, V.H.; Strohmann,C.Angew. Chem. 2007, 119, 4650; Angew. Chem., Int. Ed. 2007, 46, 4566.

6464 Organometallics, Vol. 28, No. 22, 2009 Conway et al.

nBuLi was converted to LiTMP by initial addition of theamine TMP(H) followed by TMEDA (or vice versa), andthe reaction (see Scheme 1) was completed by addition ofthe trialkylaluminium (iBu3Al) reagent. Surprisingly, thecrystalline product obtained [Li{Me2NCH2CH2N(Me)-CH2}2Al(iBu)2] (2) contains two molecules of methyl-alumi-nated TMEDA,which bridge bothmetals in a contacted ion-pair type molecular structure (Figure 2). Moreover, as 2 wasstill obtained even when a deficiency of TMEDA wasemployed (in the 1:1 stoichiometric reaction), it can beinferred that the putative intermediate [Li{Me2NCH2CH2-N(Me)CH2}(

iBu)Al(iBu)2] or [Li{Me2NCH2CH2N(Me)-CH2}(TMP)Al(iBu)2] (products of amido and alkyl basicity,respectively) is more reactive than the initial lithium TMP-aluminate “iBu3Al(TMP)Li”. Thus, the reaction was there-fore repeated in line with the stoichiometry found in 2 byadding an extra molar equivalent of TMEDA and thiscaused an increase in the yield of 2 (from 15 to 48%, seeScheme 1). This result establishes that TMP-aluminates canfunction as dual alkyl/amido bases7 even with relativelynonacidic (N)C-H bonds as ultimately one methyl groupfrom each of two molecules of TMEDA are deprotonated inthis one-pot reaction. It is important to note that on its owniBu3Al is not a strong enough base to metalate TMEDA, soboth deprotonations of this reaction are synergic in origin, asthe contacted Li appears to activate the Al-attached TMPand one of the iBu groups of the base. The molecularstructure of 2 (see Figure 2), belonging to the space group

P21/c, can be considered a contacted ion pair, with both Natoms of the metalated junction of TMEDA chiral as each isattached to four different substituents, although overall thecrystals are racemic. The anionic moiety comprises a dis-torted tetrahedral Al center (range of C-Al-C bond angles101.78(9)-117.29(10)�) made up of four C atoms, from twoterminal iBu ligands (with the shortest Al-C bonds(Al(1)-C(13) 2.017(2) A, Al(1)-C(17) 2.013(2) A]) andtwo R-deprotonated TMEDA molecules (with the longestAl-C bonds (Al(1)-C(1) 2.032(2) A, Al(1)-C(7) 2.029(2)A), in the same range as the Al-C bond length found in themetalated TMEDA units of 1 (2.0327(17) A)).7 Contact tothe cationic moiety is through the two N atoms from themetalated junction of the TMEDAmolecules, which bind toLi to complete a distorted tetrahedral coordination sphere ofN atoms (range of N-Li-N bond angles 87.35(13)-128.38-(17)�), the remaining two of which belong to the unmetalatedend of TMEDA. The Li-N bonds involving the metalatedpart of TMEDA (Li(1)-N(1) 2.060(3)A, Li(1)-N(3)2.045(3) A)) are significantly shorter than the Li-N bondsin the unmetalated part (Li(1)-N(2) 2.198(3) A, Li(1)-N(4)2.182(4) A). Overall, the Al and Li centers are held togetherin an irregularly shaped 12-membered LiNCCN-CAlCNCCN tricyclic ring, containing a smaller six-mem-bered AlNCLiNC ring, which, surprisingly, does not havethe expected chair conformation but displays a boat-shapeconformation. This probably occurs because this six-mem-bered ring is also attached to another two five-atom rings(LiNCCN) to build together the larger dodecatricyclic ring.1H, 13C, and 7Li NMR spectra recorded in benzene-d6solution were obtained for 2with only one set of resonances,supporting the diastereoselective formation of this com-pound (note that two stereogenic centers are generated,namely bothNatoms of themetalated junction ofTMEDA).All the anticipated different types of hydrogen atoms in 2 areaccounted for and well-resolved in its 1H NMR spectrum,the most significant features being the two doublet signals(AB spin system, JHA, HB = 12.9 Hz) at 1.15 and 2.02 ppmfor both protons of the metalated NCH2 unit of TMEDA.The Me2N and Me0N resonances appear at 1.63 and 2.27ppm, respectively, while all four H atoms of the two uniqueCH2 groups are inequivalent, having separate resonances at1.53, 1.80, 2.21, and 2.53 ppm, consistent with the rigid,tricyclic conformation of the structure. Similarly, the 13CNMR spectrum of 2 is easily fully assignable (see Experi-mental Section for details). The reaction filtrate that re-mained after isolating the crystalline product 2 was alsoprobed by NMR techniques. The 1H NMR spectrum in-dicated that the oily filtrate appeared to contain the afore-mentioned signals for 2 and free TMP(H) (consistent withthe amido basicity of 2) but accompanied with a complicatedmixture of products in minor yield.As we have previously demonstrated, deprotonations of

both TMEDA molecules in 2 are synergic in origin, as thecontacted Li appears to activate the Al-attached TMP andiBu bases. To probe whether the remaining terminal two iBuligands in 2 were able to bring about a metalation of a third(or even fourth) TMEDAmolecule, we decided to repeat thesynthetic process for 2 (see Scheme 1) but adding an extraexcess (3 or 4mol equiv) of TMEDA.However, this made nodifference as we always recover from these reactions[Li{Me2NCH2CH2N(Me)CH2}2Al(iBu)2] (2) as a crystallinesolid in similar yields to those found when the reactionis carried out with 2 equiv of TMEDA. Furthermore, the

Figure 2. Molecular structure of [Li{Me2NCH2CH2N(Me)-CH2}2Al(iBu)2] (2) with selective atom labeling. Hydrogenatoms have been omitted for clarity. Selected bond lengths (A)and angles (deg): Al(1)-C(1) 2.032(2), Al(1)-C(7) 2.029(2),Al(1)-C(13) 2.017(2), Al(1)-C(17) 2.013(2), Li(1)-N(1)2.060(3), Li(1)-N(2) 2.198(3), Li(1)-N(3) 2.045(3), Li(1)-N(4)2.182(4); C(1)-Al(1)-C(7) 103.83(9), C(1)-Al(1)-C(13)109.89(9), C(1)-Al(1)-C(17) 117.29(10), C(7)-Al(1)-C(13)113.91(10), C(7)-Al(1)-C(17) 101.78(9), C(13)-Al(1)-C(17)109.92(10), N(1)-Li(1)-N(2) 87.85(13), N(1)-Li(1)-N(3)113.48(16), N(1)-Li(1)-N(4) 127.99(17), N(2)-Li(1)-N(3)128.38(17), N(2)-Li(1)-N(4) 116.61(16), N(3)-Li(1)-N(4)87.35(13).

Scheme 1. Lithium-Mediated Alumination of Two Molecules of

TMEDA

Article Organometallics, Vol. 28, No. 22, 2009 6465

1H NMR analysis of the filtrate revealed the presence ofnonmetalated TMEDA and free TMP(H), showing that thetwo remaining iBu groups in 2 fail to metalate any additionalmolecule of TMEDA or even the more acidic amine TMP-(H). This lack of basicity could be related to the fact that incompound 2, lithium is coordinatively saturated and there-fore it cannot bind to another molecule of TMEDA, whichwe believe should be the first step to induce an intramolecularR-deprotonation (vide infra, Scheme 3). This was supportedby our findings when we investigated the deprotonation ofTMEDA using an alternative solvent-separated ion-pairlithium aluminate. Thus, we prepared iBu3Al(TMP)Li inTHF solution following literature procedures,1 whichwas proposed to contain solvent-separated ion pairs ofgeneral formula [{Li(THF)x}

þ{Al(TMP)n(iBu)4-n}

-]5 thenadded an equimolar amount of TMEDA to the reactionmixture. However, this procedure gave only a white oilyproduct. Its 1H NMR spectrum indicates that the oil con-tains unmetalated TMEDA. Thus, this observation seems torule out the use of separated ion pairs of general formula[{Li(THF)x}

þ{Al(TMP)n(iBu)4-n}

-] to achieve metalationof TMEDA, as the aforementioned close proximity betweenthe substrate TMEDA and the Al(TMP)n(

iBu)4-n fragment,thought necessary to promote the intramolecular deproto-nation, is no longer present.The success in the metalation of TMEDAobserved in 2 by

using iBu3Al(TMP)Li prompted us to investigate its amideenriched congener, that is, iBu2Al(TMP)2Li (previously pro-posed as a component in the dismutation process of iBu3Al-(TMP)Li in pure THF),5 obtained by mixing LiTMP withiBu2Al(TMP) (3). Organoaluminum amides (R2AlNR0

2)20

can be easily prepared from the metathesis reaction ofthe corresponding lithium amide and the dialkylaluminumhalide. For iBu2Al(TMP) 3, even though it was pre-viously found to be highly effective for the Fischer indolesynthesis21a and asymmetrization of meso-cyclic ketones,21b

to the best of our knowledge, no NMR spectroscopic datafor it have been reported. We report such data here. Inparticular, the 1H NMR spectrum is very informative,showing the presence of two well-defined sets of signals,one for the iBu groups (0.17 (4H, d, CH2-Al iBu), 1.01 (12H,d, CH3-

iBu), and 1.94 (2H, sept, CH-iBu) ppm) and the otherfor the amide TMP (1.29 (12H, s, CH3 of TMP), 1.31 (4H,m,β-TMP), 1.72 (2H,m, γ-TMP) ppm), in a 2:1 ratio. By simplymixing together a hexane solution of freshly preparedLiTMP with the diisobutylaluminium amide iBu2Al(TMP)before a molar equivalent of the Lewis base (TMEDA)was introduced, we obtained the heterobimetallic-

heterotrianionic complex [Li{Me2NCH2CH2N(Me)CH2}-(TMP)Al(iBu)2] (4) in an isolated crystalline yield of 55%.1H, 13C, and 7Li NMR spectra recorded in cyclohexane-d12solution were obtained for 4. The 1HNMR spectrum clearlyshows the expected pattern formethyl-aluminated TMEDA,mimicking that previously described for 2 (NCH2Al 1.39 and1.58 (1H each, AB spin system, d, JHA, HB = 14.0 Hz) ppm;NCH2 1.96 (2H, m), 2.73 and 2.94 (1H each, m) ppm;N(CH3)2 (2.24, 2.73, and 2.74 (3H each, s) ppm). This 1HNMR spectrum also reveals the presence of two iBu groupsand one TMP ligand (see Supporting Information). Thereaction filtrate that remained after isolating the crystallineproduct 4 was also probed by NMR techniques. The 1HNMR spectrum indicated that this reaction is almost quan-titative, as the oily filtrate appears to contain only theaforementioned signals for 4 and free TMP(H). Thus, to-ward TMEDA, the lithium dialkyldiamidoaluminateiBu2Al(TMP)2Li acts as an amido base, exhibiting onlysingle-step AMMAl (alkali-metal-mediated alumination)reactivity in contrast to the dual amido/alkyl basicity ofthe trialkylamidoaluminate iBu3Al(TMP)Li. It should benoted that, as aforementioned for 2 with iBu3Al, the orga-noaluminium amide iBu2Al(TMP) (3) is not a strong enoughbase to metalate TMEDA, so the deprotonation observed inthis reaction is also synergic in origin, as the contacted Liappears to activate one of the Al-attached TMP ligands. Themolecular structure of 4 (Figure 3) features a five-atom, four-element LiNCAlN ring, with a mixed NCH2-TMP bridgingligand set, and is completed by two terminal iBu ligands onAl and a chelating metalated TMEDA (N,N-attached) to Li.Overall, the Al and Li centers are held together in anirregularly shaped octa-LiNCCNCAlN bycyclic ring, witha Li-N hinge. The Al center displays a distorted tetrahedralgeometry (subtending bond angles from 99.57(6)�{N(3)-Al(1)-C(12)} to 121.47(6)� {N(3)-Al(1)-C(30)})made up of three C atoms, from two terminal iBu ligands(with the shortest Al-C bonds {Al(1)-C(40) 2.0319(17) A,Al(1)-C(30) 2.0340(16) A}) and one deprotonated TMEDAmolecule (with the longest Al-C bonds {Al(1)-C(12)2.0658(17) A}), as previously reported for 2, with the fourth

Figure 3. Molecular structure of [Li{Me2NCH2CH2N(Me)-CH2}(TMP)Al(iBu)2] (4) with selective atom labeling. Hydro-gen atoms have been omitted for clarity. Selected bond lengths(A) and angles (deg): Al(1)-N(3) 1.9869(13), Al(1)-C(12)2.0658(17), Al(1)-C(30) 2.0340(16), Al(1)-C(40) 2.0319(17),Li(1)-N(1) 2.000(3), Li(1)-N(2) 2.086(3), Li(1)-N(3) 2.035(3);N(3)-Al(1)-C(12) 99.57(6), N(3)-Al(1)-C(30) 121.47(7),N(3)-Al(1)-C(40) 115.57(6), C(12)-Al(1)-C(30) 106.57(7),C(12)-Al(1)-C(40) 110.56(7), C(30)-Al(1)-C(40) 102.71(7),N(1)-Li(1)-N(2) 88.58(12), N(1)-Li(1)-N(3) 114.87(14),N(2)-Li(1)-N(3) 151.46(15).

Scheme 2. Lithium-Mediated Alumination of One Molecule of

TMEDA by a Dialkyldiamidoaluminate Base

(20) (a) Mole, T.; Jeffrey, E. A. In Organoaluminium Compounds;Elsevier: Amsterdam, 1972. (b) Ooi, T.; Maruoka, K. Sci. Synth.s 2004, 7,225.(21) (a) Naruse, Y.; Yamamoto, H. Tetrahedron 1988, 44, 6021.

(b) Maruoka, K.; Oishi, M.; Yamamoto, H. J. Org. Chem. 1993, 58, 7638.

6466 Organometallics, Vol. 28, No. 22, 2009 Conway et al.

atom the N of the amido TMP (Al(1)-N(3) 1.9869(13) A),with a bond length in the same range as the Al-N distancesfound in other structures of general formula (donor 3Li(TMP)Al(iBu)3).

1b,6 The wide Al(1)-C(12)-N(1) bondangle at 116.86(10)� is marginally less than those in 2 (Al(1)-C(1)-N(1), 118.28(13)�; Al(1)-C(7)-N(3), 120.06(13)�) andthese fall in a similar range to those reported in theclosely related terminally dialuminated TMEDA comp-lexes R2AlCH2N(Me)CH2CH2N(Me)CH2AlR2 (R = Me,113.9(1)�; R = tBu, 116.3(2)/116.2(2)�), which in contrast to2 and 4were prepared by a conventional metathesis approachfrom the corresponding dilithium complex.11e Note that aneven wider Al-C(H)2-N bond angle (121.5(2)�) is found in(tBu2AlCH2N

iPr2 3LiCl)2.11f Contact from aluminum to the

lithium atom in 4 is through one amido (TMP) nitrogen atom(Li(1)-N(3) 2.035(3) A) and one nitrogen of the metalatedjunction of TMEDA (Li(1)-N(1) 2.000(3) A). The remainingnitrogen atom of TMEDA also binds to Li, displaying thelongest Li-N bond distance (Li(1)-N(2) 2.086(3) A) tocomplete a distorted trigonal coordination sphere of N atoms(subtendingbond angles from88.58(12)� {N(1)-Li(1)-N(2)}to 151.46(15)� {N(2)-Li(1)-N(3)}).After the successful isolation of the monometalated TME-

DA compound 4, which established that the lithium dialkyl-diamidoaluminate iBu2Al(TMP)2Li acts as only a single-stepamido base, we investigated whether the remaining amidobridging ligand or the terminal iBu groups were able to bringabout a metalation of a second TMEDAmolecule as seen incompound 2. Thus, we repeated the aforementioned synth-esis of 4 but adding an extra molar equivalent of TMEDA.However, this led only to recovery of [Li{Me2NCH2CH2-N(Me)CH2}(TMP)Al(iBu)2] (4) as a crystalline solidwith theexcess of unmetalated TMEDA and free TMP(H) observedin the filtrate, even when more drastic reaction conditionswere employed.22 This observation apparently rules out 4 asan intermediate in the formation of 2 and suggests that thepresence of an additional bridging TMP ligand in 4 inhibitsthe metalation of a second molar equivalent of TMEDA.A reaction sequence can be proposed (see Scheme 3) to

rationalize the formation of 2. While the structure of thestarting complex is as yet not known, the sodium congenerhas been found to adopt this structure and it is likely given

the smaller size of lithium, that in the lithium case TMEDAwill be in closer proximity to the TMP bridge to facilitate asubsequent intramolecular Me deprotonation. In the firststep, TMEDA is intramolecularlyR-aluminated by activatedTMP, with concomitant elimination of TMPH to yield theintermediate A (not isolated or detected), which is morereactive than the initial lithium TMP-aluminate “iBu3Al-(TMP)Li” as 2 was still obtained even when a deficiency ofTMEDA was employed. This putative intermediate canreact now in two different ways: (i) With an additionalequivalent of TMEDA (pathway a), where the chelatingdiamine could coordinate to lithium, because the Li 3 3 3

iBucontact inA can be expected to be relatively weak,23 allowingthe formation of intermediate24 B (not isolated or detected).In a final step, one of the neighboring iBu groups attached toAl intramoleculary deprotonates one NMe2 arm of theTMEDA, to close a six-member {LiNCAlCN} ring, afford-ing compound 2 and releasing iBuH. (ii) Alternatively, Acould reactwith the concomitant TMP(H) formed during themetalation of the first equivalent of TMEDA (pathway b),affording compound 4. In this compound, the TMP ligandforms a strong bridge between Li andAl, which is retained insolution as evidenced by the inequivalence of the fourmethylgroups of the TMP ligand in the 1H and 13C{1H} spectra (seeExperimental Section). As above-mentioned, our reactivitystudies show that 4 (prepared by reacting iBu2Al(TMP)2Liwith 1 equiv of TMEDA) fails tometalate TMEDA.This cannowbe rationalized in terms of the coordination sphere of Li,which forms three strong Li-N bonds (Li-N bond dis-tances: 2.000(3), 2.086(3), and 2. 0340(16) A), making thealkali-metal coordinatively and electronically saturated andleaving no place for the coordination of an additionalmolecule of TMEDA. Because this precoordination seemsto be crucial for the deprotonation to take place, the forma-tion of 2 must occur through the alternative reaction path-way a. The stronger Lewis basicity of TMEDAversus that of

Scheme 3. Proposed Stepwise Reaction Pathway for the Formation of 2

(22) Long reaction times at room temperature (48 h) or refluxingconditions (hexane or toluene) only yield 4 as the major product.

(23) NMR studies on the related [(THF) 3Li(TMP)Al(iBu3)] showedthat in solution the three isobutyl groups appear equivalent, suggestingthat these Li 3 3 3C bonds are forming/breaking fast in the NMR timescale, see ref 1b.

(24) A similar open contacted ion-pair motif with a tetracoordinatedlithium bridged to aluminium by an ortho-metalated benzamide incompound [(THF)3 3Li{O(dC)N(iPr)2(C6H4)}Al(iBu)3] has been pre-viously reported by us in ref 5.

Article Organometallics, Vol. 28, No. 22, 2009 6467

bulky TMP(H) and also its bidentate nature may be impor-tant factors in favoring this reaction pathway.After the success in theR-metalation ofTMEDAbymeans

of lithium(TMP)aluminates, we decided to extend this chem-istry to the tridentate ligand PMDETA, which is commonlyused as a coordinating additive to break up organolithiumaggregates and thus to aid solubility and reaction kinetics.25

As previouslymentioned, PMDETA is lithiated at a terminalor central CH3 group depending on the concentration of thealkyllithium compound.19,26 Thus, attempting to achieve R-alumination of PMDETA, the same reaction methodologyused in the synthesis of 2 was employed but substitutingTMEDA by PMDETA. However, this procedure gave onlya white oily product. In this case, a 1H NMR spectrumindicates that the oil appeared to contain metalated PMDE-TA but accompanied with a complicated mixture of pro-ducts. As no useful structural information could be gleanedfrom this oil, we decided to utilize the lithium dialkyldiami-doaluminate iBu2Al(TMP)2Li in anticipation of a directedR-alumination of PMDETA (see Scheme 4). This was indeedrealized as the solid product of the reaction was the hetero-bimetallic-heterotrianionic complex [Li{Me2NCH2CH2-N(Me)CH2CH2N(Me)CH2}(TMP)Al(iBu)2] (5) obtained ina crystalline yield of 47%.

1H, 13C, and 7Li NMR spectra recorded in cyclohexane-d12 solution were obtained for 5 with only one set ofresonances, supporting the diastereoselective formation ofthis compound (note that two stereogenic centers are gener-ated, namely the two N atoms of the metalated junction ofPMDETA). The 1H NMR spectrum clearly shows theexpected pattern for methyl-aluminated PMDETA, similarto that for TMEDA previously described for 2 and 4

(NCH2Al 1.07 and 1.82 (d, AB spin system, JHA, HB =13.4 Hz, ppm; NCH2 1.98, 2.12, and 2.95 (1H, m), 2.42 (3H,m), and 2.73 (2H, m) ppm; N(CH3)2 2.22 (6H, s), 2.34, and2.37 (3H each, s) ppm). This 1H NMR spectrum also showsthe presence of two iBu groups and one TMP ligand in 5 (seeSupporting Information). Thus, again, the lithium dialkyl-diamidoaluminate iBu2Al(TMP)2Li acts as only a single-stepamido base toward PMDETA. The reaction filtrate thatremained after isolating the crystalline product 5 was alsochecked by NMR spectroscopy. A 1H NMR spectrumindicates that this reaction is almost quantitative as the oily

filtrate appears to contain only the aforementioned reso-nances for 5 and free TMP(H). Again, we decided to check ifthe remaining amido bridging ligand or the terminal iBugroups were able to bring about a second metalation ofanother methyl group of PMDETA. Therefore, we repeatedthe aforementioned synthesis of 5 but employedmore drasticreaction conditions (refluxing themixture in toluene solutionfor 2 h). However, even by using 2 equiv of PMDETA, wealways recovered only [Li{Me2NCH2CH2N(Me)CH2-CH2N(Me)CH2}(TMP)Al(iBu)2] (5) as a crystalline materi-al. It should be mentioned that, as aforementioned for 4,the organoaluminium amide iBu2Al(TMP) (3) is not apowerful enough base to metalate PMDETA, so thedeprotonation observed in this reaction is also synergic inorigin, as the contacted Li appears to activate one of the Al-attachedTMP ligands.A low resolution crystal structure of 5confirmed its previously discussed components and theirconnectivity, featuring an undeca LiNCCNCCNCAlN tri-cyclic ring, containing two five-atom, three-elementLiNCCN rings and a five-atom, four-element LiNCAlNring, with a mixed NCH2-TMP bridging ligand set com-pleted by two terminal iBu ligands on Al and a chelatingmetalated PMDETA (N,N,N-attached) to Li, in agreementwith findings from the aforementioned NMR spectroscopicstudies (Figure 4).In conclusion, this work establishes that lithium TMP

aluminates can function as dual TMP/alkyl (for iBu3Al-(TMP)Li) or as mono amido bases (for iBu2Al(TMP)2Li)exhibiting two-fold or single-fold AMMAl (alkali-metal-mediated alumination), respectively, toward TMEDA orPMDETA. On their own, iBu3Al or iBu2Al(TMP) are notsufficiently strong bases to metalate TMEDA or PMDETA,so in 2, 4, and 5, the R-deprotonations of TMEDA andPMDETA are synergic based, as the intramolecular com-munication between Li and Al appears to activate the TMPand iBu bases. Finally, it further establishes that by usingAMMAl the normal patterns of reactivity can be reversedwith aTMEDAnonacidic C-Hbond breaking in preferenceto an acidic TMP(H) N-H bond. This special behavior canbe attributed to intramolecular proximity effects between theactive base component (TMP or iBu) and the ligatingTMEDA molecule. It remains to be established whetherthese bimetallic alumination traps can be utilized to depro-tonate other donor ligand molecules usually resistant toconventional organometallic bases.

Experimental Section

General Procedures. All reactions were carried out under aprotective argon atmosphere using standard Schlenk techni-ques. Hexane was dried by heating to reflux over sodiumbenzophenone ketyl and distilled under nitrogen prior to use.nBuLi, iBu3Al, and iBu2AlCl were purchased from AldrichChemicals and used as received. NMR spectra were recorded

Scheme 4. Lithium-Mediated Alumination of One Molecule of

PMDETA by a Dialkyldiamidoaluminate Base

Figure 4. Structure of [Li{Me2NCH2CH2N(Me)CH2CH2N(Me)-CH2}(TMP)Al(iBu)2] (5).

(25) See for example: (a) Sch€umann, U.; Kopf, J.; Weiss, E. Angew.Chem. 1985, 97, 222. Angew. Chem., Int. Ed. Engl. 1985, 24, 215.(b) Engelhardt, L. M.; Leung, W.-P.; Raston, C. L.; Salem, G.; Twiss, P.;White, A. W. J. Chem. Soc., Dalton Trans. 1988, 2403. (c) Karsch, H. H.;Zellner, K.; Mikulcik, P.; Lachmann, J.; M€uller, G.Organometallics 1990, 9,190. (d) Lappert, M. F.; Engelhardt, L. M.; Raston, C. L.; White, A. H.J. Chem. Soc., Chem. Commun. 1982, 1323.(26) Polar metalation of PMDETA has been described in: (a) Eller-

man, J.; Sch€utz,M.;Heinemann, F.W.;Moll,M.; Bauer,W.Chem. Ber.1997, 130, 141. (b) Izod, K.; Steward, J. C.; Clegg, W.; Harrington, R. H.Dalton Trans. 2007, 257.

6468 Organometallics, Vol. 28, No. 22, 2009 Conway et al.

on a Bruker DPX 400 MHz spectrometer, operating at 400.15MHz for 1H, 150.32 MHz for 7Li, and 100.62 MHz for 13C.X-ray Crystallography. Crystal data for 2: C20H48AlLiN4,

M= 378.54, monoclinic, P21/c, a= 9.2581(3), b= 15.4470(5),c= 18.2432(5) A, β = 99.457(2)�, V = 2573.50(14) A3, Z = 4,T = 223 K. Data were collected on a Nonius Kappa CCDdiffractometer with Mo KR radiation (λ = 0.71073 A), 31196reflections collected, 5063 were unique, Rint 0.053. Final refine-ment27 to convergence was on F2, R = 0.0501 (F, 3351 obsddata only) and Rw = 0.1291, F2 all data), GOF = 1.013, 293refined parameters, residual electron density max and min0.294 and -0.224 e A-3. Crystal data for 4: C23H51AlLiN3,M=403.59, monoclinic, P21/n, a=10.3165(4), b=15.3891(6),c=17.0248(7) A, β=105.505(4)�,V=2604.52(18) A3,Z=4,T = 123 K. Data were collected on an Oxford DiffractionGemini S diffractometer with Mo KR radiation (λ = 0.71073A), 23211 reflections collected, 5742 were unique, Rint 0.0441,R = 0.0457 (F, 4043 obsd data only), and Rw = 0.1136, F2 alldata), GOF = 0.999, 287 refined parameters, residual electrondensity max and min 0.337 and -0.242 e A-3.Synthesis of [Li{Me2NCH2CH2N(Me)CH2}2Al(

iBu)2] (2). Ina Schlenk tube, 4 mmol of TMEDA (0.60 mL) were added to ahexane solution of LiTMP (prepared freshly from a mixture ofnBuLi (2 mmol, 1.25 mL of a 1.6 M solution in hexane) andTMPH (2 mmol, 0.34 mL)) to give a yellow solution. After thesolution had been stirred for 30min, iBu3Al (2mmol, 2mLof a 1M solution in hexane) was introduced and the mixture changedfrom a yellow to a slightly cloudy yellow solution. This mixturewas heated gently to form a yellow solution, which was allowedto cool to ambient temperature. Freezer cooling of this solutionat -27 �C afforded colorless crystals of 2 (0.37 g, 48%). Notethat adding only 1mol equiv of TMEDAalso produced 2, but ina smaller yield. 1HNMR (400.13MHz, benzene-d6, 300K): 0.43(4H,m, CH2-Al iBu), 1.15 and 2.02 (2H each, d, AB spin system,JHA, HB = 12.9 Hz), Al-CH2N of TMEDA), 1.46 and 1.48 (6Heach, d, 3JHH=6.8 Hz, CH3-

iBu), 1.53, 1.80, 2.21, and 2.53 (2Heach,m, 4CH2-TMEDA), 1.63 (12H, broad s, 4CH3-TMEDA),2.27 (6H, s, 2 Al-CH2N(CH3) of TMEDA), 2.45 (2H, sept, 3JHH

= 6.8 Hz, CH-iBu). 13C{H} NMR (100.63 MHz, benzene-d6,300 K): 29.87 (CH of iBu), 30.36 and 30.58 (CH3 of

iBu), 31.10(Al-CH2N TMEDA), 46.87 (Al-CH2N(CH3) TMEDA), 49.92(CH3TMEDA), 58.87 and 61.96 (CH2-TMEDA). Signal forAl-CH2 of

iBu was not observed. 7Li NMR (155.50MHz, benzene-d6, 300 K, reference LiCl in D2O at 0.00 ppm): -0.13.Synthesis of iBu2Al(TMP) (3). Ten mL of hexane were added

to an oven-dried Schlenk tube. Next, 1.25mL (2mmol) of 1.6Msolution of nBuLi were added, followed by 0.34 mL (2 mmol) ofTMP(H) at room temperature. The reaction mixture was left tostir for 30min and then 0.38mL (2mmol) of iBu2AlCl were theninjected into the Schlenk tube, producing a white suspensionalmost immediately. The reaction mixture was left to stir for 45min and was then filtered through celite and glasswool, whichwas then washed with a further 10 mL of hexane. The solventwas then removed in vacuo to yield a clear oil (quantitative yieldby NMR). 1H NMR (400.13 MHz, cyclohexane-d12, 30 0K):0.17 (4H, d, 3JHH = 6.8 Hz, CH2-Al iBu), 1.01 (12H, d, 3JHH =6.8 Hz, CH3-

iBu), 1.29 (12H, s, CH3 of TMP), 1.31 (4H, m,β-TMP), 1.72 (2H, m, γ-TMP), 1.94 (1H, sept, 3JHH = 6.8 Hz,CH-iBu). 13C{H} NMR (100.63MHz, cyclohexane-d12, 300 K):18.85 (γ-TMP), 25.81 (CH2 of

iBu), 27.68 (CH3 ofiBu), 28.44

(CHof iBu), 32.45 (CH3-TMP), 39.08 (β-TMP), 50.87 (R-TMP).

Synthesis of [Li{Me2NCH2CH2N(Me)CH2}(TMP)Al(iBu)2](4). iBu2Al(TMP) was synthesized as above and added viacanula to a freshly prepared solution of LiTMP in 10 mL ofhexane (from a mixture of nBuLi (2 mmol, 1.25 mL of a 1.6 Msolution in hexane) and TMPH (2 mmol, 0.34 mL) to give anhomogeneous solution. Finally, 0.30 mL (2 mmol) of TMEDAwere injected and the reaction mixture left to stir for 30 minbefore the Schlenk tube was placed in the freezer overnight(-28 �C). A crop (0.44 g, 55%) of colorless crystals formed insolution that were suitable for X-ray crystallographic analysis.1H NMR (400.13 MHz, cyclohexane-d12, 300 K): 0.17 (4H, m,CH2-Al iBu), 0.76 and 0.89 (1H each, m, β-TMP), 0.90, 0.94,0.99, and 1.00 (3H each, d, 3JHH = 6.4 Hz, CH3-

iBu), 1.31 (6H,s, 2 CH3 of TMP), 1.34 and 1.44 (3H each, s, CH3 of TMP), 1.39and 1.58 (1H each, d, AB spin system, JHA, HB = 14.0 Hz,Al-CH2N of TMEDA), 1.69 (2H, m, β-TMP), 1.80 (1H, sept,3JHH=6.4Hz, CH-iBu), 1.96 (5H, m, 2HCH2-TMEDA, 2H γ-TMP, and 1H CH-iBu), 2.24, 2.73, and 2.74 (3H each, s, CH3-TMEDA), 2.73 and 2.94 (1H each, m, CH2-TMEDA). 13C{H}NMR (100.63 MHz, cyclohexane-d12, 300 K): 18.10 (γ-TMP),26.08 (CH of iBu), 27.06, 27.16, 27.45, and 27.67 (CH3 of

iBu),28.68, 28.80, and 29.59 (CH3-TMP), 35.39 and 36.34 (β-TMP),45.34, 46.43, and 48.07 (CH3 TMEDA), 47.39 (Al-CH2NTMEDA), 51.36 and 52.30 (R-TMP), 56.36 and 59.9 (CH2-TMEDA). Signal forAl-CH2 of

iBuwas not observed. 7LiNMR(155.50 MHz, cyclohexane-d12, 300 K, reference LiCl in D2O at0.00 ppm): 1.20.

Synthesis of [Li{Me2NCH2CH2N(Me)CH2CH2N(Me)CH2}-(TMP)Al(iBu)2] (5). Complex 5, isolated as colorless crystals(0.43 g, 47%), was prepared as previously described for 4 butusing theN-donor ligand PMDETA (2 mmol, 0.42 mL) insteadof TMEDA. 1H NMR (400.13 MHz, cyclohexane-d12, 300 K):0.23 (4H,m, CH2-Al iBu), 0.91 and 0.95 (3H each, d, 3JHH=6.4Hz, CH3-

iBu), 0.99 (6H, d, 3JHH = 6.4 Hz, CH3-iBu), 1.07 and

1.82 (1H each, d, AB spin system, JHA, HB= 13.4 Hz, Al-CH2Nof PMDETA), 1.18 and 1.71 (2H each, m, β-TMP), 1.34 and1.37 (6H each, s, CH3 of TMP), 1.71 (m, 2H, γ-TMP), 1.87 (1H,m, CH-iBu), 1.98 (2H, m, 1H of CH2-PMDETA and 1H ofCH-iBu), 2.12 and 2.95 (1H each,m, CH2-PMDETA), 2.27 (6H,CH3-PMDETA), 2.34 and 2.37 (3H each, CH3-PMDETA), 2.42(3H, m, CH2-PMDETA), 2.73 (2H, m, CH2-PMDETA). 13C-{H} NMR (100.63 MHz, cyclohexane-d12, 300 K): 18.32(γ-TMP), 26.08 (CH of iBu), 27.14, 27.22, 27.53, and 27.77(CH3 of iBu), 29.19 and 29.76 (CH3-TMP), 32.58 (b, CH2 ofiBu), 44.35 (2C, β-TMP), 46.38, 48.64, and 51.96 (CH3

PMDETA), 47.28 (Al-CH2N PMDETA), 51.96 (2C, R-TMP),54.10, 57.26, 59.00, and 61.15 (CH2-PMDETA). 7Li NMR(155.50 MHz, cyclohexane-d12, 300 K, reference LiCl in D2Oat 0.00 ppm): 0.43.

Acknowledgment. We thank the EPSRCand theRoyalSociety (International Travel Grant to E.H. and J.G.-A.and a Wolfson merit award to R.E.M.) for sponsoringthis research. In addition, E.H. thanks the Royal Societyfor a University Research Fellowship and J.G-A. thanksthe Ministerio de Ciencia y Educacion (MEC) of Spainand the European Social Fund for the award of a “Juande la Cierva” contract.

Supporting Information Available: NMR spectra for 2, 3, 4,and 5 andCIF file giving crystallographic data for compounds 2and 4. This material is available free of charge via the Internet athttp://pubs.acs.org.

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