Reactions of Electron-Rich Olefins with Proton-Active Compounds

Reactions of Electron-Rich Olefins with Proton-Active Compounds

By Jurgen Hocker and Rudolf Merten[*I

Dedicated to Professor Otto Bayer on the occasion of his 70th birthday

Proton-active substances react with certain electron-rich olefins with cleavage of the central C=C double bond to give compounds that can be formally regarded as insertion products of nucleophilic carbenes. If they satisfy certain structural conditions, they isomerize with p elimination to give open-chain compounds. Both CH-acidic compounds and compounds containing NH or OH groups can undergo this reaction. The mechanisms are discussed, and the importance of the intermediate products to biochemistry (thiamine, tetrahydrofolic acid) is indicated.

1. Introduction

The reactivity of the central C=C double bond in tetra- cyanoethylene (1) is determined by the influence of the four electron-attracting groups. The electron-poor double bond exhibits strongly electrophilic character“.

Olefins (2) with four electron-donating substituents, on the other hand, react as strong nucleophiles. These compounds are referred to as electron-rich ole fin^[**^[^ - ‘I.

NC\ ,CN A\ /A c=c, c=c, NC’ CN B’ B

( 1 ) (2) A, B = NR2, OR, SR. A and B not both SR

The ability of the substituents A and B to supply electrons increases in the order SR < OR < NR,. Whereas tetraaminoethylenes are very reactive, strongly nucleophilic compounds, tetramercaptoethylenes exhibit only weak reactivity toward electrophiles. For example, they dissolve in concentrated sulfuric acid and are reprecipitated un- changed on dilution[81. An extended Hiickel calculation shows that the charge density on the sulfur in (3) is greater than that on the olefinic C atom. Electrophiles are therefore directed to the sulfur and not to the double bond ;the chem- ical behavior of the tetramercaptoethylenes does not agrec with that of an electron-rich olefin.

[*I Dr. J. Hocker and Dr. R. Merten Wissenschaftliches Hauptlaboratorium der Bayer AG 509 Leverkusen-Bayerwerk (Germany)

[**I The term “electron-rich olefins” is sometimes also applied to ketene (thio)acetals or aminals, in which the n-electron density of the C=C double bond is also greater than 1.0 [7].

Electron-rich olefins of the type ( 4 ) or (5) react with electrophiles (E@) preferentially with cleavage of the central C=C double bond. The obvious assumption that the olefins d i ~ s o c i a t e [ ~ ~ into two resonance-stabilized carbenes was not confirmed by crossover experiments[”* ’,].

Ar Ar

A r I

,?J

I Ar

2 @$C-E

I CH,

Nevertheless, electron-rich olefins represent a source of nucleophilic carbenesrll’ ’* l3].

Characteristic reactions of the olefins of types ( 4 ) and (5 ) with electrophiles that proceed via cleavage of the molecule are those with mineral acids to form the azoli(ni)um salts (6) [6 , 1 3 - ‘’I, hydrolysis to form N-formyl compounds (7)[‘* l6], reactions with diazo compounds to form azines (8)[17-201, with azides to form monoaza and/or triaza compounds (9)[12,213221, with iso(thio, se1eno)cyanates to form stable dipoles (10) or (thio, se1eno)hydantoin deriv- atives (11)122-261, and the reaction of (4a) with ketenes, which also gives stable dipoles (l2)[”3 ”, 281.

Reactions of olefins of types (4) and (5) with proton- active compounds are discussed in detail below. The products nearly always contain one “carbene” half of the original ethylene molecule. They are formally products of the insertion[**] of a nucleophilic carbene into an XH bond, possibly with formation of a new C-C, C-N, or C-0 bond in addition to a new CH bond.

[*I The occurrence of free nucleophilic carbenes has not yet been definitely established.

[**I The expression “insertion product” carries no implications regard- ing the mechanism of the reaction.

964 Angew. Chem. internat. Edit. Vol. 11 (1972) 1 No. I 1

R R I I

(;;c=c:;)-

(4), X = N R (5). x = s

9 + R'-N, ,C=N AN=N-l-;-R1

(9) x = 0 1

P

2. Reactions with Formation of a New C-C Bond

2.1. With Aliphatic CH-Acidic Compounds (Table 1)

The first reactions of an electron-rich olefin, i. e. 1,1',3,3'- tetraphenyl-2,2-biimidazolidinylidene (4a) with CH-acidic compounds (13) were described by H.-W Wanzlick. Two types of products can be formed in these reactions:

1. a primary product (14), which may be thought of as resulting from the insertion of a nucleophilic carbene into an activated CH bond, and, in some cases,

2. a further reaction product ( Is j , which is formed from (14) by p elimination.

(4a) does not react with a-picoline and valeronitrile at 180 0C'301.

The rate of the reaction of (4a) with (13j increases with the acidity of the proton-active compound. If a mixture of

Table 1. Reaction of ( 4 a ) with aliphatic CH-acidic compounds.

Yield ( %) (13) (14 ) (15) Ref.

c301 c301 c91

45 c301 (e) C6H5-CO-CH3 little 88.5 r291

55 ~ 3 0 1

c301 67 ~ 3 1 1

[91 ( I ) Indan-1-one little 86.5 c291

c311

(a) CH,COOC2H, 57 -

(b) CH,CN 83 -

(d) NC-CH,-COOC2H, -

(fi C6H5-CH2-CO-C6H5 - ( g l C6H,-CH2-COOC,H5 89 -

(h) C,H,-CH,-CN (i) C,H,-N0,-(4)-CH2-CN -

(k) Cyclopentanone 71

(m) 5,6-Benzomorpholin-3-one 70

- I c ) CH,NOz 72

D O 1 92 -

-

-

benzyl cyanide ( I3hj and ethyl phenylacetate (13g) is allowed to act on (4a) , only the benzyl cyanide reacts[301, in agreement with known acidity series.

Biacetyl, which has two activated methyl groups, reacts via both to give a yellow compound (16j. Spectroscopic data indicate the presence of a double enol chelate. An isolable red-violet complex is obtained with iron(I1r) chloride[3 ' I .

(4a) + CH3-C-C-CH3 66

The 2-substituted imidazolidines (18a) to (18f) were obtained exclusively on reaction of SO,-activated methylene compounds (1 7a) to (1 7 f ) with (4a)13']. This reaction can be used for the synthesis of a-sulfonyl aldehydes ( I s ) , since acidic hydrolysis of the aminals ( I S j leads to the carbonyl compounds from which they are derived'321.

CfiH5

965 Angew. Chem. internat. Edit. 1 Vol. I 1 (1972) 1 No. 11

Diazo compounds (20)[33-351 containing mobile diazomethyl hydrogen also give 2-substituted imidazolidines (21) on reaction with ( 4 ~ ) “ ~ ~ . Ring cleavage with p

CsH5

elimination is ruled out here for structural reasons. Diazo compounds having an immobile diazomethyl proton react in the same way as the disubstituted diazomethanes to give a z i n e ~ [ ” - ~ ~ ! An example is the compound (22), in which the acidity of the diazomethyl proton is insufficient for the formation of a diazoaminal (21) because of the weak activating effect of the amidine group. Electrophilic attack of the terminal N atom on the double bond of (4a) followed by cleavage of the molecule yields the azine (23)[”].

7sH5 H

Very few reactions of other electron-rich olefins with CH-acidic compounds involving cleavage of the molecule and formation of a new C--C bond have been investigated. The use of ( 5 ) instead of (4a) should merely lead to a change in the reaction rate, but apart from the reaction of ( 5 ) with 3-ethyl-2-thioxooxazolidin-4-one (24) to form

HzC-0

qH-?

(25)[13’, no reactions of this olefin with CH-acidic compounds are known.

1,1’,3,3’,5,S-Hexamethyl-6,6-bi-(1,3,5-triazinylidene)-2,2’,4,4‘- tetraone (26) reacts with nitromethane to give a “CH insertion product” (27)“”.

The unbridged electron-rich olefins tetramorpholinoethyl- ene and tetrapiperidinoethylene (28) react with nitromethane with elimination of morpholine or piperidine in a

CH, - Gkti== C’ h-CH3 + 2 CH,-NOz

0 CH, H3C’

reaction sequence analogous to (13) -+( i4) -(is) to give enamine~[~’”].

n n n

(281, X = CH,, o

2.1 .I. Mechanistic Considerations

The CH acidity of the proton-active compounds and the fact that electron-rich olefins are strong 7c bases justify the assumption that the reactions are initiated by protonation of the electron-rich double bond of (4) to form (29). Decomposition of (29) affords the imidazolinium cation (30) and a nucleophilic carbene (3Z)[”]. Direct addition of the anion R- to (29) , as detected by Hoffmann and

f 4)

P Ar Ar

&-€I

!r (30)

Ar I

G e h l h a ~ s ~ ~ ~ ] in the reaction of tetramethoxyethylene with malodinitrile, has not been observed in the reactions of (4). The cation (30) now attacks R- or RH electrophilically and gives (32). This step is plausible in view of the reactions of azoli(ni)um salts with bases in the presence of CH-acidic compounds, which also yield CH insertion products. For example, 3-methylbenzothiazolium iodide reacts with (24) in the presence of Na methoxide to give (25) in 71% yield[131.

The mechanism is also supported by the reactions of the amide acetals (33)[371 with CH-acidic compounds, which probably proceed via carbonium ions (34). The dissocia- tion (33)+(34) is very fast even at - SO’C, as is shown by NMR studies on exchange experiments with N,Ndimethyl-

966 Angew. Chern. internat. Edif. Vol. I 1 (1972) 1 No. 11

formamide dimethyl a ~ e t a l [ ~ * * 3y1. The aminomethylene compound (36) is obtained with elimination of 2 moles of ethanol.

The nucleophilic carbene (31) formed as an intermediate in the reaction (29)+(30) can react in two ways :

1. Dimerization to give the starting olefin (4) . This step is irreversible, as is shown by crossover experiments" ' 3 '*I.

The olefin can take part in the reaction again.

2. Protonation to give the imidazolinium cation (30). This step is reversible, as was shown by H-D exchange experiments on the C-2 atom of imidazoli(ni)um or thiazolium salts[40-4y1. H a r t ~ l e r ' ~ ~ ~ was able to show in the reaction of the dithiocarbene"] (37) with acetophenone to form ( 3 8 ) that a carbene formed as an intermediate reacts directly with a CH-acidic compound to give the CH insertion product.

F3C\ -s, + H3C-CO-C,H, F C

F3CNc 'E + c s , - I s c:j

/c-d F3C

(37)

The overall reaction (4)+(32) is reversible, as was shown by W ~ n z l i c k [ ~ ~ ~ . Thus when (14a) is heated at 130°C and 14 torr, it is converted back into the starting olefin (4a) .

In the reaction of an active methyl or methylene compound with (4a) , two products, ( 1 4 ) and (IS), are possible. The reaction of e.g. ( 1 3 h ) with (4a) leads to a colorless sub- stance ( 1 4 h ) , whose IR spectrum contains no NH ab- sorption[30! Under certain conditions, however, (15 h) can be detected as well as ( 1 4 h ) in the NMR spectrumL3'].

[*] It should be noted, however, that dithiocarbenes react mainly electrophilically [Sl]. For the reaction of dithiolium salts or dithiocarbenes with CH-acidic compounds, cf. also [52 ] . For the reaction ofan intermediate diaminocarbene with a CH-acidiccompound, cf. [26] .

This product appears to result from a ring-chain tautomerism, which has not yet been investigated in detail.

The reaction of 4-nitrophenylacetonitrile with ( 4 a ) leads to a 70% yield of ( lS i ) , which shows no tendency to undergo ring closure either in the crystalline state or in solu- ti0n[~'1.

2.2. With Aromatic Compounds

Only preliminary studies have so far been carried out on the reactions of electron-rich olefins with phenols, the examples studied being (4a) and ( 4 b). o,o'-Disubstituted phenols (39) give the compound (40) in good yields[311. These products are cyclic aminals, and they can be hydrolyzed with hydrochloricacid to give highyieldsofp-hydroxy- benzaldehydes (41) 1311. The introduction of formyl groups with the aid of Wanzlick's olefin (4) is thus possible not only for aliphatic CH-acidic compounds (cf. cc-sulfonyl aldehydes, Section 2.1) but also for activated aromatic compounds.

(40) Ar R R'

The reaction of unsubstituted phenol and (4a) yields a 1 : 1 adduct, which probably has the structure ( 4 2 ) :

:SH5 ?sH5

C6H5 (42) b H 5

The mechanism differs only in the further reaction from that of the reactions of aliphatic CH-acidic compounds with electron-rich olefins. The first step is probably again the protonation of the electron-rich double bond of the olefin to give (29) by the proton of the hydroxyl group. (29) decomposes into the imidazolinium cation (30) and a nucleophilic carbene (31), which can either dimerize to give the starting olefm or can take part in the reaction as

Angew. Chem. internat. Edit. 1 Vol. I1 (1972) 1 No. I 1 967

such. The cation (30) reacts first with the phenoxide anion[*], probably at the point of highest electron density[**], ie. the oxygen, with reversible formation of the “OH insertion product” (43) (cf. Section 4.2), which reacts further via (44) to give the thermodynamically more stable ring-substituted product (40) .

Ar Ar

n Ar (44)

When 2,6disubstituted phenol (39 c) deuterated in the hydroxyl group is allowed to react with (4a) , a product (40c) deuterated in position 2 of the imidazolidine ring should be expected according to this mechanism. However, the deuterium content actually in position 2 is only about 60%; this is probably due to a fast H-D exchange between the phenol group of the (40c) formed and the imidazolinium cation (30) .

No reaction with ( 4 a ) was achieved either with phenetole [no possibility of protonation of the electron-rich double bond of (4a)l or with 2,4-di-tert-butyl-4-methylphenol (no possibility of electrophilic substitution on the aromatic

Heterocycles containing NH can also form “insertion products” under certain conditions. ( 4 a ) reacts with 2-methyl-2,3-dihydroindole to form an “NH insertion product” (59d) (cf. Section 3.1). The “CH insertion product” (45) is obtained, on the other hand, with 2-methyl- ind~le‘~’ ! 2,3-Dimethylindole is inert toward (4a) under the usual reaction conditions (up to 70 hours in boiling toluene).

7sH5 $kH5

2-Methylimidazole also gives a “CH insertion product” (46), whereas I-methylimidazole and 1,2dimethylimid- azole d o not react with ( 4 ~ ) ‘ ~ ’ ~ . It follows that in addition to an NH proton [for protonation of the double bond in ( 4 a ) ] , another active center must also be present (for the electrophilic substitution). The reaction mechanism is probably analogous to that of phenols.

[*I For the reaction of a dithiocarbene with phenols, cf. [50].

[**I I t IS possible that (30) attacks the para position of the phenoxide anion directly in the case of phenols with strong steric hindrance, such as ( 3 9 ~ 1 , with formation of (44) .

2.3. With Aldehydes

Though aldehydes are not proton-active compounds, their reactions with electron-rich olefins will be discussed here, since the products can also be regarded as “CH insertion products”. Benzaldehyde reacts with (4a)[679,531, (5)r’3. 531, and 3,3-dimethyl-2,2’-bi(naphtho[2,1-d] thia~olinylidene)‘~ 31

to give the products (47) to (49) ; furfural reacts with (412) to form the analogous compound (50)[6,29J. Benzoin and furoin are obtained as by-products, i. e. the electron-rich olefin or nucleophilic carbene formed in situ can replace the cyanide ion‘541 in the benzoin condensation.

CsH5

66H5

(47) CH3

f 48)

I c H3

(49)

I

c 6 H 5

f 50)

Protonation of the double bond of the electron-rich olefin can be ruled out as the initial step of this reaction, since the aldehyde proton possesses no acidity. The electrophilic

(4a) + RCHO

11

I I I H5C6 H5C6 H5C6

(47) f54) I+ RCHO

0 OH II I

(53) + R-C-CH-R +-

(56)

f 55)

carbonyl carbon atom of the aldehyde probably attacks the electron-rich double bond with formation of (51 ) . A diaminocarbene (53) is eliminated to form (52), which either

968 Angew. Chem. internat. Edit. 1 Vol. I1 (1972) 1 No. I1

1. gives the acylimidazolidine (47) by a proton shift and ketonization of the enol form (54)“’ or

2. reacts by addition of a second aldehyde molecule to form (55) , which gives the acyloin (56) by proton migration and removal of the carbene (53). The carbene (53) can either attack an aldehyde molecule nucleophilically on the carbonyl carbon atom to form (52) or can dimerize to form the starting olefin and thus undergo renewed reaction.

This mechanism of acyloin formation is similar to the classical Lapworth mechanism[54!

The carbene-catalyzed acyloin formation seems important in biochemistry. B r e s l ~ w [ ~ ~ ~ showed in 1958 that a thiazolium zwitterion (56) (the limiting resonance form of a nucleophilic carbene) is responsible for the thiamine- catalyzed acyloin formation and for the formation of “active aldehydes” (57). The reactions of the thiamine

R = H, P(O)OP(O)OH AH AH

t R‘-CHO, K‘ = C6H, I

Benzoin

zwitterion (thiamine carbene) (56) formed in situ with aldehydes‘56, ”I and a-keto which lead to C-2-acylated thiamines, hydroxyalkyl(aryl)thiamines, or acyloins has been thoroughly investigated. Benzal- dehyde is actually used as a carbene detector[631 in the reactions with 4,5-diphenyldithiolium salts. Neither the 4,5-diphenyldithiolium ion nor the carbene dimer (tetra- mercaptoethylene) can react with benzaldehyde, which reacts exclusiveiy with the dithiocarbene to form 2-acyl- 4,s-diphenyldithiols.

3. Reactions with Formation of a New C-N Bond

3.1. With Primary and Secondary Amines and Hydrazines (Table 2)

Aliphatic, aromatic, and heterocyclic amines react with the electron-rich olefins (4a) and (4c ) with initial formation of extremely readily hydrolyzed “NH insertion products” (59), which can be isolated in the case of the secondary amines, but isomerize to the aminoamidines (60) in the cases of primary amines and hydrazine~“~!

[*] A hydride shift (52 )+(47) , cf. [13, 50, 631 seems unlikely in comparison with a keto-enol tautomerism. For the formation of an enol acetate, cf. [62].

The reaction rate increases in the order aliphatic < ar- aliphatic < aromatic amines.

Ar A r

Table 2. Products (9) and (60) of the reaction of ( 4 a ) and ( 4 c ) with amines and hydrazines, respectively [64].

1,1’3,3’-Tetraphenyl-2,2’-bi(benzimidazolinylidene) (61) reacts in a similar manner with piperidine to form (62)[h51.

After the reaction of 1,1’,3,3’-tetra-(I -naphthyl)-2,2’-bi(imid- azolidinylidene) ( 4 c ) with an excess of morpholine, on the other hand, only trimorpholinomethane was isolated ; this was probably formed by transamination from (59 b,)I3’].

(4C) + H N Z O

(59b7

3.2. With Carboxamides, Sulfonamides, and Phosphoramides

The reaction of ( 4 a ) with the carboxamides (%a) to (58h) stops at the imidazolidine (591, though the structural

Angew. Chem. inrernat. Edit. 1 Vol. 11 (1972) 1 No. I 1 969

Table 3. Products (59) and (60) 1311 of the reaction of ( 4 a ) with carboxamides (58a) to (58h), carbohydrazides (58i) and (58k) , sulfonamides (581) to ( 5 8 n ) , and phosphoramides or phosphonamides (580) to (SKr) .

to form “NH insertion products” (65a) to (65m). The compounds (65a) to (65m) are more resistant to hydrolysis than (59a) to (59e).

CHO CHO COCH, COC(CH,)=CH, CO-CH=CH-CH, CO-CH=CH-C6H CO-C6H, CO-OC2H, NH-CO-CH, NH-CO-C,H,

SO,-CsH5 S02-CH,

SO2--C6HaCH3-(P) PO(OC2H5)z PO(OC6Hs)2 PS(CZH,)OC~H, PS(C2H,)OC6H,CH3, CI43.4)

[a] Contrary to our earlier belief 1191, the compound exists in the open-chain form (60).

conditions for ring cleavage are satisfied. This behavior may be due to carboxamide resonance.

Carbohydrazides (58 i ) and (58 k ) , sulfonamides (58 I ) to (58n), and phosphoramides or phosphonamides (580) to (58r), on the other hand, react via (59) with ring cleavage to give (60) (see Table 3).

Whereas the imidazolidine structure (14) was isolable in some cases as well as the open structure (15) after the reactions of active methyl and methylene compounds with ( 4 a ) (cf. Table I), the reaction with compounds containing NH groups leads exclusively to products having the formula (59) or (60). The presence of a mixture of isomers has not so far been detected.

3.3. With Lactams and h i d e s

The tetraaminoethylenes (4a) to (4c) also react with lactams (63) and imides (64) with cleavage of the molecule

3.4. With Strongly Acidic NH Compounds~66]

If the acidity of the NH compounds is increased by the introduction of two strongly electron-attracting groups, the reaction with (4) leads to the imidazolinium salts (67) because of the low nucleophilic strength of the amidate anion. Whereas amines activated by two carbonyl groups, such as phthalimide (64a), give “NH insertion products”, amines that are activated by one SO, group and one CO group, such as (66a) to (66e) form the salts (670) to (67f) with ( 4 a ) to (4~)”~. Similarly, the extremely acidic amines (66f) to (66h), which are activated by two SO, groups, and the SO,-PO activated amines (66i) to (661) give the imidazolinium salts (67g) to (67m) and (67n) to (67p) respectively. These reactions are acid-base reactions between the strong n bases (4a) to (4c) and the strong NH acids (66a) to (661).

A r

(67) A r

On the other hand, the N-acylated chloromethanesulfon- amide (68) reacts with (4a) with elimination of HC1 to

[*] The compounds ethyl 2-diazo-2-[N-(1,3-diphenyl-2-imidazolldi- y1)-N-(p-toluenesulfonyl)aminocarbonyl]acetate and 2-diazo-N-(1,3- diphenyl-2-imidazolidinyl)N-(p-toluenesulfonyl)acetoacetamide, which are described in [19], also occur in the form of the imidazolinium salts, as is shown by the spectroscopic data.

970 Angew. Chem. internat. Edit. 1 Vol. I 1 (1972) 1 No. I1

give 5-methyl-I ,3,4-oxathiazoline 3,3dioxide (70) together with 1,3diphenylimidazolinium chloride (69) in a yield of more than 90%1311['1.

3.5. Mechanistic Considerations

For the mechanism of the formation of the "NH insertion products" (59) and (60) c ~ . [ ~ ~ I . The initial step, i.e. the proton transfer from the nitrogen of the amino compound to the nucleophilic double bond of the electron-rich olefin, is followed by cleavage of the molecule into an imidazolinium cation (30) and a nucleophilic carbene (31).

In the case of the strongly acidic NH compounds (66), the reaction stops at this stage, and one obtains the imidazolinium amidates (67). In the case of more weakly acidic NH compounds, the mechanism is probably similar to that of the reactions with CH-acidic compounds[**] (cf. Section 2.1).

Tetrahydrofolic acid (THF) is important as a cofactor for the biochemical transfer of formaldehyde and formic acid to amines["I. Formylation of the THF and cyclization yields an imidazolinium cation (30') similar to (30), which then reacts with amines to give formimino-THF with formation of an "NH insertion product" (59') and ring cleavage to form ( 6 0 ) (in the case of primary amines)[*']. Since the hydrolysis of the aminoamidines (60) can be carried out in such a way that the N-formyl compound of (58) is formed in addition to dianilin~ethaner~'], the hydrolysis of ( 6 0 ) to form THF and the N-formyl compound also seems plausible. (4a) and the imidazolinium cation (30), which can be easily obtained from it, can thus serve as model substances for the biochemical formaldehyde transfer reactionf811.

I

NH-R I

(59')

1 I

[*I For the preparation of aromatically substituted 1,3,4-oxathiazoline 3.3dioxides from N-acylated chloromethanesulfonamides and bases, cf. [67,68].

[**I The ability of the azolium and dithiolium salts, when used as such, to react with amines to form "NH insertion products" is shown by the reactions of N-methylbenzothiazolium salts with piperidine [I31 and diethylamine [14], and of dithiolium salts with piperidine, benzyl- amine, dibenzylamine, ethylamine, and NH, [63, 691 and from the detailed investigations on the reactions of thiamine with bases 170-731, in which an intramolecular "NH insertion product" (free thiamine base) was obtained among other products [74-761. The reactions of the aminal esters 1771 and amide acetals [78, 791 with amines and amides to form amidines probably also proceed via a cation similar to the ion (30) 1381.

4. Reactions with Formation of a New C-0 Bond

4.1. Hydrolysis of {4a) and ( 5 )

On hydrolysis of (4a), two different products are formed, depending on the reaction conditions. Hydrolysis in pyridine/water leads with molecular cleavage to N-formyl- N,Ndiphenylethylenediamine (72), whereas hydrolysis with aqueous potassium hydroxide solution leads, with retention of the original C - C bond, to (74)16. l6].

Ho, (4a) - (29). Ar = C6H5

The hydrolysis is probably initiated by the addition of a proton to the nucleophilic double bond of the electron-rich olefin to form (29). At low concentrations of hydroxide ions (path A), (29) is split into a carbene (53) and an imidazolinium cation (30), Ar=C6H,, which adds a hydroxide ion to give (71). Compound (71) rearranges with f3 eIimination to form (72). The alkaline hydrolysis of the imidazolinium cation (30), Ar=C,H,, is plausible in view of the hydrolysis of azoli(ni)um

At high concentrations of hydroxide ions (path B), the cation (29) adds a hydroxide ion to form (73), which undergoes ring cleavage to give (74). It has been shown that (74) does not change into (72) under the reaction conditions'l61. On hydrolysis of (5), the reaction appears to stop at compound (75) [addition of water to the double b~nd]~']' '~]. Ring cleavage to give a compound correspond- ing to (74) has not been observed.

4.2. Reactions with Alcohols

Little is known about the reactions of electron-rich olefins with alcoholic or phenolic OH groups with cleavage of the molecule and formation of a new C-0 bond. (4a)

[*] Structure (75) appears more plausible from a mechanistic view- point than a structure suggested in [14,82].

Angew. Chem. internat. Edit. 1 Vol. I I (1972) 1 No. I I 971

reacts with the alcohols (76a) to (76d) in good yields to give the readily hydrolyzed cyclic aminal esters (77a) to (77 d)[841.

The reaction of tetrakis(dimethy1amino)ethylene with methanol leads with cleavage of the molecule and replace- ment of dimethylamino groups by ethoxy groups to di- methoxy(dimethylamino)methane, dimethylamine, and, with retention of the central C-C bond, 1,2-bis(dimethylamino)-1 ,I ,2-trimethoxymethane as the main products[s51. For the reaction of a phenol with formation of an “OH insertion product”, cf. Section 2.2.

The mechanism is probably similar to the hydrolysis mechanism by path A, i.e. proton transfer from the OH group to the double bond of (4a) to give the cation (29), Ar = C,H,, which decomposes into the imidazolinium ion (30), Ar=C6H5, and a nucleophilic carbene (53). The reaction of (30)[*] with alcohol yields (77) .

Thereaction (4a)$(77) is reversible. (77a) losesmethanol when heated in xylene, with regeneration of ( 4 ~ ) ~ ’ ~ ~ .

A reaction by path B is observed for ( 5 ) and for tetramethoxyethylene. An addition to the double bond leads to the compounds ( 78) fX3. 901 and (79)19” respectively.

CH3 I

H3C0,T $,OCH,

H 3 C 0 OCH, /c - c,

5. Closing Remarks

The reactivity of the electron-rich olefins (4) and ( 5 ) with proton-active compounds can be attributed in the majority of cases to the reactivities of the fragments formed by cleavage of the protonated olefm [e . g. (29)], i. e. to the azoli(ni)um ion [e. g. (30)] and the nucleophilic carbene [e . g. (31)]. Since the nucleophilic carbene formed in situ can probably dimerize to give the starting olefin or can be

[*] For the reaction of imidazolium or dithiolium salts with alcohols to form “OH insertion products“, cf. [73,86, 871. For the reaction of a dithiocarbene formed in situ with methanol, cf. [SS]. and for the reaction of the nucleophilic carbene cycloheptatrienylidene, also formed in situ, with ethanol to form an “OH insertion product”, cf. 1891.

protonated by the proton-active compound to form the azoli(ni)um ion, while the azoli(ni)um ion exists in equi- librium with the carbene in the presence of bases [cf. the base-catalyzed H-D exchange of azoli(ni)um salts, which can proceed only cia a carbene, Section 2.11, it does not seem possible to distinguish the reactions of the azoli(ni)um ion and of the carbene in these cases. The same product is obtained from both”].

Received: June 7,1972 [A 913 IE] German version : Angew. Chem. 84,1022 (1972)

Translated by Express Translation Service, London

f*] For use of azides in differentiation between the reactions of (5 ) and the carbenes formed in situ, cf. [21].

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Reactions of Electron-Rich Olefins with Proton-Active Compounds

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