Carbenes by a-elimination

The halogen-metal exchange of one of a geminal pair of bromine atoms is readily achieved -a carbenoid is formed which may have sufficient stability to react as an organolithium before decomposing to a carbene by a-elimination.. m in cyclic systems, diastereoselectivity in this process has long been known,114116 though the outcome of reactions such as that of 137 is under a mixture of kinetic and thermodynamic control.117... 

Parallel to these efforts, Wanzlick tried to prepare a stable N-heterocyclic carbene by a-elimination of chloroform from 5. The free carbene, however, could not be isolated and instead its dimer, the entetraamine 6=6, was always obtained (Scheme 1.2). Wanzlick s initially postulated cleavage of the entetraamine according to 6=62x6 could not be demonstrated conclusively. Cross-metathesis experiments with differently -substituted entetraamines failed, excluding an equilibrium between the monomer 6 and the dimer 6=6 (Scheme 1.2). " ... 

The majority of preparative methods which have been used for obtaining cyclopropane derivatives involve carbene addition to an olefmic bond, if acetylenes are used in the reaction, cyclopropenes are obtained. Heteroatom-substituted or vinyl cydopropanes come from alkenyl bromides or enol acetates (A. de Meijere, 1979 E. J. Corey, 1975 B E. Wenkert, 1970 A). The carbenes needed for cyclopropane syntheses can be obtained in situ by a-elimination of hydrogen halides with strong bases (R. Kdstcr, 1971 E.J. Corey, 1975 B), by copper catalyzed decomposition of diazo compounds (E. Wenkert, 1970 A S.D. Burke, 1979 N.J. Turro, 1966), or by reductive elimination of iodine from gem-diiodides (J. Nishimura, 1969 D. Wen-disch, 1971 J.M. Denis, 1972 H.E. Simmons, 1973 C. Girard, 1974),... 

Three possible mechanisms may be envisioned for this reaction. The first two i.e. 1) Michael addition of R M to the acetylenic sulfone followed by a-elimination of LiOjSPh to yield a vinyl carbene which undergoes a 1,2 aryl shift and 2) carbometallation of the acetylenic sulfone by R M followed by a straightforward -elimination, where discarded by the authors. The third mechanism in which the organometallic reagent acts as an electron donor and the central intermediates is the radical anion ... 

There are a number of ways of generating carbenes that will be discussed shortly. In some cases, the reactions involve complexes or precursors of carbenes rather than the carbene per se. For example, carbenes can be generated by a-elimination reactions. Under some circumstances the question arises as to whether the carbene has a finite lifetime, and in some cases a completely free carbene structure is never attained. 

Carbenes from Halides by a-Elimination. The a-elimination of hydrogen halide induced by strong base (Scheme 10.8, Entry 4) is restricted to reactants that do not have (3-hydrogens, because dehydrohalogenation by (3-elimination dominates when it can occur. The classic example of this method of carbene generation is the generation of dichlorocarbene by base-catalyzed decomposition of chloroform.152... 

The addition of dichlorocarbene, generated from chloroform, to alkenes gives dichlorocyclopropanes. The procedures based on lithiated halogen compounds have been less generally used in synthesis. Section D of Scheme 10.9 gives a few examples of addition reactions of carbenes generated by a-elimination. 

Section D illustrates formation of carbenes from halides by a-elimination. The carbene precursors are formed either by deprotonation (Entries 14 and 17) or halogen-metal exchange (Entries 15 and 16). The carbene additions can take place at low temperature. Entry 17 is an example of generation of dichlorocarbene from chloroform under phase transfer conditions. 

The chemistry of tantalum is different. The tantalum pentaalkyl complex does not exists, because it transforms easily into a carbene complex by a-elimination. This complex reacts also with silica, leading to a supported tantalum complex.262,263 Their reaction proceeds first by the addition of the silanol OH group across the tantalum-carbon double bond followed by elimination of an alkane (Scheme 7.19). 

One of the most fascinating transformations of free carbenes, generated for instance by photolysis of diazoalkanes or by a-elimination, is their insertion into aliphatic C-H bonds. This ability of carbenes is not only of theoretical interest, but also a unique tool for the synthesis of highly strained compounds such as, e.g., bicyclo[l. 1.0]butanes. 

As would be expected from their electron-deficient nature, carbenes are highly reactive. Carbenes can be generated by a-elimination reactions. 

It has been demonstrated35 that enolate trapping by the electrophilic iron-carbene complex 1 provides 2. Alkylation at sulfur followed by a-elimination again generates a carbene, which then inserts stoichiometrically 1,5 into the C —H bond to give the octahydroindenone 3. Several other applications of this cyclization have been published 3fi. 

A carbenoid-type mechanism with free or surface-bound species formed by a elimination from methanol promoted by the strong electrostatic field of zeolites was proposed first.433,456,457 Hydrocarbons then can be formed by the polymerization of methyl carbene, or by the insertion of a surface carbene (8) into a C-O bond453-455,458,459 (Scheme 3.2, route a). If surface methoxyl or methyloxonium species are also present, they may participate in methylation of carbene454,455,460,461 depicted here as a surface ylide (9) (Scheme 3.2, route b). A concerted mechanism with simultaneous a elimination and sp3 insertion into methanol or dimethyl ether was also suggested 433,454,457... 

Carbenes, R2C (Section 14-7B) are produced by a eliminations from polyhalogen compounds with organometallic reagents. The first step is halogen-metal exchange and this is followed by elimination of metal halide ... 

The silirane intermediate could also account for the formation of vinylsilane 89 when (trimethylsilyl)carbene is generated from dichloromethyl(trimethyl)silane and Na—K in a gas-phase reaction53. The transformations of (trialkylsilyl)carbenes, generated from (a-halomethyl)silanes by a-elimination with a strong base, may involve transient siliranes as well (equation 23 and Section III.E.l.b). 

In Section 5.3.7.3 the formation of a-halogen carbanions and their alkylation was discussed. If these or related intermediates are left to warm, a-elimination will usually occur to yield carbenes, which either react with the solvent, dimerize, or undergo inter- or intramolecular C-H or C-C bond insertion [291, 292, 309, 436], Because of the electron deficit at the carbene carbon atom (six valence electrons only), these intermediates are highly energetic, and their formation by a-elimination is therefore much slower than the formation of alkenes by /3-elimination. 

The absence so far of unequivocal proof of carbene formation by a-elimination of hydrogen halide from organic halides and related reactions using hydroxide and alkoxide ions or weaker bases suggests that more detailed investigations of some reported carbenoid reactions may be justified. Thus carbenoid behaviour in the reaction between benzal chloride and potassium t-butoxide in the presence of olefins seems to be critically dependent upon the nature of the solvent (McElvain and Weyna, 1959 Cawley and Westheimer, 1960) and the reaction temperature (Hodgkins et al., 1964). 

This type of reaction is better demonstrated by two examples in which the insertion reaction is a bit more obvious when there are no p hydrogens, the carbene inserts into C-H bonds a little further away in the same molecule or even in the solvent (cyclohexane in the second example). In the first case, the carbene is formed by a elimination and, in the second case, by photolysis of a diazoketone. 

A plausible intermediate of this olefination is the titanium-methylene sjtecies 4, which is formed from 1 by removal of AlMe2Cl with a Lewis base, from 2 by fragmentation with elimination of isobutene, and from 3 by a-elimination and release of methane. However, none of these three routes to titanium-carbene complexes of type 4 proved to be generally applicable. Consequently, the use of these reagents in synthesis is essentially limited to the transfer of a methylene unit 18]. From a synthetic viewpoint, a general and easy route to substituted titanium-alkylidene species and their use in carbonyl olefinations would be more desirable. 

Carbene complexes not containing hetero-atoms X and or Y but of the form, M=CHR, are known as alkylidene complexes and were first made by Schrock in 1975. A number of routes have been developed one of the most interesting is by a-elimination. Equation 17,... 

Silylenes (R2St). Silylenes, which are considerably more stable than the analogous carbenes, have a singlet ground state. With few exceptions, their reactions are analogous to those of carbenes, Silylenes are generated by a eliminations,... 

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