Carbene generation, by a-elimination

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. 

The two unshared electrons in singlet carbenes have opposite spins. Carbenes generated by a-elimination are singlets because the two unshared electrons are derived from the same MO. Triplet carbenes, in which the two unshared electrons have parallel spins, are usually generated by photolysis of diazo compounds. Triplet carbenes are discussed in Chapter 5. 

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. 

In the presence of an imidazolium salt and a base, oxidative cyclization of a Ni(0) species upon the diene and an aldehyde takes place first and forms an oxanickellacycle 25, which equilibrates with a seven-membered oxanickella-cycle 26, naturally possessing a cis double bond. cr-Bond metathesis through 26 with hydrosilane affords (Z)-allylsilane (Z)-23. The role of NHC ligand (AT-heterocyclic carbene, generated by H+ elimination from imidazolium C2H by a base) is not clear at present a Ni(0)-NHC complex is believed to effectively produce 26. 

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

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,... 

Treatment of the title compound with an aqueous base under phase-transfer catalysis conditions generates, by a-elimination, chloro(trifluoromethyl)carbene that can be trapped by cycloaddition to allylsilane and allylstannane derivatives, e.g. formation of 1 and 2. This method represents a fast and mild alternative to the Seyferth method that starts from the same alkyl halide and generates the carbene by thermal decomposition of l-bromo-l,2,2,2-tetra-fluoroethyl(phenyl)mercury (see Section 1.2.1.2.4.1.1). 

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... 

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. 

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. 

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. 

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). 

Again steric hindrance makes the reaction with electrophiles slower or difficult. Also in this case, the addition of Cu(I) salts is effective [16]. In the reactions with 1-substituted 1-stannylethene, cme-substitution is sometimes observed especially in the reaction of tins with electron-rich alkenes [69, 133-138]. This might be attributed to a participation of a Pd(0)-carbene species generated via a-elimination of regioisomeric Heck intermediate [136,139]. Cme-substitution is scarce when the electrophile is electron deficient (Scheme 29) [136,137]. Levin reported that zpso-selectivity in vinylation of a-stannylacrylate was restored by the addition of Cul [140]. 

All of these carbenes are reactive intermediates that must be generated from the appropriate precursors in the presence of the alkene (or arene) which is to be cyclopropanated. The following methods of carbene-transfer reactions to C-C double bonds will be discussed path a. from a-halo-a-metal (or alkylmetal) compounds by a-elimination path b. from iodine or sulfur ylides by thermal, photochemical or transition metal catalyzed decomposition ... 

Alkyl(or aryl)sulfanylcyclopropanes are prepared by the addition of organosulfanylcarbene (carbenoid) to an alkene. This carbene is, in turn, generated via a-elimination of hydrogen chloride from the corresponding alkyl(aryl)chloromethyl sulfides using a base (Houben-Weyl, Vol.4/3, pp248-252 and Vol.E19b, pp 1690-1691 and 1693-1695). 

Trialkylsilyl- or stannyl-substituted carbenes 2 and 3 can be generated from the title compounds by a-elimination of hydrogen halide (HX) or metal halide, and silyl- or stannylcyclopropanes can be formed by intra- or intermolecular carbene transfer to C C bonds. To this end, 1-haloalkyl(trialkyl)metal compounds can be treated with sodium (Method A, metal halide exchange followed by base-induced a-elimination of HX) or with a strong base (Method B, a-elimination of HX), and l,l-dihaloalkyl(trialkyl)metal compounds can be treated with an al-kyllithium compound (Method C, metal/halide exchange and subsequent a-elimination of a metal halide). 

A sequence of carbene additions to methylenetriangulanes can be used to build branched triangulanes. i - 3 3 When chloro(methyl)carbene generated from 1,1 -dichloroethane with butyl-lithium is used, new exocyclic double bonds for further cyclopropanation reactions can be generated by HCl elimination. For the same purpose l,l-dichloro-3-methoxypropane... 

Although these two methods have found widespread application for the synthesis of free carbenes, they failed for selected saturated imidazolidin-2-ylidenes and especially in the preparation of triazolin-5-ylidenes. In these cases the free carbene species 7 can be obtained from 2-alkoxyimidazolidines 6 [44] or 5-alkoxytriazoles [36] by thermally induced a-elimination of an alcohol (Fig. 5). In addition to 2-alkoxyimidazolidines, 2-(pentafluorophenyl)imidazolidines [45, 46] have also been used for the generation of NHCs by a-elimination. The adduct 8 eliminates acetonitrile upon heating [47] to yield the benzimidazolin-2-ylidene 9. In a more exotic procedure, imidazolium salts have been reduced electrochemically to give the free imidazolin-2-ylidenes [48]. 

This method can be extended to applications with halo-substituted allyl dichlorides. Chloro(2,2-dichlorovinyl)carbenes 6 have been formed by deprotonation with lithium tetramethylpiperid-ide, but yields were improved when sodium hexamethyldisilazanide was used (see Houben-Weyl, Vol. E19b, Table 97). 1,2,3,3-Tetrachloroprop-l-ene can also easily be deprotonated, but in this case subsequent decomposition formed two isomeric carbenes, as observed in the product ratio after cyclopropanation.2 However, treatment of 1,1,2,3,3-pentachloropropene (11) with a base gave only the [2 + 2] dimer 14 of tetrachloroallene 13, generated by jS-elimination of lithium chloride.2 Other pentahalopropenes with a fluorine in the 3-position (e.g. 15) reacted with x-elimination, forming the corresponding cyclopropanes 17 in the presence of alkenes (see Houben-Weyl, Vol. E 19b, Table 97). 

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