Carbene Formation by a-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). 

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. 

Very recently. Green has challenged the inportanoe of the cis-rearrangenent reactions, and has proposed an alternative mechanism based on the formation by a-elimination, of a carbene linking the chain to the metal this carbene would then form a metalla-cyclobutane with the inconing moncroer, and a reductive elimination would finally restore the initicd alkyl complex. 

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 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). 

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). 

Deuteration of the ionic liquid s cation has not only been applied to obtain proton-free ionic liquids for H-NMR experiments but also as direct probe for the reactivity of 1,3-dialkylimidazolium based ionic liquids vs. Ir(0) nanoclusters [70]. After addition ofD2 to [BMIM][(CF3S02)2N] in the presence of an lr(0) nanocluster Finkeand coworkers found deuterium incorporation at the 2-H, 4-H, 5-H and 8-H positions of the imidazolium cation while the control experiment in the absence of the lr(0) cluster showed no D-incorportion. The authors concluded from their H-NMR experiments that a sequence of N-heterocylic carbene formation by oxidative addition (see Section 3.1.2 for more details) of the imidazolium cation, H/D scrambling atop the nanocluster surface, followed by the reductive elimination of a C-D bond takes place. From carefiil kinetic investigations the authors concluded that the co-ordinatively unsaturated nanocluster surface acts indeed as the true catalyst for the H/D-ejadiange reaction. 

The importance of 0x0 ligands has been previously suspected by the numerous examples of oxygen containing activators. These reactions clearly demonstrated the crucial role of an 0x0 hgand. This activity could well be based on the double bond character of the M=0 Unkage. This may well facilitate carbene formation via a sequence of metal hydride reactions involving internal oxidative addition and reductive elimination. 

This chapter focuses on the generation of carbenes by extrusion of nitrogen (Tables 91.1 through 91.8). Several other methods are possible, such as the formation from tosylhydrazone salts, oxiranes, aziridines, dioxolanes, and pyrazolenines from cyclopropanes by photocydoehmination, from transition-metal carbene complexes or by a-elimination, from ot,a-dihalogen compounds, or from base treatment of N-nitrosocarbamate. ... 

The reaction of oxiranes with base can follow several paths, giving products of type (34-38 Scheme 27). (a) Formation of an oxiranyl anion (34) is rare (Section 5.05.3.5). (b) Nucleophilic ring opening to give (35) is common with unhindered bases (Section 5.05.3.4). (c) a-Elimination to give a carbene or carbenoid (36) is favored by alkyllithium bases and... 

After the initial claim of the synthesis of an oxirene (by the oxidation of propyne Section 5.05.6.3.1) this system reappeared with the claim 31LA(490)20l) that 2-chloro-l,2-diphenyl-ethanone (110) reacted with sodium methoxide to give diphenyloxirene (111), but it was later shown (52JA2082) that the product was the prosaic methoxy ketone (112 Scheme 97) (the formation of 111 from 110 would be an a-elimination carbene-type reaction). Even with strong, nonnucleophilic bases, (110) failed to provide evidence of diphenyloxirene formation (64JA4866). 

Another means of in situ metal-carbene complex formation in an ionic liquid is the direct oxidative addition of the imidazolium cation to a metal center in a low oxidation state (see Scheme 5.2-2, route b)). Cavell and co-workers have observed oxidative addition on heating 1,3-dimethylimidazolium tetrafluoroborate with Pt(PPli3)4 in refluxing THF [32]. The Pt-carbene complex formed can decompose by reductive elimination. Winterton et al. have also described the formation of a Pt-car-bene complex by oxidative addition of the [EMIM] cation to PtCl2 in a basic [EMIM]C1/A1C13 system (free CP ions present) under ethylene pressure [33]. The formation of a Pt-carbene complex by oxidative addition of the imidazolium cation is displayed in Scheme 5.2-4. 

The stereochemistry of the ring product (17) was rationalized in terms of the attraction and repulsion between the involved substituents98. The accompanying olefins may be formed via carbene intermediates (arising from a-elimination of S02 from sulfene), and the intermediacy of thiadiazoline dioxide (from sulfene and diazoalkane) explains the formation of the ketazine side-products. Thiadiazoline, on its part, may be formed directly by the cyclization of zwitterion 101. 

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