Carbenoids insertion-abstraction

Cyclic epoxides such as 124 can react in two ways with strong bases (a) via abstraction of a /3-proton to form allylic alcoholates 125 or (b) by deprotonation at the epoxide carbon atom forming the intermediate 126 and, after electrophilic substitution, the epoxides 128. If there is a suitable C—H bond in the vicinity of the C-Li moiety, intramolecular carbenoid insertion reactions to 127 may take place (equation 27) ° . ... 

E.Z-Selectivity in the insertion by unsymmetrical carbenoid 24, is specifically indicative of the transition state of the stepwise mechanism. Based on the evidence that carbenoid 24, which is generated from 42 or 43 (E Z = 84 16), exists nearly exclusively in the -configuration under the equilibrium even at —95°C,29 the observed stereoselectivity for E-isomers in the insertion products verifies that hydride abstraction takes place via an Sn2-like transition state 52 with inversion of configuration at the carbenoid carbon, followed by the recombination of menthone 40 and carbanion 53 (Scheme 19). 

Since alkyllithium compounds and their carbanions have an isoelectronic structure with alkoxides, their reaction behavior with carbenes is expected to be similar to that of alkoxides, showing enhanced reactivity in both C-H insertion and hydride abstraction.35 In this reaction, the hydride abstraction cannot be followed by recombination and, therefore, can be differentiated from the insertion. Indeed, the reaction of alkyllithium compounds 70 or nitrile anions (see Section IV.B) with ethyl(phenylthio)carbenoid, which is generated by the reaction of 1-chloropropyl sulfide 69 with BuLi, takes place at the -position of 70 more or less in a similar manner giving both insertion product 71 and hydride abstraction products 72 and 73, respectively. This again supports a general rule C-H bonds at the vicinal position of a negatively charged atom are activated toward carbene insertion reactions (Scheme 22). 

Metal hydride species can undergo an insertion reaction with an olefinic double bond, resulting in the formation of a metal alkyl species which then generates the requisite carbenoids by a-hydride abstraction [110-112] ... 

There are three general classes of mechanism most often encountered in alkane reactions (i) radical (ii) electrophilic and (iii) carbenoid. The C—bond-lneaking st in (i) and (ii) are shown in equations (2) and (3). Carbenoid reactions can go either by direct insertion into the C—bond (equation 4), which tends to happen when the carbene in question has singlet character, or by a two-stqi process (equations 5 and 6), in which H-atom abstraction precedes collapse of the radical pair, a pathway which is characteristic of triplet carbenes. ... 

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