Lithium carbenoids reactivity

The particular substitution pattern of lithium carbenoids, the fact that both an electropositive metal and an electronegative substituent X are bound to the same carbon atom, causes the ambiphilic character of this species. The chameleon-like reactivity becomes evident from the resonance formulas of the carbenoid lb (equation 1) Whereas the carbanionic character is expressed by the resonance formula la, the electrophilic character is represented by Ic. In an analogous way, the reactivity of vinylidene carbenoids 2b is expressed by the mesomeric structures 2a and 2c. 

The electrophilic reactivity of lithium carbenoids (reaction b) becomes evident from their reaction with alkyl lithium compounds. A, probably metal-supported, nucleophilic substitution occurs, and the leaving group X is replaced by the alkyl group R with inversion of the configuration . This reaction, typical of metal carbenoids, is not restricted to the vinylidene substitution pattern, but occurs in alkyl and cycloalkyl lithium carbenoids as well ". With respect to the a-heteroatom X, the carbenoid character is... 

III. LITHIUM CARBENOIDS AS REACTIVE INTERMEDIATES IN SYNTHESIS A. Generation of Lithium Carbenoids... 

Organolithium compounds which bear a lithium atom as well as a leaving group such as a halogen atom or an alkoxy group on the same carbon atom — lithium carbenoids — are a well-characterized class of compounds [1-5]. They react as nucleophiles or electrophiles depending on the chosen conditions the electrophilic reactivity is typical of carbenoids. 

The reactive intermediates under some conditions may be the carbenoid a-haloalkyllithium compounds or carbene-lithium halide complexes.158 In the case of the trichloromethyllithium to dichlorocarbene conversion, the equilibrium lies heavily to the side of trichloromethyllithium at — 100°C.159 The addition reaction with alkenes seems to involve dichlorocarbene, however, since the pattern of reactivity toward different alkenes is identical to that observed for the free carbene in the gas phase.160... 

Another feature of carbenoid-type reactivity is the cyclopropanation (reaction c). Again, this reaction does not only take place in vinylidene but also in alkyl carbenoids . On the other hand, the intramolecular shift of a /3-aryl, cyclopropyl or hydrogen substituent, known as the Fritsch-Buttenberg-Wiechell rearrangement, is a typical reaction of a-lithiated vinyl halides (reaction d) . A particular carbenoid-like reaction occurring in a-halo-a-lithiocyclopropanes is the formation of allenes and simultaneous liberation of the corresponding lithium halide (equation 3). ... 

A final carbenoid-type reactivity of a-oxygen-substituted vinylidene carbenoids has been reported for the carbamate 111. When wanned up to temperatures higher than —40 °C, a Fritsch-Buttenberg-Wiechell reanangement takes place to give the alkyne 112 (equation 60). Below that temperature, the lithium compound 111 maintains its nucleophilic reactivity . 

Mixed bimetallic reagents possess two carbon-metal bonds of different reactivity, and a selective and sequential reaction with two different electrophiles should be possible. Thus, the treatment of the l,l-bimetailic compound 15 with iodine, at — 78"C, affords an intermediate zinc carbenoid 16 that, after hydrolysis, furnishes an unsaturated alkyl iodide in 61% yield [Eq. (15) 8]. The reverse addition sequence [AcOH (1 equiv), —80 to — 40 C iodine (1 equiv)] leads to the desired product, with equally high yield [8]. A range of electrophile couples can be added, and the stannylation of 15 is an especially efficient process [Eq. (16) 8]. A smooth oxidation of the bimetallic functionality by using methyl disulfide, followed by an acidic hydrolysis, produces the aldehyde 17 (53%), whereas the treatment with methyl disulfide, followed by the addition of allyl bromide, furnishes a dienic thioether in 76% yield [Eq. (17) 8]. The addition of allylzinc bromide to 1-octenyllithium produces the lithium-zinc bimetallic reagent 18, which can be treated with an excess of methyl iodide, leading to only the monomethylated product 19. The carbon zinc bond is unreactive toward methyl iodide and, after hydrolysis, the alkene 19... 

The mechanistic outline of carbenoid/carbonyl reactivity follows the paradigm illustrated at the outset of this chapter (Scheme 1 X = halogen). The nucleophilic lithium species adds to the carbonyl compound and suffers elimination to provide the epoxide. Competition from molecular rearrangements emanating from the intermediate halohydrin or the product epoxides is sometimes a problem, particularly with cyclic ketones. Also, the initial adduct frequently fails to cyclize when the reaction is quenched at low temperature, but it is usually a simple matter to effect ring closure by treatment of the halohydrin with mild base in a separate step. 

Although these carbenoids are usually discussed in relation to insertion reactions, some of them undergo polymerization and other reactions which are similar to those of the ylid. Thus, in ylid chemistry the (CH3) 3N+ group may be considered, as a pseudo-halogen. Although it has not been shown that the ylid reacts by an insertion reaction, it is possible that the conditions under which insertion can occur have not been realized. If the ylid is considered as a carbenoid, its polymerization reactions may proceed via a lithium halide complex. Alternatively, the complex may rearrange to the bromomethyllithium which may be the reactive intermediate. 

The exact formulation of the reactive intermediate in a-elimination reactions using organolithium compounds as bases has been difficult. Apart from the free carbene, various carbenoids are possible, including the a-haloorganolithium formed on metalation, and carbene-lithium halide complexes of various degrees of association. In the case of the dichlorocarbene-trichloromethyllithium equilibrium, the... 

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