Dimetallic compounds allylation

The violence of superbasic slurries towards functionalised organic molecules means that they are at their most effective with simple hydrocarbons they also tolerate ethers and fluoro substituents. LiCKOR will deprotonate allylic, benzylic, vinylic, aromatic and cyclopropane C-H bonds with no additional assistance. From benzene, for example, it forms a mixture of mono and dimetallated compounds 496 and 497.441 ( Li/K indicates metallation with a structurally ill-defined mixture of lithium and potassium.)... 

Substituted gem-dimetallic compounds, readily obtained via allylation of alkenyl organometallics, react with aldehydes in the presence of BF3-Et20 but do not react with ketones. When alkylidenemalonates are used instead of aldehydes, the Z-olefins see (E) (Z) Isomers) are obtained with a very high stereoselectivity (Scheme 35). A transmetallation reaction with copper cyanide significantly increases the reactivity of gm-dimetallic derivatives via formation of 1,1-zinca cyanocuprates. Indeed, when these compounds react with... 

A related approach to 1,1-dimetallated compounds relies on the allylation of 1-alkenyl-magnesium, -lithium, and - uminum derivatives (23) with allylzinc bromides (22), which proceeds readily at 35 C (Scheme 7 Table 4). The resulting gem-dimetallic intermediates can be trapped by protonation (24 26,25 - 29), stannylation (24 - 27) or, more interestingly, with aldehydes in the presence of BFs OEt (e.g. 24a - 28). The dimetallated species (24) were also trapped with alkylidene malonates, l2/H30, ... 

Transmetallation of the dimetallic species 186a or 186b with CuCN led to a new organometallic species formulated as 191 which displayed remarkable thermal stability. Addition of reactive alkylating agents such as allyl bromide to 191 directly produced compound 192. Indeed, once the first alkylation occurred, the copper salts present in... 

Thus, the alkenyllithium reagent derived from the (X)-y-iodo allylic ether 205 underwent addition of crotylzinc bromide in ether at — 50 °C and, after hydrolysis of the resulting dimetallic reagent, compound 206 was obtained with high diastereoselectivity (anti/syn = 93/7) (equation 101)146. 

Combining substrate-induced diastereoselection and mutual diastereoselectivity, as illustrated for the crotylzincation of the alkenyllithium derived from 209, led to excellent results as the gewt-dimetallic species 217 was obtained in a highly stereoselective fashion. The stereochemical outcome was explained by the addition of the kinetically reactive cisoid metallotropic form of the crotylzinc reagent anti to the propyl group in the chelated allyl alkenylzinc intermediate. After hydrolysis, compound 218 was obtained as a single diastereomer (equation 106)148,149. 

Another option for the preparation of allenyllithium reagents involves the carbolithia-tion of activated conjugated enynes15. Thus, addition of BuLi to the enyne 281 led to an allenyllithium compound 282 (in metallotropic equilibrium with its propargylic counterpart). Subsequent addition of allylzinc bromide generated the allylic 1,1-dimetallic species... 

Addition reactions of three kinds of main group metal compounds, namely R—M X (carbometallation, when R are alkyl, alkenyl, aryl or allyl groups), H—M X (hydrometallation with metal hydrides) and R—M —M"—R (dimetallation with dimetal compounds) to alkenes and alkynes, are important synthetic routes to useful organometallic compounds. Some reactions proceed without a catalyst, but many are catalysed by transition metal complexes. 

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