Reactions with organosamarium reagents

Reactions of organosamarium(II) halides with aldehydes are sranewhat more complicated and synthetically less useful than those with ketones. The ability of Sm species to serve as strong reducing agents introduces a number of alternative reaction pathways. For example, reaction of EtSmI with benzalde-hyde provides a mixture of benzyl alcohol, benzoin, hydrobenzoin, and benzyl benzoate in low yields. Die first three products presumably arise from benzaldehyde ketyl, generated by single-electron transfer from the Sm reagent to benzaldehyde. The benzyl benzoate apparently is derived frmn a Tischenko-type condensation reaction between a samarium alkoxide species and benzaldehyde. 

Nitriles and esters are also unreactive in Smh-promoted Barbier reactions. A very useful procedure for lactone synthesis has been developed making use of this fact. Treatment of 7-bromobutyrates or 8-bro-movalerates with Smh in THF/HMPA in the presence of aldehydes or ketones results in generation of lactones through a Barbier-type process (equations 25 and 26). This nicely complements the -metaUo ester or homoenolate chemistry of organosamarium(III) reagents described above (Section 1.9.2.1), and also the Reformatsky-type chemistry promoted by Sml2 (Section 1.9.2.3.2). Further, it provides perhaps the most convenient route to 7- and 8-carbanionic ester equivalents yet devised. 

The Sm(OTf)3 reagent mediates the Grignard-type reaction in THF-HMPA. Alkylation, allylation and benzylation of ketones and aldehydes with alkyl, allyl or benzyl halides proceeded via stable organosamarium intermediates [69] (Scheme 29). 

The cross-coupling of alkylsamarium reagents with alkyl halides may be catalyzed by copper salts such as CuX (X=Br, Cl or I) or Li2CuCl4 [158]. The most effective catalyst for the desired cross-coupling was CuBr or Li2CuCl4. The reaction was performed at room temperature in THF with 8 equiv of HMPA. Some examples are listed in Scheme 59. In order to optimize cross-coupling products R -R2, the reaction was performed in two steps, with initial formation of the organosamarium from RxX and then addition of the catalyst and of R2X. 

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