Samarium(II) Triflate as a New Reagent for the Grignard-Type Carbonyl Addition Reaction

On treatment of a THF solution of Sm(OTf) 3 with 1 equiv of an organolithium or organomagnesium reagent at ambient temperature, the purple or deep green solution of the divalent samarium triflate [Sm(OTf) 2 ] was readily obtained. For this preparation, s -BuLi was the most effective as was evidenced by the reduction of 2-phenylethyl iodide in the presence of HMPA. The Sm(OTf) 2 reagent mediated the Grignard-type reaction effectively in THF/HMPA; alkylation and allylation of ketones or aldehydes with alkyl, allyl, or benzyl halides proceeded via organosamarium intermediates. Diastereoselectivity of the samarium−Grignard reaction was examined using 2-methylcyclohexanone, 4- tert- butylcyclohexanone, and 2-phenylpropanal and was found to be higher in each case than that with SmI 2 . With 2-methylcyclohexanone, for example, Sm(OTf) 2 gave the greatest ratio of axial alcohol: equatorial alcohol = 99:1, and SmI 2 gave a ratio of 91:9. Halides containing an ester or a silyl group were reactive in the Reformatsky- or Peterson-type reaction, respectively, using the Sm(OTf) 2 reagent.