In polar solvents (e.g. MeCN), ethyl diazoacetate reacts with [PtXMe(S,S-diop)], where X = Cl, Pr, I, to give the corresponding [PtX(CHMeCO(2)Et)(S,S-diop)] as a 2:1 mixture of diastereoisomers in high yields. The major diastereoisomer of [PtCl(CHMeCO(2)Et)(S,S-diop)] is readily separated in crystalline form, and its crystal structure reveals that the configuration at the alpha-carbon is R; it is configurationally stable in CDCl3 for at least 14 days. The factors that influence the diastereoselectivity have been examined by comparing (by P-31 NMR spectroscopy) the ratio of diastereoisomers formed in the reactions between [PtXMe(diphos*)] and N(2)CHCOR: X = Cl, Pr, I; diphos* = S,S-diop, R,R-diop, S,S-skewphos, S,S-chiraphos; R = OEt, O(l-menthyl), Ph. In MeCN, the diastereoselectivity is independent of halogen but is a sensitive function of the chiral diphosphine and diazo carbonyl, though no systematic correlations have been divined. In solvents of lower polarity (e.g. CH2Cl2), diazo carbonyls react with [PtXMe(diphos*)] to give the products derived from Pt-X insertion as well. as Pt-C insertion. When C6H6 is used as the solvent, the compounds [PtMe(CHICOR)(S,S-diop)], where R = Oft, O(l-menthyl), Ph, are formed in high yields and have been isolated. Redissolving these compounds in MeCN did not lead to isomerization to the Pt-C insertion species [PtI(CHMeCOR)(S,S-diop)]. Several trends have been found relating the extent of Pt-C insertion to the nature of the solvent and the structure of the reagents: the proportion of Pt-C insertion increases with (i) increasing polarity of the solvent (C6H6 < CHCl3 < CH2 . Cl-2 < (CH3)(2)SO), (ii) increasing nucleofugacity of the halogen (I < Pr < Cl), (iii) decreasing bite angle of the diphosphine (diop < chiraphos, skewphos), and (iv) diazo ketone < diazo ester. A mechanism which is consistent with these observations is discussed. Many of the compounds discussed here have been observed in solution only by P-31 NMR, but representative species have been isolated and fully characterized by a combination of elemental analysis, IR spectroscopy, and H-1, C-13, P-31, and Pt-195 NMR spectroscopy.
|Number of pages||10|
|Publication status||Published - Jul 1995|