Reactions of Diazo Carbonyls with [PtX(CH3)(chiral diphosphine)] (X = Cl, Br, I): Chemoselectivity and Diastereoselectivity of Pt-C and Pt-X Carbene Insertion

In polar solvents (e.g. MeCN), ethyl diazoacetate reacts with [PtXMe(S,S-diop)], where X = Cl, Br, I, to give the corresponding [PtX(CHMeCO2Et)(S,S-diop)] as a 2:1 mixture of diastereoisomers in high yields. The major diastereoisomer of [PtCl(CHMeCO2Et)(S,S-diop)] is readily separated in crystalline form, and its crystal structure reveals that the configuration at the α-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 31P NMR spectroscopy) the ratio of diastereoisomers formed in the reactions between [PtXMe(diphos*)] and N2CHCOR: X = Cl, Br, 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 = OEt, 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-Cl2< (CH3)2SO), (ii) increasing nucleofugacity of the halogen (I < Br < 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 31P NMR, but representative species have been isolated and fully characterized by a combination of elemental analysis, IR spectroscopy, and 1H, 13C, 31P, and 195Pt NMR spectroscopy.