Triflate/Mesylate Ratios and Competing C-O and S-O Bond Cleavages in Nucleophilic Vinylic Substitution

摘要

In an attempt to develop the k_(OTf)/k_(OMs) ratio as a mechanistic tool for the "addition-elimination" route in nucleophilic vinylic substitution, several pairs of vinyl mesylates and triflates were prepared. Whereas reactions of ArC(LG)=C(CO_2Et)_2 (LG = OTf, OMs) with piperidine and morpholine in MeCN or THF gave the normal substitution product with k_(OTf)/k_(OMs) ratios of 4.3-10.6, the reaction of the mesylates, Ar = p-O_2NC_6H_4, and of PhC(OMs)=C(Me)CN with MeS~- gave a ketone via an S-O bond cleavage. A related mesityl-substituted tosylate also reacted with p-MeC_6H_4X~- (X = O, S) via an S-O bond cleavage. Hence, k_(OTf)/k_(OMs) ratios cannot be used as a general mechanistic tool. Several reactivity ratios in vinylic substitution are briefly discussed.