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(CO2Et)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-O2NC6H4, 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-MeC6H4X- (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.