Structures and Reactivities of 2‑Trityl- and 2‑(Triphenylsilyl)pyrrolidine-Derived Enamines: Evidence for Negative Hyperconjugation with the Trityl Group

X-ray structures of enamines and
iminium ions derived from 2-tritylpyrrolidine
(Maruoka catalyst) and 2-(triphenylsilyl)­pyrrolidine (Bolm–Christmann–Strohmann
catalyst) have been determined. Kinetic investigations showed that
enamines derived from phenylacetaldehyde and pyrrolidine (R = H) or
2-(triphenylsilyl)­pyrrolidine (R = SiPh3) have similar
reactivities toward benzhydryl cations Ar2CH+ (reference electrophiles), while the corresponding enamine derived
from 2-tritylpyrrolidine (R = CPh3) is 26 times less reactive.
The rationalization of this phenomenon by negative hyperconjugative
interaction of the trityl group with the lone pair of the enamine
nitrogen is supported by the finding that the trityl group in the
2-position of the pyrrolidine increases the electrophilic reactivity
of iminium ions derived from cinnamaldehyde by a factor of 14. The
consequences of these observations for the rationalization of the
reactivity of the Jørgensen–Hayashi catalyst
(diphenylprolinol trimethylsilyl ether) are discussed.