title: Structures and reactivities of 2-Trityl- and 2-(Triphenylsilyl)pyrrolidine-derived enamines: evidence for negative hyperconjugation with the trityl group.
abstract: 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.
authors.affiliation: Department Chemie, Ludwig-Maximilians-Universität München , Butenandtstrasse 5-13, 81377 München, Germany.
authors.forename: Hannes
authors.initials: H
authors.lastname: Erdmann
authors.forename: Feng
authors.initials: F
authors.lastname: An
authors.forename: Peter
authors.initials: P
authors.lastname: Mayer
authors.forename: Armin R
authors.initials: AR
authors.lastname: Ofial
authors.forename: Sami
authors.initials: S
authors.lastname: Lakhdar
authors.forename: Herbert
authors.initials: H
authors.lastname: Mayr