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.

The effect of S-alkylation on organocatalytic enamine activation through imidazolidine-4-thiones

“… 13 Eqn (1) has already been used to successfully quantify the nucleophilicities of prolinol-based and MacMillan organocatalysts 14 as well as of enamines derived thereof. 15 As a consequence, the reactivity of the enamines studied in this work can straightforwardly be compared with relevant species that occur in many, more commonly used, enamine-activated reactions. 13 e ,15 Hence, a quantitative fundament for the understanding of both prebiotic relevance of ITO catalysts and their scope in organocatalytic reactions is provided in this work.…”

Reactivity scales for quantifying polar organic reactivity: the benzhydrylium methodology

“…Negative hyperconjugation of the C(OSiMe 3 ) Ph 2 substituent was found to be responsible for the increased electrophilicity of the iminium ion derived from the HayashiJørgensen catalyst compared to the iminium ion derived from the unsubstituted pyrrolidine. 28 As substrates suitable for iminium activated reactions should be nucleophilic enough to react with iminium ions but not with the precursor carbonyl compounds, it was concluded that nucleophiles with 2<N<14 should be suitable substrates for iminium activated reactions. 27 Analogous analyses have also been performed for enamine-activated reactions.…”

“…27 Analogous analyses have also been performed for enamine-activated reactions. 27,28 3.3. Why the reactivity selectivity principle cannot be a general rule Until the early 1970s decreasing selectivity with increasing reactivity was considered as a tenet of organic chemistry.…”

Molecular Structures of Enantiomerically-Pure (S)-2-(Triphenylsilyl)- and (S)-2-(Methyldiphenylsilyl)pyrrolidinium Salts

“…Concerning the first successful preparation of enantiomerically-pure (S)-2-(triphenylsilyl)pyrrolidine [(S)-1], we established an indirect synthetic route via intermediate formation of 2-(methoxydiphenylsilyl)-N-Boc-pyrrolidine [1]. Recently, a detailed structural and kinetic investigation gave new insight into the structure-reactivity relation in enamines and iminium ions derived from 2-tritylpyrrolidine [7] and 2-(triphenylsilyl)pyrrolidine [8]. 4 ], which were obtained from the respective optically-pure chloride salts [1] by treatment with perchloric acid.…”