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Mechanistic dichotomy in the asymmetric allylation of aldehydes with allyltrichlorosilanes catalyzed by chiral pyridine N-oxides
journal contributionposted on 08.08.2014, 14:30 by Andrei MalkovAndrei Malkov, Sigitas Stoncius, Mark Bell, Fabiomassimo Castelluzzo, Pedro Ramirez-Lopez, Lada Biedermannova, Vratislav Langer, Lubomir Rulisek, Pavel Kocovsky
Detailed kinetic and computational investigation of the enantio- and diastereoselective allylation of aldehydes with allyltrichlorosilanes, employing the pyridine N-oxides METHOX and QUINOX as chiral organocatalysts, indicate that the reaction can proceed via a dissociative (cationic) or associative (neutral) mechanism: METHOX apparently favors a pentacoordinate cationic transition state, while the less sterically demanding QUINOX is likely to operate via the hexacoordinate neutral complex. In both pathways, only one molecule of the catalyst is involved in the rate- and selectivity-determining step, which is supported by both experimental and computational data.
We thank the EPSRC for grant No. GR/T27051/01, the Ministry of Education and Science of Spain for a fellowship to P. R.-L., the University of Glasgow for a studentship to M. B., the University of Rome for a fellowship to F. C., the Ministry of Education of the Czech Republic (MŠMT ČR) for grant No. LC512, the European Socrates-Erasmus exchange program for a scholarship to L. B., the European Social Fund under the Global Grant measure (VP1-3.1-ŠMM- 07-K-01-030), and Dr. Alfred Bader for additional support.