Nucleophilic substitution at phosphorus (SN2@P): Disappearance and reappearance of reaction barriers

ORGN 401

Marc A. van Bochove, ma.van.bochove@few.vu.nl1, Marcel Swart, marcel.swart@udg.es2, and F. Matthias Bickelhaupt, fm.bickelhaupt@few.vu.nl1. (1) Department of Theoretical Chemistry, Vrije Universiteit Amsterdam, De Boelelaan 1083, NL-1081 HV Amsterdam, Netherlands, (2) Institut de Química Computacional, Universitat de Girona, Campus Montilivi, 17071 Girona, Spain
Pentacoordinate phosphorus species play a key role in organic and biological processes. Yet, their nature is still not fully understood, in particular, whether they are stable, intermediate transition complexes (TC) or labile transition states (TS). Through systematic, theoretical analyses of elementary SN2@C, SN2@Si and SN2@P reactions, we show how increasing the coordination number of the central atom as well as the substituents' steric demand shifts the SN2@P mechanism stepwise from a single-well potential (with a stable central TC) that is common for substitution at third-period atoms, via a triple-well potential (featuring a pre- and post-TS before and after the central TC), back to the double-well potential (in which pre- and post-barrier merge into one central TS) that is well-known for substitution reactions at carbon. Our results highlight the steric nature of the SN2 barrier but they also show how electronic effects modulate the barrier height.

 

Physical Organic Chemistry: Calculations, Mechanisms, Photochemistry, and High Energy Species
8:00 AM-11:40 AM, Tuesday, August 21, 2007 BCEC -- 254 A/B, Oral

Division of Organic Chemistry

The 234th ACS National Meeting, Boston, MA, August 19-23, 2007