Chlorophosphonium salts (CPS) have been used for a number of phosphorus functional group interconversions and as reagents in organic chemistry.[1] They had been observed to racemise in solution and we postulated that this may be via a nucleophilic substitution reaction at phosphorus. To our knowledge, barriers to nucleophilic substitution have not been examined at any element using VT-NMR before. The chirality of tri- and tetracoordinate phosphorus and the 100% abundance of 31P, makes phosphorus an ideal candidate for this. Calculations[2]indicated that similar processes to this have energy barriers that lie in ranges measureable by VT-NMR and exchange spectroscopy (EXSY).To investigate the racemisation of CPS, we designed a series of diastereomeric CPS and bromophosphonium salts (BPS) that gave clearly separated signals in NMR spectra. Using VT-NMR, the barriers to nucleophilic attack at phosphorus could be determined. Examination of steric, concentration and solvent effects on the inversion process unveiled a mechanism that contrasts with the analogous process at tetracoordinate carbon. Certain differences in behaviour between chlorophosphonium salts and bromophosphonium salts that emerged had not been anticipated. We have also tried to constrain the positively charged phosphorus atom in cyclic and bicyclic systems with interesting results. Further mechanistic details were parsed with the aid of computational methods.Acknowledgements This work was supported by Science Foundation Ireland under Grant no 09/IN.1/B2627References: [1] (a) P. A. Byrne, K. V. Rajendran, J. Muldoon, D. G. Gilheany, Org. Biomol. Chem. 2012  , 10, 3531; (b) K. V. Rajendran, D. G. Gilheany, Chem. Commun. 2012  , 48, 817; (c) R. M. Denton, X. Tang, A. Przeslak, Org. Lett., 2012  , 12, 4678; (e) R. M. Denton, J. An, P. Lindovska, W. Lewis, Tetrahedron 2012  , 68, 2899.[2] M. A.Van Bochove, M. Swart, F. M. Bickelhaupt, Phys. Chem. Chem. Phys. 2009  , 11, 259.

Dynamic NMR study of nucleophilic substitution at tetracoordinate phosphorus

248th National Meeting (2014)

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