Steric and electronic effects on the configurational stability of residual chiral phosphorus-centered three-bladed propellers: tris-aryl phosphanes.

Simona Rizzo, Tiziana Benincori, Valentina Bonometti, Roberto Cirilli, Patrizia R Mussini, Marco Pierini, Tullio Pilati, Francesco Sannicolò

Istituto di Scienze e Tecnologie Molecolari, Consiglio Nazionale delle Ricerche, via Golgi 19, 20133 Milano, Italy.

Chemistry (Weinheim an der Bergstrasse, Germany) 2013 Jan 2

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A series of tris-aryl phosphanes, structurally designed to exist as residual enantiomers or diastereoisomers, bearing substituents differing in size and electronic properties on the aryl rings, were synthesized and characterized. Their electronic properties were evaluated on the basis of their electrochemical oxidation potential determined by voltammetry. The configurational stability of residual phosphanes, evaluated by dynamic HPLC on a chiral stationary phase or/and by dynamic (1)H and (31)P NMR spectroscopy, was found to be rather modest (barriers of about 18-20 kcal mol(-1)), much lower than that shown by the corresponding phosphane oxides (barriers of about 25-29 kcal mol(-1)). For the first time, the residual antipodes of a tris-aryl phosphane were isolated in enantiopure state and the absolute configuration assigned to them by single-crystal anomalous X-ray diffraction analysis. In this case, the racemization barrier could be calculated also by CD signal decay kinetics. A detailed computational investigation was carried out to clarify the helix reversal mechanism. Calculations indicated that the low configurational stability of tris-aryl phosphanes can be attributed to an unexpectedly easy phosphorus pyramidal inversion which, depending upon the substituents present on the blades, can occur even on the most stable of the four conformers constituting a single residual stereoisomer. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Citation

Simona Rizzo, Tiziana Benincori, Valentina Bonometti, Roberto Cirilli, Patrizia R Mussini, Marco Pierini, Tullio Pilati, Francesco Sannicolò. Steric and electronic effects on the configurational stability of residual chiral phosphorus-centered three-bladed propellers: tris-aryl phosphanes. Chemistry (Weinheim an der Bergstrasse, Germany). 2013 Jan 2;19(1):182-94