The Bromination of Bulky Trialkylphosphane Selenides R 2 R′PSe (R, R′ = i Pr or t Bu) Studied by Physical and Computational Methods

Bulky trialkylphosphane selenides t Bu 3 PSe ( 1a ), i Pr 3 PSe ( 1b ), t Bu 2 ( i Pr)PSe ( 1c ) and t Bu( i Pr) 2 PSe ( 1d ) react with one equiv. of bromine providing “T‐shaped” products R 2 R′P–SeBr 2 ( 2a – d ), which contain three‐coordinate selenium atoms (10‐Se‐3). The solid compounds 2b (bimorphic), 2c and 2d exhibit different extents of distortions of the PSeBr 2 moieties and different patterns of intermolecular soft–soft interactions. In mixtures containing 1 and 2 , exemplified by the “NMR‐titration” of 1c with molecular bromine, averaged 31 P NMR singlets and their 77 Se satellites indicate rapid intermolecular bromine exchange reactions (kinetic lability of the Se–Br bonds). Calculations modelling such bromine transfer support nucleophilic attack of R 3 PSe (Se → Br) on an electrophilic Br atom of R 3 PSeBr 2 . Among the phosphane selenides 1a – d , t Bu( i Pr) 2 PSe ( 1d ) gives the largest 77 Se NMR upfield shift and t Bu 2 ( i Pr)PSe ( 1c ) the lowest, that is, 77 Se NMR shifts do not correlate with increasing numbers of tert ‐butyl groups. GIAO‐HF/962+(d) calculations on the 77 Se NMR shifts of compounds 1 allow correlation of surprising relative deshielding of 1c , compared with 1b and 1d , with its particular population of rotamers (excluding a rotamer with anti arrangement of the SePCH moiety in 1c ). Bromine addition to compounds 1 leads to line broadening and extreme deshielding in the 77 Se NMR spectroscopy. Reaction of 2b with bromine leads – inter alia – to P–Se cleavage with P ‐bromination. The structures of 1b , 2b – d and t Bu 2 ( i Pr)PBr 2 ( 3c ) were determined by X‐ray crystallography. In compounds 2b – d , intramolecular C–H ··· Br interactions determine the conformation to a large extent. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)