The Bromination of Bulky Trialkylphosphane Selenides R2R′PSe (R, R′ = iPr or tBu) Studied by Physical and Computational Methods

Abstract

AbstractBulky trialkylphosphane selenides tBu3PSe (1a), iPr3PSe (1b), tBu2(iPr)PSe (1c) and tBu(iPr)2PSe (1d) react with one equiv. of bromine providing “T‐shaped” products R2R′P–SeBr2 (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 PSeBr2 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 31P NMR singlets and their 77Se satellites indicate rapid intermolecular bromine exchange reactions (kinetic lability of the Se–Br bonds). Calculations modelling such bromine transfer support nucleophilic attack of R3PSe (Se → Br) on an electrophilic Br atom of R3PSeBr2. Among the phosphane selenides 1a–d, tBu(iPr)2PSe (1d) gives the largest 77Se NMR upfield shift and tBu2(iPr)PSe (1c) the lowest, that is, 77Se NMR shifts do not correlate with increasing numbers of tert‐butyl groups. GIAO‐HF/962+(d) calculations on the 77Se 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 77Se NMR spectroscopy. Reaction of 2b with bromine leads – inter alia – to P–Se cleavage with P‐bromination. The structures of 1b, 2b–d and tBu2(iPr)PBr2 (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)