The effect of the tert-butyl substituent at the phosphorus atom on the conformations of the 1,3,2-dioxaphosphepines with planar fragments

According to X ray crystallographic analysis of compounds 1 and 2, both molecules have a chair con formation with the equatorial tert butyl substituent. Comparison of the geometric analysis of the rings of 1,3,2 dioxaphosphepines, phosphorinanes [2], and seven membered tert butylphosphonates with oxirane and thiirane planar fragments [3] indicates that the equatorial tert Bu group has no effect on the internal geometry of the ring. However, the lack of the “fro zen” NMR spectra of compounds 1 and 2 prevented us from drawing firm conclusions about their confor mations in solutions. In this context, of considerable interest is studying the effect of the tert butyl group on the geometry and conformations of the dioxaphosphepine ring, as well as analysis of experimental NMR data with the use of quantum chemical calculations. Quantum chemical calculations were performed by the hybrid B3PW91 density functional theory method [4, 5] with the use of the 6 31G(d) basis set. In all cases, all geometrical parameters of the molecules were optimized. The cor respondence of the found structures to PES minima was proved by all positive Hessian values. The chemi cal shifts and spin–spin coupling constants (SSCCs) were calculated by the GIAO method [6]. Calculations were performed with the Gaussian 09 program package at the Joint Supercomputer Center, Kazan Branch, Russian Academy of Sciences (http://wt.knc.ru).