Relative energies of conformations and sulfinyl oxygen‐induced pentacoordination at silicon in 4‐bromo‐ and 4,4‐dibromo‐4‐silathiacyclohexane 1‐oxide: A computational study

Abstract

AbstractThe equilibrium geometries and relative energies of the chair, twist, and boat conformations of cis‐ and trans‐4‐bromo‐4‐silathiacyclohexane 1‐oxide and 4,4‐dibromo‐4‐silathiacyclohexane 1‐oxide have been calculated at the B3LYP/6‐311G(d,p) and MP2/6‐311+G(d,p) theoretical levels. The axial (SO) chair conformers of the sulfoxides are of lower energy than the chair conformers of the corresponding equatorial (SO) sulfoxides. The chair conformer of the axial (SO) trans‐4‐bromo‐4‐silathiacyclohexane 1‐oxide is only 0.10 kcal/mol more stable than the corresponding 1,4‐boat conformer which is stabilized by a transannular coordination of the sulfinyl oxygen with silicon that results in trigonal bipyramidal geometry at silicon. The 1,4‐boat structure of equatorial (SO) trans‐4‐bromo‐4‐silathiacyclohexane 1‐oxide is a transition state and is 5.77 kcal/mol higher in energy than the respective chair conformer. The 1,4‐boat conformer of axial (SO) 4,4‐dibromo‐4‐silathiacyclohexane 1‐oxide is also stabilized by transannular coordination of the sulfinyl oxygen and silicon, but it is 4.31 kcal/mol higher in energy than the corresponding chair conformer. The relatively lower stability of the 1,4‐boat conformer of 4,4‐dibromo‐4‐silathiacyclohexane 1‐oxide may be due to repulsive interactions of the axial halogen and sulfinyl oxygen atom. The relative energies of the conformers and transition states are discussed in terms of hyperconjugative interactions, orbital interactions, nonbonded interactions, and transannular sulfinyl oxygen‐silicon coordination. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005