Synthesis, stereochemistry, and crystal and molecular structure of 4-bromo-1,2,2,3-tetramethyl-1-phenylphosphetanium bromide

The synthesis of 4-bromo-2,2,3-trimethyl-1-phenylphosphetan 1-oxide was achieved by reaction of 1-bromo-3,3-dimethylbut-1-ene with phenylphonous dichloride in the presence of anhydrous aluminium chloride, followed by the addition of water. The oxide was converted into the title compound by reduction (Cl₃SiH) and then quaternization with methyl bromide. The title compound crystallizes in the monoclinic space group P2₁/n, with four molecules in a unit cell of dimensions a = 7.653(1), b = 12.662(3), c = 15.487(3) A° and β = 97.58(2°). The structure was solved by the heavy-atom method and refined to a final R value of 0.065. The four-membered ring is puckered with a dihedral angle of 34.0°. The methyl substituent at C(3) and the phenyl group have a cis-relation, whereas the bromo-substituent and phenyl ring are trans. All three of these groups occupy equatorial positions with respect to the ring.