Key Intermediates in the Synthesis of 2,2'-Biphosphole

The crystal structures of three key intermediates in the synthesis of 2,2'-biphosphole are reported. Molecules of 2,5-dibromo-3,4-dimethyl-1-phenyl-2,5-dihydrophosphole 1-oxide, C₁₂H₁₃Br₂OP, and 2,5-dibromo-3,4-dimethyl-1-phenyl-2,5-dihydrophosphole 1-sulfide, C₁₂H₁₃Br₂PS, each contain a phospholene ring with a chair conformation. In the molecules, the [alpha]-H atoms are trans to the P=O or P=S groups, thus demonstrating that the sulfurization takes place with the retention of the sterochemistry at phosphorus. In 2-bromo-3,4-dimethyl-1-phenylphosphole 1-sulfide, C₁₂H₁₂BrPS, the phosphole ring is perfectly planar.

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