Phospholes with reduced pyramidal character from steric crowding .3. NMR and X-ray diffraction studies on 1-(2,4,6-tri-isopropylphenyl)-3-methylphosphole

The 2,4,6-tri-isopropylphenyl substituent was placed on the phosphorus of a phosphole to reduce the pyramidal character. That this was accomplished was revealed by single crystal X-ray diffraction analysis; with respect to the plane of C-2-P-C-5 in the phosphole ring, the ipso carbon of the benzene ring was deflected by only 58.0 degrees, whereas the deflection is 66.9 degrees in the uncrowded 1-benzylphosphole. This proves that the concept of reducing the pyramidal character (with the goal of increasing the electron delocalization) through steric crowding can be realized. In the crystal the two rings are in orthogonal planes, but this relation is not retained in solution; NMR studies show that the two edges of the benzene ring, as well as the 2,6-isopropyl groups, are identical.