1-(2,4,6-TRI-TERT-BUTYLPHENYL)-3-METHYLPHOSPHOLE - A PHOSPHOLE WITH ASIGNIFICANTLY FLATTENED PHOSPHORUS PYRAMID HAVING PRONOUNCED CHARACTERISTICS OF AROMATICITY

Single-crystal X-ray analysis of 1-(2,4,6-tri-tert-butylphenyl)-3-meth ylphosphole synthesized to test the effect of the strongly sterically demanding P-substituent on the geometry of the molecule, revealed that the phosphorus pyramid was drastically flattened; the normal out of p lane angle of 65 degrees (formed between the P-substituent and the rin g C-2-P-C-5 plane) was reduced to 45.9 degrees. Consistent with strong electron delocalization, the C-3-C-4 bond length was dramatically sho rtened relative to that for other phospholes, and the Bird index of ar omaticity was 56.5, almost the equivalent of that in pyrrole (59). The phosphole ring, normally resisitant to electrophilic substitutions, u nderwent reaction with acetyl chloride-aluminum chloride, consistent w ith considerable cyclic electron delocalization.