PHOTOINDUCED SKELETAL REARRANGEMENT OF 1,1-DIARYLSPIROPENTANES

The photochemical reactivities of 1,1-diarylspiropentanes la-c have be en investigated under various photolysis conditions. Upon direct photo lysis or acetone-sensitized photolysis, 1 underwent skeletal rearrange ment to afford methylidenecyclobutanes 2, 3 and 4. A mechanism involvi ng diradical intermediates has been proposed. Also studied were the ph otochemical and thermal reactions of the electron donor-acceptor (EDA) complexes of 1 with tetracyanoethylene (TCNE). Photoirradiation of th e charge-transfer (CT) absorption bands of the EDA complexes resulted in the skeletal rearrangement of 1 to 2 and 3, with concomitant format ion of TCNE adducts 11b,c and 12a-c. The X-ray structure of Ile is rep orted. A mechanism involving ion radical pairs [1'(+), TCNE'(-)] is pr oposed for the photoreaction. In contrast, no skeletal rearrangement w as observed in the thermal reaction of the EDA complexes although 11b and 11c were produced.