The Photochemistry of Isobenzofuran. I. Structure of the Dimers Resulting From Ultraviolet Irradiation of Isobenzofuran in Acetone and Ether Solution

Abstract

Irradiation of isobenzofuran (1) in acetone solution yields a symmetrical [π8s+π8s] dimer (24) involving bonding at the peri -position of the furan moiety in each molecule. The anti-stereochemistry of this dimer was established by a novel application of lanthanide induced shift spectroscopy. In contrast, irradiation of isobenzofuran (1) in ether solution yielded the unsymmetrical dimer (34) as the major product, together with lesser amounts of the symmetrical dimer (24), and small amounts of a new dimer (50) resulting from [π8s+π4s] cycloaddition , where the 8π system of the isobenzofuran of one molecule reacts with the carbocyclic 4π diene of the other. The structure of the unsymmetrical dimer (34) was confirmed by synthesis. No evidence for the Dewar form (7e) of isobenzofuran could be obtained in these reactions conducted at -60° and monitored by 1H n.m.r. spectroscopy. However, the small, but persistent, production of o-phthalaldehyde may implicate an intermediate derived from (7e).