[2 + 2]-Cycloadditions to Strained Bridgehead Olefins. I. 1, 1-Dichloro-2,2-difluoroethene

The strained bridgehead olefins bicyclo 3.3.1]non-1-ene ( 1 ), bicyclo 4.2.1] non--1-(8) ( 2 ), and bicyclo 4.2.1]non-1-ene ( 3 ) react rapidly with 1,1-dichloro-2, 2--difluoroethene ( 5 ) to yield mixtures of regioisomeric dichlorodifluorocyclobutanes 8/9, 10/11 and 12/13 , respectively. On the contrary, the reaction of 5 with the model compound (E)-l-methylcyclooctene ( 4 ) is completely regioselective. The structure of the cycloadducts has been elucidated mainly by ¹⁹F-NMR. and ¹³C-NMR. spectroscopy.