Diaryloxycarbenes from oxadiazolines

Symmetric and unsymmetric 2,2-diaryloxy-5,5-dimethyl-Delta (3)-1,3,4-oxadia zolines were synthesized by oxidative cyclization of aryloxycarbonyl hydraz ones of acetone with lead tetraacetate and subsequent treatment of the prod uct mixture with a phenol in acidic solution. Thermolysis of the oxadiazoli nes in benzene solution at 110 degreesC afforded carbonyl ylide intermediat es that cyclize, in part, to the corresponding 2,2-diaryloxyoxirane interme diates. The oxiranes, which were not observed, are required to account for the 1,1-diaryloxy-2-methylpropenes (ketene acetals) that were isolated. Mos t of the carbonyl ylides fragment to acetone and diaryloxycarbenes. The lat ter form dimers (tetraaryloxyethenes) or they can be trapped with phenols t o form orthoformates. Diphenoxycarbene was also trapped with dimethyl acety lenedicarboxylate (DMAD). The method appears to be the first for generating the parent diphenoxycarbene under relatively mild conditions in solution, and the only one to date for generating unsymmetrically substituted diarylo xycarbenes. Minor competing fragmentations of the oxadiazolines to 2-diazop ropane and the appropriate diaryl carbonates, were also observed.