. Can. J. Chem. 65, 1457Chem. 65, (1987. Crystals of 8-methyl-6,6-diphe~yl-5,7-dioxa-8-azonia-6-borata-6,7-dihydro-5H-be11~0~ycloheptene are triclijc, a = 12.917(1), b = 13.285(1), c = 10.567(1) A, a = 94.183(8), P = 90.094(5), y = 72.176(6)", Z = 4, space group P 1. The structure was solved by direct methods and was refined by full-matrix least-squares procedures to R = 0.045 and R,,, = 0.042 for 2914 reflections with I 2 2u(I). The envelope-like conformation adopted by the seven-membered chelate ring places one B-phenyl group above the plane of the fused-ring system facilitating a thermally-induced 1,4-migration of a B-phenyl group to the nitrone carbon atom. Mean bond lengths (corrected for libration) are (N)O-B = 1.559 (6)