N-Phenyl-C,C-diphenylketenimine adds to -1-dibutylboryl-2-diphenylphos
phino-2-phenylethene to give aphenyl-1-aza-5-phosphonia-2-boratacycloh
ex-3-ene, which rearranges into ino-1,1,3,5-tetraphenyl-3-aza-4-borade
ca-1,5-diene upon heating. Its crystal structure has been established
by X-ray analysis.