New Insights in Frustrated Lewis Pair Chemistry with Azides

The geminal frustrated Lewis pair (FLP) tBu(2)PCH(2)BPh(2) (1) reacts with phenyl‐, mesityl‐, and tert‐butyl azide affording, respectively, six, five, and four‐membered rings as isolable products. DFT calculations revealed that the formation of all products proceeds via the six‐membered ring structure, which is thermally stable with an N‐phenyl group, but rearranges when sterically more encumbered Mes−N(3) and tBu−N(3) are used. The reaction of 1 with Me(3)Si−N(3) is believed to follow the same course, yet subsequent N(2) elimination occurs to afford a four‐membered heterocycle (5), which can be considered as a formal FLP‐trimethylsilylnitrene adduct. Compound 5 reacts with hydrochloric acid or tetramethylammonium fluoride and showed frustrated Lewis pair reactivity towards phenylisocyanate.