Barriers to the intramolecular N- to N-transfer of different groups in pyrazoles: Prototropy vs. elementotropy

Barriers to the N- to N-transfer of different groups in pyrazole have been calculated at different levels (B3LYP and MP2 with 6-31G*, 6-311G*, and CEP-121 basis sets). Ten groups studied were H, BH2, CH3, CHO, AlH2, SiH3, GaH2, GeH3, SnH3, and HgH. Two types of different transition states were found: The most common is a triangular situation with the group symmetrically linked to both N atoms. For metals M of the group 13 (B, Ga, Al) that situation is a second minimum while the TS corresponds to a rotation about the N-M bond plus a displacement of the migrating group to yield a nonsymmetric TS. A relationship between the barrier and the geometry has been found. (c) 2005 Wiley Periodicals, Inc.