Unexpected formation of bis-pyrazolyl derivatives by solid support coupled with microwave irradiation

Starting from 1,3-dimethyl-5-aminopyrazole 1 and p-substituted benzaldehydes 2 (R=H, CH3, NO2), four different compounds have been obtained: the Schiff bases 4, a bis-pyrazolyl Schiff base 6a (R=H), the expected bispyrazolo[3,4-b;4′,3′-e]pyridine 7b (R=CH3) and the carbinol derived from the Schiff base 8c (R=NO2). The products have been characterised by MS, NMR (1H and 13C) and X-ray crystallography (in the case of 6a). A proposal for the relationships between the different compounds and a possible mechanism is presented.