Initial experiments surveyed common hydrogen transfer
catalysts, in both the presence and absence of a hydrogen
acceptor (crotonitrile). No pyrazole formation was observed in
the presence of [Cp*
IrCl2]2, Milstein’s catalyst, or
RuCl2(PPh3)3 (Table 1, entries 1−3). [Ru3(CO)12] and
Xantphos in tert-amyl alcohol, which was utilized by Beller
and co-workers for vicinal diol dehydrogenation,11e provided
9% of the desired pyrazole. RuH2(PPh3)3CO and Xantphos in
the presence of piperidinium acetate, a system developed by
Williams for transfer of hydrogen from alcohols to crotonitrile,
11b,15 successfully yielded 68% of pyrazole 3a (entry 5).
Notably, omission of piperidinium acetate results in no desired
product formation. Substitution of piperidinium acetate with
AcOH boosted the yield to 75% (entry 7). The importance of
acidic additives may be due to acceleration of both aldehyde
condensation steps. Under otherwise identical conditions, but
with a decreased catalyst loading, a 40% yield of pyrazole 3a
was isolated (entry 8). Omission of Xantphos led to unreacted
diol 1a, without any formation of the desired pyrazole (entry