Rhodium-catalyzed reaction of N-(2-pyridinyl)piperazines with CO and ethylene. A novel carbonylation at a C-H bond in the piperazine ring

The reaction N-(2-pyridinyl)piperazines with CO (15 atm) and ethylene in the presence of a catalytic amount of Rh₄(CO)₁₂ in toluene at 160°C resulted in a novel carbonylation reaction, which involves dehydrogenation and carbonylation at a C-H bond. The carbony-lation takes place regioselectively at a C-H bond α to the nitrogen atom substituted by a pyridine. The presence of an additional nitrogen functionality at the 4-position of the piperazine ring is also essential for the reaction to proceed. The electronic nature of subsituents, both on the 4-nitrogen and in the pyridine ring, affects significantly the reactivity of the substrates. The substitution of an electron-donating group on the 4-nitrogen causes an increase in reactivity, as does the substitution of an electron-withdrawing group in the pyridine ring. It is found that the reaction involves two discrete reactions: (i) dehydrogenation of the piperazine ring and (ii) carbonylation at a C-H bond in the resulting olefin. The reaction proceeds via two cleavages of the C-H bond, first at the sp³ C-H bond and then at the sp² C-H bond. The reaction stops at the dehydrogenation step by replacement of the pyridinyl group with a phenyl group. Rhodium complexes are the only active catalysts for the present reaction.