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 of N-(2-pyridinyl)piperazines with CO (15 atm) and ethyle ne in the presence of a catalytic amount of Rh-4(CO)(12) in toluene at 160 degrees C resulted in a novel carbonylation reaction, which invol ves dehydrogenation and carbonylation at a C-H bond. The carbonylation takes place regioselectively at a C-H bond a to the nitrogen atom sub stituted by a pyridine. The presence of an additional nitrogen functio nality at the 4-position of the piperazine ring is also essential for the reaction to proceed. The electronic nature of substituents, both o n the 4-nitrogen and in the pyridine ring, affects significantly the r eactivity 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) dehyd rogenation of the piperazine ring and (ii) carbonylation at a C-H bond in the resulting olefin. The reaction proceeds via two cleavages of t he C-H bond, first at the sp(3) C-H bond and then at the sp(2) C-H bon d. The reaction stops at the dehydrogenation step by replacement of th e pyridinyl group with a phenyl group. Rhodium complexes are the only active catalysts for the present reaction.