Silylformylation catalyzed by Rh and Rh-Co mixed metal complexes and its application to the synthesis of pyrrolizidine alkaloids

Reactions of hydrosilanes with 1-hexyne catalyzed by Co₂Rh₂(CO)₁₂, Rh₄(CO)₁₂, (^tBuNC)₄RhCo(CO)₄, and Rh(acac)(CO)₂ at 25°C and atmospheric pressure to 10 atm of carbon monoxide give (Z)-1-silyl-2-formyl-1-hexenes (1), which are the products of "silylformylation", and/or (E)-1-silyl-1-hexenes (2). The ratio of silylformylation vs. hydrosilylation products depends on the electronic nature of hydrosilane used, e.g., PhMe₂SiH gives 1 almost exclusively whereas (MeO)₃SiH favors the formation of 2. When trialkylsilanes such as Et₃SiH and EtMe₂SiH are used, the reaction catalyzed by Co₂Rh₂(CO)₁₂ or (^tBuNC)₄RhCo(CO)₄ gives 2,5-bis(n-butyl)-3-silylcyclopent-2-en-1-one (3) as a major product, which is a unique silylcarbocyclization product. Mechanism of the formation of 3 is discussed on the basis of deuterium-labeling experiments. Chemoselective silylformylations of alkenynes, a dialkyne, and an alkynyl nitrile proceed in high yields in which alkene and nitrile functionalities are inert for the reaction. Silylformylation is successfully applied to the syntheses of pyrrolizidine alkaloids, (±)-isoretronecanol and (±)-trachelanthamidine, from 5-ethynyl-2-pyrrolidinone (6) in combination with amidocarbonylation.