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-degrees-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 ana 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, (+/-)-isoretronecailol and (+/-)-trachelanthamidine, from 5-ethynyl-2-pyrrolidinone (6) in combination with amidocarbonylation.