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 atmos pheric 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. hydrosil ylation products depends on the electronic nature of hydrosilane used, e.g., PhMe2SiH gives 1 almost exclusively whereas (MeO)3SiH favors th e formation of 2. When trialkylsilanes such as Et3SiH ana EtMe2SiH are used, the reaction catalyzed by Co2Rh2(CO)12 or (tBuNC)4RhCo(CO)4 giv es 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 fo rmation of 3 is discussed on the basis of deuterium-labeling experimen ts. Chemoselective silylformylations of alkenynes, a dialkyne, and an alkynyl nitrile proceed in high yields in which alkene and nitrile fun ctionalities are inert for the reaction. Silylformylation is successfu lly applied to the syntheses of pyrrolizidine alkaloids, (+/-)-isoretr onecailol and (+/-)-trachelanthamidine, from 5-ethynyl-2-pyrrolidinone (6) in combination with amidocarbonylation.