ChemInform Abstract: 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 Co 2 Rh 2 (CO) 12 , Rh 4 (CO) 12 , ( t BuNC) 4 RhCo(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., PhMe 2 SiH gives 1 almost exclusively whereas (MeO) 3 SiH favors the formation of 2 . When trialkylsilanes such as Et 3 SiH and EtMe 2 SiH are used, the reaction catalyzed by Co 2 Rh 2 (CO) 12 or ( t BuNC) 4 RhCo(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.