Highly efficient synthesis of aldenamines from carboxamides by iridium-catalyzed silane-reduction/dehydration under mild conditions

摘要

The combination of IrCl(CO)(PPh_3)_2 with 1,1,3,3-tetramethyl-disiloxane or poly(methylhydrosiloxane) (PMHS) is found to be an efficient catalyst system for the preparation of aldenamines from carboxamides; in particular, facile removal of silicone and indium residues from the product can be achieved by the use of PMHS.rnEnamines are of great utility in organic synthesis because they act as isolable nucleophiles equivalent to enolates. Enamines are generally synthesized by acid assisted condensation of secondary amines and carbonyl compounds. Although catalytic dehydrogenation of amines, cross-coupling between amines and alkenyl halides, and hydroamination of alkynes are also effective methods, these require long reaction times at high temperatures. Recently, Belanger et al. have reported the 4 A molecular sieves (MS 4A)-promoted condensation of aldehydes and amines. The reaction proceeds even at 0 ℃ within a few hours, however, this method requires large amounts of MS 4A (1.16 g mmol~(-1) of carbonyl compound). Furthermore, in most of the above methods, distillation or chromatography is necessary for removing the residual materials, which results in the lowering of the yield of enamine. Thus, the development of a new catalyst system, which shows high catalytic activity under mild conditions and realizes a facile isolation of labile enamines, is an important target in organic synthesis.