Hydrogenation and Hydrosilylation of Quinoxaline by Homogeneous Rhodium Catalysts

Hydrogenation of quinoxalines by a homogeneous rhodium catalyst gives tetrahydroquinoxalines.The catalytic process is applicable for hydrosilylation of quinoxallnes.2-Methylquinoxaline is asymmetrically hydrosilylated by a chiral catalyst.Catalytic homogeneous hydrogenation has been used as an important procedure in organic synthesis.It is the greatest merit that the catalyst can be modified for asymmetric synthesis.There have been reported various examples o f asymmetric hydrogenation of olefins, dehydroamino acids, and carbonyl compounds and. in some cases, a perfect enantioselectivity has been achieved.'O n the contrary, a few examples have been reported on the application for imines and nitrogen heterocyclic c o m ~o u n d s .~ In this paper, we report homogeneous hydrogenation and hydrosilylation of quinoxalines with an easily available rhodium complex and its application for asymmetric reduction.Hydrogenation of 2-methylquinoxaline [ll with a hydridorhodium catalyst containing 1.4-bisldipheny1phosphino)butane IDPPB) ligand [(DPPB)RhHI proceeded in ethanol under Hz (7-10 kg/cm2) at 25 ' C to give 2-methyltetrahydroquinoxaline [ 2 1 .Triphenylphosphine, 1.2-bis(dipheny1phosphina)ethane.and 1.3-bis(dipheny1-phosphin0)propane were ineffective as the ligand, and the best ligand was ( + ) - 2,3-Q-isopropylidene-2,3-dihydroxy-1,4-bisdipheny1ph0sphin0butane [ (+I-DIOPI.~The hydrogenation was applicable for quinoxaline and 2-propylquinoxaline.The catalyst [(+I-(D1OP)RhHl was also effective for hydrosilylation of these quinoxalines by dlphenylsilane and were obtained lj,lj'-bis(diphenylsily1)tetra- hydroquinoxalines which were easily desilylated to tetrahydroquinoxalines by