A highly enantioselective synthesis of chiral fluorinated propargylamines was developed through phosphoric acid and ruthenium-catalyzed chemoselective biomimetic hydrogenation of the carbon-nitrogen double bond of fluorinated alkynyl ketimines in the presence of a carbon-carbon triple bond. This reaction features high chemoselectivity and slow background reaction. In addition, selective transformations of the chiral fluorinated propargylamines were also reported.

A series of tunable and regenerable biomimetic hydrogen sources, 4,5-dihydropyrrolo[1,2-a]quinoxalines, have been synthesized and applied in biomimetic asymmetric hydrogenation of 3-aryl-2H-benzo [b] [1,4]oxazines and 1-alkyl-3-aryl-quinoxalin-2(1H)-ones, providing the chiral amines with up to 92% and 89% ee, respectively.B iomimetic approaches of asymmetric-transfer hydrogenation (ATH) reactions have emerged as a preeminent synthetic method for the preparation of chiral molecules in the chemists' repertoire. 1 Since pioneering reports in the 1980s, Hantzsch ester (HEH) or related compounds 2,3 were the only superior biomimetic hydride source for a long time, until Akiyama and co-workers demonstrated another hydride transfer reagent, benzothiazoline ( Figure 1). 4 However, in ordinary, stoichiometric or excessive amount of HEH or benzothiazoline was needed and a substantial number of dehydrogenation wastes generated in these transformations, which obviously limits the application of these specific hydrogen sources in both industry and academia. Consequently, the development of ATH reactions with regenerable hydrogen source is strongly desired.Very recently, our group discovered that Hantzsch ester 5 or dihydrophenanthridine (DHPD) 6 could be regenerated in situ by Ru(II) complexes under hydrogen gas, which had been employed in the biomimetic asymmetric hydrogenation of heteroaromatics and cyclic imines with excellent enantioselectivities. Remarkably, the demand for hydrogen source could be reduced to a catalytic amount (10 mol %). Although such progress has been achieved, the harsh regeneration conditions of HEH and limited derivatization possibility of DHPD impelled us to seek for easy tunable and regenerable versatile hydrogen sources.The foregoing results have demonstrated that development of a regenerable biomimetic hydrogen source should fulfill the following requirements: (i) regenerate under mild conditions as well as with high hydride transfer ability and (ii) easy control of the reaction enantioselectivity and simultaneously with various derivatization possibilities. Based on these guidelines, we began our studies through investigating the transfer hydrogenation ability of 4,5-dihydropyrrolo[1,2-a]quinoxalines, which are easily obtained through the mild partial hydrogenation of corresponding pyrrolo[1,2-a]quinoxalines (Scheme 1). In addition, the latter compounds could be easily prepared and derived from the simple starting materials. 7 The readily available imine 3-phenyl-2H-benzo [b][1,4]-oxazine 3a 8 was selected as the model substrate for condition optimization (Table 1). Gratifyingly, the exposure of ketimine 3a with pyrrolo[1,2-a]quinoxaline 1a (10 mol %) in the presence of chiral phosphoric acid 5a and [Ru(p-cymene)I 2 ] 2 at room temperature furnished amine 4a with 88% ee and 75% of conversion (entry 1). Notably, the reaction failed to proceed in the absence of 1a (entry 7). Through screening the reaction

Homogeneous Pd-catalyzed asymmetric hydrogenation of 3-phthalimido substituted quinolines was successfully developed, providing facile access to chiral substituted tetrahydroquinolines bearing two contiguous stereogenic centers with up to 90% ee.