Frustrated Lewis pairs (FLPs) has become one type of non-metallic catalysts for the catalytic hydrogenation since their seminal discovery in 2006.[5,6] Especially, the FLP-catalyzed asymmetric hydrogenation has witnessed a rapid growth over the past 15 years.[7] A wide range of unsaturated compounds containing C=N, C=O, and C=C bonds have proven to be valuable substrates for this metal-free reaction.[8] Interestingly, FLPs exhibit some unique catalytic activity and selectivity during the hydrogenation of N-heteroarenes, such as quinolines[9] and quinoxalines.[10] Strangely, phenanthridines have seldom been employed as substrates for the FLP-catalyzed hydrogenation. Herein, we wish to report our efforts on the metal-free asymmetric hydrogenation of phenanthridines with chiral boranes (Scheme 1).
Results and Discussion
Initially, we subjected 6-methylphenanthridine (1a ) into the asymmetric hydrogenation with the use of chiral borane derived from chiral diene 3a (Scheme 2). A 48% conversion was observed but only led to a racemic product, which might be attributed to the small steric hindrance and the little difference between the two sides of the N-ring. When a tert -butyl substituent was introduced at 3-position of phenanthridine (1b ), the conversion was improved to 83% along with a 11% ee. Furthermore, when 1c was employed as a substrate, the desired product 4c was obtained in 83% conversion with 16% ee.
Scheme 2 Initial studies on the asymmetric hydrogenation of phenanthridines