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