a All reactions were carried out with1c (0.1 mmol),
HB(C6F5)2 2(0.01 mmol), chiral alkene 5a (0.01 mmol), and
H2 (40 bar) in solvent (0.5 mL) at 30 °C for 24 h.b Determined by crude 1H NMR.c Determined by chiral HPLC.d With 1.0 mL solvent.e With 0.2 mL solvent.f For 16 h.
Under optimal conditions, the scope of substrates for the asymmetric
hydrogenation was investigated. As illustrated in Scheme 4, a variety of
electron-donating and withdrawing substituents on the phenyl motif were
well tolerated for this reaction, delivering the desired
dihydrophenanthridines 4c-r in 81-96% yields with 57-93%
ee’s. Notably, a fluorine substituent at the 3-position of
phenanthridine (1o ) led to the corresponding product4o a promising 93% ee. The absolute configuration of compound4o was designed as R according to its X-ray structure
analysis.[11] The absolute configuration of other
analogs was tentatively assigned accordingly. When the substitution at
the 6-position of phenanthridine was replaced with an ethyl group
(1s ), product 4s was yielded with 78% ee.
Furthermore, when methyl groups were used instead of tert -butyl
groups at the 7- and 9- positions (1t ), a decreased 23% ee was
obtained. While, changing with phenyl groups, the desired product4u was obtained in a 94% yield and 83% ee were obtained.
Unfortunately, bulky phenanthridines 1v -x were still
inert substrates for the current catalytic system. Further efforts on
the utilization of chiral products 4 as chiral hydrogen sources
for the transfer hydrogenation of various unsaturated compounds failed,
which may be attributed to the bulky steric hindrances of them.
Scheme 4 Asymmetric hydrogenation of phenanthridines