A. Lei, M. Chen, M. He, X. Zhang
SHORT COMMUNICATION
reduced while the carbonyl group remained intact. Hydro-
genation products 8a and 8b were obtained in 69.1% ee and
65.5% ee, respectively (Scheme 4). The obtained hydroge-
nated products are functionalized piperidines, which are im-
portant building blocks for organic synthesis.
Experimental Section
General Procedure for the Asymmetric Hydrogenation of Pyridine
Derivatives: To a solution of the vinylogous amide substrate
(0.2 mmol) in dichloromethane (3.0 mL) in a glove box was added
[Rh{(S,S,R,R)-TangPhos}NBD][SbF6] (0.004 mmol). The hydro-
genation was performed at 80 °C under 1500 psi of hydrogen pres-
sure for 72 h. After the hydrogen was released, the reaction mixture
was passed through a short silica gel column to remove the catalyst.
The (R) configuration was assigned by comparison with (S)-nipec-
otic acid ethyl ester. Enantiomeric excesses were determined by
HPLC with Chiralcel AD and OJ-H columns (1 mL/min, hexane/
iPrOH = 95:5).
Supporting Information (see footnote on the first page of this arti-
cle): General procedures for the preparation of N-carbamate and
N-acyl vinylogous amides as well as their asymmetric hydrogena-
tion reactions. Characterization information of represented com-
pounds.
Scheme 4. Asymmetric hydrogenation of N-methoxyl carbamate
vinylogous amides 6.
Partial hydrogenation of ethyl nicotinate in the presence
of 1.2 equiv. of Ac2O with THF as the solvent was investi-
gated, and we are pleased to find that the reaction produced
3a in high yield and excellent selectivity. Further experimen-
tation revealed that this reaction could be carried out with-
out the use of any solvent in a 100 g-scale of the nicotinate
ester [Equation (3)]. The product after filtration (removal of
the Pd/C catalyst) and washing (removal of AcOH and
Ac2O) could be directly employed in the asymmetric hydro-
genation step.
Acknowledgments
We would like to acknowledge the Start-up Fund of Green Cataly-
sis Institute from Wuhan University, National Natural Science
Foundation of China (20502020), and X. Z. thanks the NIH and
NSF grants.
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