Communication
Organic & Biomolecular Chemistry
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We are grateful for the financial support from the National
Natural Science Foundation of China (21572231 and
21521002).
Fig. 1 A comparison of two boron Lewis acids.
Notes and references
1
For leading reviews, see: (a) J. G. de Vries and C. J. Elsevier,
The Handbook of Homogeneous Hydrogenation, Wiley-VCH,
Weinheim, 2007; (b) G. Ertl, H. Knözinger, F. Shcüth and
J. Weitkamp, Handbook of Heterogeneous Catalysis, Wiley-
VCH, Weinheim, 2008.
afford the corresponding amines 7a–i in 84–99% yields with
4
5–79% ee values. The hydrogenation of imines 6j–l bearing
substituents at p- and m-positions gave amines 7j–l in 91–97%
yields with 72–89% ee values. Imines 6m and 6n were suitable
substrates to furnish the desired products 7m and 7n with
2
3
For a seminal work, see: G. C. Welch, R. R. San Juan,
J. D. Masuda and D. W. Stephan, Science, 2006, 314, 1124.
For leading reviews, see: (a) D. W. Stephan and G. Erker,
Angew. Chem., Int. Ed., 2010, 49, 46; (b) T. Soós, Pure Appl.
Chem., 2011, 83, 667; (c) G. Erker, Pure Appl. Chem., 2012,
6
6% and 65% ee, respectively. Moreover, imines 6o–t with
p-methoxylphenyl, p-methylphenyl, or o-methoxylphenyl
groups as the N-protecting groups were also effective substrates
for the metal-free asymmetric hydrogenation to give amines
84, 2203; (d) D. W. Stephan, Org. Biomol. Chem., 2012, 10,
7
o–t in high yields with 55–73% ee values.
5740; (e) J. Paradies, Angew. Chem., Int. Ed., 2014, 53, 3552;
A comparison of chiral boron Lewis acids derived from
(
(
f) D. W. Stephan, Acc. Chem. Res., 2015, 48, 306;
g) D. W. Stephan and G. Erker, Angew. Chem., Int. Ed.,
chiral diene and chiral alkene 4i with the same substituents at
the 3,3′-positions was conducted for the asymmetric hydrogen-
ation of imine 6a. Chiral boron Lewis acid 2′ gave 49% ee in
favor of R absolute configuration, while 5i gave 65% ee in favor
of S absolute configuration (Fig. 1). Preliminary theoretical
studies were carried out to clarify some differences between
chiral boron Lewis acids 2′ and 5i utilizing the M06-2X
2015, 54, 6400; (h) D. W. Stephan, J. Am. Chem. Soc., 2015,
137, 10018; (i) D. W. Stephan, Science, 2016, 354, aaf7229.
4
5
D. Chen and J. Klankermayer, Chem. Commun., 2008, 2130.
For leading reviews on asymmetric hydrogenations, see:
(
(
(
(
(
a) Y. Liu and H. Du, Huaxue Xuebao, 2014, 72, 771;
b) X. Feng and H. Du, Tetrahedron Lett., 2014, 55, 6959;
c) L. Shi and Y.-G. Zhou, ChemCatChem, 2015, 7, 54;
d) J. Paradies, Top. Organomet. Chem., 2017, 62, 193;
e) W. Meng, X. Feng and H. Du, Acc. Chem. Res., 2018, 51,
9
10
method at the 6-31G(d) level. The dihedral angles for 2′ and
i were 109.50(5) and 69.25(3) degrees, respectively. ADCH
(
5
1
1
atomic dipole moment corrected Hirshfeld population) ana-
lysis showed that atomic charges of borons are 0.191560 a.u.
for 5i, 0.129395 and 0.084396 a.u. for 2′, respectively, which
indicates that chiral boron Lewis acid 5i possesses a stronger
Lewis acidity. The detailed asymmetric induction process is
still not clear at present and awaits further mechanistic study.
1
91.
6
(a) D. Chen, Y. Wang and J. Klankermayer, Angew. Chem.,
Int. Ed., 2010, 49, 9475; (b) V. Sumerin, K. Chernichenko,
M. Nieger, M. Leskelä, B. Rieger and T. Repo, Adv. Synth.
Catal., 2011, 353, 2093; (c) Z. M. Heiden and
D. W. Stephan, Chem. Commun., 2011, 47, 5729;
(
d) D. W. Stephan, S. Greenberg, T. W. Graham, P. Chase,
J. J. Hastie, S. J. Geier, J. M. Farrell, C. C. Brown,
Z. M. Heiden, G. C. Welch and M. Ullrich, Inorg. Chem.,
2011, 50, 12338; (e) G. Ghattas, D. Chen, F. Pan and
J. Klankermayer, Dalton Trans., 2012, 41, 9026; (f) D. Chen,
V. Leich, F. Pan and J. Klankermayer, Chem. – Eur. J., 2012,
18, 5184; (g) M. Mewald and M. Oestreich, Chem. – Eur. J.,
2012, 18, 14079; (h) J. Hermeke, M. Mewald and
M. Oestreich, J. Am. Chem. Soc., 2013, 135, 17537;
(i) M. Lindqvist, K. Borre, K. Axenov, B. Kótai, M. Nieger,
M. Leskelä, I. Pápai and T. Repo, J. Am. Chem. Soc., 2015,
137, 4038; ( j) J. Lam, B. A. R. Günther, J. M. Farrell,
P. Eisenberger, B. P. Bestvater, P. D. Newman, R. L. Melen,
Conclusions
In summary, a variety of readily available chiral alkenes were
prepared in one step from chiral binaphthols. Using chiral
boron Lewis acids generated in situ by the hydroboration of
chiral alkenes with Piers’ borane, a metal-free asymmetric
hydrogenation of imines has been successfully realized, to give
a variety of optically active amines in 84–99% yields with
4
5–89% ee values. Further efforts on searching for more
effective chiral catalysts, exploring the mechanism, and
expanding the application of these chiral boron Lewis acids in
other asymmetric reactions are underway in our laboratory.
8688 | Org. Biomol. Chem., 2018, 16, 8686–8689
This journal is © The Royal Society of Chemistry 2018