A. C. Fernandes, C. C. Roma˜o / Tetrahedron Letters 46 (2005) 8881–8883
8883
fluoro, chloro, ester or a nitro groupwere chemoselec-
tively reduced to the corresponding amines.
In summary, we have developed a novel method for the
reduction of imines to the corresponding amines in
excellent to moderate yields. This result confirms the
ability of MoO2Cl2 as an effective catalyst for hydrosilyl-
ation reactions. This novel catalytic system is suitable
for chemoselective reduction of imines bearing fluoro,
chloro, ester and nitro groups. Other organic reductions
with this system as well as mechanistic studies are now
under investigation in our group.
In a typical procedure, to a solution of MoO2Cl2
(10 mol %) in dry THF (5 ml) were added the imine
(1.0 mmol) and PhSiH3 (1.0 mmol) under nitrogen
atmosphere. The reaction mixture was stirred at reflux
temperature (the reaction times are indicated in Table
2) and monitored periodically by TLC. Upon comple-
tion, the reaction mixture was diluted with hexane,
loaded directly on to a silica-gel column and chromato-
graphed with the appropriate mixture of n-hexane and
ethyl acetate. All the compounds were characterized
by spectroscopy (IR, NMR) in many instances against
the literature data.
References and notes
1. Hutchins, R. O.; Hutchins, M. K. Comprehensive Organic
Synthesis; Pergamon Press: Oxford, 1991; Vol. 8, pp 25–
78.
Several experiments were carried out in order to ascer-
tain some fundamental mechanistic aspects of this
reduction. No reaction was observed when imine 1
was treated with 10 mol % of MoO2Cl2 without phen-
ylsilane or when imine 1 was treated with excess of
phenylsilane without catalyst in refluxing THF after
24 h. These results show that MoO2Cl2 catalyzes the
reaction by activation of the silane and this rapid reac-
tion is still under investigation. At this point it is not
clear how this activation takes place. A similar activa-
tion of silanes by (PPh3)2Re(O)2I produces a Re–H that
was observed by NMR.17 Unfortunately, all our experi-
ments to detect a Mo–H species in situ were unsuccess-
ful at various concentrations, and temperatures and
with different silanes. However, it is inescapable that a
functional equivalent of a hydride (Mo–H) is produced
since the reaction of imine 1 with dimethylphenylsi-
lane-d, under the same experimental conditions, resulted
in the incorporation of the deuterium only in the carbon
2. Shibata, I.; Moriuchi-Kawakami, T.; Tanizawa, D.; Suwa,
T.; Sugiyama, E.; Matsuda, H.; Baba, A. J. Org. Chem.
1998, 63, 383–385.
3. Minato, M.; Fujiwara, Y.; Ito, T. Chem. Lett. 1995, 647–
648.
4. Mizushima, E.; Yamaguchi, M.; Yamagishi, T. Chem.
Lett. 1997, 237–238.
5. Aida, T.; Kuboki, N.; Kato, K.; Uchikawa, W.; Matsuno,
C.; Okamoto, S. Tetrahedron Lett. 2005, 46, 1667–
1669.
6. Freifelder, M. Catalytic Hydrogenation in Organic Syn-
thesis Procedures and Commentary; John Wiley and Sons:
New York, 1978; pp 65–77.
7. Nishibayashi, Y.; Takei, I.; Uemura, S.; Hidai, M.
Organometallics 1998, 17, 3420–3422.
8. Hashimoto, H.; Aratani, I.; Kabuto, C.; Kira, M.
Organometallics 2003, 22, 2199–2201.
9. Nishibayashi, Y.; Segawa, K.; Singh, J. D.; Fukuzawa,
S.-I.; Ohe, K.; Uemura, S. Organometallics 1996, 15, 370–
379.
10. Verdaguer, X.; Lange, U. E. W.; Reding, M. T.; Buch-
wald, S. L. J. Am. Chem. Soc. 1996, 118, 6784–6785.
11. Hansen, M. C.; Buchwald, S. L. Org. Lett. 2000, 2, 713–
715.
12. Field, L. D.; Messerle, B. A.; Rumble, S. L. Eur. J. Org.
Chem. 2005, 2881–2883.
13. Lipshutz, B. H.; Shimizu, H. Angew. Chem., Int. Ed. 2004,
43, 2228–2230.
14. Ireland, T.; Fontanet, F.; Tchao, G.-G. Tetrahedron Lett.
2004, 45, 4383–4387.
1
of the double bond, as confirmed by H NMR. This re-
sult is consistent with the hydrosilylation of the imine to
give an intermediate N-silylamine, followed by a rapid
hydrolysis, probably, due to the presence of a trace of
water in the imine or, more likely, due to hydrolysis of
the N–Si bond on the silica-gel during work-upand
purification. This mechanism is analogous to that pro-
posed for hydrosilylation of aldehydes and ketones
where a silyl ether is formed that can be later hydrolyzed
to the alcohol. However, nothing can yet be said as to
the nature of the intermediates that are involved in these
hydrogen transfers and the following elimination of the
silylimine from the catalyst.
15. Fernandes, A. C.; Fernandes, R.; Romao, C. C.; Royo, B.
˜
Chem. Commun. 2005, 213–214.
16. Holm, R. H. Chem Rev. 1987, 87, 1401–1449.
17. Kennedy-Smith, J. J.; Nolin, K. A.; Gunterman, H. P.;
Toste, F. D. J. Am. Chem. Soc. 2003, 125, 4056–4057.