nitroalkanes in good to excellent selectivity under operation-
ally straightforward conditions.
withdrawing groups on mono-N-tosylated diamines led to
remarkable improvement in selectivity and reactivity. In
particular, stilbene diamine ligands derivatized as perfluori-
nated sulfonamides were notable (Table 1, entries 2-7).
Finetuning of the aryl-groups and re-examination of the
sulfonyl-group led to a modest but significant increase in
selectivity (Table 1, entry 7). Interestingly, this reactivity
trend stands in sharp contrast to what is observed with
the typical Ru(II) based catalyst systems and molecular
hydrogen as well as the Ir(III) based catalysts in water
and formic acid.2b,6
There is ample precedence for the use of diamine derived
Ru complexes in transfer hydrogenation reactions, as exem-
plified by the catalyst systems developed by Noyori and
Ikariya.1b,2 In contrast, there was little precedence available
to guide our initial investigations with Ir catalysts in water.
Recent studies by Ogo and Fukuzumi document water-
soluble [Cp*Ir(bpy)(H2O)](SO4) catalysts that perform achiral
reductions of simple ketones in water at pH 5.3 Yet, whether
the ligands in the parent complex could be replaced by chiral
donors without altering the stability or reactivity of the Ir
complex was unclear at the time that we commenced our
studies.6 Initial screening and optimization studies relied upon
preparation of iridium(III) complexes derived from optically
active diamines.7 To our delight, the corresponding com-
plexes 3 are simply prepared by combining the known Ir(III)
trihydrate complex 1 with a donor ligand 2 of choice in
aqueous methanol at ambient temperature. Solvent evapora-
tion furnished aqua Ir complexes as air stable solids in
quantitative yields (Scheme 1).
We then proceeded to examine the scope of nitroalkenes
which would undergo selective and efficient reduction. As
displayed in Table 2, reduction with catalyst 5 provided
Table 2. Substrate Scope
Since many of the studies about asymmetric transfer
hydrogenations are limited to the reduction of acetophe-
nones and related compounds,1,2,6 we were interested in
expanding the scope of this powerful methodology to other
substrate classes, such as conjugate reduction of activated
double bonds. We choose nitroalkenes8 as valuable targets
since these substrates have only been studied to a limited
extent, namely in enzymatic,5a-c metal-catalyzed,5d-f and
organocatalytic5g reductions. Our initial studies commenced
with nitroolefin 4 as a test substrate and a broad range of
ligands, with aqua Ir(III) complexes in water. Monotosylated
1,2-diphenyldiamines provided a leading result and warranted
closer scrutiny (Table 1, entry 1). The use of strongly electron
entrya b
R
mol % cat.
yield,c %
ee,d e
%
,
,
1
2
3
4
5
6
7
8
C6H5
1.0
1.0
1.0
1.0
1.0
1.0
1.5
1.0
1.0
1.5
90
82
92
92
94
78
94
87
77
56
90
94
90
92
91
90
92
92
89
92
p-F-C6H4
p-Cl-C6H4
p-Br-C6H4
m-Cl-C6H4
p-CH3-C6H4
p-CH3O-C6H4
p-CN-C6H4
p-tBu-C6H4
2-naphthyl
Table 1. Initial Catalyst Screening
9
10f
a Reactions carried out with 0.5 mmol of nitroalkene. b 1.0 M formic
acid solutions were utilized. c Isolated yields. d Determined by chiral HPLC.
e Absolute configuration established by correlation to known compounds,
see the Experimental Section in the Supporting Information. f 40 °C.
excellent yields and good selectivity for a variety of
nitroalkenes, including those substituted with electron with-
drawing and donating groups (Table 2, entries 6-8). A series
entrya
R1
R2
pH
convn,b
%
ee,c %
(3) (a) Ogo, S.; Makihara, N.; Watanabe, Y. Organometallics 1999, 18,
5470. (b) Ogo, S.; Makihara, N.; Kaneko, Y.; Watanabe, Y. Organometallics
2001, 20, 4903. (c) Abura, T.; Ogo, S.; Watanabe, Y.; Fukuzumi, S. J. Am.
Chem. Soc. 2003, 125, 4149. (d) Ogo, S.; Uehara, K.; Abura, T.; Fukuzumi,
S. J. Am. Chem. Soc. 2004, 126, 3020.
1
2
3
4
5
6
7
H
H
H
H
H
Tol
2.0
2.0
3.5
2.0
3.5
3.5
2.0
72
95
91
100
100
94
50
82
86
79
82
85
89
1
CF3
CF3
C6F5
C6F5
C6F5
CF3
(4) For discussions of water as a reaction medium see: (a) Lindstro¨m,
U. M. Chem. ReV. 2002, 102, 2751. (b) Li, C. J. Chem. ReV. 2005, 105,
3095. For examples of ATH in water see: (c) Wu, X.; Liu, J.; Li, X.; Zanotti-
Gerosa, A.; Hancock, F.; Vinci, D.; Ruan, J.; Xiao, J. Angew. Chem., Int.
Ed. 2006, 45, 6718. (d) Li, X.; Blacker, J.; Houson, I.; Wu, X.; Xiao, J.
Synlett 2006, 8, 1155. (e) Wu, X.; Li, X.; King, F.; Xiao, J. Angew. Chem.,
Int. Ed. 2005, 44, 3407. (f) Wu, X.; Li, X.; Hems, W.; King, F.; Xiao, J.
Org. Biomol. Chem. 2004, 2, 1818. (g) Wu, J.; Wang, F.; Ma, Y.; Cui, X.;
Cun, L.; Zhu, J.; Deng, J.; Yu, B. Chem. Commun. 2006, 1766.
2,4-di-F
2,4-di-F
93
b
a Reactions carried out with 0.1 mmol of ketone. Determined by H
NMR of unpurified product. c ee was determined by chiral HPLC.
Org. Lett., Vol. 11, No. 18, 2009
4197