Communications
Table 1: Asymmetric cyclopropanation of substituted olefins using CMOF cata-
respectively. Similarly, 1 and 2 exhibit an uptake of
lysts.[a]
0.4 and 12.4 wt% of crystal violet, respectively.
These results additionally confirm the different
catenation behaviors for 1 and 2.
Treatment of 1 and 2 with strong reducing agents
such as LiBEt3H (superhydride) or NaB(OMe)3H
led to a color change from dark green to dark red,
thus suggesting the reduction of the RuIII centers to
RuII centers in the CMOFs. The reduced CMOFs,
1R and 2R, exhibited very different UV/Vis/NIR
diffuse reflectance spectra from those of 1 and 2
(Figure 2b). Upon reduction of 1 and 2, the
characteristic RuIII/salen ligand-to-metal charge-
transfer (LMCT) bands at 771 nm disappeared,
with concomitant appearance of new peaks at
520 nm that are indicative of the RuII/salen
1MLCT bands.[17] These results were confirmed by
the solution absorption spectra of the corresponding
CMOFs dissolved in pyridine (Figure 2c). Interest-
ingly, the reduction of 1 and 2 occurred in a single-
Entry Catalyst
R
Mol% Yield
d.r.
ee [%] ee [%]
cat.
[%]
(trans)
(cis)
1
2
2R
Ph
Ph
Ph
Ph
Ph
3
7.8
4.2
7
–
65
91
–
51
84
–
2R[b]
3
3
54
1
3
2
4
1R
3
1
1.2
9.6
10.9
2
2.6
1.7
1.7
–
33
92
98
61
77
25
29
–
47
85
92
67
79
13
24
–
5
6
7
8
9
10
11
[Ru(L-Me2)(py)2]
1
1
2
2
2
28
53
20
37
27
47
<1
[Ru(L-Me2)(py)2][b] Ph
2R[b]
OEt
[Ru(L-Me2)(py)2][b] OEt
2R[b]
CH3(CH)2
[Ru(L-Me2)(py)2][b] CH3(CH)2
2
Zn-BPDC
Ph
n/a[c]
[a] For reaction conditions see Experimental section. [b] With NaBH(OMe)3 in
solution. [c] n/a=not applicable.
crystal to single-crystal fashion. 1R and 2R retained their
single-crystal nature with the same space groups and similar
cell parameters as those of 1 and 2, respectively. Single-crystal
X-ray structure studies additionally indicated that the struc-
tures of 1R and 2R are essentially identical to those of 1 and
2, respectively. Even more remarkably, 1R and 2R can be re-
oxidized in air to afford dark green CMOF crystals of 1’ and
2’, which is supported by the diffuse reflectance and solution
UV/Vis/NIR absorption spectra. Single-crystal to single-
crystal reduction/reoxidation processes in 1 and 2 are thus
entirely reversible. PXRD and unit-cell determinations
indicated that 1’ and 2’ remained single crystals with the
same structures as 1 and 2 (Figure 2d and see the Supporting
Information). This work represents the first example of totally
reversible single-crystal to single-crystal reduction/oxidation
processes in MOFs.[8,9]
Nguyen et al. elegantly demonstrated that the [Ru-
(salen)(py)2] complex is a competent homogeneous cyclo-
propanation catalyst that transfers the carbene fragment from
ethyldiazoacetate to various olefins with excellent enantio-
and diastereoselectivities.[18] We hypothesized that the pres-
ent ruthenium/salen-derived CMOFs could catalyze the
cyclopropanation reactions heterogeneously. Using styrene
and other substituted alkenes as test substrates, we have
evaluated the activity of 1R and 2R towards enantio- and
diastereoselective cyclopropanation reactions.
and thus deactivated during the cyclopropanation reactions.
Consistent with this hypothesis, the control reaction with 2 as
the catalyst did not produce any cyclopropane product
(Table 1, entry 3). To prevent the 2R catalyst from oxidizing
and deactivating, we carried out the cyclopropanation reac-
tion in the presence of NaBH(OMe)3. A much improved yield
(54%) of cyclopropane products was obtained under these
reaction conditions, with a d.r. of 7 in favor of the trans
products and ee values of 91% and 84% for the trans and cis
products, respectively (Table 1, entry 2). Interestingly, a
similar cyclopropanation reaction with 1R as the catalyst
gave the desired products in less than 1% yield and with
modest ee values (Table 1, entry 4). The framework inter-
penetration in 1R significantly reduced the open channel
sizes, thus preventing the diffusion of the reagents into the
RuII/salen active sites in the interior of 1R. The beneficial
effect of a reducing agent for the cyclopropanation reaction
was also observed for the homogeneous control reaction with
the [Ru(L-Me2)(py)2] catalyst: The cyclopropane products
were obtained in a yield of 28% and 53% in the absence or
presence, respectively, of NaBH(OMe)3 (Table 1, entries 5
and 6). Finally, we have shown that 2R is also an active
catalyst for the cyclopropanation of both 1,3-pentadiene and
ethyl vinyl ether, albeit in lower yields and with lower d.r. and
ee values, as is the case for [Ru(L-Me2)(py)2]-catalyzed cyclo-
propanation reactions (Table 1, entries 7–10).
After reduction of 2 with LiBEt3H, the resulting 2R was
washed repeatedly with anhydrous THF and then with
dichloromethane. The cyclopropanation reaction between
styrene and ethyldiazoacetate was carried out in the presence
of 3 mol% of 2R under anaerobic conditions for 24 hours.
Disappointingly, the 2R-catalyzed cyclopropanation reaction
afforded the cyclopropane products in less than 8% yield with
a trans/cis diastereomeric ratio (d.r.) of 4.2 (Table 1, entry 1).
The enantiomeric excesses (ee) for the trans- and cis-cyclo-
propane products were 65% and 51%, respectively. We
noticed that the color of the 2R suspension always turned
dark green during the cyclopropanation reactions, and we
reasoned that the RuII centers in 2R were readily oxidized
We have also tested the heterogeneity of the CMOF
catalysts. The supernatants from the cyclopropanation reac-
tions after filtration through a 0.45 mm filter did not afford
additional cyclopropane products. The CMOF catalysts were
recyclable and the recovered CMOF catalysts showed activity
in subsequent runs of cyclopropanation reactions, albeit in
reduced yields and stereoselectivities (see the Supporting
Information). We believe that the reduced yields and
stereoselectivities of the recovered CMOFs are a result of
the intrinsic instability of the RuII/salen cyclopropanation
catalyst but not the dissolution or decomposition of the
CMOFs. Leaching experiments were performed on both 1
and 2. UV/Vis and ICP-MS analyses indicated that less than
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Angew. Chem. Int. Ed. 2011, 50, 8674 –8678