Angewandte
Chemie
common organic solvents and are stable in the open
atmosphere in solution (for at least 1 week in CDCl3) and in
the solid state (for at least 1 month), as revealed by 1H NMR
spectroscopy and ESIMS analysis. The X-ray crystal structure
of 1b revealed that the axially coordinated NHC ligands are
roughly orthogonal to the porphyrin plane (Figure 1).[6] The
[Ru(Por)(NHC)2] complexes showed significantly higher
reactivity towards alkene cyclopropanation. Lowering of the
catalyst loading of 1 f to 0.004 mol% led to complete EDA
consumption within 20 min with the formation of cyclopro-
pane 2a in 78% yield (see Table S2), thus indicating
a turnover frequency of 1950 minÀ1.
We examined other alkenes (Scheme 2) and found that
the catalytic cyclopropanation reaction proceeded smoothly
for styrenes bearing electron-donating (p-Me and p-OMe)
Figure 1. Perspective view of [Ru(4-F-TPP)(IMe)2] (1b). Hydrogen
atoms are omitted for clarity. Non-hydrogen atoms are represented by
thermal ellipsoids drawn at the 30% probability level.
Ru–C(NHC) distances are both 2.076 ꢀ and thus comparable
À
À
to those reported for Ru C single bonds (2.07–2.15 ꢀ for Ru
C(NHC)).[7]
As representative examples, the conversion of [Ru(4-F-
TPP)CO] into 1b (Scheme 1) led to a red shift of the Soret
band and a blue shift of the b band (see Figure S1 in the
Supporting Information). The 13C NMR spectra of 1a–1g all
Scheme 2. Alkene cyclopropanation with EDA under the catalysis of
1 f. Reaction conditions: alkene (1.6 mmol), EDA (0.8 mmol), 1 f
(0.05 mol%), CH2Cl2 (2 mL). The yields given are for the isolated
product (as based on EDA). The trans/cis and exo/endo ratios were
determined by H NMR spectroscopic analysis of the crude reaction
mixture.
[1d,7]
1
À
showed (Ru C) signals at d = 140–150 ppm.
The IR
oxidation-state marker bands of 1a–g all fell into the range
of 998–1005 cmÀ1, consistent with the RuII oxidation state.[8]
The electrochemistry of complexes 1a–f and [Ru-
(Por)CO] was examined. The first oxidation couple of
[Ru(Por)(NHC)2] is much less anodic as compared to that
of [Ru(Por)CO] (see Table S1 in the Supporting Informa-
tion). For example, in the case of 1b and [Ru(4-F-TPP)CO],
the oxidation couple of 1b at E1/2 = À0.04 V (with reference
to Ag/AgNO3 (0.1m in CH3CN)) was less anodic by 660 mV
than that of [Ru(4-F-TPP)CO] (E1/2 = 0.70 V; see Figure S2).
This change is attributed to the oxidation of RuII to RuIII. The
remarkably low E1/2 value reflects substantial stabilization of
the RuIII species by the two strong s-donor NHC ligands. The
second oxidation couple was tentatively assigned to porphy-
rin-centered oxidation.
and electron-withdrawing (p-Cl, p-Br, and p-F) substituents:
Cyclopropanes 2b–f were obtained in 93–98% yield with
excellent selectivity. Reactions of gem-disubstituted alkenes
proceeded smoothly to give 2g and 2h. Cyclopropanation of
aliphatic 4-phenyl-1-butene with EDA gave exclusively trans-
2i, which was isolated in 96% yield. cis-Stilbene also under-
went cyclopropanation smoothly with EDA to give 2j in 95%
yield with an exo/endo ratio of 10:1. In all reactions in
Scheme 2, EDA was completely consumed in 20 min, and no
EDA coupling product was detected.
À
We further expanded our study to carbene C H insertion
(Table 1). The reaction of 1,4-cyclohexadiene (3a; 4 mmol),
methyl phenyldiazoacetate (4; 0.4 mmol), and 1 f (2 mol%) in
CH2Cl2 (2 mL) at reflux for 24 h afforded 5a, the product of
To evaluate the catalytic activity of the [Ru(Por)(NHC)2]
complexes, we examined the cyclopropanation of styrene with
ethyl diazoacetate (EDA; see Table S2). A solution of EDA
(0.8 mmol) in CH2Cl2 was added to a mixture of styrene
(1.6 mmol) and the catalyst (0.05 mol%) in CH2Cl2 at room
temperature under a N2 atmosphere over a period of 10 min
by the use of a syringe pump. The mixture was then stirred for
a further 10 min at room temperature. The best result was
obtained by using [Ru(4-F-TPP)(BIMe)2] 1 f as the catalyst,
which led to the formation of 2a in 98% yield with a trans/cis
ratio of 20:1 (Scheme 2; see Table S2). As compared to
[Ru(TPP)CO] (slow addition of EDA over 8 h plus additional
stirring for 8 h led to the product in 65% yield; see Table S2),
À
insertion into a vinyl C H bond, in 81% yield (Table 1,
entry 1). The reaction also proceeded under neat conditions
to give 5a in 78% yield (Table 1, entry 1). No carbene dimer
or cyclopropane was detected in the crude product mixture,
and the diazo compound 4 was even added in one portion. In
contrast, [Ru(TPP)CO] was inactive towards this reaction
under the same reaction conditions. We compared other
metalloporphyrin catalysts (see Table S3) and found that the
catalytic activity of 1 f was superior to that of a range of RuII,
FeIII, CoII, and MnIII metalloporphyrin catalysts, all of which
are well-documented to be effective in catalyzing carbene
Angew. Chem. Int. Ed. 2014, 53, 2982 –2987
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2983