Communications
approximately 55% D on the a-methylene position, but with
only 7% D on the d-methyl positions (see Figure S1 in the
Supporting Information).
The carbon isotope effect of the coupling reaction was
measured by employing Singletonꢀs NMR technique at
natural abundance.[13] The most pronounced carbon isotope
=
effect was observed on the b-carbon atom of (E)-C6H5CH
CHCONEt2; the 13C ratio of the recovered substrate to that of
the virgin sample [(13C(recovered)/13C(virgin)] was Cb = 1.018
(average of 3 runs, at 75%–80% conversion; see Table S1 in
the Supporting Information). These results indicate that the
olefin insertion into an a,b-unsaturated carbonyl substrate is
the rate-limiting step of the coupling reaction.
Scheme 1. Proposed mechanism of the formation of the tetrasubsti-
tuted olefin. rds=rate-determining step.
In an effort to trap catalytically relevant species, the
reaction of complex 1 (5.0 mmol) with a naphthyl-substituted
amide (25 mmol), cyclopentene (5 equivalents), and H2O
(10 equivalents) in CD2Cl2 was monitored by 1H and 31P NMR
spectroscopy [Eq. (2)]. The formation of the Ru/allyl complex
3 was detected after 5 hours at room temperature. In a
preparatory scale reaction, the complex 3 was isolated from
the reaction of the tetrameric complex [{(PCy3)(CO)RuH}4-
(m-O)(m-OH)2] with the naphthyl-substituted amide,
HBF4·OEt2, and cyclopentene in wet CH2Cl2, and its structure
was unequivocally established by X-ray crystallography (see
Figure S2 in the Supporting Information).[12]
well established that both the olefin bond polarity and the
chelation of the carbonyl group are important in directing
regioselective insertion of enamides and a,b-unsaturated
carbonyl compounds.[16] In our case, an electrophilic Ru
center should also promote the regioselective olefin insertion
in the formation of the carbonyl-chelated species 5. The
carbon isotope effect study provides strong support for the
rate-limiting olefin insertion step. In light of the recent
deuterium labeling study on the alkene dimerization and
isomerization reactions,[17] the olefin isomerization step is
expected to be facile in yielding the tetrasubstituted olefin
product 2 with the regeneration of 4. The successful isolation
of the catalytically active Ru/allyl complex 3 suggests species
5 as a possible intermediate, which can undergo dehydroge-
nation and then trapping by a water molecule. An alternative
oxidative coupling mechanism has also been considered for
the coupling reaction. Although we cannot rule out this
mechanism at this time, the oxidative coupling mechanism
cannot readily explain both the deuterium labeling pattern
and the stereoselective formation of the tetrasubstituted Z-
olefin products.[18]
The complex 3 was found to exhibit virtually the same
=
activity as 1 in mediating the coupling reaction of (E)-PhCH
In summary, a novel catalytic method for the synthesis of
tetrasubstituted olefins has been developed from the con-
jugate addition of unactivated olefins to a,b-unsaturated
carbonyl compounds. The preliminary kinetic and spectros-
copic studies provide supporting evidence for a mechanistic
pathway that involves a rate-limiting olefin insertion to the
a,b-unsaturated carbonyl substrate and rapid olefin isomer-
ization steps. Efforts are currently underway to establish the
detailed mechanism as well as to extend the scope of the
coupling reaction.
CONHMe and propene under the conditions stipulated in
Equation (1); the reaction gave 2k in a > 90% yield after 2h.
When this reaction was performed in the presence of
1.5 equivalents of H2O, a substantially lower product con-
version (75% after 2 hours) was observed.[14] To further
establish catalytic relevance of the complex, the reaction of 3
with 1 equivalent of (E)-PhCH CHCONHMe was moni-
tored by H NMR spectroscopy. The reversible coordination
of the a,b-unsaturated carbonyl substrate was observed at
=
1
room temperature to form a 2:1 ratio of 3 and the carbonyl-
À
coordinated complex, but no new Ru H species was detected,
even after heating at 608C. Although a more careful study is
needed to establish the exact mechanism, the preliminary
results suggest that both the a,b-unsaturated carbonyl com-
pound and excess alkene substrates are required for the
conversion of the complex 3 into a catalytically active
species.[15]
Experimental Section
Representative procedure of the catalytic reaction. In a glove box,
complex 1 (10 mg, 17.4 mmol) and ethyl cinnamate (0.58 mmol) were
dissolved in CH2Cl2 (2.0 mL) in a 25 mL Schlenk tube equipped with
a Teflon stopcock and a magnetic stirring bar. The Schlenk tube was
brought out of the box, and was cooled in a dry ice/acetone bath.
Excess propene (2.9 mmol) was condensed into the reaction tube by a
vacuum transfer, and the reaction mixture was stirred in an oil bath
for 2 h at 708C. The reaction mixture was cooled to room temperature
and then opened to air. After filtering through a small pad of silica gel
(hexanes/EtOAc = 2:1), the solution was analyzed by GC methods.
Analytically pure product 2a was isolated after column chromatog-
raphy (hexanes/EtOAc = 20:1 to 4:1).
These results support a mechanism involving the cationic
À
Ru H species 4, which is initially formed from the ligand
exchange reaction of
1 with the carbonyl substrate
(Scheme 1). We propose that the chelate-directed regioselec-
tive alkene insertion and b-hydride elimination steps would
form the cationic Ru/alkene/hydride species 5. It has been
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 1692 –1695