COMMUNICATION
DOI: 10.1002/chem.200802412
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Hydrogenation of C C Multiple Bonds Mediated by [PdACTHNURTGENN(UG NHC)ACHTUGNTREN(NUNG PCy3)]
(NHC=N-Heterocyclic Carbene) under Mild Reaction Conditions
Vꢀclav Jurcˇꢁk,[a, b] Steven P. Nolan,[a, b] and Catherine S. J. Cazin*[a, b]
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The reduction of C C multiple bonds by transition-metal-
catalyzed hydrogenation is one of the most important reac-
tion in organic chemistry.[1] Efforts are continuously being
undertaken to improve catalyst efficiency, selectivity and
versatility. In this regard, homogeneous systems that have
been developed to address these criteria are notably based
on Rh, Ru, and Ir. In contrast, less success has been attained
with homogeneous palladium-based catalytic systems,[2]
which might be imputed to their propensity to decompose
under hydrogenation conditions.[3] Ligands that are known
to stabilize palladium catalysts are N-heterocyclic carbenes
(NHCs).[4] However, it has been observed that hydrido–pal-
ladium complexes of NHC are susceptible to decomposition
through reductive elimination.[5] Furthermore, Cloke and co-
Scheme 1. Complex 1 [PdACHTUNGTERNNU(G SIPr)ACHTUGNTRENN(GNU PCy3)] in the presence of H2.
blet at d=À4.90 ppm was observed, with a coupling con-
2
stant of 17.4 Hz, consistent with a JHP coupling.[9,10] The
31P{1H} NMR spectrum contains two singlets, at d=48.1 and
66.7 ppm, attributed to [Pd
ACHTUGNTRENU(NNG SIPr)ACHTUNRTEGGNN(UN PCy3)] and [Pd(H)2ACHTUNGTRENNUNG(SIPr)-
workers showed that the homoleptic complex [Pd
G
ACHTUNGTRENNUNG
(ItBu=N,N’-di-tert-butylimidazol-2-ylidene) decomposes in
the presence of H2 to form palladium black and 1,3-bis-tert-
butylimidazolidine.[6] In contrast, Elsevier showed that in
situ- and pre-formed mono-NHC complexes of Pd0, stabi-
lized by h2-alkenes, are stable under hydrogenation condi-
tions and can promote the semi-reduction of alkynes.[7]
Herein, we report the behavior of a Pd0–NHC complex
under hydrogen and its outstanding catalytic performance in
position to palladium black was observed, indicating that
complex 1 might be stable under hydrogenation conditions.
This prompted us to evaluate the catalytic potential of 1 in
hydrogenation.
Solvent optimization was conducted using trans-stilbene
(2a), 1 mol% of 1, under 1 bar of H2, at room temperature
(Table 1). In nonpolar solvents such as toluene and methyl-
cyclohexane, poor reduction of 2a was observed (Table 1,
entries 1 and 2). In polar solvents such as THF, diethyl ether
and tBuOMe, moderate to good conversions to 1,2-diphenyl-
ethane were obtained (Table 1, entries 3–5), while in ace-
tone, the reaction barely proceeded (Table 1, entry 6). Alco-
hol solvents were found appropriate for this reaction as
quantitative yields of the hydrogenation product were ob-
tained (Table 1, entries 7–9). As such solvents can be a
source of proton in transfer hydrogenation reactions,[1] back-
ground reactions were conducted in the absence of H2 in al-
cohols. In all cases, no alkane product could be detected, il-
lustrating the innocent role of alcohols in this system. The
catalyst loading was then decreased, and, in both methanol
and isopropanol, 0.25 mol% Pd was found to be the opti-
mum loading for the quantitative reduction of trans-stilbene
to 1,2-diphenylethane (Table 1, entries 10–15). Further de-
creasing the catalyst loading was feasible when using a more
reactive substrate (Table 1, entries 16 and 17). Indeed, with
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the hydrogenation of C C multiple bonds.
In the presence of H2, [Pd
G
G
(bis(2,6-diisopropylphenyl)imidazolidine),[8] seems to under-
go an oxidative addition leading to the trans-dihydride spe-
cies [Pd(H)
2ACHTUNGTRENNUNG(SIPr)ACHTUNGTRENNUNG(PCy3)] (Scheme 1). The presence of this
1
species was supported by H NMR spectroscopy, as a dou-
ˇ
[a] Dr. V. Jurcꢀk, Prof. Dr. S. P. Nolan, Dr. C. S. J. Cazin
Institute of Chemical Research of Catalonia (ICIQ)
Av. Paꢁsos Catalans 16, 43007 Tarragona (Spain)
ˇ
[b] Dr. V. Jurcꢀk, Prof. Dr. S. P. Nolan, Dr. C. S. J. Cazin
School of Chemistry, University of St Andrews
North Haugh, St Andrews, KY16 9ST (UK)
Fax : +44 (0)1334 463808
Supporting information for this article is available on the WWW
Chem. Eur. J. 2009, 15, 2509 – 2511
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2509