DOI: 10.1002/chem.201603825
Communication
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Homogeneous Catalysis
General, Simple, and Chemoselective Catalysts for the
Isomerization of Allylic Alcohols: The Importance of the Halide
Ligand
Elis Erbing+,[a, b] Ana Vꢀzquez-Romero+,[a, b] Antonio Bermejo Gꢁmez,[a, b] Ana E. Platero-
Prats,[a, c] Fabian Carson,[b, c] Xiaodong Zou,[b, c] Pꢂivi Tolstoy,[d] and Belꢃn Martꢄn-Matute*[a, b]
high catalyst loadings, chlorinated or aromatic solvents, activa-
Abstract: Remarkably simple IrIII catalysts enable the iso-
tors, or high temperatures. Also, the sophisticated ligands in
the majority of these reports require additional synthetic
effort. With a few exceptions,[5a,8] each catalyst isomerizes ex-
clusively either primary or sec-allylic alcohols efficiently, and
tend to follow distinct isomerization mechanisms: whereas sec-
alcohols form enone intermediates,[1,5a,9] primary ones isomer-
ize through migratory insertion/b-hydride elimination sequen-
ces.[10–12] For the latter, transition metal hydrides have given ex-
cellent results under mild conditions, enabling enantioselective
isomerizations.[4a,10–14]
merization of primary and sec-allylic alcohols under very
mild reaction conditions. X-ray absorption spectroscopy
(XAS) and mass spectrometry (MS) studies indicate that
the catalysts, with the general formula [Cp*IrIII], require
a halide ligand for catalytic activity, but no additives or ad-
ditional ligands are needed.
Isomerization reactions involving functional-group interconver-
sions are highly important in organic synthesis. This is the case
for transition metal-catalyzed isomerization of allylic alco-
hols.[1,2]Allylic alcohols are therefore masked synthons for pre-
paring carbonyl compounds, including functionalized ones.[3]
From a total synthesis perspective, one can take advantage of
the distinct reactivity of these two functional groups in the
design of synthetic routes, or can use available naturally occur-
ring allylic alcohols as carbonyl precursors.
We report here the isomerization of primary and sec-allylic
alcohols by using remarkably simple, commercially available P/
N-ligand-free IrIII complexes. Allylic alcohols with mono-, di-,
and trisubstituted double bonds were isomerized in aqueous
solvents and even at room temperature and under an atmos-
phere of air. Insights into the structure of the active catalyst
were obtained by mass spectrometry (MS) and X-ray absorp-
tion spectroscopy (XAS). Mechanistic investigations are also
presented.
Despite the prior development of several protocols,[4–11]
a general and simple catalytic system able to isomerize selec-
tively and efficiently both primary and sec-allylic alcohols
under mild conditions has remained a challenge. The scope is
commonly limited to molecules with few substituents, in par-
ticular for sec-allylic alcohols. An important example was re-
cently reported,[8] whereby a palladium hydride mediated the
isomerization of primary and sec-allylic alcohols and remotely
functionalized olefins. The established methods usually need
We have previously reported the synthesis of a-halocarbon-
yls from allylic alcohols catalyzed by [Cp*IrIII] complexes.[3]
However, the simple isomerization reaction did not take place
or represented a minor pathway. To investigate whether the
isomerization could occur under similar conditions, we carried
out the reactions of 1a with catalysts I, II, and III (Table 1).
Under the conditions previously used with [Cp*Ir(H2O)3]SO4 (I)
and [(Cp*Ir)2(OH)3]OH (II),[3c] but without halogenating agent,
unreacted 1a was recovered (Table 1, entries 1–4). In contrast,
the reaction with [{Cp*IrCl2}2] (III) in THF or acetone/H2O mix-
ture afforded 2a in conversions of 85 and >99%, respectively,
in 30 min (Table 1, entries 5 and 6).
[a] E. Erbing,+ Dr. A. Vꢀzquez-Romero,+ Dr. A. Bermejo Gꢁmez,
Dr. A. E. Platero-Prats, Prof. B. Martꢂn-Matute
Department of Organic Chemistry, Stockholm University
Stockholm, 10691 (Sweden)
[b] E. Erbing,+ Dr. A. Vꢀzquez-Romero,+ Dr. A. Bermejo Gꢁmez, Dr. F. Carson,
The substrate scope was then evaluated (Table 2). Excellent
yields (95–99%) were obtained at RT with terminal aliphatic
sec-allylic alcohols 1a–g, and functional groups such as ke-
tones, ethers, and esters were tolerated (2d,e,g). The isomeri-
zation of olefin-functionalized 1 f indicated selectivity towards
allylic alcohols, which was further confirmed with homoallylic
1h. Aromatic 1i–k afforded the products in moderate to good
yields. Alcohols with 1,2-disubstituted double bonds (1l–p)
were also isomerized in excellent yields. sec-Allylic alcohols
with 1,1-disubstituted double bonds, of which reported exam-
ples are rare,[4a,5] afforded 2q–s in excellent yields at 608C for
2q–r, and at 1008C for 2s. Other challenging substrates with
a 1,1,2-trisubstituted double bond (1t) and cyclic 1u were iso-
Prof. X. Zou, Prof. B. Martꢂn-Matute
Berzelii Center EXSELENT on Porous Materials
10691 Stockholm (Sweden)
[c] Dr. A. E. Platero-Prats, Dr. F. Carson, Prof. X. Zou
Department of Materials and Environmental Chemistry
Stockholm University Stockholm, 10691 (Sweden)
[d] Dr. P. Tolstoy
Cambrex Karlskoga AB, 691 85, Karlskoga (Sweden)
[+] These authors contributed equally this work.
Supporting information and ORCID number(s) for the author(s) for this arti-
Chem. Eur. J. 2016, 22, 1 – 6
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ꢅ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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