DOI: 10.1002/chem.201501677
Communication
&
Homogeneous Catalysis
CaII-Catalyzed Alkenylation of Alcohols with Vinylboronic Acids
David Lebœuf, Marc Presset, Bastien Michelet, Christophe Bour, Sophie Bezzenine-LafollØe,
and Vincent Gandon*[a]
to generate carbocation intermediates,[10] so they could be rel-
evant catalysts for new CÀC bond-forming reactions. They are
Abstract: Direct alkenylation of a variety of alcohols with
air- and moisture-stable and also weakly toxic. Herein, we de-
vinylboronic acids has been accomplished using the air-
scribe our preliminary investigations towards a calcium(II)-cata-
stable calcium(II) complex Ca(NTf2)2 under mild conditions
lyzed alkenylation of a wide range of allyl, propargyl, and
with short reaction times. For reluctant transformations,
benzyl alcohols, with a series of vinylboronic acids and related
an ammonium salt was used as an additive to circumvent
reactants (Scheme 1).
the reactivity issue.
Cross-coupling reactions are of paramount importance to pro-
vide access to highly diverse molecules.[1] Among the different
cross-coupling partners, boronic acids are very popular re-
Scheme 1. Alkenylation of alcohols with vinylboron derivatives.
agents because they are readily available, easy-to-handle
solids, weakly toxic, and compatible with a broad range of
functional groups.[2] On the other hand, the synthesis of the
We first investigated the reactivity of vinylbenzyl alcohol 1a
second coupling partner (an organohalide for instance) often
requires several tedious steps. The cross-coupling bears also its
own limitations due to the use of a transition-metal complex
as the catalyst, which can be expensive, toxic, difficult to pre-
pare, and sensitive to oxygen and moisture. Regarding the
substrates, it would be especially appealing to use readily
available alcohols for CÀC bond-forming transformations with
boronic acids. However, the poor leaving ability of the hydrox-
yl group remains a major impediment. Over the last decade,
several groups have reported a few transition-metal-catalyzed
cross-couplings of allyl alcohols with arylboronic acids.[3,4] The
transition-metal-catalyzed cross-coupling of alcohols with vinyl-
boronic acids has also been sporadically reported.[5,6,7] Most of
these approaches involve the formation of p-allyl metal com-
plexes, yet, the versatility of this process remains to be demon-
strated. Currently, there are only a few methods reported for
the direct alkenylation of alcohols using simple styrene deriva-
tives or vinyl silanes by the formation of a carbocation inter-
mediate; however, these methods require either harsh condi-
tions, long reaction times, or suffer from a limited substrate
scope.[8,9] For these reasons, a general and mild reaction be-
tween alcohols and vinylboronic acids that avoids the use of
noble transition-metals would be even more elegant. In this
regard, calcium(II) salts display a high affinity towards alcohols
with (E)-styrylboronic acid 2a (Table 1). While the use of
Ca(OTf)2 did not lead to any reaction (Table 1, entry 1), we
were pleased to find that, with Ca(NTf2)2,[11] the linear product
3a was formed selectively in 73% yield after 30 min at room
temperature in CH2Cl2 (entry 2). It is noteworthy that the same
result was obtained under argon or in air using distilled CH2Cl2.
In addition, the branched cross-coupled product 3’’a was not
detected, which can be explained by the steric hindrance at
the benzylic position. The use of nitromethane, 1,4-dioxane, or
acetone as a solvent significantly decreased the reactivity lead-
ing to lower yields or no reaction (entries 3–5). Other Group 2
metal salts such as Mg(NTf2)2 and Ba(NTf2)2 were also tested,
but the reaction proceeded at a slower rate and 3a was isolat-
ed in 55 and 30% yield, respectively (entries 6 and 7). We,
then, examined the influence of additives (entries 8–10). Sever-
al reports have recently shown that the use of ammonium or
potassium salts, such as nBu4NBF4, nBu4NPF6, or KPF6,[12] could
be beneficial to reactions involving Ca(NTf2)2 as a catalyst.
Indeed, the ammonium or potassium salts of weakly coordinat-
ing anions can promote an anion metathesis to generate the
heteroleptic complex Ca(NTf2)(X) (X=BF4, PF6), which is more
Lewis acidic than Ca(NTf2)2.[11f] Surprisingly, the reaction was
less efficient and proceeded at a slower rate (entries 8–10).
Moreover, 3a was obtained admixed with the conjugated
diene 3’a, which was not detected in our previous experi-
ments. On the other hand, the use of the Brønsted acid HNTf2
as the catalyst led to a significant decrease of the yield
(entry 11). A control experiment, carried out in the absence of
Ca(NTf2)2, led to the recovery of the starting materials
(entry 12). The coupling was also tested with other boron re-
agents, which promoted either the decomposition of the alco-
hol (BPin, entry 13), led to a complex mixture (BF3K, entry 14),
[a] Dr. D. Lebœuf, Dr. M. Presset, B. Michelet, Dr. C. Bour,
Dr. S. Bezzenine-LafollØe, Prof. Dr. V. Gandon
ICMMO (UMR CNRS 8182)
UniversitØ Paris-Sud
Bâtiment 420, 91405 Orsay cedex (France)
Supporting information for this article is available on the WWW under
Chem. Eur. J. 2015, 21, 11001 – 11005
11001
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim