COMMUNICATION
DOI: 10.1002/chem.201203623
Asymmetric Counteranion-Directed Catalytic Hosomi–Sakurai Reaction
Manuel Mahlau, Pilar Garcꢀa-Garcꢀa, and Benjamin List*[a]
Enantioselective carbonyl allylations are useful carbon–
carbon bond forming reactions and have frequently been
utilized to evaluate the potential of new concepts for asym-
metric synthesis.[1] Several very effective stoichiometric
methods using enantiomerically pure, nucleophilic allyl
transfer reagents,[2] as well as catalytic variants with a varie-
ty of achiral reagents such as allyl stannanes,[3] allyl boron
reagents,[4] allyl silanes,[5–7] and others, have been develop-
influence of the counteranion on the non-enantioselective
Hosomi–Sakurai reactions catalyzed by trimethylsilyl tri-
fluoromethanesulfonate (Me3SiOTf) or bis(trifluorometh-
AHCTUNGTRENNUNG
anesulfonyl)imide (Me3SiNTf2),[14] we hypothesized that it
should be possible to control the stereochemical course of
the usually undesired trimethylsilylium ion-catalyzed path-
way by ion-pairing with a chiral disulfonimide anion.[15]
Strong encouragement for this concept came from our previ-
ous success in catalyzing related Si-transfer processes with
chiral disulfonimides.[11–13]
ACHTUNGTRENNUNG
ed.[1c,8] Amongst these, the Hosomi–Sakurai reaction, which
employs inexpensive and nontoxic allyl trimethylsilane and
its derivatives, has aroused significant interest but remained
particularly challenging. Previously developed variants uti-
lize cyclic alkoxy boranes (CAB) developed by Yamamoto
et al.,[6] or Ti-based catalysts,[7] the most versatile of which
was developed by the Carreira group,[7b] and give the corre-
sponding homoallylic alcohols with moderate to good yields
and enantioselectivities. However, these methods for the
Hosomi–Sakurai reaction with allyl trimethylsilane (deriva-
tives) require exhaustive exclusion of air and moisture and
are typically limited in scope. In general, catalytic enantiose-
lective Hosomi–Sakurai reactions struggle with a highly effi-
cient though non-enantioselective silylium catalysis back-
ground reaction. This process is readily initiated by a chiral
Lewis acid and can lead to the requirement of high catalyst
loadings and diminished enantioselectivity.[9] We now sug-
gest an alternative approach that relies upon this inherent
silylium activation. We report herein a new chiral disulfoni-
mide that catalyzes efficient Hosomi–Sakurai reactions of
substituted allyl silanes with various aldehydes in high enan-
tioselectivity.
We started our investigations by testing the methallylation
of benzaldehyde (2a) with methallyltrimethylsilane (3a)
using disulfonimide catalyst 1b (Scheme 1), which had
proven to be the best catalyst in our studies on the Mu-
kaiyama aldol reaction.[11,12]
Scheme 1. Comparison of the optimal catalyst 1a of the Hosomi–Sakurai
reaction versus that for Mukaiyama aldol reactions (1b).[11,12]
Recently we embarked upon the expansion of asymmetric
counteranion-directed catalysis (ACDC)[10] to silylium-based
Lewis acid organocatalysis and realized highly efficient
chiral disulfonimide-catalyzed Mukaiyama aldol,[11] vinylo-
gous and bisvinylogous Mukaiyama aldol reactions,[12] as
well as hetero Diels–Alder reactions.[13] We have suggested
that these transformations proceed via silyloxycarbenium–
chiral disulfonimide ion pairs, of which the chiral counteran-
ion induces the asymmetry. In light of the established strong
Utilizing catalyst 1b we obtained the methallylation prod-
uct 4a with an encouraging enantiomeric ratio (e.r.) of
83.5:16.5. Starting from these initial results, an extensive
screening of reaction conditions and catalysts revealed that
the reaction is best performed with the even more electron-
deficient disulfonimide 1a at À788C in toluene (see the Sup-
porting Information).
Gratifyingly, our new catalyst methallylates a variety of
aldehydes with high efficiency and selectivity (Table 1). Re-
actions are best performed by first silylating catalyst 1a with
a catalytic amount of the TMS–ketene acetal of methyl iso-
butyrate 6. Analyzing steric effects of aryl aldehyde substitu-
ents revealed that para substitution has no significant influ-
ence on the enantioselectivity (entries 1 and 2, Table 1). In-
troducing substituents in the meta position was found to
[a] M. Mahlau, Dr. P. Garcꢀa-Garcꢀa, Prof. Dr. B. List
Max-Planck-Institut fꢁr Kohlenforschung
Kaiser-Wilhelm-Platz 1, 45470 Mꢁlheim an der Ruhr (Germany)
Fax : (+49)208-306-2982
Supporting information for this article is available on the WWW
Chem. Eur. J. 2012, 18, 16283 – 16287
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
16283