Pd-Catalyzed Coupling Reactions
COMMUNICATION
yields (67–85%) could still be achieved with slightly higher
catalyst loading. When the 2,4,6 positions of benzenesulfinic
acid were substituted with methyl groups, that is, 1 f, the
product yield with styrene was reduced dramatically to
52%, probably due to the steric hindrance. However, the re-
action of 1 f proceeded smoothly with acrylic esters, produc-
ing 3 fd and 3 ff in yields of 78% and 73%, respectively.
Moreover, the reaction of 2-naphthylsulfinic acid was also
effective to give the corresponding cross-coupling products
3ga, 3gd, and 3gf in good yields (66–82%).
As seen from Table 2, only one isomer with trans-configu-
ration was identified in each case, reflecting the extremely
high regioselectivity and stereoselectivity of our Heck-type
reaction. In a conventional Heck reaction, a base and a
ligand are usually required for the conversion of the hydri-
dopalladium(II) complex to the active palladium(0) catalyst
and the stabilization of the regenerated palladium(0) spe-
cies, respectively.[1–4] In sharp contrast, our Heck-type reac-
tion did not require any base or ligand.
tion of Pd0 to ArSO2Cl is involved.[7] Elimination of SO2
from complex A generates ArPdX, which inserts into the
olefin to give intermediate B. A b-H elimination from inter-
mediate B furnishes the final coupling product 3 and Pd0
species. Oxidation of Pd0 by Cu
(OAc)2. A palladium(II)-catalyzed pathway with CuACHTUNGTRENNUNG
ACHTUNGTRENNUNG
T
as an oxidant has also been described for the Heck-type re-
action of aryl boronic acids with alkenes.[9]
In the conventional Heck reaction, organic halides usually
cannot contain b-hydrogens because they are easily elimi-
nated to give olefins.[1] Therefore, the use of aliphatic hal-
ides as coupling partners in the Heck reaction is rare and
challenging; aliphatic carboxylic and phosphonic acids as
the alkyl source have not been described.[5,6] We attempted
the Pd-catalyzed reaction of benzylsulfinic acid with 4-
chloroACHTNUTRGNEsNUG tyrene and that with butyl acrylate under our stan-
dard conditions, and obtained the desired coupling products
4ac and 4af only in 6% yield and in trace amounts, respec-
tively. When tricyclohexylphosphine as ligand was added to
Pioneering work by the Myers group on the Pd-catalyzed
decarboxylative Heck reaction is certainly a landmark. Nev-
ertheless, at least one ortho substituent for aryl carboxylic
acids was necessary for the successful decarboxylative palla-
dation to occur.[5] It was found that a high loading of palladi-
and CuACTHNGUTERNU(NG OAc)2 was removed from the reaction systems, the
yields were improved to 22% and 17%, respectively
(Scheme 3).[10] Although the yields were still low, our results
demonstrated the feasibility of the SO2 extrusion from ali-
phatic sulfinic acids to form alkyl palladium and subsequent
coupling with alkenes.
um and silver, excess acid additive, or carbene ligand for the
[5]
À
C C bond fission, and the unusual oxidant Me3NO with
[6]
À
TBAF as the activator for the C P bond cleavage were
crucial in the previous work. Therefore, our protocol, which
employed aryl sulfinic acids with catalytic Pd
ACHTUNGRTENUN(NG OAc)2 and in-
expensive Cu(OAc)2, has its own merits and is complemen-
ACHTUNGTRENNUNG
tary to the existing methodologies.[5,6] Notably, the reaction
of sodium p-toluenesulfinate with alkenes in the presence of
a stoichiometric amount of a palladium(II) salt has been re-
ported,[8] yet our catalytic version is much more efficient.
Although the mechanism of this reaction is not clear right
now, we propose a possible pathway compatible with our
data (Scheme 2). The first step is presumably a ligand ex-
Scheme 3. Palladium-catalyzed reaction of benzylsulfinic acid with al-
kenes.
In conclusion, we have developed an efficient protocol for
the desulfitative Heck-type reaction of aryl sulfinic acids
with a variety of alkenes in the presence of a catalytic
change between PdACHTUNGTRENNUNG(OAc)2 and the aryl sulfinic acid provid-
ing complex A. This step is mechanistically distinct from the
first step in the Pd-catalyzed desulfitative Heck-type reac-
tion of aryl sulfonyl chlorides, in which the oxidative addi-
amount of PdACTHNUGRTENNUG(OAc)2 and cheap CuACHTUNGTRNEN(GUN OAc)2 as oxidant. Our
method doesn’t require a ligand or a base, broadening the
scope of Pd-catalyzed coupling reactions. The investigation
of aryl sulfinic acids as the aryl source in other coupling re-
actions is underway.
Experimental Section
General procedure for the Heck-type reaction of aryl sulfinic acids with
alkenes catalyzed by Pd
a suspension of aryl sulfinic acid (1a–g, 0.25 mmol), Pd
0.025 mmol), and Cu(OAc)2·H2O (50.0 mg, 0.25 mmol) in dioxane
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
(1.5 mL) with magnetic stirring. The mixture was stirred at 1008C for
24 h. After completion of the reaction, the solvent was evaporated to dry-
ness in vacuo. The residual was separated on a silica gel column with pe-
troleum ether/ethyl acetate 15/1 as the eluent to get the desired product
3aa–3gf.
Scheme 2. Proposed reaction mechanism.
Chem. Eur. J. 2011, 17, 5787 – 5790
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5789