BHA was successfully carried out by using palladium catalysts,
but such coupling required the activation of the hydroxyl group
with acetate or carbonate.4 The use of nonactivated BHA would
be more desirable, particularly from the point of view of atom
economy.5a,b Recently Darses, Genet, et al. reported the synthesis
of trisubstituted olefins using homogeneous rhodium catalyst
via an unexpected mechanism involving 1,4-addition/â-hydroxy
elimination steps.1c
Rhodium Fluorapatite Catalyst for the Synthesis
of Trisubstituted Olefins via Cross Coupling of
Baylis-Hillman Adducts and Arylboronic Acids
M. Lakshmi Kantam,* K. B. Shiva Kumar, and B. Sreedhar
Inorganic and Physical Chemistry DiVision, Indian Institute of
Chemical Technology, Hyderabad 500 007, India
Despite the advantages of homogeneous metal complex
catalysts, difficulties in recovering the expensive catalyst metals
and ligands from the reaction mixture severely limit their
industrial applications. Apatites are metal basic phosphates for
which the chemical formula is M10(PO4)6(X)2 [M ) divalent
metal, X ) monovalent anion] and various kinds of cations and
anions can be readily introduced into their framework due to
their large ion exchange ability and such exchanged apatites
are already in use in several organic transformations.5c,6,7 Very
recently, we reported the preparation of recyclable heterogeneous
Cu-exchanged fluorapatite and copper exchanged tert-butoxya-
patite catalysts, for N-arylation of imidazoles and other hetero-
cycles with iodo-, bromo-, chloro-, and fluoroarenes (EW) with
good to excellent yields.8a,b We also reported PdFAP catalyst
(Fluorapatite-supported palladium catalyst) for the Suzuki and
Heck coupling reaction.8c Thus in continuation of our work with
fluorapatite we herein report the preparation of recyclable
heterogeneous rhodium-exchanged fluorapatite catalyst (RhFAP)
for cross coupling of Baylis-Hillman adducts with arylboronic
acids to yield trisubstituted olefins.
Calcium Fluorapatite (Ca/P) of 1.56 FAP (Ca10(PO4)6(F)2)
was synthesized according to the literature procedure9 and
exchanged with RhCl3, yielding the RhFAP as a yellowish
orange powder. The rhodium content was measured as 0.1 mmol
g-1 with ICP-AES. XRD and FTIR of the RhFAP were similar
to those of parent FAP. The (Rh+Ca)/P ratio of the RhFAP
was 1.56, which shows the occurrence of a substitution of Rh
for Ca2+ at the column of Ca and O atoms parallel to the
hexagonal axis. XPS spectra were recorded for the RhFAP
catalyst. A high-resolution narrow scan of Rh showed peaks at
ReceiVed September 10, 2007
Treatment of fluorapatite (prepared by incorporating basic
species F- in apatite in situ by coprecipitation) with an
aqueous solution of RhCl3 resulted in rhodium-exchanged
fluorapatite catalyst (RhFAP), which successfully promoted
cross coupling of Baylis-Hillman adducts with arylboronic
acids to yield trisubstituted olefins. A variety of arylboronic
acids and Baylis-Hillman adducts were converted to the
corresponding trisubstituted olefins, demonstrating the ver-
satility of the reaction. The reaction is highly stereoselective.
RhFAP was recovered quantitatively by simple filtration and
reused with almost consistent activity.
Rhodium-catalyzed carbon-carbon bond formation via cross
coupling of organoboron compounds and organic electrophiles
is one of the most important reactions in organic synthesis.1
The Baylis-Hillman reaction of an aldehyde and an activated
olefin produces Baylis-Hillman adducts (BHA), an interesting
class of disubstituted olefins possessing both allylic alcohol and
R,â-unsaturated ester moieties in which the carbon-carbon
double bond is an integral part of both allyl alcohol and R,â-
unsaturated ester moieties, which makes it valuable in a number
of stereoselective processes2 and offers multiple opportunities
for further transformations.3 Arylation with trifluoro(organo)-
borates and organosilanes to R,â-unsaturated ester moieties of
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10.1021/jo701982m CCC: $40.75 © 2008 American Chemical Society
Published on Web 12/12/2007
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