C O M M U N I C A T I O N S
Scheme 2. Oxidative Esterification of Aliphatic Aldehydesa
that a siloxane functions as the hydride acceptor when acetonitrile
is used as solvent (see Supporting Information). The reaction
proceeds under both air and nitrogen atmosphere which is in
agreement with the proposed catalytic cycle; that is, oxygen can
be ruled out as the hydride acceptor. We found that during the
course of the esterification the aldehyde signal disappears as a new
NMR signal at 4.6 ppm is formed which corresponds to pentavalent
HSiF(OMe)3. The siloxane thus fulfills three important functions:
it generates a Lewis-acidic silicate that activates the aldehyde and
it serves as both the methoxy group donor and the hydride acceptor.
It is likely that the hydride ultimately reacts with tetrabutylammo-
nium via Hoffmann elimination or with the solvent, in particular
when protic solvents such as isopropyl alcohol and water are used.
In conclusion, selective chemodivergent conversion of aldehydes
toward esters and secondary alcohols has been realized using
siloxanes and POPd as a reaction switch under otherwise almost
identical conditions. Both reactions are applicable to a range of
aldehydes. The palladium phosphinous acid-catalyzed oxidative
esterification procedure generates esters in a single step in high
yields under mild conditions. The TBAF-promoted arylation of
aldehydes with arylsiloxanes provides the corresponding secondary
alcohols in better yields than previously reported methods while
avoiding the use of a transition metal catalyst and arylfluorosilanes.
a On a 1 g scale.
Scheme 3. Pd(II)-Catalyzed Oxidative Esterification of Aldehydes
CsF were ineffective additives; however, we found that the reaction
proceeds with excellent yields in the presence of TBAF. Fluoride
additives are known to activate arylsiloxanes through formation of
pentavalent silicates, which are likely to play a key role during
arylation. The phenyl transfer may be further facilitated by an
increase of the electrophilicity of the aldehyde through coordination
to the Lewis acidic countercation which would explain the different
results obtained with metal and ammonium fluoride salts. In contrast
to reactions with Grignard and organolithium reagents, our method
is compatible with halides, cyano, and nitro groups (entries 5, 6,
8-10). As expected, other arylsiloxanes such as 3-furyl-, 2-thiophene-,
and p-tolylsiloxane undergo 1,2-addition to benzaldehyde with up
to 89%, whereas vinylsiloxane gave less than 40% yield (see SI).
The POPd-catalyzed conversion of aldehydes to the correspond-
ing methyl esters incorporates an oxidation and an esterification
step into a single process under mild reaction conditions. The pro-
cedure is also suitable for aliphatic aldehydes. Phenylacetaldehyde,
3-phenylpropanal, cinnamaldehyde, phenylglyoxal, and cyclo-
hexanecarboxaldehyde were converted on a 1 g scale to the
corresponding esters at 50 °C with up to 89% yield (Scheme 2).
Other siloxanes such as tetraethyl and tetrabutyl orthosilicate give
the corresponding esters in similar yields. While one-pot oxidative
esterifications of aldehydes with hydrogen peroxide or oxone in
alcoholic solution are known, few practical examples of transition
metal-catalyzed processes can be found in the literature.7 Our
method might become particularly useful for oxidation of aldehyde
groups in multifunctional compounds when carboxylic acid inter-
mediates need to be avoided or when sensitive electron-rich
heteroatoms that do not tolerate commonly used oxidizing reagents
are present.8
A proposed mechanism for the palladium-catalyzed one-pot
esterification of aldehydes is shown in Scheme 3. POPd appears
to be the catalytically active species and is regenerated at the end
of the catalytic cycle. We have been able to recover POPd after
completion of the esterification of benzaldehyde and confirmed the
structure by crystallographic analysis. Coordination of the aldehyde
to the oxophilic pentavalent silicate generated in situ from TBAF
and tetramethyl orthosilicate will increase its electrophilicity and
facilitate transfer of a methoxide group.9 Transmetalation from
silicon to the palladium catalyst is then followed by â-hydride
elimination and formation of the corresponding ester. The hydride
might initially be bonded to the palladium catalyst but both proton
and deuterium NMR studies with deuterated benzaldehyde showed
Acknowledgment. We thank CombiPhos Catalysts, Inc., New
Supporting Information Available: All experimental procedures
and product characterization. This material is available free of charge
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