Journal of the American Chemical Society
ARTICLE
by atomic oxygen adsorbed on the gold surface to form adsorbed
methoxy and water, and no iodine-containing promoter is required.
Further, in the work reported here, CO inserts into the CH3OꢀAu
bond to form adsorbed methoxycarbonyl (CH3OCO), whereas in
the homogeneous process CO inserts into the CH3ꢀRh bond to
form acetyl. Lastly, in the gold-catalyzed methoxycarbonylation
reported here a second nucleophile adds to the methoxycarbonyl
to produce the products (e.g., carbonates, carbamates, Scheme 2),
whereas reductive elimination of MeCOI with subsequent hydrolysis
yields acetic acid in the Rh-catalyzed reaction. Though the mechan-
ism of the carbonylation reaction on the supported gold catalyst is
not yet known in detail, it appears strongly derivative of the
homogeneous system. Iodide coordinated to Au is proposed to be
an integral part of the active site for the carbonylation reaction,35 and
the products of the reaction are methyl acetate and acetic acid. Thus,
the mechanism of carbonylation reported here differs fundamentally
from that of previously reported work.
Metallic gold has the added advantage that it is recoverable
and reusable. The mechanism established by our work provides a
more direct route that could enhance the efficiency of these
reactions and reduce the production of undesirable side pro-
ducts. Our work clearly establishes the need to determine the
practical potential of gold for carbonylation processes through
investigation under working catalytic conditions.
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’ CONCLUSIONS
We demonstrate that the oxygen-activated Au surface effi-
ciently mediates direct carbonylation of methoxy via a unique
two-step nucleophilic attack mechanism. Methoxy nucleophili-
cally attacks the carbon atom in CO to form a stable surface inter-
mediate, methoxycarbonyl. Methoxycarbonyl is subsequently
attacked by residual methoxy, leading to the formation of di-
methyl carbonate, a green methylation agent. Other nucleophiles
introduced to surface after methoxycarbonyl formation also react
to yield, e.g., methyl ethyl carbonate (CH3OC(dO)OC2H5) or
CH3OC(dO)N(CH3)2, demonstrating the generality of this
pathway for methoxy carbonylation. The gold-mediated direct
methoxycarbonylation has the potential for streamlining carbox-
ymethylation and transesterification reactions, obviating the
need for dimethyl carbonate.
’ ASSOCIATED CONTENT
S
Supporting Information. Procedures for product iden-
b
tification using mass spectrometry, assignments of vibrational
peaks, DFT calculational details, and XPS reference spectra and
assignments. This material is available free of charge via the
’ AUTHOR INFORMATION
Corresponding Author
(31) Angelici, R. J. J. Organomet. Chem. 2008, 693, 847.
(32) Nielsen, I.; Taarning, E.; Egeblad, K.; Madsen, R.; Christensen, C.
Catal. Lett. 2007, 116, 35.
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Sci. 2003, 544, 5.
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(35) Goguet, A.; Hardacre, C.; Harvey, I.; Narasimharao, K.; Saih, Y.;
Sa, J. J. Am. Chem. Soc. 2009, 131, 6973.
’ ACKNOWLEDGMENT
We gratefully acknowledge the support of the U.S. Depart-
ment of Energy, Basic Energy Sciences, under Grant DE-FG02-
84-ER13289 (C.M.F.), and the National Science Foundation,
Division of Chemistry, Analytical and Surface Science (R.J.M.)
CHE-0952790. B.X. also acknowledges the Harvard University
Center for the Environment for support through the Graduate
Consortium in Energy and the Environment.
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dx.doi.org/10.1021/ja207389z |J. Am. Chem. Soc. 2011, 133, 20378–20383