Journal of the American Chemical Society
Article
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(10) Natural bond orbital (NBO) analysis on the DFT-optimized
ground state structure of I′, a simplified model (PAr2 = PPh2; [Si] =
SiMe3) of rhodium silyl dihydride I, yields a bond order of 0.4211 for
the Si−H bond. See the Supporting Information for details.
(11) Because the ligand is chiral, the two phosphorus atoms and two
hydrides would be inequivalent, even if the silyl group assumed an
idealized axial position.
iridium-catalyzed borylation of arenes. C−H bond cleavage in
the borylation processes occur by Rh(III) boryl and Ir(III)
boryl intermediates.8,32 We presume that the change in rate-
limiting step from generation of the reactive intermediate that
cleaves the C−H bond (as in silylation) to cleavage of the C−H
bond (as in borylation) is a function of the different oxidation
states of the boryl and silyl complexes that cleave the C−H
bonds. Cleavage of a C−H bond by a Rh(III) or Ir(III) species
is likely less facile than cleavage of a C−H bond by a Rh(I) silyl
species.
Extensive computational data are needed to gain information
on the C−H bond cleavage step and the relationship between
the main-group-assisted C−H bond cleavage reactions by boryl
complexes33 and the role of silicon in the C−H bond cleavage
step of this Rh-catalyzed reaction. Such computational studies
and an assessment of the effect of ligands on the individual
steps as a means to create more active catalysts will be the
studies of future work in our laboratory.
ASSOCIATED CONTENT
■
S
* Supporting Information
Text, figures, tables, and a CIF file giving experimental
procedures, computational details, kinetics data, and crystallo-
graphic information. This material is available free of charge via
(12) The silylation of 1,3-xylene is also zero order in the
concentration of the arene (see the Supporting Information),
suggesting that reactions of electron-rich and electron-deficient arenes
occur by the same mechanism.
AUTHOR INFORMATION
■
Corresponding Author
(13) For a quantitative estimation of the KIE for partially reversible
reactions, see the Supporting Information.
Notes
The authors declare no competing financial interest.
(14) Deuterium exchange among the silane, cyclohexene, and 1,3-
bis(trifluoromethyl)benzene (1) could complicate the KIE results
because this process lowers the deuterium content in the starting
arene. However, because the KIE experiments were conducted with
excess arenes, and initial reaction rates (up to 5% conversion) were
measured, the experimental results should be close to the true relative
rates for reaction of 1,3-bis(trifluoromethyl)benzene and 1,3-
bis(trifluoromethyl)benzene-d.
(15) Ozawa and co-workers have shown that the barriers for C−Si
reductive elimination from Pt(II) alkynyl silyl complexes are higher for
more electron deficient alkynyl groups.
(16) Ozawa, F.; Mori, T. Organometallics 2003, 22, 3593−3599.
(17) Control experiments with 1,3-bis(trifluoromethyl)benzene (1)
run without cyclohexene (which would not generate VI) or without
the ligand gave no silylarene product or deuterated starting arene,
suggesting that neither II (or I) nor an unligated rhodium species is
responsible for arene C−H activation.
(18) The reaction with D[Si] was run with cyclohexene-d10 and 5-D-
1,3-bis(trifluoromethyl)benzene to avoid erosion of the deuterium
content of the silane caused by H−D exchange with cyclohexene and
the arene.
(19) Modern Rhodium-Catalyzed Organic Reactions, 1st ed.; Wiley-
VCH: Weinheim, Germany, 2005.
(20) Pathway 2 requires the alkene hydrogenation step to proceed
through pathway A (Scheme 6), which we disfavor because of
unfavorable binding of cyclohexene to I.
(21) (a) Duckett, S. B.; Perutz, R. N. Organometallics 1992, 11, 90−
98. (b) Sakaki, S.; Sumimoto, M.; Fukuhara, M.; Sugimoto, M.;
Fujimoto, H.; Matsuzaki, S. Organometallics 2002, 21, 3788−3802.
(22) Only the 4- and 5-positions are available for silylation. The 3-
and 6-positions are not available because these C−H bonds have
substituents ortho to them.
ACKNOWLEDGMENTS
■
We thank the NSF (CHE-1213409) for funding, the Molecular
Graphics and Computation Facility (CHE-0840505) of College
of Chemistry, University of California, Berkeley, for computa-
tional resources, Dr. A. DiPasquale for X-ray crystallography,
and the M. B. Francis group for use of a centrifuge.
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