Journal of the American Chemical Society
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The ratio of product yields for the borylation of THF com-
AUTHOR INFORMATION
1
pared to other cyclic ethers was determined by GC analysis.
Corresponding Author
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To determine the relative rates for borylation of cyclic ethers
with varying structures, we conducted competition experi-
ments. The relative ratios of products from the borylation of
tetrahydrofuran versus cyclic ethers containing different ring
sizes and substitution patterns are shown in Scheme 4. The
borylation of the five-membered cyclic ether occurs preferen-
tially over the borylation of six- or seven-membered cyclic
ethers. This trend correlates with the slightly higher Lewis
basicity of the oxygen atom of the five-membered cyclic ether,
compared to that of the oxygen in the six- and seven-
membered cyclic ethers.17,18 Also, the borylation of THF oc-
ACKNOWLEDGMENT
We thank the NSF (CHE-1156496) for support of this work,
Johnson Matthey for [Ir(COD)OMe]2, and [Ir(COD)Cl]2, and Al-
lyChem and BASF for B2pin2. C.W.L. thanks Abbott Laboratories
and the NSF graduate research fellowship program for predoctoral
fellowships.
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curred
faster
than
the
borylation
of
2,2-
dimethyltetrahydrofuran, but by only a 2.4:1 ratio. This ratio is
again consistent with the greater Lewis basicity of the oxygen
atom in THF towards Lewis acids compared to 2,2-
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14) See Supporting Information for details of the ligands
examined.
15) Clauti, G.; Zassinovich, G.; Mestroni, G. Inorg. Chim. Acta
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H
O
O
H
Bpin
Bpin
Bpin
Bpin
N
N
N
N
Ir
Ir
Bpin
Bpin
Figure 1. Proposed origin of selectivity in secondary C-H boryla-
tion.
In summary, we have identified an Ir complex that catalyzes
the borylation of secondary C-H bonds for the first time in
good yield. Moreover, this C-H borylation reaction occurs
with unique selectivity for the 3-position of cyclic ethers. A
series of isotopic labeling experiments are consistent with high
selectivity for direct C-H cleavage at the 3-position, rather
than initial cleavage of the weaker C2 C-H bond, followed by
isomerization. Computational studies are currently underway
to help elucidate the details of the mechanism and origin of
selectivity of this reaction.
16) This combination of ligand and iridium precursor also led to
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that it encompassed the directed silylation of unactivated
primary C-H bonds; Simmons, E. M.; Hartwig, J. F. Nature
2012, 483, 70.
17) Searles, S. J.; Tamres, M. In The Chemistry of the Ether
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1998, 925.
ASSOCIATED CONTENT
Supporting Information
Experimental procedures and spectra for all new com-
pounds. This material is available free of charge via the
ACS Paragon Plus Environment