C O M M U N I C A T I O N S
Scheme 1
96% yield based on the quantity of 1. After 4 h, the HBpin was
also consumed to form PhBpin in an overall 88% yield based on
the number of equivalents of the Bpin unit in 1. Reaction of HBpin
with benzene under the conditions used for the borylation by 1
gave PhBpin in 80% yield (eq 2).
HBpin + PhH
(60 equiv)
1/2[IrCl(COD)] /bpy (3 mol %)8
+ H (2)
2
2
PhBpin
80%
80 °C/16 h
1
Careful monitoring by H NMR and GC/mass spectroscopy of
the early stages of the reaction between benzene-d6 and 1 at 60 °C
revealed an induction period during which the cyclooctadiene ligand
was reduced to cyclooctene-d2. Thus, we evaluated reactions con-
ducted with [IrCl(COE)2]2 as precursor. These reactions showed
no induction period even when we employed lower reaction temper-
atures. In fact, the reaction between 1 and benzene in the presence
of [IrCl(COE)2]2 and 4,4′-di-tert-butyl-2,2′-bipyridine (dtbpy), which
generates more soluble iridium complexes, produced PhBpin in 83%
yield after 4.5 h at room temperature (eq 3). This reaction of 1
with benzene constitutes the first catalytic C-H borylation at room
temperature. Moreover, this catalyst system provides remarkably
high turnover numbers for a hydrocarbon functionalization process.
Reaction of 1 at 100 °C in benzene catalyzed by 0.02 mol % of
iridium formed 2 equiv of PhBpin for each diboron reagent in 80%
yield (8000 turnovers) (eq 4). This value surpasses previously
published catalyst efficiencies by more than an order of magnitude.
Dissolution of tris-boryl 4 in C6D6 generated 3 equiv of C6D5-
Bpin in 80% yield within minutes at room temperature. Thus, 4 is
chemically and kinetically competent to be an intermediate in the
catalytic process. Reaction of 4 with a mixture of C6H6 and C6D6
revealed a kinetic isotope effect of 3.6 ( 0.2. Catalytic reactions
conducted in this solvent mixture showed an essentially identical
kH/kD ) 3.8 ( 0.4. Reaction of 4 in a mixture of toluene and
trifluoromethylbenzene showed selectivity that was also identical
with that of the catalytic reactions. These similar selectivities further
substantiated the involvement of 4 in the catalytic process.
In conclusion, Ir(I) complexes containing readily available and
air-stable 2,2′-bipyridine catalyze the borylation of arenes by bis-
(pinacolato)diboron even at room temperature. The method provides
a simple and direct route for the synthesis of arylboronates that
traditionally have been prepared by transmetalation between aryl-
lithium or arylmagnesium reagents and trialkyl borates.
1/2[IrCl(COE) ] /dtbpy (5 mol %)8
+ H
(3)
2
2
B2pin2 +
B2pin2 +
2PhH
(60 equiv)
2PhBpin
83%
2
room temperature/4.5 h
1/2[IrCl(COE) ] /dtbpy (0.02 mol %)8
+ H (4)
2
2
2PhH
(60 equiv)
2PhBpin
80%
2
100 °C/16 h
Acknowledgment. J.F.H. and N.R.A. thank the National Sci-
ence Foundation (CHE-9986236 for J.F.H.) for support of this work.
Supporting Information Available: Experimental procedures and
spectral analyses of all reaction products (PDF). This material is avail-
Our studies subsequently focused on isolating likely Ir-boryl
intermediates. To do so, we generated the dtbpy- and COD-ligated
Ir(I) complexes [IrCl(dtbpy)(COD)] and [Ir(dtbpy)(COD)]OTf (2).
Addition of 1.1 equiv of diboron 1 to the triflate formed the Ir(III)
bis-boryl complex cis-[Ir(dtbpy)(COD)(Bpin)2]OTf (3) (eq 5).23
Reaction of the chloride did not generate an isolable boryl complex.
Although bis-boryl 3 catalyzed the reaction between C6D6 and B2-
pin2 to form PhBpin in 80% yield after 5 h at 80 °C, thermolysis
of 3 in C6D6 in the absence of added diboron regent, a reaction
that would complete the catalytic cycle, did not form PhBpin.
Instead it simply extruded B2pin2 in 75% yield and generated the
starting [Ir(dtbpy)(COD)]OTf.24
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1
catalytic reaction that contained high catalyst loadings showed a
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B2pin2 in mesitylene solvent, which dissolved the resulting complex
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JA0173019
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