Neopentylglycolborylation of aryl mesylates and tosylates catalyzed by Ni-based mixed-ligand systems activated with Zn

Article abstract of DOI:10.1021/ja910808x

(Chemical Presented) The mixed-ligand system NiCl₂(dppp)/dppf is shown to be an effective catalyst for the neopentylglycolborylation of ortho-, meta-, and para-substituted electron-rich and electrondeficient aryl mesylates and tosylates. The ad

Full text of DOI:10.1021/ja910808x

Published on Web 01/22/2010
Neopentylglycolborylation of Aryl Mesylates and Tosylates Catalyzed by
Ni-Based Mixed-Ligand Systems Activated with Zn
Daniela A. Wilson, Christopher J. Wilson, Costel Moldoveanu, Ana-Maria Resmerita,
Patrick Corcoran, Lisa M. Hoang, Brad M. Rosen, and Virgil Percec*
Roy & Diana Vagelos Laboratories, Department of Chemistry, UniVersity of PennsylVania,
Philadelphia, PennsylVania 19104-6323
Received December 22, 2009; E-mail: percec@sas.upenn.edu
Table 1. Comparison of NiCl2(dppp)/dppf-Catalyzed
Neopentylglycolborylation of Aryl Mesylates and Tosylates with
and without Zn
Arylboronic acids and arylboronate esters are critical intermedi-
ates in organic synthesis, and their roles as elements of functional
materials and as therapeutic agents are emerging.1 Transition-metal
catalysis provides a mild and robust approach to the preparation of
diversely functionalized arylboronate esters that overcomes the
technical limitations and limited functional group tolerance associ-
ated with traditional hard-metalation conditions. Re,2 Rh,3 and Ir4
catalysts for the direct C-H borylation of arenes using tetraalkoxy-
diboron and dialkoxyborane have been developed. Although it is a
powerful technique, direct C-H borylation cannot always provide
the desired regioisomers. Pd-catalyzed borylation is a complemen-
tary tool for the regiospecific conversion of aryl triflates, iodides,
and bromides to the corresponding arylboronate esters.5 Recently,
Buchwald-type ligands have expanded Pd-catalyzed borylation to
aryl chlorides.6 The borylation of aryl mesylates and tosylates has
remained a challenge in this field, although it would greatly expand
the scope of transition-metal-catalyzed borylation. An efficient
solution to this challenge is reported here.
Following an early report that provided two examples of Ni-
catalyzed pinacolborylation,7 our laboratory reported an efficient,
cost-effective Ni-catalyzed borylation using in situ-prepared neo-
pentylglycolborane.8 Ni-catalyzed borylation of aryl iodides and
bromides was successfully paired with sequential Ni-catalyzed8a,b
or complementary one-pot Pd-catalyzed8b cross-coupling for rapid
access to biaryls. Earlier, our laboratory determined the improved
performance of mixed-ligand systems in Ni-catalyzed cross-
coupling.9 Development of mixed-ligand catalytic systems, notably
NiCl2(dppp)/dppf, has expanded the scope of Ni-catalyzed neo-
pentylglycolborylation to aryl chlorides8c and ortho-substituted aryl
halides.
As NiCl2(dppp)/dppf demonstrated superior performance to other
ligand systems for the borylation of less-reactive aryl chloride and
ortho-substituted substrates, we tested its performance in the
borylation of aryl mesylates and tosylates (Table 1). Aryl mesylates
and tosylates are incompatible with single-ligand Ni catalysts for
borylation, and no reports have suggested their applicability in Pd-
catalyzed systems. Once again, it was found that the NiCl2(dppp)/
dppf mixed-ligand system excels in comparison to single-ligand
systems, providing borylation of all substrates tested. However, high
yields were obtained only for phenyl methanesulfonate (entry 1),
2-methanesulfonylnaphthalene (entry 2), and 4-cyanophenyl meth-
anesulfonate (entry 6). The yields obtained for ortho- and para-
substituted aryl mesylates were significantly lower, although
somewhat better yields were observed for meta-substituted aryl
mesylates and tosylates (entries 4, 10, 11, 14, and 15).
a Conversion calculated from GC. b Yield determined by GC. Isolated
yield in parentheses. c Using 2 equiv of Zn powder. d Isolated as the
solid trifluoroborate since the corresponding arylboronate is a liquid.
Previously, our laboratory utilized Zn as a reductant in Ni-
catalyzed homo- and cross-coupling of aryl mesylates and tosy-
lates.10 Astonishingly, introduction of 2.0 equiv of Zn in the Ni-
catalyzed neopentylglycolborylation of aryl mesylates and tosylates
provided excellent yields. In addition to the enhanced yield, the
reaction time to achieve maximum conversion was dramatically
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1800 J. AM. CHEM. SOC. 2010, 132, 1800–1801
10.1021/ja910808x  2010 American Chemical Society

C O M M U N I C A T I O N S
reduced from 1-4 days in the absence of Zn to 1-3 h in its
presence (Table 1 and Table ST1 in the Supporting Information).
(entry 2). Further reduction in the catalyst loading level to 2 mol
% NiCl2(dppp) and 4 mol % dppf provided quantitative yield only
after 7 h (entry 3). Effective borylation could also be achieved with
reduced Zn levels as low as 0.5 equiv (entries 4-6). However,
decreasing the Zn level gave a corresponding decrease in yield.
The borylation of substrates containing aldehydes, ketones, and
pyridines in the presence of Zn is accompanied by side reactions
and will be reported in a scope and limitations manuscript together
with mechanistic studies.
Table 2. Catalyst Screening in the Ni-Catalyzed
Neopentylglycolborylation of Aryl Mesylates Using Zn
convna/yieldb (%)
entry
catalyst (%)
ligand (%)
1 h
2 h
4 h
Acknowledgment. Financial support by the National Science
Foundation (DMR-0548559 and DMR-0520020) and the P. Roy
Vagelos Chair at the University of Pennsylvania is gratefully
acknowledged. We also thank Professor G. A. Molander of the
University of Pennsylvania for reading the final version of the
manuscript and for suggestions.
1
2
3
NiCl2(dppp) (5)
NiCl2(dppp) (5)
NiCl2(dppp) (5)
dppf (10)
PPh3 (10)
PTol3 (10)
100/100
79/79
61/61
-
93/93
83/83
-
100/100
97/97
a Conversion calculated from GC. b Yield determined by GC.
Other mixed-ligand systems can also mediate effective neopen-
tylglycolborylation of aryl mesylates and tosylates (Table 2), though
longer reaction times are needed to obtain maximum yield. With 5
mol % NiCl2(dppp) and 10 mol % dppf, quantitative borylation of
methyl 4-methanesulfonyloxybenzoate in 1 h was achieved (entry
1). Using 10 mol % PPh3 (entry 2) or PTol3 (entry 3) as the coligand
provided 100 and 97% yield, respectively, after 4 h.
Supporting Information Available: Experimental details and
compound characterization. This material is available free of charge
via the Internet at http://pubs.acs.org.
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NiCl2(dppp)/dppf-Catalyzed Neopentylglycolborylation of Aryl
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convna/yieldb (%)
entry
catalyst (%)
ligand (%) equiv of Zn
1 h
2 h
4 h
7 h
1
2
3
4
5
6
7
8
NiCl2(dppp)(5) dppf (10)
NiCl2(dppp)(3) dppf (6)
NiCl2(dppp)(2) dppf (4)
NiCl2(dppp)(5) dppf (10)
NiCl2(dppp)(5) dppf (10)
NiCl2(dppp)(5) dppf (10)
2
2
2
1.5
1
100/100
-
-
-
-
-
46/46
-
-
-
0/0
-
95/95 100/100
63/63 86/86 100/100
100/98
-
-
100/95
100/91
0/0
-
-
0.5
2
2
-
-
-
-
-
0/0
-
0/0
30/30
NiCl2(dppp)(5)
-
a Conversion calculated from GC. b Yield determined by GC.
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A control experiment was performed in which only Zn, neo-
pentylglycolborane, Et3N, and methyl 4-methanesulfonyloxyben-
zoate were heated in toluene (Table 3, entry 7). Without the Ni
catalyst, no borylation was observed, indicating the role of Zn as
an additive, presumably as a reductant. As expected, subsequent
addition of the Ni catalyst to this system allowed the borylation to
commence (entry 8). Ideally, low catalyst and reductant loadings
are desired. With 5 mol % NiCl2(dppp), 10 mol % dppf, and 2
equiv of Zn, quantitative borylation of methyl 4-methanesulfony-
loxybenzoate was achieved in 1 h (entry 1). Reducing the catalyst
loading level to 3 mol % NiCl2 (dppp) and 6 mol % dppf resulted
in a slower reaction that achieved complete conversion after 4 h
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