Bull. Chem. Soc. Jpn. Vol. 79, No. 12 (2006)
ꢀ 2006 The Chemical Society of Japan 1981
Table 2. Isomer Distributions for C–H BorylationaÞ
ium(I) complexes catalyzed the dehydrogenative coupling of
arenes 2 with pinacolborane (1). Above all, [Rh(TpMe2)-
(cod)] and [Ir(Tp)(cod)] showed high catalytic activities in
the aromatic C–H borylation; however, the mechanism for this
borylation is unclear at the present stage. Further investiga-
tions of this and related C–H functionalizations are currently
underway in our laboratory.
Yield % (o:m:p)b)
[Rh(TpMe2)(cod)] [Ir(Tp)(cod)]
Entry Product 3
1
R = Me
(3b)
R = OMe
(3c)
R = CF3
(3d)
74c)
(0:62:38)
57
(4:63:33)
54
81
(0:64:36)
74
(3:66:31)
99
(0:68:32)
R
2
3
BPin
Experimental
All the experiments were carried out under a nitrogen atmo-
sphere in oven-dried (120 ꢁC) glassware. Pinacolborane (1) and
all arenes 2 were purchased from commercial sources and purified
by distillation before use. [{RhCl(cod)}2], [{IrCl(cod)}2], TpꢂKþ,
(0:70:30)
OMe
4
5
6
7
30
10
72 [66]d)
86 [84]d)
65 [61]d)
88 [85]d)
PinB
OMe
(3e)
ꢀ
ꢀ
BpꢂKþ, TpMe2ꢂKþ, [{RhCl2(Cp )}2], and [{IrCl2(Cp )}2] were
purchased and used without purification. [Rh(TpMe2)(cod)]12 and
[Ir(Tp)(cod)]13 were prepared by literature methods.
CF3
PinB
Typical Procedure for Synthesis of Dialkoxyarylborane.
[Ir(Tp)(cod)] or [Rh(TpMe2)(cod)] (5 mmol) was added to a reseal-
able Schlenk tube. The tube was evacuated and backfilled with
nitrogen and then charged with arene 2 (5 mmol) and pinacol-
borane (1) (0.50 mmol). After being stirred at 120 ꢁC for 18 h,
the reaction mixture was analyzed by GC and GC-MS. The product
was extracted with ether, washed with brine, dried over Na2SO4,
and concentrated. The residue was purified by Kugelrohr distilla-
tion to afford the desired dialkoxyarylborane 3. 3a: identical
spectroscopic data to those previously described.3b 3b: the above
procedure afforded an inseparable mixture of m-, p-3b, and
PhCH2BPin. By comparing with the retention time of the prepared
authentic 3b (isomer mixture, m:p = 69:31)7a and PhCH2BPin,13
the product distribution was determined by GC and GC-MS anal-
ysis of the crude product. 3c: the above procedure afforded an
inseparable mixture of o-, m-, and p-3c. By comparing with the
retention time of the prepared authentic mixture of 3c (isomer
mixture, o:m:p = 1:74:25),7a the product distribution was deter-
mined by GC and GC-MS analysis of the crude product. 3d: the
above procedure afforded an inseparable mixture of m- and p-
3d. By comparing with the retention time of the prepared authen-
tic 3d (isomer mixture, m:p = 70:30),7a the product distribution
was determined by GC and GC-MS analysis of the crude product.
3e: identical spectroscopic data to those previously described.5b
3f: identical spectroscopic data to those previously described.5b
3g: identical spectroscopic data to those previously described.7a
3h: 1H NMR (CDCl3): ꢂ 1.36 (s, 12H), 6.98 (br s, 1H), 7.10 (br s,
1H), 7.39 (br s, 1H). 13C NMR (CDCl3): ꢂ 24.77, 84.21, 116.54
(dd, J ¼ 8:3 and 26.9 Hz), 119.69 (dd, J ¼ 9:3 and 24.8 Hz),
122.22 (dd, J ¼ 9:3 and 22.8 Hz), 158.84 (d, J ¼ 242:1 Hz),
162.92 (d, J ¼ 247:3 Hz). HR-MS (EI): m=z calcd for C12H15-
O2F2B [Mþ]: 240.1133; found: 240.1131.
CF
3 (3f)
Cl
trace
29
PinB
PinB
Cl
F
(3h)
F
a) Reaction conditions: 1 (0.5 mmol), 2 (5 mmol), catalyst
(0.005 mmol), 120 ꢁC, 16 h. b) Yield and isomer ratio were
determined by GC analysis. c) A 23% yield of PhCH2BPin
was also obtained. d) Isolated yield.
was required (Entries 6 and 7). In contrast to the rhodium(I)-
catalyzed reactions, the use of TpMe2 gave quite lower yield
(Entry 10), whereas Bp was found to be an effective ligand for
this reaction (Entry 9). Presumably, the formation of the ꢁ2
isomer is essential for the Ir(Tp) system to catalyze the C–H
borylation, and the steric hindrance around the iridium center
would have a profound influence on the catalytic activity.
The results obtained with arenes 2, giving arylpinacol-
boranes 3 similarly as above, are listed in Table 2. The yields
and product ratios were determined by GC analysis of crude
reaction mixtures. The presence of functional groups did not
interfere with the outcome of the iridium(I)-catalyzed reaction.
However, the rhodium(I)-catalyzed reaction lacked wide appli-
cability to functionalized substrates 2, presumably due to coor-
dination of hetero atoms to the rhodium metal center yielding
an inactive species (Entries 2–6). Furthermore, the present
C–H borylation using Tp complexes showed the following
common features: (i) benzylic activation of toluene (2b)
increased for the rhodium catalyst system versus the iridium
analogue (Entry 1);4b,7f (ii) the reaction of monosubstituted
arenes 2b–2d resulted in a 2:1 mixture of meta and para iso-
mers and electronic characteristics of the substituent on 2 hard-
ly affected the statistical meta/para ratios (Entries 1–3); and
(iii) disubstituted arenes 2e–2h were functionalized regioselec-
tively for steric reasons (Entries 4–6).4b,5b,7a–d,8
References
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2
3
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