JOURNAL OF CHEMICAL RESEARCH 2016 395
eg
N
N
B
H
O
O
H
=
Ru
HB
Beg
B
N
N
H
TS7-8
N
N
50
40
30
20
10
0
49.5
N
Beg
Ru
N
TS8-9
8
H
TS5-6
38.3
HB
37.3
N
N
H
32.1
N
N
N
N
N
N
N
N
Ru
HB
6
H
14.7
4
9.1
4
3.9
N
0.0
N
Beg
Ru
H
-10
HB
N
N
–10.7
eg
B
H
–13.9
N
N
Beg
N
N
N
N
N
N
N
7
H
Ru
Ru
HB
HB
H
H
N
N
H2
N
N
N
5
9
Fig. 2 Reaction pathway with calculated relative free energies (kcal mol–1).
Ru-catalysed borylation of arenes; general procedure
contains a coordinated 3a. Finally, the elimination of the
product 3a and H2 from 9 regenerates 4.
[RuCl2(p-cymene)]2 (2.3 mg, 3.8 μmol) and TpMe K (2.5 mg, 7.5 μmol)
were placed in a resealable Schlenk tube. The tube was evacuated,
backfilled with dinitrogen and then charged with the arene 2 (5 mmol).
After stirring the mixture at room temperature for 1 h, pinacolborane
(1; 36 μL, 0.25 mmol) was added. The reaction mixture was then
stirred at 120 °C for 16 h. After the reaction, the mixture was analysed
by GC and GC−MS. The volatile material was removed in vacuo, and
the residue was purified by Kugelrohr distillation.
2
As shown in Fig. 2, the transition state for the electrophilic
attack of the borenium moiety TS7–8 is the highest point on
the free energy profile of the catalytic cycle. The electrophilic
aromatic substitution involving the carbon’s hybridisation
change from sp2 to sp3 would be in agreement with the inverse
secondary isotope effect as shown in Scheme 2.12,13 Furthermore,
we believe that the electrophilic attack step, 7→TS7–8, should
greatly contribute to determining the product selectivity.
4,4,5,5-tetramethyl-2-phenyl-1,3,2-dioxaborolane (3a):6 yield 82%;
1H NMR (500 MHz, CDCl3) δ 1.35 (s, 12H), 7.37 (t, J = 7.2 Hz, 2H),
7.46 (tt, J = 7.4, 1.5 Hz, 1H), 7.81 (dd, J = 7.7, 1.1 Hz, 2H); 13C NMR
(125 MHz, CDCl3) δ 24.86, 83.75, 127.69, 131.23, 134.72.
Conclusions
In conclusion, we have demonstrated that the ruthenium
2-[3,5-bis(trifluoromethyl)phenyl]-4,4,5,5-tetramethyl-1,3,2-
complex prepared in situ from [RuCl2(p-cymene)]2 and TpMe
K
1
dioxaborolane (3b):6 yield 58%; H NMR (500 MHz, CDCl3) δ 1.37 (s,
2
12H), 7.94 (s, 1H), 8.23 (s, 2H); 13C NMR (125 MHz, CDCl3) δ24.84, 84.84,
123.49 (q, J= 33.3 Hz), 124.7 (br s), 130.89 (q, J= 247.3 Hz), 134.64 (br s).
2-(3,4-dimethylphenyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (3c):6
catalysed the C–H borylation of arenes 2 with pinacolborane
(1). Theoretical calculations support that the catalytic cycle
involves a borenium cation equivalent. The calculations as well
as KIE experiments suggest that the electrophilic attack of the
borenium ion on the aromatic carbon is the rate-determining
step of the catalytic cycle. Further studies are currently
underway to obtain detailed mechanistic insights.
1
yield 77%; H NMR (500 MHz, CDCl3) δ 1.38 (s, 12H), 2.27 (s, 3H),
2.27 (s, 3H), 7.15 (d, J = 7.5 Hz, 1H), 7.55 (d, J = 7.5 Hz, 1H), 7.58 (s, 1H);
13C NMR (125 MHz, CDCl3) δ 19.44, 19.99, 24.54, 24.81, 83.55, 129.13,
132.37, 135.90, 140.11.
2-(2,5-difluorophenyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (3d):6
yield 71%; 1H NMR (500 MHz, CDCl3) δ 1.36 (s, 12H), 6.98 (br s, 1H),
7.10 (br s, 1H), 7.39 (br s, 1H). 13C NMR (125 MHz, CDCl3) δ 24.77, 84.21,
116.54 (dd, J = 8.3 and 26.9 Hz), 119.69 (dd, J = 9.3 and 24.8 Hz), 122.22
(dd, J = 9.3 and 22.8 Hz), 158.84 (d, J = 242.1 Hz), 162.92 (d, J = 247.3 Hz).
3e: the above procedure afforded an inseparable mixture of o-, m-,
and p-3e. By comparing with the retention time of a prepared authentic
mixture of 3e (isomer mixture, o:m:p = 4:63:33),6 the product
distribution was determined by GC analysis of the crude product.
3f: the above procedure afforded an inseparable mixture of m- and
p-3f. By comparing with the retention time of a prepared authentic
mixture of 3f (isomer mixture, m:p = 62:38),6 the product distribution
was determined by GC analysis of the crude product.
Experimental
General information
All experiments were carried out under a nitrogen atmosphere using
oven-dried (120 °C) glassware. NMR spectra were recorded on a JEOL
JNM-A500 spectrometer (1H, 500 MHz; 13C, 125 MHz). GLC analyses
were carried out with a Shimadzu GC-14B instrument equipped with
a glass column (OV-17 on Chromosorb W, 2 m) and with a capillary
column (DB-1, 0.53 mm I.D., 30 m). GLC yields were determined
using suitable hydrocarbons as internal standards. GC–MS analyses
were performed on a Shimadzu GC/MS QP-5000 spectrometer at an
ionisation potential of 70 eV.
4,4,5,5-Tetramethyl-1,3,2-dioxaborolane (1; Aldrich) and all arenes
2a–g (TCI) were purchased from commercial sources and purified by
distillation before use. [RuCl2(p-cymene)]2 (Kanto Chemical Co., Inc.)
3g: the above procedure afforded an inseparable mixture of m- and
p-3g. By comparing with the retention time of a prepared authentic
mixture of 3g (isomer mixture, m:p = 70:30),6 the product distribution
was determined by GC analysis of the crude product.
and TpMe K (TCI) were purchased and used as received.
2