Organic Letters
Letter
We recently studied the rhodium-catalyzed diboron-mediated
transfer hydrogenation reaction of alkenes and carbonyls, and
high reactivity of the catalytic system was observed.15 Given
the unique reactivity and selectivity of rhodium in hydro-
genation, we envisioned the rhodium/diboron system might
work for the selective transfer hydrogenation of functionalized
arenes. During the course of our study, the Glorius group
reported the transfer hydrogenation of arenes and heteroarenes
with ammonia borane/TFE (2,2,2,-trifluoroethanol) as the
hydrogen donor, and several types of functionalized arenes
were efficiently hydrogenated.16 Herein, we report the transfer
hydrogenation of functionalized arenes using B2(OH)4/EtOH
as the hydrogen donor. This new protocol shows good
functional group tolerance, operational simplicity, and control-
lable chemoselectivity (Scheme 1).
the Wilkinson’s catalyst, were not active (entries 4−6). The
commercial Rh/C was found to be active for the transfer
hydrogenation but showed a lower efficiency (entry 7).
Different diborons were next investigated with [Rh(OH)-
(cod)]2 as the catalyst, and bis(catecholate) diboron B2(cat)2
was found to be ineffective while both bis(pinacolate) diboron
B2(pin)2 and bis(neopentylglycolate) diboron B2(neop)2 gave
low yield of the desired product (entries 8−10). Thus,
[Rh(OH)(cod)]2 was chosen as the catalyst and B2(OH)4 was
chosen as the diboron reagent for further studies.
With the established conditions for transfer hydrogenation
of arylboronic acid pinacol ester in hand (Table 1, entry 3), the
substrate scope was then studied. As shown in Scheme 2, a
e
Scheme 2. Transfer Hydrogenation of Aryl Boronate Esters
We commenced the investigation with phenylboronic acid
pinacol ester 1a (Table 1). With cyclooctodiene (cod)-
Table 1. Study on the Transfer Hydrogenation of Phenyl
a
Boronate Ester 1a
conv
b
c
entry
catalyst and additive
[RhCl(cod)]2
[Rh(OH)(cod)]2
diboron
(%)
yield (%)
1
2
3
4
5
6
7
8
9
10
B2(OH)4
B2(OH)4
B2(OH)4
B2(OH)4
B2(OH)4
B2(OH)4
B2(OH)4
B2(cat)2
80
>95
>95
<5
7
<5
55
<5
27
24 (23)
44 (41)
81 (74)
<5
<5
<5
55 (53)
<5
<5
[Rh(OH)(cod)]2, pinacol
[Rh(OH)(binap)]2, pinacol
[Rh(OH)(dppe)]2, pinacol
RhCl(PPh3)3, pinacol
Rh/C, pinacol
[Rh(OH)(cod)]2, pinacol
[Rh(OH)(cod)]2, pinacol
[Rh(OH)(cod)]2, pinacol
B2(pin)2
B2(neop)2
28
17
a
Reaction conditions: 1a (0.20 mmol), diboron (0.90 mmol), catalyst
(5 mol % Rh), pinacol (1.0 mmol, if added), EtOH (1.0 mL), 50 °C,
b
1
18 h. Conversion of 1a, determined by H NMR analysis of the
crude reaction mixture with 1,1,2,2-tetrachloroethane (0.20 mmol) as
c
1
the internal standard. Yield of 2a based on the crude H NMR with
1,1,2,2-tetrachloroethane (0.20 mmol) as the internal standard.
Isolated yield is shown in parentheses.
a
b
iPrOH instead of EtOH. Et3N (0.30 mmol) was added.
c
d
[Rh(OH)(cod)]2 (10.0 μmol) was used. B2(OH)4 (0.80 mmol)
e
was used. General conditions: aryl boronate ester (0.20 mmol),
coordinated rhodium(I) chloride as the catalyst, the desired
cyclohexylboronic acid pinacol ester 2a was obtained using
tetrahydroxydiboron as the reducing reagent in ethanol, and
the conversion was higher by using the hydroxorhodium
catalyst (Table 1, entries 1 and 2). In contrast to the high
conversion of 1a in these two reactions, the yield of 2a was
low. Given that other arylboronic esters undergo trans-
metalation with rhodium catalyst much easier than the
corresponding pinacol ester17 and no side product was
B2(OH)4 (0.90 mmol), [Rh(OH)(cod)]2 (5.0 μmol), pinacol (1.0
mmol), EtOH (1.0 mL), 50 °C, 18 h. Conversion and NMR yields are
given (1,1,2,2-tetrachloroethane as the internal standard), and isolated
yields are shown in parentheses.
range of arylboronic acid pinacol esters bearing diverse
substitutions (2b−2g) and substitutions at different positions
(2h−2i) were all tolerated, giving the cis products as the major
isomer. When the naphthyl substrates were employed, the
unsubstituted ring was preferentially hydrogenated (semi:full
>20:1) to give the tetrahydro-naphthylboronic acid pinacol
esters (2j−2k) in high yields, and no fully hydrogenated
product was observed. Note that the lower isolated yields of
some products than NMR yields were mainly due to their
instability on the silica gel column.
1
detected by H NMR analysis of the crude reaction mixture,
we reasoned that solvolysis of 1a/2a with EtOH followed by
rhodium-catalyzed protodeboronation can be the possible side
reaction to give volatile side products. Additional pinacol was
then added to minimize the possible solvolysis of substrate/
product, and gratifyingly the isolated yield of 2a was improved
to 74% (entry 3). Rhodium phosphine complexes, including
1911
Org. Lett. 2021, 23, 1910−1914