Rhodium-catalyzed addition of arylboron compounds to nitriles, ketones, and imines

Article abstract of DOI:10.1016/j.jorganchem.2006.02.022

The rhodium-catalyzed addition reactions of sodium tetraphenylborate and arylboronic acids to nitriles, ketones, and imines were examined. The reaction of nitriles could be carried out efficiently in the presence of a catalyst system of [RhCl(cod)]₂-dppp and H₂O to give the corresponding monoarylated products selectively. Although unactivated ketones and imines are known to be poor electrophiles for rhodium-catalyzed arylation, the phenylation of them with use of sodium tetraphenylborate proceeded smoothly in the presence of [RhCl(cod)]₂ and Rh(acac)(cod) as catalysts, respectively. The addition of NH₄Cl was found to be crucial to effectively conduct the reaction of ketones and imines.

Full text of DOI:10.1016/j.jorganchem.2006.02.022

Journal of Organometallic Chemistry 691 (2006) 2821–2826
www.elsevier.com/locate/jorganchem
Rhodium-catalyzed addition of arylboron compounds
to nitriles, ketones, and imines
*
*
Kenji Ueura, Sawako Miyamura, Tetsuya Satoh , Masahiro Miura
Department of Applied Chemistry, Faculty of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
Received 26 January 2006; received in revised form 15 February 2006; accepted 16 February 2006
Available online 6 March 2006
Abstract
The rhodium-catalyzed addition reactions of sodium tetraphenylborate and arylboronic acids to nitriles, ketones, and imines were
examined. The reaction of nitriles could be carried out efficiently in the presence of a catalyst system of [RhCl(cod)]2–dppp and H2O
to give the corresponding monoarylated products selectively. Although unactivated ketones and imines are known to be poor electro-
philes for rhodium-catalyzed arylation, the phenylation of them with use of sodium tetraphenylborate proceeded smoothly in the pres-
ence of [RhCl(cod)]2 and Rh(acac)(cod) as catalysts, respectively. The addition of NH4Cl was found to be crucial to effectively conduct
the reaction of ketones and imines.
ꢀ 2006 Elsevier B.V. All rights reserved.
Keywords: Rhodium catalyst; Arylation; Arylboron compounds; Nitriles; Ketones; Imines
1. Introduction
ing imine, and the product successively undergoes ortho-
phenylation via C–H bond cleavage under the conditions
employed (path A in Scheme 2) [16]. This is a rare example
of rhodium-catalyzed intermolecular nucleophilic addition
to nitriles [10,11]. We report herein our new findings that
the reaction of nitriles to yield monophenylation products
can be performed selectively under appropriate conditions
with suppressing the ortho-arylation (path B in Scheme 2).
Moreover, a number of ketones and unactivated imines
have also been found to undergo phenylation by the rho-
dium catalysis.
The rhodium-catalyzed nucleophilic addition reactions
of organoboron and -stannane reagents to carbon-hetero-
atom multiple bonds are now recognized to be highly use-
ful tools for C–C bond formation [1–6]. In these reactions,
the mild, weakly nucleophilic organometallic reagents are
effectively activated under rhodium catalysis to react read-
ily with aldehydes and activated imines possessing electron-
withdrawing groups, such as sulfonyl substituents, on their
nitrogen (Scheme 1). However, in contrast to the additions
toward the reactive electrophiles, those to ketones [4,7,8],
unactivated imines [3,4,9], and nitriles [10,11] have been
considered to be sluggish.
2. Results and discussion
Meanwhile, in the course of our study of rhodium-cata-
lyzed arylation reactions [12–15], we observed that treat-
ment of benzonitrile with sodium tetraphenylborate can
bring about the efficient addition of the phenylboron
reagent to the C–N triple bond to produce the correspond-
As reported previously, treatment of sodium tetraphe-
nylborate (1) (0.5 mmol) with benzonitrile (2a) (2 mmol)
in the presence of [RhCl(cod)]2 (cod = 1,5-cyclooctadiene,
0.005 mmol) and dppp (1,3-bis(diphenylphosphino)pro-
pane, 0.01 mmol) in o-xylene at 120 ꢁC for 44 h under
nitrogen gave benzophenone imine (3a) in 21% yield, along
with di- (0.09 mmol, 36% based on the amount of 1 used)
and triphenylated products (0.02 mmol, 11%) (Scheme 2
and Entry 1 in Table 1) [16]. To our surprise, addition of
*
Corresponding authors. Tel.: +81 6 6879 7361; fax: +81 6 6879 7362.
E-mail addresses: satoh@chem.eng.osaka-u.ac.jp (T. Satoh), miura@
chem.eng.osaka-u.ac.jp (M. Miura).
0022-328X/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2006.02.022

2822
K. Ueura et al. / Journal of Organometallic Chemistry 691 (2006) 2821–2826
R'
C H
YH
[RhCl(cod)]
(0.005 mmol)
dppp (0.01 mmol)
2
Rh-cat.
R
M
+
R' C H
R
Y
+
NaBPh
RCN
PhCOR
4
M = B, Sn
Y = O, N EWG
o-xylene/H O
2
1
4b-e
2b-e
(2 mmol)
o
120 C, 1-2 h
Scheme 1. Addition of organoboron and -stannane reagents to aldehydes
and activated imines.
(0.5 mmol)
a
Product, Yield (%)
2
+
2b: R = 4-CH C H
4b: R = 4-CH C H , 98
NaBPh
PhCN
3
6
4
3 6 4
4
2c: R = 4-ClC H
4c: R = 4-ClC H , 63
6 4
6
4
Ph
Ph
NH
Ph
2d: R = PhCH
2e: R = Et
4d: R = PhCH , 120
2
2
A
b
4e: R = Et, 73
+
+
Ph CNH
2
C NH
C
a
b
o-xylene
Ph
Isolated yield. With 2e (5 mmol).
[RhCl(cod)]
2
Scheme 3. Phenylation of nitriles.
dppp
B
Ph CNH
Ph CO
2
2
same conditions. Thus, benzylcyanide (2d) and propionit-
rile (2e) reacted smoothly to afford benzyl phenyl ketone
(4d) and propiophenone (4e) in 120% and 73% yields,
respectively. In the former case, the yield exceeding 100%
means that more than one phenyl group in 1 can be
utilized.
o-xylene
H O
2
Scheme 2. Reaction of sodium tetraphenylborate with benzonitrile.
H2O (0.5 mL) completely suppressed the ortho-phenylation
[9] and significantly promoted the reaction to afford 3a in
91% yield within 1 h (Entry 2). Under the conditions with
H2O, part of 3a was hydrolyzed to benzophenone (4a).
During the post-treatment for product isolation by column
chromatography, the imine 3a was completely hydrolyzed
to give 4a in 85% yield. Decreasing the amount of 2a some-
what reduced the product yield (Entry 3). The omission of
dppp (Entry 4) or the use of PPh3 in place of it (Entry 5)
resulted in low efficiency of the phenylation.
The reaction using other nitriles 2b–e with 1 was next
examined (Scheme 3). 4-Methylbenzonitrile (2b) underwent
phenylation by 1 to give the corresponding ketone 4b in
98% isolated yield. The reaction of 4-chlorobenzonitrile
(2c) also proceeded efficiently, but in this case, ortho-
phenylation could not be suppressed completely even in
the presence of H2O to give a minor amount of dipheny-
lated product (ca. 20%) other than monophenylated one
4c (63%). Not only aromatic but also aliphatic nitriles
underwent phenylation upon treatment with 1 under the
Arylboronic acids could be used instead of borate 1 with
employing an appropriate base for their activation. Thus,
treatment of phenylboronic acid (5a) (0.5 mmol) with
2a (2 mmol) in the presence of [RhCl(cod)]2–dppp
(0.005 mmol and 0.01 mmol, respectively) and CsF
(2 mmol) as base in o-xylene at 120 ꢁC for 2 h afforded 4a
in 54% yield (Scheme 4). In contrast to the case using 1,
addition of water significantly retarded the reaction. Under
the same conditions, 4-methyl- (5b) and 4-chlorophenylbo-
ronic acids (5c) reacted with 2a to give 4b and 4c, respec-
tively. The reaction of 5c was relatively slow, and a small
amount of diarylated product (ca. 10%) was detected by
GC–MS analysis as in the reaction of 1 with 2c (Scheme 3).
A plausible mechanism for the reaction of arylboron
reagents 1 and 5 to nitriles 2 is illustrated in Scheme 5.
The reaction proceeds via nucleophilic addition of an aryl-
rhodium(I) intermediate, which is generated by transmetal-
lation of Rh(I)X species with 1 or 5, to 2. The resulting
imidorhodium species may undergo protonolysis by water,
Table 1
Reaction of sodium tetraphenylborate (1) with benzonitrile (2a)a
[RhCl(cod)]2
dppp
NaBPh4 + PhCN
Ph2CNH + Ph2CO
o-xylene/H2O
2a
3a
4a
1
Entry
L (mmol)
Time (h)
Yield of 3a + 4a (%)b
1c
2
3e
4
dppp (0.01)
dppp (0.01)
dppp (0.01)
–
44
1
1
21d
91 (85)
76
1
32
5
PPh3 (0.02)
2
41
a
Reaction conditions: 1 (0.5 mmol), 2a (2 mmol), [RhCl(cod)]2 (0.005 mmol) in o-xylene/H2O (9:1, 5 mL) at 120 ꢁC under N2.
GC yield based on the amount of 1 used. Value in parentheses indicates isolated yield of 4a.
In o-xylene (5 mL).
Di- and triphenylated products were also formed (0.09 and 0.02 mmol, respectively).
With 2a (1 mmol).
b
c
d
e

K. Ueura et al. / Journal of Organometallic Chemistry 691 (2006) 2821–2826
2823
up to 96% (Entry 9). The reaction using 5a in place of 1
was less efficient (Entry 10).
[RhCl(cod)]
(0.005 mmol)
dppp (0.01 mmol)
2
The reaction of ketones seems to proceed via a similar
sequence to that for the reaction of nitriles (Scheme 3).
Thus, a phenylrhodium intermediate generated by trans-
metallation is considered to react with a ketone to afford
an alkoxyrhodium species, which releases an alcohol or
alkoxide salt via protonolysis or transmetallation, depend-
ing on the reaction conditions employed. Although the
exact role of the weak acid, NH4Cl, is not clear at the pres-
ent stage, it is possible that it promotes the nucleophilic
addition step by protonation of the carbonyl oxygen,
and/or acts as the effective source for the alcoholic hydro-
gen at the protonolysis step.
ArB(OH) + PhCN
ArCOPh
2
CsF (2 mmol)
o-xylene
5a-c
(0.5 mmol)
4a-c
2a
(2 mmol)
o
120 C, 2 h
a
5
Product, Yield (%)
4a: Ar = Ph, 54
4b: Ar = 4-MeC H , 66
5a: Ar = Ph
5b: Ar = 4-MeC H
6
4
6 4
5c: Ar = 4-ClC H
4c: Ar = 4-ClC H , 16
6
4
6
4
a
GC yield.
Scheme 4. Arylation of benzonitrile.
Other aromatic ketones, 4-methyl- (6b) and 4-chloroac-
etophenone (6c), benzophenone (6d), and bis(4-chlorophe-
nyl)ketone (6e), underwent phenylation upon treatment
with 1 under the same conditions with those for the reac-
tion of 6a to give the corresponding alcohols in fair to good
yields (Entries 1–4, Table 3). Aliphatic ketones such as
cyclohexanone (6f) and 2-nonanone (6g) also reacted with
1 efficiently (Entries 5 and 6).
Ar BNa
Ar B+NaCl
4
3
or ArB(OH)
or ClB(OH)
2
2
RCN
2
Rh-Cl
Rh-Ar
Ar B+NaOH
3
or B(OH)
3
Ar BNa
4
or ArB(OH)
2
The phenylation system using [RhCl(cod)]2–NaBPh4
(1)-NH4Cl as catalyst, phenyl source, and promoter,
respectively, was found to be effective for the reaction of
unactivated aryl imines. When N-benzylideneaniline (8a)
(1 mmol) was treated with 1 (0.5 mmol) in the presence of
[RhCl(cod)]2 (0.005 mmol) and NH4Cl (1 mmol) in o-
xylene (5 mL) under N2 at 160 ꢁC (bath temperature) for
4 h, N-(diphenylmethyl)aniline (9a) was formed in 78%
yield (Entry 1 in Table 4). It should be noted that this is
a rare example for the rhodium-catalyzed arylation of
poorly electrophilic imines. The phenylation of 8a using
trimethylphenylstannane under rhodium catalysis has been
reported, as a sole precedent [3], to our knowledge, to pro-
duce 9a with a moderate yield. Unexpectedly, the reaction
efficiency was significantly improved by using Rh(acac)(-
cod) in place of [RhCl(cod)]2. Thus, 9a was obtained in
135% yield, which indicates the reaction of more than
one phenyl group of 1 (Entry 2). Other Rh–acac complexes,
Rh(acac)(CO)2 and Rh(acac)(C2H4)2, did not show any
activity (Entries 3 and 4). The reaction also did not proceed
at all without the addition of NH4Cl (Entry 5). Under the
conditions using Rh(acac)(cod) and NH4Cl, not only N-
benzylidene-4-chloroaniline (8b) but also more electron-
rich N-benzylidene-4-anisidine (8c) underwent phenylation
efficiently (Entries 6 and 7). Other benzylideneanilines, N-
(4-chlorobenzylidene)-4-chloroaniline (8d) and N-(4-meth-
ylbenzylidene)aniline (8e) also reacted with 1 under the
same conditions to give the corresponding amines 9d and
9e in good yields (Entries 8 and 9).
Ar
R
Rh-OH
Ar
N
Rh
R
Ar
R
H O
2
N
O
4
H O
2
H
3
Scheme 5. Plausible mechanism.
which is either added or generated in situ by condensation
of 5, to give an imine 3 and a Rh(I)OH species. The latter
undergoes transmetallation to regenerate the arylrhodium
intermediate. Under the wet conditions, part of 3 once pro-
duced tends to be hydrolyzed to form 4.
As described above, the rhodium-catalyzed arylation of
ketones is known to be sluggish. Actually, it was recently
reported that an attempted phenylation of acetophenone
(6a) by trimethylphenylstannane failed to occur [6]. In the
present system, however, 6a was found to undergo pheny-
lation. Thus, treatment of 6a (1 mmol) with 1 (0.5 mmol) in
the presence of [RhCl(cod)]2 (0.005 mmol) and dppp
(0.01 mmol) in o-xylene at 120 ꢁC under nitrogen for 25 h
gave 1,1-diphenylethanol (7a) in 47% yield (Entry 1, Table
2). An increase in the amount of 6a added slightly
decreased the yield of 7a (Entry 2). In contrast to the case
using 2a as electrophile (Table 1), the omission of dppp did
not affect the reaction efficiency (Entry 3). Other rhodium
species examined as catalysts, Rh(acac)(cod) (acac = acet-
ylacetonate), [Rh(OH)(cod)]2, [RhCl(nbd)]2 (nbd = nor-
bornadiene), and [RhCl(C2H4)]2 were less effective than
[RhCl(cod)]2 (Entries 4–7). Addition of water decreased
the product yield (Entry 8). Fortunately, however, use of
NH4Cl (1 mmol) as an additive significantly improved it
In summary, we have demonstrated that the rhodium-
catalyzed intermolecular phenylation of nitriles, ketones,
and unactivated imines can be performed efficiently with
use of sodium tetraphenylborate. Arylboronic acids can
also be employed as aryl sources, albeit less effective than
the borate. It has also been found that the addition of

2824
K. Ueura et al. / Journal of Organometallic Chemistry 691 (2006) 2821–2826
Table 2
Reaction of sodium tetraphenylborate (1) with acetophenone (6a)a
Ph
Ph
Me
Rh-cat.
Ph
Me
NaBPh
+
4
OH
7a
o-xylene
O
6a
1
Entry
Rh-cat.
Time (h)
Yield of 7a (%)b
1c
2c,d
3
4
5
[RhCl(cod)]2
[RhCl(cod)]2
[RhCl(cod)]2
25
25
25
25
25
25
6f
47
40
44
16
32
5
Rh(acac)(cod)e
[Rh(OH)(cod)]2
[RhCl(nbd)]2
[RhCl(C2H4)2]2
[RhCl(cod)]2
[RhCl(cod)]2
[RhCl(cod)]2
6
7
0
8g
9h
10i
a
5f
23
96 (93)
37
25
2f
Reaction conditions: 1 (0.5 mmol), 6a (1 mmol), Rh-cat. (0.005 mmol) in o-xylene (5 mL) at 120 ꢁC under N2.
GC yield based on the amount of 1 used. Value in parentheses indicates isolated yield.
With the addition of dppp (0.01 mmol).
With 6a (1.5 mmol).
With Rh(acac)(cod) (0.01 mmol).
The yield of 7a did not increase even if the reaction time was elongated.
In o-xylene/H2O (9:1, 5 mL).
With the addition of NH4CI (1 mmol).
b
c
d
e
f
g
h
i
5a (2 mmol) and CsF (2 mmol) were used in place of 1.
Table 3
Reaction of sodium tetraphenylborate (1) with ketones 6b–ga
R1
R2
Rh-cat.
R1
Ph
R2
NaBPh4
+
OH
7b-g
o-xylene
O
6b-g
1
Entry
6
Product, yield (%)b
1
2
3
4
5
6
a
6b: R1 = Me, R2 = 4-CH3C6H4
6c: R1 = Me, R2 = 4-ClC6H4
6d: R1 = R2 = Ph
7b, 74 (50)
7c, 169 (129)
7d, 61 (36)
7e, 103 (78)
7f, 139 (97)
7g, 55 (47)
6e: R1 = R2 = 4-ClC6H4
6f: R1R2 = –(CH2)5–
6g: R1 = Me, R2 = n-C7H15
Reaction conditions: 1 (0.5 mmol), 6 (1 mmol), [RhCl(cod)]2 (0.005 mmol), NH4Cl (1 mmol) in o-xylene (5 mL) at 120 ꢁC for 25 h under N2.
GC yield based on the amount of 1 used. Value in parentheses indicates isolated yield.
b
H2O and NH4Cl is crucial to smoothly conduct the reac-
tion of nitriles and of the latter two substrates, respectively.
a CBP-1 capillary column (i.d. 0.25 mm · 25 m). Imines
8b–e were prepared according to the published method
[18]. Other reagents were commercially available. Products
4a [19], 4b [20], 4c [20], 4d [21], 4e [22], 7a [23], 7b [24], 7c
[25], 7d [26], 7e [27], 7f [28], 7g [29], 9a [30], 9b [31], 9c [31],
9d [32], and 9e [9] are known.
3. Experimental
3.1. General
Reactions were carried out in a 20 mL two-neck flask
under N2. [Rh(OH)(cod)]2 was prepared according to the
published method [17]. 1H and 13C NMR spectra were
recorded at 400 and 100 MHz, respectively, for CDCl3
solutions. MS data were obtained by EI. GC analysis
was carried out using a silicon OV-17 column (i.d.
2.6 mm · 1.5 m) or a CBP-1 capillary column (i.d.
0.25 mm · 25 m). GC–MS analysis was carried out using
3.2. Typical procedure for the rhodium-catalyzed reaction
of sodium tetraphenylborate (1) with benzonitrile (2a)
(Entry 2 in Table 1)
A mixture of 1 (0.5 mmol, 171 mg), 2a (2 mmol,
206 mg), [RhCl(cod)]2 (0.005 mmol, 2.5 mg), dppp
(0.01 mmol, 4.1 mg), and dibenzyl (ca. 50 mg) as internal
standard was stirred in o-xylene/H2O (9/1, 5 mL) at

K. Ueura et al. / Journal of Organometallic Chemistry 691 (2006) 2821–2826
2825
Table 4
Reaction of sodium tetraphenylborate (1) with imines 8a–ea
R1
HN
R1
Ph
R2
Rh-cat.
NaBPh4
+
N
o-xylene
R2
1
9a-e
8a-e
Entry
8
Rh-cat.
Time (h)
Product, yield (%)b
c
1
2
3
8a: R1 = R2 = Ph
8a: R1 = R2 = Ph
8a: R1 = R2 = Ph
8a: R1 = R2 = Ph
8a: R1 = R2 = Ph
[RhCl(cod)]2
4
5.5
4
4
6
3.5
4
5
6
4
9a, 78
9a, 135 (85)
–
–
–
9b, 139 (108)
9c, 103 (78)
9d, 121 (93)
9e, 121 (100)
9a, 26
Rh(acac)(cod)
Rh(acac)(CO)2
Rh(acac)(C2H4)2
Rh(acac)(cod)
Rh(acac)(cod)
Rh(acac)(cod)
Rh(acac)(cod)
Rh(acac)(cod)
Rh(acac)(cod)
4
5d
6
8b: R1 = Ph, R2 = 4-ClC6H4
7
8
9
10e
a
8c: R1 = Ph, R2 = 4-CH3OC6H4
8d: R1 = R2 = 4-ClC6H4
8e: R1 = 4-CH3C6H4, R2 = Ph
8a: R1 = R2 = Ph
Reaction conditions: 1 (0.5 mmol), 8 (1 mmol), Rh-cat. (0.01 mmol), NH4Cl (1 mmol) in o-xylene (5 mL) at 160 ꢁC under N2.
GC yield based on the amount of 1 used. Value in parentheses indicates isolated yield.
With [RhCl(cod)]2 (0.005 mmol).
b
c
d
e
Without NH4Cl.
5a (0.5 mmol) and CsF (0.5 mmol) were used in place of 1.
120 ꢁC under N2 for 1 h. After cooling, the reaction mix-
References
ture was extracted with Et2O and dried over sodium sul-
fate. Product 4a (77 mg, 85%) was isolated by column
chromatography on silica gel using hexane–ethyl acetate
as eluent.
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[8] Reactions involving the intramolecular addition of vinylrhodium
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3.3. Typical procedure for the rhodium-catalyzed reaction of
sodium tetraphenylborate (1) with acetophenone (6a)
(Entry 9 in Table 2)
In a flask was placed NH4Cl (1 mmol, 54 mg), which
was then dried at 150 ꢁC in vacuo for 2 h. Then, 1
(0.5 mmol, 171 mg), 6a (1 mmol, 120 mg), [RhCl(cod)]2
(0.005 mmol, 2.5 mg), 1-methylnaphthalene (ca. 50 mg) as
internal standard, and o-xylene (5 mL) were added and
the resulting mixture was stirred at 120 ꢁC under N2 for
25 h. After cooling, the reaction mixture was extracted with
Et2O and dried over sodium sulfate. Product 7a (92 mg,
93%) was isolated by thin-layer chromatography on silica
gel using hexane–ethyl acetate as eluent.
3.4. Typical procedure for the rhodium-catalyzed reaction of
sodium tetraphenylborate (1) with N-benzylideneaniline
(8a) (Entry 2 in Table 4)
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A mixture of 1 (0.5 mmol, 171 mg), 8a (1 mmol,
181 mg), Rh(acac)(cod) (0.01 mmol, 3.1 mg), NH4Cl
(1 mmol, 54 mg), and dibenzyl (ca. 50 mg) as internal
standard was stirred in o-xylene (5 mL) at 160 ꢁC under
N2 for 5.5 h. After cooling, the reaction mixture was
extracted with Et2O and dried over sodium sulfate.
Product 9a (111 mg, 85%) was isolated by column chro-
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