Synthesis and reactivity of boryloxorhodium complexes. Relevance to intermolecular transmetalation from boron to rhodium in Rh-catalyzed reactions

Article abstract of DOI:10.1039/d1dt00440a

The synthesis of a dimeric boryloxorhodium complex having the Rh-O-Bpin scaffold from the reaction of [(cod)Rh(OMe)]₂or [(cod)Rh(OH)]₂with an arylboronate has been achieved. The obtained dirhodium complex is converted into mononuclear complex [(cod)Rh(OBpin)(PPh₃)], which reacts with arylboronic acid to afford the complex with an Rh-aryl bondviatransmetalation from boron to rhodium. The dimeric boryloxorhodium complex catalyzes the 1,4-addition of arylboronic acid to cyclohexene-2-one.

Full text of DOI:10.1039/d1dt00440a

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PAPER
Synthesis and reactivity of boryloxorhodium
complexes. Relevance to intermolecular
transmetalation from boron to rhodium in
Rh-catalyzed reactions†
Cite this: Dalton Trans., 2021, 50,
3610
Yasushi Nishihara, *‡ Yasuhiro Nishide and Kohtaro Osakada
*
The synthesis of a dimeric boryloxorhodium complex having the Rh–O–Bpin scaffold from the reaction
Received 9th February 2021,
Accepted 16th February 2021
of [(cod)Rh(OMe)]2 or [(cod)Rh(OH)]2 with an arylboronate has been achieved. The obtained dirhodium
3
complex is converted into mononuclear complex [(cod)Rh(OBpin)(PPh )], which reacts with arylboronic
DOI: 10.1039/d1dt00440a
acid to afford the complex with an Rh-aryl bond via transmetalation from boron to rhodium. The dimeric
rsc.li/dalton
boryloxorhodium complex catalyzes the 1,4-addition of arylboronic acid to cyclohexene-2-one.
plexes by several groups led to isolation of the complexes with
Ar–B(OH)–O–M bonds. Chemical properties of these arylboro-
Introduction
Arylboronic acids and their derivatives were employed as the nate complexes are of interest with relevance to intermediates
key substrates in synthetic organic reactions catalyzed by tran- and mechanism of the catalytic reactions. Studies of the com-
sition metal complexes, such as palladium-catalyzed Suzuki– plexes, however, have been much less common than those
1
9
Miyaura coupling, and rhodium-catalyzed 1,4-addition to with hydroxo, alkoxo, and aryloxo ligands, and with dialkoxy-
2
10
α,β-unsaturated carbonyl compounds. The organoboron com- boryloxo
pounds such as arylboronic acids, arylboronates, and aryltri- remained less unexplored. We have previously disclosed that
fluoroborates are used as sources of aryl groups in the cata- transmetalation of Pt complexes having a Pt–O–BAr(OH) with
ligand-containing
complexes,
M–O–B(OR)2,
3
4
5
lytic reactions. Their advantages as a substrate for the organo- arylboronic acids gives arylplatinum(II) complexes (Scheme 1
metallic reactions reside in their high stability in polar sol-
vents, including H O, and in smooth formation of the aryl-
2
transition metal intermediate via transmetalation. Hayashi
et al. conducted NMR studies of the elemental reactions relat-
ing to the catalysis and observed the formation of [Rh(Ar)
(
PPh )(binap)] from the stoichiometric reaction of arylboronic
3
6
acid with [Rh(μ-OH)(binap)] at 25 °C. Recent studies on the
2
reaction by theoretical calculations suggested that the OH
ligand bonded to Rh(I) assisted complexation of arylboronic
7
acid and induced formation of an Rh-aryl bond. Reactions of
arylboronic acid with chlorogold(I) complex and of arylboronic
esters with alkoxo complexes of base transition metals were
reported to cause direct transmetalation to form the corres-
8
ponding aryl complexes. Carefully controlled stoichiometric
reactions of arylboronic acids with late transition metal com-
Laboratory for Chemistry and Life Science, Tokyo Institute of Technology,
4
259 Nagatsuta, Yokohama 226-8503, Japan.
E-mail: osakada.k.aa@m.titech.ac.jp, ynishiha@okayama-u.ac.jp
Electronic supplementary information (ESI) available. CCDC 2001091–2001093.
For ESI and crystallographic data in CIF or other electronic format see DOI:
†
10.1039/d1dt00440a
‡
Current address: Research Institute for Interdisciplinary Science, Okayama
University, 3-1-1 Tsushimanaka, Kita-ku, Okayama 700–8530, Japan.
Scheme 1 Transmetalation of the aryl groups from boron to metals.
3610 | Dalton Trans., 2021, 50, 3610–3615
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1
1
(
i)). Hartwig and co-workers reported that Rh complexes with
the arylboronate ligands produce the corresponding arylrho-
dium complexes via intramolecular transfer of an aryl group
1
2
from boron to rhodium (Scheme 1 (ii)). Denmark’s group
revealed the presence of Pd–O–BAr(OH) complexes that cause
1
3
transmetalation (Scheme 1 (iii)).
Thus, transmetalation
of arylboronic acids proceeds via an arylboronate complex
formed via activation of OH bonds by transition metals.
Reports on the mechanism of transmetalation of arylboronates
have been much less common. Recently, Denmark conducted
experimental and theoretical studies on the reaction of various
arylboronates with hydroxopalladium complexes to cause an
Fig. 1 ORTEP drawing of complex 1 with 50% thermal ellipsoidal plot-
ting. Hydrogen atoms were omitted for clarity. Selected bond distances
intermolecular interaction of the OH ligand on palladium with [Å] and angles [°] for 1: Rh1–O1 2.100(3), B1–O1 1.344(6), B1–O2
.372(6), B1–O3 1.362(6); Rh1–O1–Rh1 88.4(1), Rh1–O1–B1 134.6(3),
1
the B–O moiety of arylboronates prior to the transfer of aryl
groups from boron to palladium.
1
4
O1–B1–O2 124.8(5), O1–B1–O3 122.8(5), O2–B1–O3 112.5(4).
In this paper, we report synthesis of mono- and dinuclear
rhodium complexes with Rh–O–Bpin (Bpin = pinacolboryl)
bonds, and their structures, stoichiometric reaction with an
arylboronic acid, causing intermolecular transmetalation, and
catalysis using the complexes.
tion. Even cooling 1 to −95 °C resulted in no change of the
broadened signals of the 1,5-cyclooctadiene ligands.
Both [(cod)Rh(OMe)] and [(cod)Rh(OH)] appeared to react
2 2
similarly to give complex 1, despite more stable Me–O bond
than H–O bond of the ligands. Therefore, we investigated in
2
detail the reaction using [(cod)Rh(OMe)] , and found the for-
Results and discussion
mation of complex 1 in 78% isolated yield, and byproducts,
including methanol (46%), toluene (36%), and 2-(4-methyl-
phenyl)-2-propanol (62%) (Scheme 3). This experimental result
suggests the following reaction mechanism. When [(cod)Rh
2
Heating an equimolar mixture of [(cod)Rh(OH)] and 4-methyl-
phenylboronic acid pinacolate in acetone for 16.5 h at 50 °C
resulted in the formation of the boryloxorhodium complex,
(
OMe)] is first converted to [(cod)Rh(OH)] by water present in
2 2
2
[(cod)Rh(OBpin)] (1) as a pale yellow crystalline solid in 93%
acetone, methanol is formed. The generated [(cod)Rh(OH)]
2
yield. Similarly, complex 1 was obtained in 85% yield from the
reaction with [(cod)Rh(OMe)]2 (Scheme 2). This chemistry
reflects both the acidity of the boron centre and the thermo-
dynamically stable B–O bond formation. As a result, the reac-
tivity shown here provides a novel synthetic route of transition
metal complexes with the M–O–Bpin fragment.
Complex 1 is stable in air and is soluble in benzene and
toluene. When 1 was dissolved in CHCl3 or CH Cl , [(cod)
reacts with ArBpin to form an arylrhodium(I) complex and
HOBpin, both of which react further to give complex 1 and
toluene. On the other hand, the arylrhodium(I) complex adds
to acetone and undergoes subsequent hydrolysis to produce
1
5
2-(4-methylphenyl)-2-propanol, regenerating [(cod)Rh(OH)] .
2
However, we cannot rule out a pathway in which the arylrho-
dium(I) complex may undergo direct hydrolysis by water
present in the reaction system to produce toluene and regener-
2
2
2
RhCl] was formed. The structure of 1 was determined by X-ray
2
ate [(cod)Rh(OH)] .
analysis of single crystals grown from a toluene/hexane
solution. As shown in Fig. 1, the molecule of 1 contains two
square-planar rhodium atoms, bridged by the boryloxo
ligands. The B1–O1 bond is slightly shorter, but the other B–O
bonds (B1–O2 1.372(6), B1–O3 1.362(6) Å) are normal com-
pared to those of the trigonal planar BO3 group of the organo-
As shown in Scheme 4, with the addition of PPh3
(
1.0 equiv./Rh) to 1 in toluene, the originally pale yellow solu-
tion immediately turned dark red, exhibiting the formation of
3
1
1
the monomeric rhodium complex 2. The P{ H} NMR spec-
trum of 2 in CD Cl shows a doublet at 20.0 ppm, indicating
2
2
1
6
1
that the phosphorus atom is coupled to the rhodium centre
J(RhP) = 159 Hz). Fig. 2 shows an ORTEP diagram of the
boron compound. The H NMR spectrum of 1 in toluene-d8
(
indicates that all the ligands on the Rh atom are inert in solu-
Scheme 2 Preparation of complex 1.
2
Scheme 3 Reaction of [(cod)Rh(OMe)] with p-tolylboronate.
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Scheme 4 Preparation of complex 2.
Scheme 5 Preparation of complex 3.
(Scheme 5). In contrast, we have previously shown that the reac-
tion of the aryloxorhodium complex [(PMe Rh(OC Me-4)]
3
)
3
6 4
H
with arylboronic acid led to the formation of cationic rhodium
−
complexes with tetraarylpentaborates, [B O Ar ] (Ar = C H OMe-
5
6
4
6 4
20
6 3 2
4, C H Me -2,6) as counteranions.
1
13
Complex 3 was characterized by comparison of the H,
C
1
31
1
{
H}, and P{ H} NMR spectra as well as X-ray diffraction as
having a structure similar to [(nbd)Rh(C Me -2,6)(PPh )]
(Fig. 3). The formation of the
6
H
3
2
3
2
1
(
nbd = norbornadiene)
Fig. 2 ORTEP drawing of complex 2 with 50% thermal ellipsoidal plot- complex 3 can be considered to proceed by the transfer of the
ting. One of the two crystallographically independent molecules is aryl group via cleavage of the C–B bond in the arylboronic
shown. Hydrogen atoms were omitted for clarity. Selected bond distances
acid. This is related to the catalytic 1,4-addition of organo-
[
Å] and angles [°] for 2: Rh1–P1 2.303(5), 2.335(4), Rh1–O1 2.02(1), 2.07(1),
boron compounds to activated unsaturated organic molecules,
which can be effectively catalyzed by similar rhodium com-
plexes. The reaction mechanism of the envisioned catalytic
process involves initial transmetalation between the organo-
boron compound and the Rh–OR (R = Me or H) complex, 1,4-
addition of the formed arylrhodium species to unsaturated
organic compounds to furnish rhodium enolate, subsequent
hydrolysis to yield 1,4-adducts, and regeneration of the cata-
B1–O1 1.34(2), 1.28(2), B1–O2 1.48(2), 1.51(2), B1–O3 1.28(2), 1.31(2); Rh1–
O1–B1 132(1), 137(1), O1–B1–O2 118(2), 124(2), O1–B1–03 129(2), 127(2),
O2–B1–O3 112(2), 109(2), P1–Rh1–O1 86.0(4), 88.2(4).
monomeric boryloxorhodium complex 2. The structure of 2 is
almost square planar with Rh1–O1–B1 bond angles of 132(1)
and 137(1)°, which is slightly larger than that of [Rh(OBpin)
2
2
lyst. Therefore, we investigated the 1,4-addition of 2,6-di-
methylphenylboronic acid and the corresponding pinacol bor-
onate to cyclohexen-2-one catalyzed by complexes 1–3 and
(
(
PEt
11)°).
3
)
3
] (131.0(2)°) and [trans-Rh(OBpin)(CO)(PEt
3
)
2
] (126.69
1
5f
In terms of both steric and electronic factors, the
closest analogue of the boryloxo ligand would be the alkoxo
1
7
[(cod)Rh(OH)] as a reference. The reaction in the presence of
2
ligand. The terminal B1–O1 bonds (1.34(2), 1.28(2) Å) in 2
are found to be similar to those (1.304(4)–1.324(2) Å) observed
complex 1 (3 mol%) produced the 1,4-adducts in 70% and
5e 62% yields, respectively, showing catalytic activity comparable
1
in [Rh(OBpin)(PEt ) ] and [trans-Rh(OBpin)(CO)(PEt3)2].
3
3
2
to that of [(cod)Rh(OH)] (70% and 67%, respectively). In con-
Both complexes 1 and 2 are highly stable and exhibited no
trast, complexes 2 and 3 had no catalytic activity under similar
conditions (Table 1). This is probably because the bulky PPh3
ligand of complexes 2 and 3 retards further reactions of the
boronate or aryl ligands with the substrates.
signs of decomposition in the solid state or even in solution
unless they are heated. The B NMR resonances of boryloxor-
hodium complexes 1 and 2 show broad singlets at 23.6 and
1
1
2
1.7 ppm, respectively. For comparison, the rhodium boronate
1
3
complex [Rh{OB(OH)(Ph)}(PEt ) ]
exhibited a signal at
3
3
2
5.6 ppm, while the resonance for the rhodium complexes
having Rh–O–Bpin moieties such as [RhH(OBpin)(4-C N)
CN Bu)(PEt ) ] and [trans-Rh(OBpin)(CO)(PEt ) ] appear at
2.8 and 21.0 ppm, respectively.
The successful synthesis of the monomeric boryloxorho-
5 4
F
t
(
2
3
2
3 2
1
5d,e
dium complex 2 prompted us to compare directly its behavior
with that of the corresponding alkoxo, aryloxo, and hydroxo
1
8
complexes. Complex 2 is an ideal complex to study the reac-
tivity of the Rh–OB(OR) functionality because its reaction is
2
severely confined to this moiety. For instance, the transmetala-
Fig. 3 ORTEP drawing of complex 3 with 50% thermal ellipsoidal plot-
ting. Hydrogen atoms were omitted for clarity. Selected bond distances
tion reactivity of 2 to boronic acid was elucidated to compare
1
9
with those of the alkoxo analogues. The reaction of complex
with 2,6-dimethylphenylboronic acid was conducted in acetone.
To our surprise, the arylrhodium complex, [(cod)Rh(C Me -2,6)
PPh )] (3), was obtained in 71% yield, even at room temperature 89.1(2), C1–Rh1–C14 87.9(2).
[
Å] and angles [°] for 3: Rh1–P1 2.322(1), Rh1–C1 2.049(4), Rh1–C9
.244(5), Rh1–C10 2.203(5), Rh1–C13 2.185(5), Rh1–C14 2.187(5); P1–
Rh1–C1 89.9(1), P1–Rh1–C9 97.1(1), P1–Rh1–C10 96.0(2), C1–Rh1–C13
2
2
6 3
H
2
(
3
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Table 1 Rh-Catalyzed conjugate addition cyclohexen-2-one with aryl- and BF
2
·OEt (δ 0), respectively. Elemental analyses were
3
a
boronic acid and boronate
carried out with a Yanaco MT-5 CHN autocorder. The IR
spectra were recorded on a Shimadzu FTIR-8100A spectro-
meter in KBr. CCDC 2001091 (1), 2001092 (2), and 2001093
(
3)† contain the supplementary crystallographic data for this
paper [(cod)Rh(OH)] and [(cod)Rh(OMe)] were prepared
2
2
2
3
according to the literature methods.
b
Synthetic procedures
Yield (%)
2
Preparation of [(cod)Rh(OBpin)] (1) (Scheme 1). To a yellow
Rh catalyst
B(OR)
2
= B(OH)
2
2 2 2 4
B(OR) = BO C Me
suspension of [(cod)Rh(OMe)] (291 mg, 0.6 mmol) in 30 mL
2
Complex 1
Complex 2
Complex 3
72
0
0
62
0
0
of anhydrous acetone was added 4-methylphenylboronic pina-
colate (262 mg, 1.2 mmol) at room temperature. Upon heating
at 50 °C for 21 h, the color of the reaction mixture became pale
yellow. Removal of volatiles gave yellow precipitates, which
were washed with hexane (10 mL × 2), and collected by fil-
[
(cod)Rh(OH)]
2
70
67
a
Conditions: cyclohexen-2-one (2.5 mmol), arylboron compound
3.75 mmol), Rh catalyst (0.075 mmol), acetone (6 mL), water (4 mL),
(
b
5
0 °C, 1 h. NMR yields using diphenylmethane as an internal tration, and dried in vacuo to give [(cod)Rh(OBpin)] (1) as pale
2
standard.
yellow powder (362 mg, 0.51 mmol, 85%). Complex 1 crystal-
lizes from toluene-hexane as pale yellow crystals.
Elemental Analysis (%) calcd for C H B O Rh : C, 47.49;
2
8
48
2
6
2
1
6 6
H, 6.83; found: C, 47.15; H, 6.74. H NMR (300 MHz, C D ,
Conclusions
2
8
5 °C): δ 1.00 (s, 24H, Me), 1.46 (partially collapsed d, J = 8 Hz,
H, CH of cod), 2.30 (brs, 8H, CH of cod), 4.49 (brs, 8H, CH
Arylboronate causes smooth substitution of the bridging
ligands of [Rh(μ-OH)(cod)] by pinacolboryloxo ligand to afford
2
2
1
2
1
1
13
1
of cod); B{ H} NMR (160.4 MHz, C
NMR (75.3 MHz, C , 25 °C): δ 24.9 (Me), 31.0 (CH
3.3 (d, J = 14 Hz, CH of cod), 80.5 (OCMe); IR (KBr): v(B–O)
6
D
6
, 25 °C): δ 23.6; C{ H}
the dinuclear boryloxorhodium complex 1, which suggests
higher stability of the Rh–OBpin bond than the Rh–OH bond.
The reaction does not form a complex with either arylboronate
nor aryl ligand via direct transmetalation. Mononuclear
complex 2 is easily obtained by addition of phosphine to the
dirhodium complex 1, and undergoes stoichiometric transme-
talation by addition of arylboronic acid. The complex 1 cata-
lyzed 1,4-addition of cyclohexanone by arylboronic acid.
Activities of the complex both for stoichiometric transmetala-
tion and for the catalysis suggest a catalytic cycle involving the
complexes with a Rh–O–B(OR)2 bond as an intermediate.
Extensive exploration of the reactions of boryloxo complex 1 to
illuminate the role of new types of boryloxo complexes
in metal–catalyzed organic reactions is underway in our
laboratory.
6
D
6
2
of cod),
7
1
366, 1377, 1387 cm− . It decomposed at 189 °C turning from
1
yellow to brown. Similarly, the reaction with [(cod)Rh(OH)]
0 °C for 16.5 h afforded 1 in 93% yield.
The above reaction in CHCl . To a yellow solution of [(cod)
Rh(OMe)] (97 mg, 0.2 mmol) in 10 mL of CHCl , which was
dehydrated by adding calcium hydride overnight, was added
-methylphenylboronic pinacolate (87 mg, 0.4 mmol) at room
2
at
5
3
2
3
4
temperature. The reaction mixture was stirred at 50 °C for
12 h. Removal of volatiles gave brown oily substances, which
were extracted with 80 mL of hexane. The solvent was evapor-
ated under vacuum to furnish yellow powder, whose H NMR
and elemental analysis is identical with those of [(cod)RhCl]2.
Isolated yield was 59%.
1
Preparation of (cod)Rh(OBpin)(PPh
toluene (20 mL) solution of [(cod)Rh(OBpin)]2 (1) (283 mg,
.4 mmol) was added triphenylphosphine (210 mg, 0.8 mmol)
at room temperature. Stirring the yellow suspension at room
temperature for 3 h caused the color change of the solution to
3
) (2) (Scheme 2). To a
0
Experimental
Materials and methods
All manipulations were carried out under a nitrogen or an dark yellowish orange. Removal of the volatiles under reduced
argon atmosphere using standard Schlenk techniques under pressure and addition of 7.5 mL of hexane to the resulting
an argon or a nitrogen atmosphere. Unless otherwise noted, yellow oily residue led to the formation of a yellow solid, which
all other chemicals were used as received from commercial was washed with 7.5 mL of hexane, collected by filtration, and
1
13
suppliers without further purification. NMR spectra ( H,
{
C
dried in vacuo to give yellow powder of 2 (442 mg, 0.72 mmol,
H}, and P{ H}) were recorded on JEOL GX-500 (500 MHz), 90%). Complex 2 crystallizes from toluene-hexane as dark
JEOL EX-400 (400 MHz), or Varian Mercury 300 (300 MHz) yellow crystals.
spectrometers. Residual peaks of solvent were used as the Elemental analysis (%) calcd for C32
1
31
1
H
39BO
3
PRh: C, 62.36;
1
1
reference for H NMR (δ 5.32 CD Cl , δ 7.15 C D , δ 7.26 H, 6.38; found: C, 62.04; H, 6.42. H NMR (300 MHz, CD Cl ,
2
2
6
6
2
2
1
3
1
CDCl
CD Cl
3
, and δ 2.09 (CD of cod),
3
)
2
CO) and for C{ H} NMR (δ 53.8 25 °C): δ 1.00 (s, 12H, Me), 1.83–1.89 (m, 2H, CH
2
3
1
1
11
1
2
2
, δ 77.0 CDCl , and δ 128.0 C D ). P{ H} and B{ H} 1.94–2.01 (m, 2H, CH of cod), 2.31–2.47 (m, 4H, CH of cod),
3
6
6
2
2
NMR signals were referenced with external 85% H PO (δ 0) 2.78 (brs, 2H, CH of cod), 5.53 (brs, 2H, CH of cod), 7.34–7.45
3
4
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1
1
1
(
(
m, 9H, PPh
3
), 7.45–7.76 (m, 6H, PPh
3
);
B{ H} NMR
Notes and references
3
1
1
160.4 MHz, C D , 25 °C): δ 21.7; P{ H} NMR (121.5 MHz,
6
6
1
3
1
CD
(
(
2
Cl
2
, 25 °C): δ 20.0 (d, J(RhP) = 159 Hz); C{ H} NMR
Cl , 25 °C): 24.2 (Me), 28.2 (CH of cod), 32.7
CH of cod), 66.1 (d, J = 13 Hz, of cod), 78.5 (OCMe), 104.2 (d,
1 (a) A. Suzuki, Pure Appl. Chem., 1986, 58, 629–638;
(b) N. Miyaura and A. Suzuki, Chem. Rev., 1995, 95, 2457–
2483; (c) N. Miyaura, Adv. Met.-Org. Chem., 1998, 6, 187–
243; (d) A. Suzuki, J. Organomet. Chem., 1999, 576, 147–
168.
2 (a) M. Sakai, H. Hayashi and N. Miyaura, Organometallics,
1997, 16, 4229–4231; (b) Y. Takaya, M. Ogasawara,
T. Hayashi, M. Sakai and N. Miyaura, J. Am. Chem. Soc.,
1998, 120, 5579–5580; (c) T. Hayashi, Synlett, 2001, 879–
887; (d) T. Hayashi and K. Yamasaki, Chem. Rev., 2003, 103,
2829–2844; (e) T. Hayashi, Pure Appl. Chem., 2004, 76, 465–
475; (f) H. Tajuddin, L. Shukla, A. C. Maxwell, T. B. Marder
and P. G. Steel, Org. Lett., 2010, 12, 5700–5703.
3 (a) M. Sakai, M. Ueda and N. Miyaura, Angew. Chem., Int.
Ed. Engl., 1998, 37, 3279–3281; (b) S. Sakuma, M. Sakai,
R. Itooka and N. Miyaura, J. Org. Chem., 2000, 65, 5951–
5955; (c) M. Ueda and N. Miyaura, J. Org. Chem., 2000, 65,
4450–4452; (d) T. Senda, M. Ogasawara and T. Hayashi,
J. Org. Chem., 2001, 66, 6852–6856; (e) S. Sakuma and
N. Miyaura, J. Org. Chem., 2001, 66, 8944–8946;
(f) M. Kuriyama, K. Nagai, K. Yamada, Y. Miwa, T. Taga
and K. Tomioka, J. Am. Chem. Soc., 2002, 124, 8932–8939;
(g) R. Itooka, Y. Iguchi and N. Miyaura, J. Org. Chem., 2003,
68, 6000–6004; (h) A. Kina, H. Iwamura and T. Hayashi,
J. Am. Chem. Soc., 2006, 128, 3904–3905; (i) T. Miura,
M. Shimada and M. Murakami, Chem. – Asian J., 2006, 1,
868–877.
75.3 MHz, CD
2
2
2
2
J = 8 Hz, CH of cod) 127.7 (d, J = 10 Hz, CH of PPh
CH of PPh ), 131.5 (d, J = 38 Hz, CH of PPh ), 134.4 (d, J = 11
Hz, CH of PPh ); IR (KBr): v(B–O) 1364, 1372, 1383 cm
3
), 129.6 (s,
3
3
−
1
.
3
6 3 2 3
Preparation of (cod)Rh(C H Me -2,6)(PPh ) (3) (Scheme 4).
To an acetone solution (15 mL) of 2 (309 mg, 0.5 mmol) was
added 2,6-dimethylphenylboronic acid (75 mg, 0.5 mmol) at
room temperature. After the reaction mixture was stirred for
3
h, the solvent was removed under reduced pressure to give
orange powder, which was purified by column chromato-
graphy on activated alumina with hexane/EtOAc (1 : 1).
Concentration of orange fraction followed by recrystallization
from dichloromethane-hexane gave dark orange crystals of 3
(207 mg, 0.357 mmol, 71%).
Elemental analysis (%) calcd for C34
.27; found: C, 70.74; H, 6.13%. H NMR (400 MHz, CDCl ,
H
36PRh: C, 70.59; H,
1
6
2
3
5 °C): δ 2.02–2.10 (m, 2H, CH
2
of cod), 2.15–2.22 (m, 2H, CH
of cod), 2.42–2.51 (m, 8H, Me
2
of cod), 2.28–2.40 (m, 2H, CH
2
and CH of cod), 3.69–3.72 (m, 2H, CH of cod), 4.64–4.66 (m,
2
2
7
2
2
H, CH of cod), 6.52 (d, J = 7 Hz, 2H), 6.64 (t, J = 7 Hz, 2H),
3
1
1
.19–7.33 (m, 18H, PPh
5 °C): 25.7 (d, J = 180 Hz); C{ H} NMR (100 MHz, CDCl ,
3 3
); P{ H} NMR (121.5 MHz, CDCl ,
1
3
1
3
2 2
5 °C): δ 26.4 (Me), 30.6 (CH of cod), 31.1 (CH of cod), 86.4
(
(
d, J = 7 Hz, CH of cod), 93.9 (t, J = 10 Hz, CH of cod), 122.0
Cpara of xylyl), 124.9 (s, Cmeta of xylyl), 127.6 (d, J = 9 Hz, CH of
PPh
34.0 (d, J = 12 Hz, Cipso–P of PPh
Rh of a xylyl group was not observed.
Catalytic 1,4-addition reaction of 2,6-dimethylphenylboronic
acid and cyclohexen-2-one: representative procedure with
complex 1 (Table 1). To a mixture of acetone (6 mL), water
3
), 129.1 (d, J = 21 Hz, CH of PPh
3
), 133.8 (s, CH of PPh
3
),
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(
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h, H NMR was measured to calculate the NMR yield of 1,4-
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There are no conflicts to declare.
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This work was partially supported by a Grant-in-aid for
Scientific Research for Young Chemists No. 13740412 and for
Scientific Research on Priority Areas, “Molecular Physical
Chemistry” No. 11166221, from the Ministry of Education,
Culture, Sports, Science and Technology, Japan.
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