Novel rhodium-1,3-dialkyl-3,4,5,6-tetrahydropyrimidin-2-ylidene complexes as catalysts for arylation of aromatic aldehydes

Article abstract of DOI:10.1016/j.jorganchem.2005.07.085

Six new rhodium-tetrahydropyrimidin-2-ylidene complexes (2a-f) have been prepared and characterized by C, H, N analysis, ¹H NMR and ¹³C NMR. Phenylboronic acid reacts with aldehydes in the presence of a catalytic amount of the new rhodium(I)-carbene complexes, RhCl(COD)(1,3- dialkyl-3,4,5,6-tetrahydropyrimidin-2-ylidene), (2a-f), to give the corresponding secondary aryl alcohols in good yields (72-96%).

Full text of DOI:10.1016/j.jorganchem.2005.07.085

Journal of Organometallic Chemistry 690 (2005) 5849–5855
www.elsevier.com/locate/jorganchem
Novel rhodium-1,3-dialkyl-3,4,5,6-tetrahydropyrimidin-2-ylidene
complexes as catalysts for arylation of aromatic aldehydes
a,
a
b
b
Ismail Ozdemir *, Serpil Demir , Bekir C¸ etinkaya , Engin C¸ etinkaya
¨
a
Ino¨nu¨ University, Faculty Science and Art, Department of Chemistry, 44280 Malatya, Turkey
b
Ege University, Faculty Science, Department of Chemistry, 35100 Bornova-Yzmir, Turkey
Received 30 May 2005; accepted 15 July 2005
Available online 12 September 2005
Abstract
Six new rhodium-tetrahydropyrimidin-2-ylidene complexes (2a–f) have been prepared and characterized by C, H, N analysis, 1H
NMR and 13C NMR. Phenylboronic acid reacts with aldehydes in the presence of a catalytic amount of the new rhodium(I)-carbene
complexes, RhCl(COD)(1,3-dialkyl-3,4,5,6-tetrahydropyrimidin-2-ylidene), (2a–f), to give the corresponding secondary aryl alco-
hols in good yields (72–96%).
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Tetrahydropyrimidin-2-ylidene; N-heterocyclic carbene; Arylation; Rhodium
1. Introduction
esis is that NHCs are strong r-donors with negligible p-
accepting ability, and so they resemble donor phosphine
ligands rather than classical Fischer- or Schrock-type
carbenes [6] and thereby also lead to electron-rich metal
centers [7]. However, in contrast to metal phosphine
complexes, they can form metal complexes that have
high stability towards heat, moisture, and oxygen [8].
The ancillary ligand (NHC) coordinated to the metal
center has a number of important roles in homogeneous
catalysis such as providing a stabilizing effect and govern-
ing activity and selectivity. The number, nature and posi-
tion of the substituents on the nitrogen atom(s) and/or
NHC ring have been found to play a crucial role in tuning
the catalytic activity. Whilst many modifications to the
five-membered ring of the ligand aryl substituent have
been described, relatively little attention has been paid
to the effect of the ring size [9]. For the present study, we
selected 3,4,5,6-tetrahydropyrimidin-2-ylidene precur-
sors (2). This choice was guided by several considerations.
An important characteristic of the carbene ligands in ac-
tive complexes is their strong-electron donating effect, pri-
marily a r-effect. We have previously reported the use of
The first metal complexes of N-heterocyclic carbenes
(NHCs) were reported independently in 1968 by Wanz-
¨
lick [1] and Ofele [2] and subsequently, electron-rich alk-
enes were used as a source of various NHC complexes
by Lappert and co-workers [3,4]. Since the 1991 isola-
tion and crystallographic characterization of stable of
N-heterocyclic carbenes by Arduengo [5], increasing
attention has been focused on using these compounds
as ancillary ligands for transition-metal complexes.
Interestingly, most studies focusing on catalysts incor-
porating NHC ligands have revolved around the plati-
num metal groups. In numerous instances simple
substitution reaction routes involving replacement of
phosphines by NHC ligands lead to higher catalytic
activity as well as improved thermal stability of the
resulting organometallic complex. The working hypoth-
*
Corresponding author. Tel.: +904223410010; fax: +904223410037.
¨
E-mail address: iozdemir@inonu.edu.tr (I. Ozdemir).
0022-328X/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.07.085

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I. Ozdemir et al. / Journal of Organometallic Chemistry 690 (2005) 5849–5855
5850
an in situ formed imidazolidin-2-ylidenepalladium(II)
system which exhibits high activity in various coupling
reactions of aryl bromides and aryl chlorides [10]. In order
to obtain a more stable, efficient and active system, we
have also investigated benzo-annelated derivatives [11].
Due to their six-membered ring geometry, tetrahydopyr-
imidine-2-ylidenes are stronger r-donating ligands when
compared to five-membered analogues [9].
Although, rhodium-carbene complexes have been
extensively studied, there are few reports on the catalytic
activity of rhodium-carbene complexes in rhodium-med-
iated processes [12,13]. Buchmeiser and Herrmann have
addressed the synthesis and catalytic properties of 1,3-
dimesityltetrahydropyrimidine-2-ylidene complexes of
palladium and rhodium [14,15]. Miyaura reported that
rhodium catalyse the addition of aryl and alkenylbo-
ronic acids to aldehydes giving secondary alcohols.
The reactions were facilitated by the presence of an elec-
tron withdrawing group on the aldehyde and an electron
donating group on the arylboronic acid, suggesting that
the mechanism involves a nucleophilic attack of the aryl
group on the aldehyde [16]. The finding that these reac-
tions were run with sterically hindered and strongly ba-
standard Schlenk-type flasks. The solvents used were
purified by distillation over the drying agents indicated
and were transferred under Ar: THF, Et2O (Na/K al-
loy), CH2Cl2 (P4O10), hexane, toluene (Na). Flash chro-
matography: Merck silica gel 60 (230–400 mesh). Test
reactions for the catalytic activity of catalysts in the
addition of phenylboronic acid to aldehydes were car-
ried out in air. The complex [RhCl(COD)]2[19] and 1
were prepared according to known methods [20]. All re-
agents were purchased from Aldrich Chemical Co. All
1H and 13C NMR were performed in CDCl3. 1H
NMR and 13C NMR spectra were recorded using a
Varian As 400 Merkur spectrometer operating at
399.9 MHz (1H), 100.5 MHz (13C). Chemical shifts (d)
are given in ppm relative to TMS, coupling constants
(J) in Hertz. Infrared spectra were recorded as KBr pel-
lets in the range 400–4000 cmÀ1 on a ATI UNICAM
1000 spectrometer. Melting points were measured in
open capillary tubes with an Electrothermal-9200 melt-
ing point apparatus and are uncorrected. Elemental
analyses were performed by TUBITAK (Ankara, Tur-
key) Microlab.
sic ligands attracted the attention of Furstner who
¨
2.1. General procedure for the preparation of the rhodium
carbene complexes (2a–f)
subsequently applied N-heterocyclic carbene ligands.
An in situ generated catalytic system for the addition
of phenylboronic acid to aldehydes is prepared by com-
bination of rhodium salt, 1,3-dialkylimidazolium chlo-
ride and base [17]. Recently our group reported that
novel complexes of rhodium(I) based on 1,3-dialkyimi-
dazolidin-2-ylidenes give good yields for the addition
of phenylboronic acid to aldehydes [18].
Based on these findings and our continuing interest in
developing more efficient and stable catalysts, we wished
to examine whether we could influence the catalytic
activity of rhodium-1,3-dialkyl-3,4,5,6-tetrahydropyrim-
idin-2-ylidene complexes for the addition of phenylbo-
ronic acid to aldehydes (Scheme 1).
A solution of bis(1,3-dialkyl-3,4,5,6-tetrahydropyr-
imidin-2-ylidene) (1) (0.5 mmol) and [RhCl(COD)]2
(0.5 mmol) in toluene (20 mL) were heated under reflux
for 2 h. Upon cooling to room temperature, yellow-
orange crystals of 2a–f were obtained. The crystals were
filtered, washed with diethyl ether (3 · 15 mL) and dried
under vacuum. The crude product was recrystallized
from CH2Cl2/Et2O.
2.2. Chloro(g4-1,5-cyclooctadiene){1,3-bis(2,4,6-tri-
methylbenzyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene}-
rhodium(I) (2a)
We now report: (i) the straightforward preparation of
new RhCl(COD)(1,3-dialkyl-3,4,5,6-tetrahydropyrimi-
din-2-ylidene) complexes and (ii) their efficient catalysis
of the addition of phenylboronic acid to aldehydes.
Yield: 0.517 g (87%). Anal. Calc. for C32H44N2ClRh:
C, 64.59; H, 7.45; N, 4.71. Found: C, 64.55; H, 7.47; N,
1
4.70%; m.p. 236–237 ꢁC, m(CN) = 1619 cmÀ1. H NMR
(399.9 MHz, CDCl3) d 6.86 (s, 4H, CH2C6H2(CH3)3-
2,4,6), 6.62 and 5.33 (d, J = 14.0 Hz, 4H, CHCOD),
4.94 and 3.40 (m, 4H, CH2C6H2(CH3)3-2,4,6), 2.62 (t,
J = 4.4 Hz, 4H, NCH2CH2CH2N), 2.51(m, 4H,
CH2COD), 2.40 (s, 12H, CH2C6H2(CH3)3-2,6), 2.26 (s,
2. Experimental
All reactions for the preparation of 1 and 2 were
carried out under Ar in flame-dried glass-ware using
OH
O
[RhClCOD(NHC)]
2
+
C
B(OH)2
CH
DME / water,80˚C
Rn
Rn
H
Scheme 1.

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I. Ozdemir et al. / Journal of Organometallic Chemistry 690 (2005) 5849–5855
5851
6H, CH2C6H2(CH3)3-4), 1.93 (m, 4H, CH2COD), 1.65
(quint, J = 4.4 Hz, 2H, NCH2CH2CH2N). 13C NMR
(100.5 MHz, CDCl3) d 211.9 (d, J = 45.7 Hz, Ccarbene),
138.9, 137.7, 129.5 and 129.2, (CH2C6H2(CH3)3-2,4,6),
97.0 and 69.6 (d, J = 6.8 Hz and J = 15.3 Hz, CHCOD),
55.4 (CH2C6H2(CH3)3-2,4,6), 43.1 (NCH2CH2CH2N),
32.9 and 28.9 (CH2COD), 22.1 (NCH2CH2CH2N), 21.1
(CH2C6H2(CH3)3-4), 21.0 (CH2C6H2(CH3)3-2,6).
2.5. Chloro(g4-1,5-cyclooctadiene){1-methoxyethyl-3-
(3,4,5-trimethoxybenzyl)-3,4,5,6-tetrahydropyrimidin-2-
ylidene}rhodium(I) (2d)
Yield: 0.489 g (86%). Anal. Calc. for C25H38N2O4Cl-
Rh: C, 52.78; H, 6.73; N, 4.92. Found: C, 52.75; H, 6.70;
N, 4.93%; m.p. 219–220 ꢁC, m(CN) = 1592 cmÀ1
.
1H
NMR (399.9 MHz, CDCl3) d 6.90 (s, 2H, CH2C6H2-
(OCH3)3-3,4,5), 6.81 (d, J = 14.0 Hz, 2H, CHCOD),
4.88 (m, 2H, CH2C6H2(OCH3)3-3,4,5), 4.83 (d,
J = 14.0 Hz, 2H, CHCOD), 3.91 (s, 3H, CH2CH2OCH3),
3.87 (s, 6H, CH2C6H2(OCH3)3-3,5), 3.83 (s, 3H,
CH2C6H2(OCH3)3-4), 3.35 and 3.17 (t, J = 5.6 Hz, 4H,
NCH2CH2CH2N), 2.89 (m, 4H, CH2CH2OCH3), 2.32
and 1.87 (m, 8H, CH2COD), 1.74 (quint, J = 5.6 Hz,
2H, NCH2CH2CH2N). 13C NMR (100.5 MHz, CDCl3)
2.3. Chloro(g4-1,5-cyclooctadiene){1,3-bis(3,4,5-
trimethoxybenzyl)-3,4,5,6-tetrahydropyrimidin-2-
ylidene}rhodium(I) (2b)
Yield: 0.629 g (91%). Anal. Calc. for C32H44N2O6-
ClRh: C, 55.62; H, 6.42; N, 4.05. Found: C, 55.59; H,
6.45; N, 4.01%; m.p. 217–218 ꢁC, m(CN) = 1592 cmÀ1
.
d
206.9 (d, J = 46.5 Hz, Ccarbene), 153.7, 137.7,
1H NMR (399.9 MHz, CDCl3) d 6.91 (s, 4H, CH2C6-
H2(OCH3)3-3,4,5), 6.83 (d, J = 14.0 Hz, 2H, CHCOD),
4.89 (m, 2H,CH2C6H2(OCH3)3-3,4,5), 4.83 (d, J =
14.0 Hz, 2H, CHCOD), 3.88 (s, 12H, CH2C6H2-
(OCH3)3-3,5), 3.83 (s, 6H, CH2C6H2(OCH3)3-4), 3.36
(m, 2H, CH2C6H2(OCH3)3-3,4,5), 2.89 (t,J = 5.6 Hz,
4H, NCH2CH2CH2N), 2.33 (m, 4H, CH2COD), 1.87 (m,
4H, CH2COD), 1.74 (quint, J = 5.6 Hz, 2H, NCH2CH2-
CH2N). 13C NMR (100.5 MHz, CDCl3) d 206.9 (d,
J = 45.7 Hz, Ccarbene), 153.6, 137.7, 132.6 and 106.2
(CH2C6H2(OCH3)3-3,4,5), 97.1 and 69.6 (d, J = 6.9 Hz
and J = 15.3 Hz, CHCOD), 62.8 (CH2C6H2(OCH3)3-
3,4,5), 61.0 (CH2C6H2(OCH3)3-4), 56.7 (CH2C6H2-
(OCH3)3-3,5), 43.3 (NCH2CH2CH2N), 32.8 and 29.0
(CH2COD), 21.3 (NCH2CH2CH2N).
132.6 and 106.2 (CH2C6H2(OCH3)3-3,4,5), 97.13 and
69.6 (d, J = 6.9 Hz and J = 15.2 Hz, CHCOD), 66.0
(CH2CH2OCH3), 62.8 (CH2C6H2(OCH3)3-3,4,5), 61.0
(CH2CH2OCH3), 59.3 (CH2CH2OCH3), 56.9 (CH2C6-
H2(OCH3)3-4), 56.7 (CH2C6H2(OCH3)3-3,5), 43.3 and
41.8 (NCH2CH2CH2N), 32.9 and 29.0 (CH2COD), 21.4
(NCH2CH2CH2N).
2.6. Chloro(g4-1,5-cyclooctadiene){1-(2,4,6-trimethyl-
benzyl)-3-methyl-3,4,5,6- tetrahydropyrimidin-2-ylidene}
rhodium(I) (2e)
Yield: 0.405 g (85%). Anal. Calc. for C23H34N2ClRh:
C, 57.93; H, 7.19; N, 5.87. Found: C, 57.90; H, 7.21; N,
1
5.85%; m.p. 203–204 ꢁC, m(CN) = 1535 cmÀ1. H NMR
(399.9 MHz, CDCl3) 6.87 (s, 2H, CH2C6H2(CH3)3-
2,4,6), 5.94 and 5.40 (d, J = 14.0 Hz, 4H, CHCOD),
4.90 (m, 2H, CH2C6H2(CH3)3-2,4,6), 3.96 (s, 3H,
NCH3), 3.13 and 3.06 (m, 4H, NCH2CH2CH2N), 2.36
(m, 4H, CH2COD), 2.32 (s, 6H, CH2C6H2(CH3)3-2,6),
2.25 (s, 3H, CH2C6H2(CH3)3-4), 1.84 (m, 4H, CH2COD),
1.64 (quint, J = 6.4 Hz, 2H, NCH2CH2CH2N). 13C
NMR (100.5 MHz, CDCl3) d 209.5 (d, J = 45.8 Hz,
Ccarbene), 139.1, 137.8, 129.4 and 129.1 (CH2C6H2-
(CH3)3-2,4,6), 96.6 and 67.5 (d, J = 6.1 Hz and
J = 15.3 Hz, CHCOD), 54.7 (CH2C6H2(CH3)3-2,4,6),
46.9 and 46.5 (NCH2CH2CH2N), 41.2 (NCH3), 32.2
and 28.3 (CH2COD),21.1 (CH2C6H2(CH3)3-4), 20.9
(CH2C6H2(CH3)3-2,6), 15.5 (NCH2CH2CH2N).
2.4. Chloro(g4-1,5-cyclooctadiene){1-(2,4,6-trimethyl-
benzyl)-3-cyclohexyl-3,4,5,6-tetrahydropyrimidin-2-
ylidene}rhodium(I) (2c)
Yield: 0.457 g (84%). Anal. Calc. for C28H42N2ClRh:
C, 61.71; H, 7.77; N, 5.14. Found: C, 61.75; H, 7.78; N,
1
5.17%; m.p. 221–222 ꢁC, m(CN) = 1510 cmÀ1. H NMR
(399.9 MHz, CDCl3) 6.86 (s, 2H, CH2C6H2(CH3)3-
2,4,6), 5.91 and 5.41 (d, J = 13.6 Hz, 4H, CHCOD),
4.89 (m, 2H, CH2C6H2(CH3)3-2,4,6), 3.46 (quint, J =
7.2 Hz, 1H, NCH(CH2)4CH2), 3.37 and 2.97 (t,J =
4.8 Hz, 4H, NCH2CH2CH2N), 2.36 (s, 6H, CH2C6H2-
(CH3)3-2,6), 2.31 (m, 4H, CH2COD), 2.27 (s, 3H,
CH2C6H2(CH3)3-4), 1.85 (m, 4H, CH2COD), 1.75 (quint,
J = 4.8 Hz, 2H, NCH2CH2CH2N), 1.22 (m, 10H, NCH-
(CH2)4CH2). 13C NMR (100.5 MHz, CDCl3) d 209.0 (d,
J = 45.8 Hz, Ccarbene), 139.0, 137.6, 129.4 and 129.3
(CH2C6H2(CH3)3-2,4,6), 95.8 and 68.2 (d, J = 6.9 Hz
and J = 9.2 Hz, CHCOD), 66.9 (CH2C6H2(CH3)3-2,4,6),
54.7 (NCH(CH2)4CH2), 43.1 and 39.3 (NCH2CH2-
CH2N), 32.9 and 28.7 (CH2COD), 26.6, 26.3 and
26.0 (NCH(CH2)4CH2), 21.1 (CH2C6H2(CH3)3-4), 20.9
(CH2C6H2(CH3)3-2,6), 14.3 (NCH2CH2CH2N).
2.7. Chloro(g4-1,5-cyclooctadiene){1-(2,3,5,6-tetramethyl-
benzyl)-3-methyl-(3,4,5,6-tetrahydropyrimidin)-2-
ylidene}rhodium(I) (2f)
Yield: 0.397 g (81%). Anal. Calc. for C24H36N2ClRh:
C, 58.72; H, 7.39; N, 5.71. Found: C, 58.70; H, 7.41; N,
5.68%; m.p. 250–251 ꢁC, m(CN) = 1536 cmÀ1.1H NMR
(399.9 MHz, CDCl3) 6.96 (s, 1H, CH2C6H(CH3)4-

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I. Ozdemir et al. / Journal of Organometallic Chemistry 690 (2005) 5849–5855
5852
2,3,5,6), 5.97 and 5.47 (d, J = 13.6 Hz, 4H, CHCOD), 4.87
(m, 2H, CH2C6H(CH3)4-2,3,5,6), 3.96 (s, 3H, NCH3),
3.36 and 3.10 (m, 4H, NCH2CH2CH2N), 2.35 (m, 4H,
CH2COD), 2.27 (s, 6H, CH2C6H(CH3)4-2,6), 2.24 (s,
6H, CH2C6H(CH3)4-3,5), 1.96 (m, 4H, CH2COD), 1.62
(quint, J = 5.2 Hz, 2H, NCH2CH2CH2N). 13C NMR
(100.5 MHz, CDCl3) d 209.6 (d, J = 45.8 Hz, Ccarbene),
135.3, 134.2, 132.1 and 131.8 (CH2C6H(CH3)3-2,3,5,6),
96.5 and 67.5 (d, J = 6.9 Hz and J = 15.2 Hz, CHCOD),
55.0 (CH2C6H(CH3)4-2,3,5,6), 47.0 and 46.6 (NCH2-
CH2CH2N), 41.3 (NCH3), 32.2 and 28.3 (CH2COD),
21.9 (CH2C6H(CH3)4-3,5), 20.7 (CH2C6H(CH3)4-2,6),
16.6 (NCH2CH2CH2N).
2.8. General procedure for rhodium-carbene catalyzed
addition of phenylboronic acid to aldehydes
Phenylboronic acid (1.20 g, 9.8 mmol), KOtBu
(4.9 mmol), substituted aldehydes (4.9 mmol), rhodium
carbene catalyst (1 mmol%) and dimethoxyethane
(15 mL) were introduced into a Schlenk tube and then
water (5 mL) was added. The resulting mixture was
heated for 4–10 h at 80 ꢁC, cooled to ambient tempera-
ture and extracted with ethyl acetate (30 mL). After dry-
ing over MgSO4 the organic phase was evaporated and
the residue was purified by flash chromatography (hex-
ane/ethyl acetate, 5/1).
OMe
N
N
Rh
Rh
N
N
Cl
Cl
(2a)
OMe
MeO
OMe
OMe
R
R'
N
MeO
(2d)
N
OMe
CH3
1/2
N
N
R
N
Rh
R'
N
N
N
Rh
Cl
Cl
1
+
(2b)
(2e)
OMe
OMe
1/2 [RhCl(COD)]2
MeO
CH3
N
Rh
Cl
N
N
N
Rh
Cl
(2f)
(2c)
Scheme 2. Synthesis of rhodium-carbene complexes (2a–f).

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I. Ozdemir et al. / Journal of Organometallic Chemistry 690 (2005) 5849–5855
5853
Table 1
Selected analytical data for the new rhodium-carbene complexes (2a–f)
Complex
Isolated yield (%)
m.p. (ꢁC)
m
(NCN)(cmÀ1
)
13C NMR Rh–C d ppm (1J(Rh–C), Hz)
2a
2b
2c
2d
2e
2f
87
91
84
86
85
81
236–237
217–218
221–222
219–220
203–204
250–251
1619
1592
1510
1592
1535
1536
211.9 (45.7)
196.9 (45.7)
209.0 (45.8)
206.9 (46.5)
209.5 (45.8)
209.6 (45.8)
3. Results and discussion
2-ylidene) (1), with the binuclear [RhCl(COD)]2 com-
plex proceeded smoothly in refluxing toluene to give
the RhCl(COD)(1,3-dialkyl-3,4,5,6-tetrahydropyrimi-
din-2-ylidene) complexes (2a–f) as crystalline solids in
81–91% yields (Scheme 2, Table 1). Although com-
pounds 2a–f are available by the reactions of [Rh(OMe)
(COD)]2 with the appropriate 1,3-dialkyl-3,4,5,6-tetra-
hydropyrimidinium salts. The purification procedure
3.1. Synthesis and characterization of RhCl(COD)NHC
The bis(1,3-dialkyl-3,4,5,6-tetrahydropyrimidin-2-yli-
dene) (1), was synthesized using a method similar to
that reported by Lappert and co-workers [20]. The
reaction of bis(1,3-dialkyl-3,4,5,6-tetrahydropyrimidin-
Table 2
Rhodium-carbene catalyzed addition of phenylboronic acid to aldehydes
OH
O
[RhClCOD(NHC)]
2
+
C
B(OH)2
CH
DME / water,80˚C
Rn
Rn
H
Entry
Catalyst
Rn
Time (h)
Yield (%)a,b,c,d
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
a
2a
2b
2c
2d
2e
2f
p-Cl
p-Cl
p-Cl
p-Cl
p-Cl
p-Cl
4
4
8
4
8
5
4
6
8
6
8
8
4
4
7
7
5
5
6
6
6
6
6
6
4
4
7
4
10
10
93
89
72
89
74
77
94
74
72
90
89
74
87
92
85
95
81
94
91
89
87
96
93
89
92
86
76
80
87
79
2a
2b
2c
2d
2e
2f
2,4,6-(CH3)3
2,4,6-(CH3)3
2,4,6-(CH3)3
2,4,6-(CH3)3
2,4,6-(CH3)3
2,4,6-(CH3)3
3,4,5-(OMe)3
3,4,5-(OMe)3
3,4,5-(OMe)3
3,4,5-(OMe)3
3,4,5-(OMe)3
3,4,5-(OMe)3
2,5-(OMe)2
2,5-(OMe)2
2,5-(OMe)2
2,5-(OMe)2
2,5-(OMe)2
2,5-(OMe)2
p-C(CH3)3
p-C(CH3)3
p-C(CH3)3
p-C(CH3)3
p-C(CH3)3
p-C(CH3)3
2a
2b
2c
2d
2e
2f
2a
2b
2c
2d
2e
2f
2a
2b
2c
2d
2e
2f
Reactions conditions. Phenylboronic acid (9.8 mmol), aldehydes (4.9 mmol), rhodium-carbene (1 mmol%), KOtBu (4.9 mmol), dimethoxyethane
(15 mL).
b
Isolated yield.
c
Yields based on aldehydes.
All reactions were monitored by GC.
d

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I. Ozdemir et al. / Journal of Organometallic Chemistry 690 (2005) 5849–5855
5854
is more complicated than this procedure with these N-
substituted.
method for the addition of phenylboronic acid to aldehydes
is presented. The procedure is simple and efficient towards
various aryl aldehydes and does not require induction peri-
ods. Future investigations are aimed at the development of
an asymmetric version of this process.
Complexes (2a–f), are very stable in the solid state
and were characterized by analytical and spectroscopic
techniques (Table 1). The rhodium complexes exhibit a
characteristic t(NCN) band (Table 1) typically at 1510–
1619 cmÀ1 [21]. 13C chemical shifts, which provide a use-
ful diagnostic tool for metal carbene complexes, show
Acknowledgment
that Ccarb is substantially deshielded. Values of d(13Ccarb
)
This work was financially supported by the Techno-
logical and Scientific Research Council of Turkey
are in the range 196.9–211.9 ppm and are similar to those
found in other carbene complexes [18]. Coupling con-
stants J(103Rh–13C) for the new rhodium complexes
(2a–f) are comparable with those found for carbene rho-
dium(I) complexes (Table 1). These new complexes show
typical spectroscopic signatures which are in line with
those recently reported for RhCl(COD)(1,3-dialkylbenz-
imidazolidin-2-ylidine) complexes [21].
_
¨
TUBITAK [TBAG-2474 (104T085)] and Ino¨nu Univer-
¨
sity Research Fund (BAP:2005/42).
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