Novel azolinium/rhodium system catalyzed addition of arylboronic acids to aldehydes

Article abstract of DOI:10.3987/COM-06-10731

There novel 1,3-dialkylperhydrobenzimidazolinium (2a-c) and two 1,3-dialkylimidazolinium salts (4a,b) as NHC precursors were synthesized from N,N′-dialkyl-1,2-cyclohexanediamine dihydrochloride and 1,2-dialkylpropanediamine dihydrochloride. The in situ prepared three component system [RhCl(COD)]₂ / imidazolinium salts (2, 4) and KOBu^t catalyses the addition of phenylboronic acid to sterically hindered aldehydes affording the corresponding arylated secondary alcohols in good yields.

Full text of DOI:10.3987/COM-06-10731

HETEROCYCLES, Vol. 68, No. 7, 2006
1371
HETEROCYCLES, Vol. 68, No. 7, 2006, pp. 1371 - 1379. © The Japan Institute of Heterocyclic Chemistry
Received, 13th March, 2006, Accepted, 22nd May, 2006, Published online, 23rd May, 2006. COM-06-10731
NOVEL AZOLINIUM/RHODIUM SYSTEM CATALYZED
ADDITION OF ARYLBORONIC ACIDS TO ALDEHYDES
Ismail Özdemir a, *, Murat Yiğit a, Engin Çetinkaya,b and Bekir Çetinkayab
a İnönü University, Faculty of Science and Arts, Department of Chemistry, 44280
Malatya, Turkey
b
Department of Chemistry, Ege University, 35100, Bornova-İzmir, Turkey
* Corresponding author; Fax: +90 422 341 0037, e-mail: iozdemir@inonu.edu.tr
Abstract- There novel 1,3-dialkylperhydrobenzimidazolinium (2a-c) and two
1,3-dialkylimidazolinium salts (4a,b) as NHC precursors were synthesized from
N,N’-dialkyl-1,2-cyclohexanediamine
dihydrochloride
and
1,2-
dialkylpropanediamine dihydrochloride. The in situ prepared three component
system [RhCl(COD)]2 / imidazolinium salts (2, 4) and KOBut catalyses the
addition of phenylboronic acid to sterically hindered aldehydes affording the
corresponding arylated secondary alcohols in good yields.
INTRODUCTION
N-Heterocyclic carbenes (NHCs) have emerged as an important class of ligands for transition metals and
are beginning to play a role in transition-metal catalysis.1 Öfele and Wanzlick first pioneered the
metalation of imidazol-2-ylidenes, better known as N-heterocyclic carbens, from imidazolium salts in
1968.2,3 Lappert and co-workers followed this work with the investigation of N-heterocyclic carbens
complexes synthesized from electron-rich olefins.4,5 However, it was not until the isolation of the first
free carbene by Arduengo , in 1991, that significant interest was given to the area.6 NHC ligands,
predominantly regarded as σ-donors such as tertiary phosphines, are able to stabilize various oxidation
states and coordinatively unsaturated intermediates occuring in catalytic cycles. In addition, ligand
dissociation as experienced in the use of phosphines is less likely to appear in a considerable manner
when NHCs are used. Employment of sterically demanding N-heterocyclic carbenes, which resemble
bulky phosphines with respect to their bonding, has led to the synthesis of new robust ruthenium, rhodium
and palladium catalysts, which are less oxygen sensitive in comparison to the phosphine analogues.7
Transition-metal complexes with these ligands have proven to be effective in the Heck reaction,8 Suzuki

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and Sonogashira coupling,9,10 hydrosilylation,11 Kumada coupling,12 hydrogenation,13 olefin
cyclopropanation,14 intramolecular cyclization of (Z)-3-methylpent-2-en-4-yn-1-ol into 2,3-
dimethylfuran,15,16 cycloisomerisation,17 amination of aryl halides18 and, of course, olefin metathesis.19
Diarylmethanols are important intermediates for the synthesis of biologically and pharmaceutically active
substances.20,21 The addition of organometallic reagents to aldehydes has been one of the general methods
for the synthesis of diarylmethanols. Of those organometallic reagents, organolithium and
organomagnesium compounds are most frequently used for this purpose, but tolerate only a few
electrophilic groups on themselves.22,23 Examples of using other functionalized organometallic species24,25
such as organocopper, organochromium, organotin, especially organozinc, have been described. However,
these organometallic reagents are usually toxic and sensitive to air and moisture.
Miyaura reported that rhodium catalyzes the addition of aryl and alkenylboronic acids to aldehydes giving
secondary alcohols. The reactions were facilitated by the presence of an electron 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.26 The finding that these reactions were
run with sterically hindered and strongly basic ligands attracted the attention of Fürstner who
subsequently applied N-heterocyclic carbene ligands. A in-situ generated catalytic system for the addition
of phenylboronic acid to aldehydes is prepared combination of rhodium salt, 1,3-dialkylimidazolium
chloride and base.27-31 Buchmeiser reported rhodium- and iridium-tetrahydropyrimidin-2-ylidene
complexes are effective of addition of phenylboronic acid to aldehydes,32 and also Rh(I) and nitrogen
containing ligands are used for the aryl transferation to aldehydes.33
Although the nature of the NHC ligand on complexes has a tremendous influence on the rate of catalyzed
reactions, the use of saturated NHC ligands in addition of phenylboronic acid to aldehydes reaction is a
neglected area. In order to find more efficient rhodium catalysts we have prepared a series of new
perhydrobenzimidazolinium and imidazolinium chlorides LHX, (2a-c) and (4a,b) (Scheme 1), containing
a saturated imidazole ring and we report here in-situ rhodium-carbene based catalytic system for the
addition of phenylboronic acid to aldehydes.
RESULTS AND DISCUSSION
Dialkylperhydrobenzimidazolinium and imidazolinium salts, (2a-c and 4a,b) are conventional NHC
precursors. The reaction of 1 and 334,35 with triethyl orthoformate yielded the symmetrical
perhydrobenzimidazolinium (2a-c) and imidazolinium (4a,b) salts (Scheme 1). The salts are air- and
moisture stable both in the solid state and in solution. The structures of 2 and 4 were determined by their
characteristic spectroscopic data and elemental analyses.
13C NMR chemical shifts were consistent with the proposed structure, the imino carbon appeared as a

HETEROCYCLES, Vol. 68, No. 7, 2006
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1
typical singlet in the H-decoupled mode in the 162.63, 162.4, 162.04, 158.74 and 159.16 ppm
respectively for imidazolinium salts (2a-c) and (4a,b). The 1H NMR spectra of the
perhydrobenzimidazolinium and imidazolinium salts furher supported the assigned structures; the
resonances for C(2)-H were observed as sharp singlets in the 10.42, 10.75, 10.56, 10.23 and 9.00 ppm
respectively for 2a-c and 4a,b. The IR data for perhydrobenzimidazolinium and imidazolinium salts (2a-
c) and (4a,b) clearly indicate the presence of the –C=N- group with a ν(C=N) vibration at 1665, 1585,
1603, 1654, and 1644 cm-1 respectively for 2a-c and 4a,b. The NMR and IR values are similar to those
found for other 1,3-dialkylimidazolinium salts.34,35
OMe
OMe
OMe
OMe
OMe
OMe
N
+
N
+
Cl-
Cl-
N
N
OMe
OMe
OMe
OMe
OMe
OMe
2a
4a
R
R
OMe
NH.HCl
NH.HCl
N
+
N
CH(OEt)3
Cl-
NH.HCl
R
NH.HCl
R
OMe
3
1
2b
O
O
O
N
+
N
+
N
Cl-
Cl-
N
O
2c
4b
Scheme 1. Synthesis of 1,3-dialkylperhydrobenzimidazolinium and 1,3-dialkylimidazolinium salts
Although the addition of carbon nucleophiles to aldehydes is usually a facile process, limits are
encountered that functionalized organometallic reagents required. Recent publications describing the

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HETEROCYCLES, Vol. 68, No. 7, 2006
addition of arylboronic acid derivatives to aldehydes in the presence of the catalytic amounts of Rh(I) and
phosphine derivatives deserve particular mention.26,27 Originally [Rh(acac)(CO)2] in combination with
bidentate phosphine ligand such as dppf [1,1’-bis(diphenylphosphino)ferrocene] has been recommended
for the in situ preparation of the yet elusive catalyst.36
Here, various perhydrobenzimidazolinium and imidazolidinium salts (2a-c and 4a,b) were compared as
ligand precursors under the same reaction conditions. To survey the reaction parameters for the addition
of phenylboronic acid to aldehydes, we chose to examine Cs2CO3, K2CO3, and KOBut as base and
DME/H2O (3:1) as solvent. We found that the reactions performed in DME/H2O (3:1) with Cs2CO3 or
KOBut as the base at 25 oC and 60 oC appeared to be best. We started our investigation with the addition
of phenylboronic acid to p-chlorobenzaldehyde, in the presence of [RhCl(COD)]2/2-4. Table 1
summarizes the results obtained in the presence of 2a-c and 4a,b (Table 1 , Entries 1-5).
Table 1. Rhodium-carbene catalyzed addition of phenylboronic acid to aldehydes.
OH
C
O
[RhClCOD]2, LHX
DME/ H2O, KOBut
B(OH)2
+
C H
R
H
R
Entry
Yielda-d(%)
R
LHX
2a
2b
2c
1
p-Cl
p-Cl
p-Cl
p-Cl
p-Cl
H
95
82
90
92
88
94
84
92
93
90
89
77
85
87
82
2
3
4
4a
4b
2a
2b
2c
5
6
7
H
8
H
9
H
4a
4b
2a
2b
2c
10
11
12
13
14
15
H
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
4a
4b

HETEROCYCLES, Vol. 68, No. 7, 2006
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16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
p-C(CH3)3
2a
2b
2c
4a
4b
2a
2b
2c
4a
4b
2a
2b
2c
4a
4b
91
79
84
88
82
80
71
76
77
73
75
65
73
74
69
p-C(CH3)3
p-C(CH3)3
p-C(CH3)3
p-C(CH3)3
2,5(OCH3)2
2,5(OCH3)2
2,5(OCH3)2
2,5(OCH3)2
2,5(OCH3)2
3,4,5(OCH3)3
3,4,5(OCH3)3
3,4,5(OCH3)3
3,4,5(OCH3)3
3,4,5(OCH3)3
aIsolated yield (purity of yield checked by NMR and GC), b Yields are based on aldehydes, cAll reactions
d
were monitored by TLC, 60oC, 5h.
Control experiment indicated that the addition of phenylboronic acid to p-chlorobenzaldehyde reaction
did not occur in the absence of 2a. Under the determined reaction conditions, a wide range of aryl
aldehydes bearing electron-donating or electron-withdrawing groups can react with phenylboronic acid
affording the addition products in excellent yields (Table 1 Entries 1, 6, 9, 11, 16, 21 and 26). A
systematic study on the substituent effect in the imidazolidinium salts (2a, 4a) indicated that the
introduction of 3,4,5-trimethoxybenzyl substituent on the N-atoms notably increased the reaction rate and
the yield of the product. In summary, we have demonstrated that in situ generated imidazolidin-2-ylidene
complexes of rhodium are very effective for the addition of phenylboronic acid to aldehydes.
Although the chemistry of NHC is limited as compare to phosphine chemistry. The NHCs ligands are
more stable than corresponding phosphine ligands. Furthermore milder experimental conditions exist for
preparation of secondary alcohols by NHCs ligands.
In conclusıon, we are pleased to find that among the various NHC precursurs,
perhydrobenzimidazolinium and imidazolidinium salts (2, 4) are excellent ligand precursors for the
addition of phenylboronic acid to aldehydes reaction. Also a convenient and highly user-friendly method

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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 periods.
EXPERIMENTAL
All reactions for the preparation of perhydrobenzimidazolinium and imidazolinium salts (2ac and 4a,b)
were carried out under argon using standart Schlenk-type flasks. Test reactions for the catalytic activity
of catalysts in the addition of phenylboronic acid to aldehydes reactions were carried out in air. The
complex [RhCl(COD)]237 and 1 and 3 were prepared according to known methods. 34,35 All reagents were
13
1
purchased from Aldrich Chemical Co. All 1H and C-NMR were performed in DMSO-d6. H NMR and
13C NMR spectra were recorded using a Bruker AC300P FT spectrometer operating at 300.13 MHz (1H),
75.47 MHz (13C). Chemical shifts (δ) are given in ppm relative to TMS, coupling constants (J) in Hz.
Infrared spectra were recorded as KBr pellets in the range 400-4000 cm-1 on a ATI UNICAM 1000
spectrometer. Melting points were measures in open capillary tubes with an Electrothermal-9200 melting
point apparatus and uncorrected. Elemental analyses were performed by TUBITAK (Ankara, Turkey)
Microlab.
Preparation of 1,3-bis(3,4,5-trimethoxybenzyl)perhydrobenzimidazolinium chloride (2a).
A mixture of N,N’-bis(3,4,5-trimethoxybenzyl) 1,2-cyclohexanediamine dihydrochloride (5 g; 9.14
mmol), in triethyl orthoformate (50 mL) was heated in a distillation apparatus until the distillation ethanol
o
ceased. The temperature of the reaction mixture reached 110 C. Upon cooling to rt a colorless solid
precipitated which was collected by filtratiton, and dried in vacuum. The crude product was recrystallized
from absolute ethanol to give colourless needles, and the solid was washed with ether (2x10 mL), dried
1
under vacuum. Yield: 4.42 g, 93%. mp 161oC, IR (KBr) ν= 1665 cm-1 (-CH=N-). H-NMR(CDCl3) δ:
1.18, 2.07, 3.25 (m,10 H, NCH(CH2)4CHN); 3.80, 3.87 (s, 18H, CH2C6H2(OCH3)3); 4.71, 4.87 (d, J 14.8
Hz, 4H, CH2C6H2(OCH3)3); 6.68 (s, 4H, CH2C6H2(OCH3)3); 10.42 (s, 1H, 2-CH). 13C- NMR(CDCl3):
23.85, 27.88, 51.55 (NCH(CH2)4CHN); 56.68, 61.02 (CH2C6H2(OCH3)3); 67.44 (CH2C6H2(OCH3)3);
106.16, 129.14, 138.55, 153.94 (CH2C6H2(OCH3)3); 162.63 (2-CH). Anal. Calcd for C27H37N2O6Cl: C,
62.24; H, 7.10; N, 5.37. Found: C, 62.52; H, 7.28; N, 5.12.
Preparation of 1,3-bis(p-methoxybenzyl)perhydrobenzimidazolinium chloride (2b).
Compound (2b) was prepared in the same way as 2a from N,N’-bis(p-methoxybenzyl) 1,2-
cyclohexanediamine dihydrochloride (5.5 g; 12.88 mmol), in triethyl orthoformate (50 mL) to give white
crystals of 2b. Yield: 4.64 g, 90%. mp 186oC. IR ν= 1585 cm-1 (-CH=N-). 1H-NMR(CDCl3) δ: 1.11, 1.74,
3.13 (m, 10 H, NCH(CH2)4CHN); 3.65 (s, 6H, CH2C6H4OCH3); 4.64, 4.89 (d, J 14.8 Hz, 4H,

HETEROCYCLES, Vol. 68, No. 7, 2006
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13
CH2C6H4OCH3); 6.55, 7.28 (d, J 8.4 Hz, 8H, CH2C6H4OCH3); 10.75 (s, 1H, 2-CH). C- NMR(CDCl3):
23.81, 27.76, 50.58 (NCH(CH2)4CHN); 55.30 (CH2C6H4OCH3); 67.07 (CH2C6H4OCH3); 96.28, 114.60,
125.51, 130.15, 159.95 (CH2C6H4OCH3); 162.40 (2-CH). Anal. Calcd for C23H29N2O2Cl: C, 68.91; H,
7.24; N, 6.99. Found C, 68.72; H, 7.38; N, 6.84.
Preparation of 1,3-bis(p-benzyloxybenzl)perhydrobenzimidazolinium chloride (2c).
Compound (2c) was prepared in the same way as 2a from N,N’-bis(p-benzyloxybenzyl) 1,2-
cyclohexanediamine dihydrochloride (6 g; 8.85 mmol), in triethylorthoformate (50 mL) to give white
crystals of 2c. Yield: 5.22 g, 87%. mp 164oC. IR ν= 1603 cm-1 (-CH=N-). 1H-NMR(CDCl3) δ: 1.18, 1.85,
2.76 (m,10 H, NCH(CH2)4CHN); 4.64, 4.89 (d, J 11.2 Hz, 4H, CH2C6H4OCH2C6H5); 5.04 (s, 4H,
CH2C6H4OCH2C6H5); 6.91, 7.32 (m, 18H, CH2C6H4O CH2C6H5); 10.56 (s, 1H, 2-CH). 13C-
NMR(CDCl3): 23.82, 27.70, 50.57 (NCH(CH2)4CHN); 67.04, 70.29 (CH2C6H4OCH2C6H5); 115.67,
125.63, 127.71, 128.26, 128.79, 130.26, 136.83, 159.32 (CH2C6H4OCH2C6H5); 162.04 (2-CH). Anal.
Calcd for C35H37N2O2Cl: C, 76.01; H, 6.69; N, 5.06. Found C, 76.32; H, 6.53; N, 5.21.
Preparation of 1,3-bis(3,4,5-trimethoxybenzyl)-4-methylimidazolinium chloride (4a).
Compound (4a) was prepared in the same way as 2a from 1,2-bis(3,4,5-trimethoxybenzylamino)propane
dihydrochloride (5.8 g, 11.43 mmol), in triethyl orthoformate (50 mL) to give white crystals of 4a. Yield:
4.83 g, 88%. mp 111oC. IR ν= 1654 cm-1 (-CH=N-). 1H-NMR(CDCl3) δ: 4.06 (m, 1H, NCH(CH3)CH2N);
3.24, 3.88 (t, J 8.4 Hz, 2H, NCH(CH3)CH2N); 1.30 (d, J 6.4 Hz, 3H, NCH(CH3)CH2N); 3.76, 3.82, 3.83
(s, 18H, CH2C6H2(OCH3)3); 4.35, 5.07 (d, J 14.8 Hz, 2H, CH2C6H2(OCH3)3); 4.71,4.75 (d, J 14.4 Hz, 2H,
CH2C6H2(OCH3)3); 6.66, 6.68 (s, 4H, CH2C6H2(OCH3)3); 10.23 (s, 1H, 2-CH). 13C- NMR(CDCl3): 18.49,
50.19, 52.84 (NCH(CH3)CH2N); 54.73, 55.71, 56.75, 56.85 (CH2C6H2(OCH3)3); 60.97
(CH2C6H2(OCH3)3); 106.43, 128.53, 138.62, 138.65, 153.96, 153.99 (CH2C6H2(OCH3)3); 158.74 (2-CH).
Anal. Calcd for C24H33N2O6Cl: C, 59.93; H, 6.86; N, 5.82. Found C, 59.71; H, 6.97; N, 5.92.
Preparation of 1,3-bis(p-benzyloxybenzl)-4-methylimidazolinium chloride (4b).
Compound (4b) was prepared in the same way as 2a from 1,2-bis(p-benzyloxybenzlamino)propane
dihydrochloride (5.3 g, 9.83 mmol), in triethyl orthoformate (50 mL) to give white crystals of 4b. Yield:
4.28 g, 85%. mp 158oC. IR ν= 1644 cm-1 (-CH=N-). 1H-NMR(CDCl3) δ: 4.02 (m, 1H, NCH(CH3)CH2N);
3.25, 3.82 (t, J 11.2 Hz, 2H, NCH(CH3)CH2N); 1.20 (d, J 6.4 Hz, 3H, NCH(CH3)CH2N); 4.46, 4.75 (d, J
14.8 Hz, 2H, CH2C6H4OCH2C6H5); 4.60 (s, 2H, CH2C6H4OCH2C6H5); 5.09 (s, 4H, CH2C6H4OCH2C6H5);
13
7.04, 7.36 (m, 18H, CH2C6H4O CH2C6H5); 9.00 (s, 1H, 2-CH). C- NMR(CDCl3): 18.49, 48.42, 50.99
(NCH(CH3)CH2N); 54.98, 56.37, 70.00 (CH2C6H4OCH2C6H5); 115.80, 126.56, 126.82, 128.39, 128.56,
129.13, 130.72, 130.76, 137.62, 157.84 (CH2C6H4OCH2C6H5); 159.16 (2-CH). Anal. Calcd for 129.13,

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74.92; H, 6.43; N, 5.46. Found C, 74.79; H, 6.64; N, 5.62.
General Procedure for Rhodium-Carbene Catalyzed Addition of Phenylboronic Acid to Aldehydes.
Phenylboronic Acid (1.20 g, 9.8 mmol), KOBut (4.9 mmol), substituted aldehydes (4.9 mmol),
[RhCl(COD)]2 (1 mol%), imidazolinium salts (2-4) (2 mol), dimethhoxyethane (15 mL) were introduced
o
in to Schlenk tube and then water (5 mL) was added. The resulting mixture was heated for 5 h at 60 C,
cooled to ambient temperature, exracted with ethyl acetate (30 mL). After drying over MgSO4 the organic
phase was evaporated and the residue was purified by flash chromatography. Isolated yield (yields based
on aldehydes) is checked by NMR and GC, all reactions were monitored by TLC.
ACKNOWLEDGEMENTS
This work was financially supported by the Technological and Scientific Research Council of Turkey TÜBİTAK
[TBAG-2474 (104T085)] and Inönü University Research Fund (BAP:2005/42).
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