In situ preparation of rhodium/N-heterocyclic carbene complexes and use for addition of arylboronic acids to aldehydes

Article abstract of DOI:10.1002/jhet.5570440111

(Chemical Equation Presented) The in situ prepared three component system [RhCl(COD)]₂/imidazolidinium 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. Four novel 1,3-dialkylimidazolidinium (2-4) salts as NHC precursors were synthesized from N,N′-dialkylethylenediamine.

Full text of DOI:10.1002/jhet.5570440111

Jan-Feb 2007
69
In situ Preparation of Rhodium/N-Heterocyclic Carbene
Complexes and use for Addition of Arylboronic Acids to Aldehydes
Rafet Kılınçarslana, Murat Yiꢀitb, ꢁsmail Özdemirb*, Engin Çetinkayaa, Bekir Çetinkayaa
a Pamukkale University,Faculty Science and Arts, Chemistry Department, Denizli, Turkey
b Inönü University,Faculty Science and Arts, Chemistry Department, 44280 Malatya, Turkey
Received February 18, 2006
OH
O
[RhClCOD]2, LHX
B(OH)2
2
C
H
C
H
+
KOBut
R
R
DME/ H2O
R
N
)
X-
LHX:
+
N
R
The in situ prepared three component system [RhCl(COD)]2/imidazolidinium salts (2, 4) and KOBut
catalyses the addition of phenylboronic acid to sterically hindered aldehydes affording the corresponding
arylated secondary alcohols in good yields. Four novel 1,3-dialkylimidazolidinium (2-4) salts as NHC
precursors were synthesized from N,N'-dialkylethylenediamine.
J. Heterocyclic Chem., 44, 69 (2007).
INTRODUCTION
[14], arylation of aromatic aldehydes [15], palladium
catalyzed arylation of benzaldehyde [16], aerobic alcohol
oxidation [17] and allylic alkylation and etherification
[18].
N-Heterocyclic carbenes (NHCs) are currently
receiving much research attention for their wide
applicability in coordination chemistry and catalysis [1].
Access to new ligand types is a critical component of the
design of novel homogeneous transition metal catalysts.
Because drastic changes in catalytic activity can result
from apparently minor modifications of ligand structure
[2], ligand designs that allow systematic variation of steric
and electronic properties are particularly valuable.
In general, metal complexes of “saturated”
(nonaromatic) imidazolidin-2-ylidenes [19] reveal higher
catalytic activity than the analogous complexes of
“unsaturated” (aromatic) imidazolin-2-ylidenes [20] or
benzannulated benzimidazol-2-ylidenes [21]. This is
regarded as a consequence of the stronger basicity and
nucleophilicity of the nonaromatically stabilized carbene
ligands.
Since Arduengo's discovery of stable imidazol-2-
ylidenes in 1991 [3], substantial efforts have focused on
the design and catalytic application of diaminocarbene
ligands [4], primarily “N-heterocyclic carbenes” (NHCs)
derived from imidazolium cations [5]. Diaminocarbenes
function as exceptionally strong ꢀ-donors to late transition
metals, with binding energies exceeding those of the most
basic phosphines by as much as 10 kcal/mol [6], and they
lack the electrophilic reactivity of Fischer-type carbenes
[4]. The high catalytic activities achieved by a number of
NHC-based catalysts have been attributed in part to this
strong donor ability, in combination with the unique steric
demands [6,7] and enhanced thermal stability [8] of these
ligands compared with phosphines. Catalyst advances
with particular promise for synthetic applications have
been achieved using NHC ligands in ruthenium-catalyzed
ring-closing olefin metathesis [8], intramolecular
cyclization of (Z)-3-methylpent-2-en-4-yn-1-ol into 2,3-
dimethylfuran [9,10], cycloisomerisation [11], amination
of aryl halides [12], palladium-catalyzed Suzuki-Miyaura
cross coupling [13], rhodium catalyzed hydrosilylation
Diarylmethanols are important intermediates for the
synthesis of biologically and pharmaceutically active
substances [22,23].
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
[24]. 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 [25].
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

70
R. Karaaslan, M. Yiꢀit, ꢁ. Özdemir, E. Çetinkaya, B. Çetinkaya
Vol 44
phenylboronic acid to aldehydes reaction is a neglected
area. In order to find more efficient rhodium catalysts we
have prepared a series of new 1,3-dialkylimidazolidinium
chlorides LHX, 2-5 containing a imidazolidine ring and
we report here the synthesized of rhodium-carbene based
catalytic system generated in-situ in the presence of base
for the addition of phenylboronic acid to aldehydes
(Scheme 1).
resonances for C(2)-H were observed as sharp singlets in
the 8.17, 8.53, 9.32 and 7.94 respectively for 2-5. The IR
data for imidazolidinium salts 2-5 clearly indicate the
presence of the –C=N- group with a ꢀ(C=N) vibration at
1667, 1610, 1624 and 1608 cm-1 respectively for 2-5. The
NMR and IR values are similar to those found for other
1,3-dialkylimidazolidinium salts [26,27].
Although the addition of carbon nucleophiles to
aldehydes is usually
a facile process, limits are
OH
O
encountered that functionalized organometallic reagents
required. Recent publications describing the addition of
arylboronic acid derivatives to aldehydes in the presence
of the catalytic amounts of Rh(I) and phosphine
derivatives deserve particular mention [24,25]. 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 [28].
Here, various imidazolidinium salts (2-5) 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
[RhClCOD]2, LHX
B(OH)2
2
C
H
C
H
+
DME/ H2O, KOBut
R
R
Scheme 1. Addition of phenylboronic acid to aldehydes.
RESULTS AND DISCUSSION
Imidazolidinium salts, (2-5) are conventional NHC
precursors. The reaction of 1 [26,27] with triethyl
orthoformate yielded the 2-5 salts (Scheme 2). The salts
are air- and moisture stable both in the solid state and in
solution. The structures of 2-5 were determined by their
characteristic spectroscopic data and elemental analyses.
Me2N
N
+
N
N
+
N
)
Cl-
)
Cl-
NH4Cl
CH(OEt)3
NH4Cl
CH(OEt)3
Me2N
2
3
R'
R'
NHR
NHR
Et2N
1
NH4PF6 / NaI
CH(OEt)3
NH4Cl
OMe
CH(OEt)3
N
+
N
N
+
N
)
I-
)
Cl-
OMe
Et2N
5
4
Scheme 2. Synthesis of 1,3-dialkylimidazolidinium salts.
13C NMR chemical shifts are consistent with the
proposed structure, the imino carbon appeared as a typical
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 °C and 60 °C appeared to be best. We
started our investigation with the addition of phenyl-
boronic acid to p-chlorobenzaldehyde, in the presence of
1
singlet in the H-decoupled mode in the 163.7, 157.9,
160.8, and 157.4 ppm respectively for imidazolidinium
1
salts 2-5. The H NMR spectra of the imidazolidinium
salts further supported the assigned structures; the

Jan-Feb 2007
In situ Preparation of Rhodium/N-Heterocyclic Carbene Complexes
71
from Aldrich Chemical Co. All H and 13C-NMR were performed
in DMSO-d6. 1H 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 (ꢀ) 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 an ATI UNICAM
1000 spectrometer. Melting points were measures in open capillary
tubes with an Electrothermal-9200 melting point apparatus and are
uncorrected. Elemental analyses were performed by TUBITAK
(Ankara, Turkey) Microlab.
1
[RhCl(COD)]2/2-5. Table
1 summarizes the results
obtained in the presence of 2-5 (Table 1, entries 1-4).
Control experiment indicated that the addition of
phenylboronic acid to p-chlorobenzaldehyde reaction did
not occur in the absence of 2. 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, 5, 6, 9, 13,
17 and 21).
Preparation of 1,3-Bis(4-dimethylaminophenyl)imidazol-
idinium chloride (2). Triethyl orthoformate (10 ml) was added
onto N,N'-bis(4-dimethylaminophenyl)ethane-1,2-diamine (1.49
In conclusion, four 1,3-dialkylimidazolidinium salts (2-
5) have been prepared and characterized. We are pleased
Table 1
Rhodium-carbene catalyzed addition of phenylboronic acid to aldehydes.
OH
O
[RhClCOD]2, LHX
B(OH)2
2
C
H
C
H
+
DME/ H2O, KOBut
R
R
Yielda-d(%)
Entry
LHX
Yielda-d(%)
R
R
Entry
LHX
1
2
3
4
5
6
7
8
9
p-Cl
p-Cl
p-Cl
p-Cl
H
H
H
2
3
4
5
2
3
4
5
2
3
4
5
96
95
92
89
87
87
85
83
76
74
73
71
13
14
15
16
17
18
19
20
21
22
23
24
p-C(CH3)3
p-C(CH3)3
p-C(CH3)3
2
3
4
5
2
3
4
5
2
3
4
5
81
79
73
70
84
80
76
72
73
70
64
66
p-C(CH3)3
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
H
2,4,6(CH3)3
2,4,6(CH3)3
2,4,6(CH3)3
2,4,6(CH3)3
10
11
12
[a] Phenylboronic acid (9.8 mmol), aldehydes (4.9 mmol), [RhCl(COD)]2 (1 mmol%), LHX (2 mmol), KOtBu (4.9 mmol),
dimethoxyethane (15 mL), [a] Isolated yield (purity of yield checked by NMR and GC). [b]Yields are based on aldehydes. [c] All
reactions were monitored by TLC. [d] 60°C.
g, 5 mmol) and NH4Cl (0.27 g, 5 mmol). The mixture was
stirred for 6 h in oil bath at 140 °C under argon atmosphere.
Meanwhile EtOH, formed as a by product, removed from the
medium via distillation. The resulting yellow solid was collected
by filtration, dissolved in MeOH (10 ml) and treated with
Na2CO3 (0.5 M, 20 ml). The resulting colorless solid was
collected by filtration and was crystallized from methanol/
diethyl ether to give colourless needles, and the solid was
washed with diethyl ether (2x10 mL), dried under vacuum, and
the yield was 1.47 g (85 %), mp 221-223 °C; ir: 1667 cm-1 (N-C-
to find that among the various NHC precursurs, imidazol-
idinium salts (2-5) are excellent ligand precursors for the
addition of phenylboronic acid to aldehydes reaction.
Also a convenient and highly user-friendly 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 periods.
Detailed investigations, focusing on imidazolin-2-ylidene
and benzimidazolin-2-ylidene substituent effects,
functional group tolerance and catalytic activity in this
and other coupling reactions are ongoing.
1
N). H-NMR(CDCl3): ꢀ 2.79 (s,12H, 4-N(CH3)2C6H4), 3.89 (s,
4H, NCH2CH2N), 6.67, 6.97 (d, 8H, J=8.8 Hz, 4-N(CH3)2C6H4),
8.17 (s, 1H, 2-CH). 13C- NMR (CDCl3): ꢀ 40.8 (4-
N(CH3)2C6H4), 43.8 (NCH2CH2N), 113.0, 126.9, 129.7, 150.1
(4-N(CH3)2C6H4), 163.7 (2-CH). Anal. Calcd. for C19H25N4Cl:
C, 66.17, H, 7.31, N, 16.25. Found C, 66.22, H, 7.39, N, 16.20.
Preparation of 1,3-Bis(2-biphenyl)imidazolidinium chloride
(3). This compound was prepared in the same manner as 2 using
N,N'-bis(2-biphenyl)ethane-1,2-diamine (1.82 g, 5 mmol) and
NH4Cl (0.27 g, 5 mmol), in triethyl orthoformate (10 ml) to give
colorless crystals of 3. Yield: 1.78 g (87 %), mp 254-256 °C;
ir: 1610 cm-1 (N-C-N). 1H-NMR(CDCl3): ꢀ 3.90 (s, 4H,
EXPERIMENTAL
All reactions for the preparation of imidazolidinium salts (2-5)
were carried out under argon using standart Schkenk-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)]2 [29] and 1 were prepared
according to known methods [26,27]. All reagents were purchased

72
R. Karaaslan, M. Yiꢀit, ꢁ. Özdemir, E. Çetinkaya, B. Çetinkaya
Vol 44
NCH2CH2N), 7.20-7.29, 7.37-7.46 (m, 16H, 2-C6H5C6H4), 8.12
(d, 2H, J=8.2 Hz, 2-C6H5C6H4), 8.53 (s, 1H, 2-CH). 13C-
NMR(CDCl3): ꢀ 52.3 (NCH2CH2N), 127.5, 128.7, 128.9, 129.5,
129.9, 130.0, 131.4, 133.6, 137.3, 137.9 (2-C6H5C6H4), 157.9 (2-
CH). Anal. Calcd. for C27H23N2Cl: C, 78.91, H, 5.64, N, 6.82.
Found C, 78.96, H, 5.69, N, 6.79.
REFERENCES AND NOTES
Correspondence to: ꢁsmail Özdemir, Inönü University,
Faculty Science and Arts, Chemistry Department, 44280 Malatya,
Turkey. Tel. +90 4223410212; Fax: +90 4223410037; E-mail address:
iozdemir@inonu.edu.tr (ꢁ. Özdemir)
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*
Preparation of 1,3-Bis(2-methoxyethyl)-4,5-bis(2,4,6-tri-
methylphenyl)imidazolidinium iodide (4). To a solution of
1,3-bis(2-methoxyethyl)-4,5-bis(2,4,6-trimethylphenyl)imidazol-
idinium hexafluorophosphate (2.84 g, 5 mmol), in acetone (15
ml) was added NaI (0.75 g, 5 mmol). The reaction mixture was
refluxed for 3 h. The solvent was removed under vacuum.
CH2Cl2 (5 ml) was added to the resulting crude and filtered.
Volume of the solution was reduced to ca 2-3 ml under vacuum
and diethy ether (15 ml) was added to complete precipitation.
The resulting colorless solid was collected by filtration and was
crystallized from methanol/diethyl ether to give colourless
needles, and the solid was washed with diethyl ether (2x10 mL),
dried under vacuum, and the yield was 2.25 g (82 %), mp 173-
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1
175 °C; ir: 1624 cm-1 (N-C-N). H-NMR (CDCl3): ꢀ 2.13, 2.14
(s, 18H, 2,4,6-(CH3)3C6H2), 3.34-3.37, 3.93-3.98 (m, 4H,
NCH2CH2OCH3), 3.72-3.77 (m, 4H, NCH2CH2OCH3), 6.05 (s,
2H, NCHCHN), 6.61, 6.66 (s, 4H, 2,4,6-(CH3)3C6H2), 9.32 (s,
1H, 2-CH). 13C- NMR (CDCl3): ꢀ 20.9, 21.3, 21.6 (2,4,6-
(CH3)3C6H2), 46.9 (NCH2CH2OCH3), 69.6 (NCH2CH2OCH3),
59.3 (NCH2CH2OCH3), 124.2, 130.4, 132.0, 137.3, 139.1 (2,4,6-
(CH3)3C6H2), 160.8 (s, 1H, 2-CH). Anal. Calcd. for C27H39N2O2I:
C, 58.91, H, 7.14, N, 5.09. Found C, 58.89, H, 7.18, N, 5.12.
Preparation of 1,3-Bis(2-methyl-4-dethylaminophenyl)-
imidazolidinium chloride (5). This compound was prepared in
the same manner as 2 using N,N'-bis(2-methyl-4-dethylamino-
phenyl)ethane-1,2-diamine (1.91 g, 5 mmol) and NH4Cl (0.27 g, 5
mmol), in triethyl orthoformate (10 ml). The colorless crystals 5
formed were stored under argon since they were sensitive toward
air. Yield: 0.94 g, 84%, mp 163-165 °C; ir: 1608 cm-1 (N-C-N).
1H-NMR(CDCl3) ꢀ: 1.07 (t, 12H, J=6.81 Hz, N(CH2CH3)2C6H3),
3.22 (q, 8H, J=6.41 Hz, N(CH2CH3)2C6H3), 2.28 (s, 6H, 2-CH3-4-
N(CH2CH3)2C6H3), 4.53 (s, 4H, NCH2CH2N), 6.37 (s, 2H,
N(CH2CH3)2C6H3), 6.43 (d, 2H, J=8.65 Hz, N(CH2CH3)2C6H3),
7.46.43 (d, 2H, J=8.66 Hz, N(CH2CH3)2C6H3), 7.94 (s, 1H, 2-CH).
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13C- NMR(CDCl3):
ꢀ
12.7 (N(CH2CH3)2C6H3), 44.7
(N(CH2CH3)2C6H3), 18.8 (2-CH3-4-N(CH2CH3)2C6H3), 53.7
(NCH2CH2N), 110.4, 113.3, 122.5, 128.3, 134.7, 148.8
(N(CH2CH3)2C6H3), 157.4 (2-CH). Anal. Calcd. for C25H37N4Cl:
C, 66.99, H, 8.69, N, 13.06. Found C, 69.91, H, 9.8.79, N, 13.01.
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%) (with respect to
aldehyde), imidazolidinium salts (2-5) (2 mol%), dimethoxy-
ethane (15 mL) were introduced in to Schlenk tube and then
water (5 mL) was added. The resulting mixture was heated for 6
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 chroma-
tography. Isolated yield (yields based on aldehydes) is checked
by NMR and GC, all reactions were monitored by TLC.
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Acknowledgement. This work was financially supported
by the Technological and Scientific Research Council of
Turkey TÜBꢁTAK [TBAG-2474 (104T085)] and Inönü
University Research Fund (BAP:2005/42).
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Jan-Feb 2007
In situ Preparation of Rhodium/N-Heterocyclic Carbene Complexes
73
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