Dialkylamino cyclopentadienyl ruthenium(II) complex-catalyzed α-alkylation of arylacetonitriles with primary alcohols

Article abstract of DOI:10.1039/b917805h

Aminocyclopentadienyl ruthenium complexes, [(η⁵-C ₅H₄NMe₂)Ru(PPh₃)₂(CH ₃CN)]⁺BF₄^- and [(η⁵- C₅H₄NEt₂)Ru(PPh₃) ₂(CH₃CN)]⁺BF₄^-, are moderately active catalysts for α-alkylation of arylacetonitriles with primary alcohols; on the other hand, the analogous unsubstituted cyclopentadienyl ruthenium complex [(η⁵-C₅H ₅)Ru(PPh₃)₂(CH₃CN)] ⁺BF₄^- shows very low catalytic activity. On the basis of experimental results and theoretical calculations, rationalization for the much higher catalytic activity of the aminocyclopentadienyl complexes over that of the unsubstituted Cp complex is provided. In the catalytic systems with the former, it is possible to regenerate the active solvento complexes via protonation of the metal hydride intermediates and subsequent ligand substitution; this process is, however, very nonfacile in the catalytic system with the latter. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/b917805h

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PAPER
www.rsc.org/dalton | Dalton Transactions
Dialkylamino cyclopentadienyl ruthenium(II) complex-catalyzed a-alkylation
of arylacetonitriles with primary alcohols†
Hung Wai Cheung,a Juan Li,b Wenxu Zheng,b Zhongyuan Zhou,a Yu Hin Chiu,a Zhenyang Lin*b and
Chak Po Lau*a
Received 28th August 2009, Accepted 15th October 2009
First published as an Advance Article on the web 23rd November 2009
DOI: 10.1039/b917805h
5
-
Aminocyclopentadienyl ruthenium complexes, [(h -C5H4NMe2)Ru(PPh3)2(CH3CN)]+BF4 and
[(h -C5H4NEt2)Ru(PPh3)2(CH3CN)]+BF4 , are moderately active catalysts for a-alkylation of
5
-
arylacetonitriles with primary alcohols; on the other hand, the analogous unsubstituted
5
-
cyclopentadienyl ruthenium complex [(h -C5H5)Ru(PPh3)2(CH3CN)]+BF4 shows very low catalytic
activity. On the basis of experimental results and theoretical calculations, rationalization for the much
higher catalytic activity of the aminocyclopentadienyl complexes over that of the unsubstituted Cp
complex is provided. In the catalytic systems with the former, it is possible to regenerate the active
solvento complexes via protonation of the metal hydride intermediates and subsequent ligand
substitution; this process is, however, very nonfacile in the catalytic system with the latter.
C–C bond formation reactions, we studied catalytic a-alkylation
Introduction
of nitriles with primary alcohols with these ruthenium complexes;
however, they were found to be inactive or very poor catalysts
for the reactions. Fortunately, we later found that a couple of
aminocyclopentadienyl-ruthenium complexes are active catalysts
for the reactions; we report here the findings of our work with these
catalytic systems. We also provide, with the help of theoretical
calculations, an explanation for the low catalytic activity of the
analogous cyclopentadienyl-ruthenium species.
a-Alkylated nitriles are an important class of compounds for
their potential as versatile building blocks in the synthesis of
amides, amidines, carboxylic acids, ketones, and biologically
active compounds.1 Traditional synthesis of these nitriles requires
usage of alkyl halides and a stoichiometric amount of inorganic
base; toxicity of the former constitutes a major drawback of
this synthetic method. Direct catalytic alkylation of nitriles
thus represents an attractive green reaction from both an eco-
nomical and environmental point of view. Few examples of
direct alkylation of nitriles with alcohols which are catalyzed
by transition metals are known. The early ones being the Ru-
and Rh-catalyzed reactions;2 more recent ones include reactions
Experimental
Materials and general methods
3
catalyzed by the iridium complex (Cp*IrCl2)2 and a novel Ru-
grafted hydrotalcite.4 In these reactions, aryl- and heteroaryl
nitriles were used. Closely related reactions involving the addition
of acetonitrile and other alkyl nitriles to aldehydes to yield b-
hydroxynitriles have been reported; these reactions are catalyzed
by ruthenium5 and rhodium6 complexes. b-hydroxynitriles are
potential precursors for pharmaceutically important substances.7
We have recently reported that b-alkylation of secondary alco-
hols with primary alcohols are catalyzed by a number of ruthenium
complexes, by virtue of their being able to affect respectively
oxidation of the primary and secondary alcohols to aldehydes and
methyl ketones, which then undergo aldol condensation under
basic conditions.8 Continuing our interest in ruthenium-catalyzed
All manipulations were carried out under a nitrogen atmosphere
using standard Schlenk techniques. Solvents were freshly distilled
under nitrogen from sodium-benzophenone (tetrahydrofuran),
sodium (diethyl ether, hexane and toluene), calcium hydride
(dichloromethane, and acetonitrile) or P2O5 (C6D6 and CDCl3);
they were degassed prior to use. Elemental analyses were per-
formed by M-H-W Laboratories, Phoenix, AZ (USA). 1H NMR
spectra were obtained from a Bruker DPX-400 spectrometer at
400 MHz; chemical shifts (d, ppm) were reported relative to
1
residual peaks of the deuterated solvents used. 13C{ H}NMR
spectra were recorded with a Bruker DPX-400 spectrometer at
100.61 MHz; chemical shifts were internally referenced to CDCl3
(d = 77.7 ppm), C6D6 (d = 128.1 ppm) or (CD3)2CO (d =
1
206.26, 29.84ppm). 31P{ H}NMR spectra were recorded on a
aDepartment of Applied Biology and Chemical Technology, The Hong Kong
Polytechnic University, Hung Hom, Kowloon, Hong Kong, People’s Republic
of China. E-mail: bccplau@polyu.edu.hk
Bruker DPX-400 spectrometer at 161.70 MHz; chemical shifts
were externally referenced to 85% H3PO4 in D2O. All spectra were
obtained at ambient probe temperature unless stated otherwise.
Mass spectrometry was carried out with a Finnigan MAT 95S
mass spectrometer with the samples dissolved in dichloromethane
bDepartment of Chemistry, The Hong Kong University of Science and
Technology, Clear Water Bay, Kowloon, Hong Kong, People’s Republic of
China. E-mail: chzlin@ust.hk
† Electronic supplementary information (ESI) available: Molecular struc-
ture of complex M3+. CCDC reference numbers 745961 & 745962. For
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/b917805h
5
5
or acetone. The complexes (h -C5H5)Ru(PPh3)2Cl,9 and (h -
C5H5)Ru(PPh3)2(CH3CN)BF4 (M3),10 were prepared according to
literature methods. The organic products described in Table 3 are
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5
◦
5
Table 1 Crystal data and structure refinement of [(h -C5H4NMe2)-
˚
Table 2 Selected bond distances (A) and angles ( ) for [(h -C5H4NMe2)-
Ru(PPh3)2(CH3CN)]+BF4 (M1)
-
-
Ru(PPh3)2(CH3CN)]+BF4 (M1)
Empirical formula
C45H43BF4N2P2Ru
Bond distances
Formula weight
Temperature
Wavelength
Crystal system
Space group
Unit cell dimensions
858.61
296(2) K
0.71073 A
Orthorhombic
Ru(1)-C(1)
Ru(1)-C(2)
Ru(1)-C(3)
Ru(1)-C(4)
Ru(1)-C(5)
Ru(1)-N(1)
Ru(1)-P(1)
Ru(1)-P(2)
2.363(3)
2.226(3)
2.172(4)
2.167(4)
2.224(3)
2.059(3)
2.3413(9)
2.3662(9)
C(1)-C(2)
C(1)-C(5)
C(2)-C(3)
C(3)-C(4)
C(4)-C(5)
N(1)-C(8)
N(11)-C(1)
1.435(4)
1.429(6)
1.418(5)
1.390(5)
1.433(5)
1.145(5)
1.351(5)
˚
Pca21
a = 90◦
b = 90◦
g = 90◦
˚
˚
˚
a = 20.6849(5) A
b = 10.1795(2) A
c = 19.2592(5) A
3
˚
Volume
4055.25(16) A
Z
4
Density (calculated)
Absorption coefficient
F(000)
1.406 Mg m-3
0.518 mm-1
1756
Bond angles
C(1)-C(5)-C(4)
C(1)-N(11)-C(6)
C(1)-N(11)-C(7)
C(3)-C(2)-C(1)
C(3)-C(4)-C(5)
C(4)-C(3)-C(2)
C(5)-C(1)-C(2)
107.6(3)
120.6(3)
119.0(3)
108.3(3)
108.6(3)
108.4(3)
106.5(4)
C(6)-N(11)-C(7)
C(8)-N(1)-Ru(1)
N(1)-C(8)-C(9)
N(1)-Ru(1)-P(1)
N(1)-Ru(1)-P(2)
P(1)-Ru(1)-P(2)
116.7(4)
176.3(3)
178.1(5)
86.52(8)
92.71(8)
105.47(3)
Crystal size
Theta range for data
collection
0.28 ¥ 0.24 ¥ 0.20 mm3
1.97 to 27.38◦.
Index ranges
-26 ≤ h ≤ 26, -13 ≤ k ≤ 13,
-24 ≤ l ≤ 23
Reflections collected
Independent reflections
Completeness to theta =
27.44◦
33629
8863 [R(int) = 0.0993]
99.7%
˚
Hydrogen bond distances/A
Absorption correction
Semi-empirical from
equivalents
Max. and min. transmission
Refinement method
1.000 and 0.736
Full-matrix least-squares on
F2
D-H ◊ ◊ ◊ A
d(D-H)
0.98
0.98
d(H ◊ ◊ ◊ A)
2.34
2.66
d(D ◊ ◊ ◊ A)
3.277(5)
3.202(5)
C(3)-H(3A) ◊ ◊ ◊ F(1)
C(4)-H(4A) ◊ ◊ ◊ F(3)
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2s(I)]
R indices (all data)
Absolute structure parameter -0.05(4)
Largest diff. peak and hole
8863/1/496
1.001
R1 = 0.0493, wR2 = 0.0891
Hydrogen bond angles (◦)
R1 = 0.1001, wR2 = 0.1044
D-H ◊ ◊ ◊ A
C(3)-H(3A) ◊ ◊ ◊ F(1)
C(4)-H(4A) ◊ ◊ ◊ F(3)
∠(DHA)
158.9
115.3
-3
˚
1.279 and -1.099 e A
known and were characterized by comparing their 1H NMR data
with the reported ones.
(t, J = 5 Hz, –C-N(CH3)2), d 43.88 (s, –N(CH3)2). ESI-MS: m/z
734.15, [M]+.
(g5-C5H4NEt2)Ru(PPh3)2H (M8). To a solution of diethyl-
amine (0.36 mL, 3.45 mmol) in freshly degassed THF (10 mL)
cooled in an ice bath was added slowly n-butyllithium (1.7 mL,
2.7 mmol, 1.6 M in hexane); the solution was then allowed
to warm to room temperature, and stirring was continued for
Syntheses and reactions
(g5-C5H4NMe2)Ru(PPh3)2H (M4). A sample of (h -C5H5)-
5
Ru(PPh3)2Cl (0.50 g, 0.69 mmol) was added to a two-necked
round bottom flask equipped with a dropping funnel, which was
then evacuated and flushed with nitrogen for four cycles. Freshly
degassed THF (50mL) was added to the flask. LiN(Me)2 (0.18 g,
3.53 mmol) was added to the dropping funnel, followed by 20 mL
of THF to dissolve it, the resulting solution was slowly added to
the flask. The mixture was stirred at room temperature for 2 h. At
the end of this period, 0.1 mL of H2O was added to the reaction
mixture. It was then evaporated to dryness under reduced pressure
to produce a yellow colloidal material, 20 mL of freshly degassed
toluene was then added, and the insoluble material was filtered
off. The solvent was removed from the filtrate under reduced
pressure to give a yellow paste; pre-cooled hexane (15 mL) was
added to the residue, with stirring, to produce a yellow solid. The
solid was collected by filtration and dried under vacuum at room
5
30 min. A sample of (h -C5H5)Ru(PPh3)2Cl (0.50 g, 0.69 mmol)
was added to a two-necked round bottom flask, which was then
evacuated and flushed with nitrogen for four cycles. Freshly
degassed THF (50 mL) was added to the flask. Upon complete
5
dissolution of (h -C5H5)Ru(PPh3)2Cl, the lithium diethylamide
solution just prepared was added and the mixture was stirred at
room temperature for 2 h. At the end of this period, 0.1 mL of
H2O was added to the reaction mixture. It was then evaporated
to dryness under reduced pressure to produce a yellow colloidal
material; 20 mL of freshly degassed toluene was added, and the
insoluble material was filtered off. The solvent of the filtrate were
removed under reduced pressure to give a yellow paste; pre-cooled
hexane (15 mL) was added to the residue, with stirring, to produce
a yellow solid. The solid was collected by filtration and dried
under vacuum at room temperature. Yield: 0.38 g (73%). 1H NMR
1
temperature. Yield: 0.41 g (82%). H NMR (400 MHz, C6D6,
25 ◦C): d 7.74 (m, 12H; PPh3-H), d 7.15-7.03 (m, 18H; PPh3-H), d
4.80, d 3.00 (s, 2H, 2H; C5H4), d 2.34 (s, 6H; –N(CH3)2), d -10.30
◦
(400 MHz, C6D6, 25 C): d 7.85-7.75 (m, 12H; PPh3-H), d 7.11-
◦
(t, J = 32 Hz, 1H; Ru-H). 31P{ H}NMR (161 MHz, C6D6, 25 C):
7.04 (m, 18H; PPh3-H), d 4.69, d 3.27 (s, 2H, 2H; C5H4), d 2.83
(q, J = 7 Hz, 4H; –N(CH2CH3)2), d 1.09 (t, J = 7 Hz, 6H;
1
d 68.59 (s). 13C{ H}NMR (100.61 MHz, C6D6, 25 ◦C): d 66.54
1
266 | Dalton Trans., 2010, 39, 265–274
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Table 3 Ru(II)-catalyzed a-alkylation of arylacetonitriles with primary alcoholsa
nitrile
alcohol
products
% conversionb
entry
1
2
3
4
mol% [Ru]
M1
M2
38 (32)
M3
1
2
3
4
5
6
7
8
1a R1 = Ph
2a R2 =Ph
3a (4a)
3b (4b)
3c (4c)
1%
1%
1%
2%
1%
2%
1%
1%
2%
2%
1%
2%
1%
2%
1%
37 (18)
50 (19)
16 (0)
33 (24)
22 (0)
33 (11)
57 (6)
40 (5)
49 (4)
31 (7)
25 (0)
36 (0)
26 (0)
35 (0)
36 (31)
7 (0)
4 (0)
4 (0)
15 (0)
3 (0)
15 (0)
4 (0)
3 (0)
7 (0)
11 (0)
5 (0)
1a
1a
2b R2 = 4-FC6H4
58 (21)
27 (10)
41 (14)
35 (8)
37 (15)
67 (7)
49 (9)
58 (9)
23 (33)
31 (0)
40 (0)
34 (0)
39 (0)
40 (38)
2c R2 = 2-OMeC6H4
1a
2d R2 = 4-OMeC6H4
3d (4d)
1a
1a
2e R2 = thiophen-2-yl
3e (4e)
3f (4f)
2f R2 = 2-furyl
9
10c
11
12
13
14
15
1a
2g R2 = Pr
3g (4g)
3h (4h)
1b R1 = 4-OMeC6H4
2a
16 (0)
6 (0)
22 (trace)
6 (0)
1b
2b
3i (4i)
1c R1 = 4-FC6H4
2a
3j (4j)
a Reaction conditions: catalyst (0.02 mmol), nitrile (1 or 2 mmol depend on mol% of cat.), alcohol (6 mmol), DBU (0.4 mmol), 120 ◦C, 24 h. b Conversion
(based on nitrile) determined by 1H NMR spectroscopy with 1,1,2,2-tetrachloroethane as an internal standard. Values in parenthesis indicate the
conversion of the corresponding saturated products 4. c 48 h.
1
–N(CH2CH3)2), d -10.23 (t, J = 33 Hz, 1H; Ru-H). 31P{ H}NMR
(q, J = 7 Hz, 4H;–N(CH2CH3)2), d 2.20 (s, 3H; Ru-NCCH3), d
◦
1
1.11 (t, J = 7 Hz, 6H; –N(CH2CH3)2). 31P{ H}NMR (161 MHz,
(161 MHz, C6D6, 25 C): d 69.04 (s). 13C{ H}NMR (100.61 MHz,
1
(CD3)2O, 25 ◦C): d 46.45 (s). 13C{ H}NMR (100.61 MHz,
◦
1
C6D6, 25 C): d 65.70(t, J = 4 Hz,–C-N(CH2CH3)2), d 47.20 (s,
(CD3)2O, 25 ◦C): d 59.95 (t, J = 6 Hz, –C-N(CH2CH3)2), d 45.85
(s, –C–N(CH2CH3)2), d 13.55 (s, –C–N(CH2CH3)2). ESI-MS: m/z
762.19, [M-CH3CN]+.
–C–N(CH2CH3)2), d 13.73 (s, –C–N(CH2CH3)2). ESI-MS: m/z
762.21, [M]+.
[(g5-C5H4NMe2)Ru(PPh3)2(CH3CN)]+BF4 (M1). A sample
of (h -C5H4NMe2)Ru(PPh3)2H (0.10 g, 0.14 mmol) was added
-
(g5-C5H4NMe2)Ru(PPh3)(Ph)(CO) (M5). A sample of [(h -
5
5
to a two-necked round bottom flask which was degassed and
flushed with nitrogen four times, freshly degassed THF (20 mL)
was added to dissolve the complex, followed by the addition of
HBF4·Et2O (25 mL, 1.5 mmol) and CH3CN (1 mL). The solution
was stirred at room temperature for 1 h, and evaporated to
dryness under reduced pressure to yield a yellow paste. Hexane
(5 mL) was added to the residue, with stirring, to produce a
yellow solid. The solid was filtered off and washed with diethyl
ether (2 ¥ 2 mL). It was collected and dried under vacuum
at room temperature. Yield: 89 mg (76%). Anal. Calcd (%) of
C45H43BF4N2P2Ru: C, 62.73; H, 5.03; N, 3.25. Foun◦d: C, 62.55;
C5H4NMe2)Ru(PPh3)2H (0.50 g, 0.68 mmol) was loaded into a
two-necked round bottom flask, which was then evacuated and
flushed with nitrogen for four cycles. Degassed toluene (10 mL)
and benzaldehyde (0.35 mL, 2.07 mmol) were then added and
the resulting mixture was refluxed with stirring for 24 h. At the
end of this period, the solution was cooled to room temperature
and was evaporated to dryness under reduced pressure to yield a
pale yellow paste. Hexane (5 mL) was added to the residue, with
stirring at -78 ◦C, to produce a yellow solid. The solid was filtered
off, and thoroughly dried under vacuum to give M5 which was
contaminated by free triphenylphosphine and phosphine oxide.
M5 was characterized by NMR spectroscopy. 1H NMR (400 MHz,
C6D6, 25 ◦C): d 7.79-7.04(m, 20H; PPh3-H, Ru–C6H5), d 4.87, d
4.53 (d, J = 2 Hz, 1H, 1H; C5H4), d 4.18, d 4.16 (s, 1H, 1H; C5H4),
1
H, 5.09; N, 3.19. H NMR (400 MHz, (CD3)2O, 25 C): d 7.46-
7.39 (m, 6H; PPh3-H), d 7.31-7.25 (m, 24H; PPh3-H), d 4.28, d
3.27 (s, 2H, 2H; C5H4), d 2.88 (s, 6H; N(CH3)2), d 2.28 (s, 3H;
◦
◦
Ru-NCCH3). 31P{ H}NMR (161 MHz, (CD3)2O, 25 C): d 46.22
d 2.23 (s, 6H; N(CH3)2).31P{ H}NMR (161 MHz, C6D6, 25 C):
1
1
◦
◦
1
1
(s) 13C{ H}NMR (100.61 MHz, (CD3)2O, 25 C): d 60.42 (t, J =
6 Hz, –C-N(CH3)2), d 40.12 (s, –N(CH3)2). ESI-MS: m/z 734.15,
[M-CH3CN]+.
d 59.88 (s) 13C{ H}NMR (100.61 MHz, C6D6, 25 C): d 207.70
2
2
(d, JPC = 19 Hz, Ru-CO); d 152.31 (d, JPC = 12 Hz, ipso C of
Ru–C6H5).
-
-
[(g5-C5H4NEt2)Ru(PPh3)2(CH3CN)]+BF4 (M2).
A
proce-
In situ Preparation of HDBU+BF4 . The compound was
5
dure similar to that for the synthesis of [(h -C5H4NMe2)Ru-
synthesized according to the literature method with a slight
modification.11 HBF4·Et2O (0.58 mL, 3.6 mmol) was added to an
ice-cooled solution of DBU (0.5 mL, 3.3 mmol) in Et2O (10 mL).
The reaction mixture was allowed to stir in the ice bath for 30 min.
At the end of this period, the mixture was evaporated to dryness
under reduced pressure to give a pale yellow oil. The resulting oil
was washed with pentane (2 ¥ 3 mL). It was collected and dried
under vacuum at room temperature. Yield: 0.66 g (83%). 1H NMR
(PPh3)2(CH3CN)]+BF4- was followed, except that (h -C5H4NEt2)-
5
5
Ru(PPh3)2H (0.10 g, 0.13 mmol) was used in place of (h -
C5H4NMe2)Ru(PPh3)2H. Yellow solid; yield: 83 mg (71%). Anal.
Calcd (%) of C47H47BF4N2P2Ru: C, 63.45; H, 5.32; N, 3.15. Found:
C, 63.38; H, 5.39; N, 3.11. 1H NMR (400 MHz, CDCl3, 25 ◦C): d
7.37-7.30 (m, 6H; PPh3-H), d 7.27-7.21(m, 12H; PPh3-H), d 7.19-
7.11(m, 12H; PPh3-H), d 4.08, d 3.04 (s, 2H, 2H; C5H4), d 3.15
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(400 MHz, CDCl3, 25 ◦C): d 7.97 (s, 1H; NH), d 3.58-3.53 (m,
4H), d 3.41-3.39 (m, 2H), d 2.69-2.67 (m, 2H), d 2.09-2.037 (m,
2H), d 1.76-1.70 (m, 6H).
1996) program based on Fourier coefficient fitting was applied.
The crystal structure was solved by Patterson function methods
and expanded by difference Fourier synthesis, then refined by full-
matrix least-squares on F2 using the Bruker Smart and Bruker
SHELXTL program packages. All non-hydrogen atoms were
refined anisotropically. Hydrogen atoms were placed in ideal
positions and refined as riding atoms. The R and Rw values of
M1 are 0.0493 and 0.0891, respectively. Further crystallographic
details and selected bond distances and angles for M1 can be found
in the Results and discussion section.
General procedure of catalytic a-alkylation of arylacetonitriles.
The reactions were carried out in a 10 mL round-bottomed flask
equipped with a reflux condenser topped with a nitrogen bypass.
In a typical run, ruthenium complex (0.02 mmol) was loaded
into the flask; it was then evacuated and filled with nitrogen
for four cycles. Arylacetonitriles (1 or 2 mmol depending on
mol% of cat.), primary alcohol (6 mmol) and DBU (0.4 mmol)
were added to the flask via syringes and needles. The flask was
heated in a silicon oil bath at 120 ◦C for 24 h. At the end of
this period, the system was cooled to room temperature and
25mL 1,1,2,2-tetrachloroethane was added as internal standard; a
Computational details. In the DFT calculations, PMe3 was
used as a model for PPh3. Geometry optimizations and frequency
calculations have been performed for all species involved in the
reaction at the Becke3LYP12 level of density functional theory
(no imaginary frequencies for an equilibrium structure and one
imaginary frequency for a transition structure). The intrinsic
reaction coordinate (IRC)13 analysis was also carried out to
confirm that all stationary points are smoothly connected to each
other. Gibbs free energy at 298.15 K was obtained on the basis of
the frequency calculations. The Ru and P atoms were described
using the LANL2DZ basis set, a double-z valence basis set with the
Hay and Wadt effective core potential (ECP).14 For all other atoms,
the 6-31G basis set was used.15 Polarization functions were added
for N (zd = 0.864) and for P (zd = 0.387). For those H bonded
to Ru, polarization functions were also added (zp = 1.100).16 All
calculations were performed with Gaussian 03 packages.17
1
0.1 mL aliquot of the solution was removed and analyzed by H
NMR spectroscopy (in CDCl3). Conversions of the reactions were
obtained by measuring the integrations of the characteristic peaks
of the products with reference to distinct peaks of the internal
standard.
Monitoring of [(g5-C5H4NMe2)Ru(PPh3)2(CH3CN)]+BF4 (M1)
-
and [(g5-C5H5)Ru(PPh3)2(CH3CN)]+BF4 (M3)-catalyzed a-alkyl-
-
31
1
ation of benzyl cyanide with benzyl alcohol using P{ H}NMR
spectroscopy. The reactions were carried out in 5 mm NMR tubes
capped with rubber septa. In a typical run, the ruthenium complex
(0.01 mmol, 1 mol%) was loaded to a tube; it was evacuated and
filled with nitrogen for four cycles. Benzyl cyanide (1 mmol), benzyl
alcohol (3 mmol) and DBU (0.2 mmol) were added to the tube via
syringes and needles. The resulting solution was heated in a silicon
oil bath at 120 ◦C. At different time intervals, the NMR tube
Results and discussion
In view of the catalytic activity of ruthenium complexes in
b-alkylation of secondary alcohols with primary alcohols, we
studied catalytic a-alkylation of nitriles with primary alcohols
using a number of Cp-Ru and Tp-Ru complexes (Tp = hy-
drotris(pyrazolyl)borate); however, they were found be inactive
or very poor catalysts.
1
was rapidly cooled down to room temperature and 31P{ H}NMR
spectra of the solution were taken. The relative concentrations of
the species present were obtained by comparing the integrations
1
of their signals in the 31P{ H}NMR spectra.
Reactions of the metal hydrides M4 and M6 with DBUH+ in
the presence of CH3CN. A weighted amount of hydride complex
(0.01 mmol) was loaded into a 5 mm NMR tube capped with a
septum; it was then evacuated and filled with nitrogen for four
We then prepared the aminocyclopentadienyl-Ru com-
5
-
5
plexes [(h -C5H4NMe2)Ru(PPh3)2(CH3CN)]+BF4 (M1) and [(h -
C5H4NEt2)Ru(PPh3)2(CH3CN)]+BF4 (M2), and found that they
-
are active for arylacetonitrile alkylation reactions (eqn (1)).
cycles. Freshly prepared HDBU+BF4 (0.12 g, 0.5 mmol) in THF
-
(0.45 mL) was then added to the tube via syringe and needle. After
complete dissolution of the complex, CH3CN (26 mL, 0.5 mmol)
was added. The resulting solution was heated in a silicon oil bath at
60 ◦C. At different time intervals, the NMR tube was cooled down
(1)
1
to room temperature and 31P{ H} NMR spectra of the solution
Preparation and characterization of complexes M1 and M2, and
X-ray structure of M1
were taken. The relative concentrations of the complexes present
were obtained by comparing the integrations of their signals in the
1
31P{ H}NMR spectra.
The aminocyclopentadienyl-ruthenium complexes M1 and M2
were prepared by protonation of the hydride precursors with
HBF4·Et2O in the presence of acetonitrile; the hydride complexes
Crystallographic structure analysis of [(g5-C5H4NMe2)Ru-
(PPh3)2(CH3CN)]+BF4 (M1). Yellow crystals suitable for the
-
5
X-ray diffraction study for M1 were obtained by layering of
n-hexane onto a dichloromethane solution of the complex. A
suitable crystal of the complex was mounted on a Bruker CCD
area detector diffractometer and subjected to Mo-Ka radiation
were formed by reacting (h -C5H5)Ru(PPh3)2Cl with LiNR2 (R =
CH3, C2H5) in THF (Scheme 1).
Aminocyclopentadienyl-Fe
C5H4NR2)Fe(L1)(L2)H (R = CH3, C2H5; L1 = CO, L2
5
hydride
complexes
(h -
=
(l = 0.71073 A) from a generator operating at 50 kV and 30
PR3; L1 = L2 = PR3) were prepared in a similar manner.18 The
1H NMR spectra of M1 and M2 show signals of the acetonitrile
ligands at d 2.28 and 2.20 ppm, respectively. A singlet that
corresponds to the amino methyl groups in M1 is seen at d
2.88 ppm; on the other hand, the existence of amino ethyl groups
˚
mA. The intensity data of M1 was collected in the range q =
1.97–27.38◦, with oscillation frames of y and w in the range 0–
180◦. A total of 1756 frames in M1 were taken in four shells.
An empirical absorption correction of the SADABS (Sheldrick,
268 | Dalton Trans., 2010, 39, 265–274
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Scheme 1
atoms of the tetrafluoroborate anion and two of the Cp hydrogen
of M2 is confirmed by the observation of quartet (d 3.15 ppm)
and triplet (d 1.11 ppm) signals which are present in a 2 : 3 ratio.
The phosphine ligands of M1 and M2 appear as singlets at d
˚
˚
atoms (H(3A) ◊ ◊ ◊ F(1), 2.34 A; H(4A) ◊ ◊ ◊ F(3), 2.66 A).
46.22 and 46.45 ppm, respectively in their 31P{ H} NMR spectra.
1
Catalytic a-alkylation of arylacetonitriles with alcohols
Yellow crystals of M1 suitable for X-ray diffraction study were
obtained by layering hexane onto a CH2Cl2 solution of the com-
plex. Fig. 1 shows the molecular structure of M1. The crystal data
and refinement details are given in Table 1. Selected bond distances
and angles are given in Table 2. The amino moiety is linked to the
The major products of the M1- and M2-catalyzed reactions are the
unsaturated nitriles, and the saturated nitriles in some cases only
appear as very minor products (Table 3). Product distributions in
our study are quite different from those of the studies carried
out by others; in their studies, the saturated nitriles are the
overwhelming products.
˚
Cp ring via a short C(1)–N(11) bond (1.351(5) A) which is shorter
2
3
˚
than the standard C(sp )–N(sp ) single bonds (1.40–1.44 A) but
5
-
The Cp complex [(h -C5H5)Ru(PPh3)2(CH3CN)]+BF4 (M3)
shows very low activity. Generally speaking, the complex with the
diethylamino substituent on the Cp ring (M2) is more active than
the one in which the Cp ring carries the dimethylamino group
(M1). Benzyl alcohols with electron-donating substituents give
lower overall conversions (entries 3–6), and the one containing
an electron-withdrawing fluoro group affords higher conversion
(entry 2). Alcohols containing heteroatoms seem to be less active
than benzyl alcohol (entries 7–9), and alkyl alcohol is even
less reactive (entry 10). Attachment of an electron-withdrawing
fluoro group to the arylacetonitrile modestly increases the overall
conversion (entry 15); on the other hand, the presence of an
electron-donating substituent lowers the activity of the system
(entries 11–14).
2
2
19
˚
=
longer than typical C(sp ) N(sp ) double bonds (1.25–1.28 A).
The distance from Ru to the N(CH3)2-substituted Cp carbon (Ru–
C(1), 2.363(3) A) and to the two carbons in b positions from C(1)
(Ru–C(3), 2.172(4) A; Ru–C(4), 2.167(4) A) are significantly longer
and shorter, respectively, than the Ru–C(Cp) bonds found in the
˚
˚
˚
unsubstituted Cp complex [(h -C5H5)Ru(PPh3)2(CH3CN)]+BF4
5
-
(M3) (Ru–C(Cp) ª 2.21 A).20 The nitrogen atom is nearly coplanar
with its substituents (sum of the angles around N, RN = 356.3◦); it
˚
˚
sits 0.156 A above the C1–C6–C7 plane, which makes a small angle
of 4.4◦ with the Cp ring. The structural properties of M1+ is in line
with the structural data reported for the aminocyclopentadienyl-
Fe complex (h -C5H4NEt2)Fe(PPh(OEt)2)(CO)Br18 and for 1,1¢-
5
bis(dimethylamino)-titanocene dichloride.21 The structure of M1
shows hydrogen-bonding interactions between two of the fluorine
We are interested in understanding the large difference in cat-
alytic activity between the systems based on the aminocyclopenta-
dienyl complexes M1, M2 and that based on the unsubstituted
Cp complex M3. In a separate experiment, we learned that
benzyl cyanide reacts, in the presence of DBU (40 mol%), with
benzaldehyde to give the unsaturated nitrile 3a in 88% conversion;
no metal catalyst is needed for the reaction. The reaction, however,
does not proceed in the absence of the base. It therefore seems
that in the M1- and M2-catalyzed arylacetonitrile a-alkylation
reactions, the major function of the metal complex is to affect
dehydrogenation of the alcohol to yield the aldehyde, which then
undergoes base-catalyzed condensation with the arylacetonitrile.
In the presence of DBU, the complex reacts with alcohol to
generate metal alkoxide, and subsequent b-elimination gives the
aldehyde and the metal hydride species.
NMR monitoring of M1- and M3-catalyzed alkylation of benzyl
cyanide with benzyl alcohol; comparison of rates of conversion of
the metal hydrides to M1 and M3
We monitored the M1- and M3-catalyzed alkylation of benzyl
1
cyanide with benzyl alcohol with 31P{ H}NMR spectroscopy. In
5
the M1-catalyzed reaction, it was found that the metal hydride (h -
C5H4NMe2)Ru(PPh3)2H (M4) was rapidly formed and it remained
the major metal-containing species throughout the experiment;
5
Fig. 1 ORTEP view (30% probability) of [(h -C5H4NMe2)Ru(PPh3)2-
(CH3CN)]+BF4 (M1) showing the atom-labeling scheme.
-
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small amounts of free phosphine and phosphine oxide due to
complex decomposition were detected, their amounts increased
with time. In addition, minute amounts of the carbonyl species
5
(h -C5H4NMe2)Ru(PPh3)(Ph)(CO) (M5) and an unknown species
were also detected. In the M3-catalyzed alkylation reaction,
5
in which conversion was very low, the hydride complex (h -
C5H5)Ru(PPh3)2H (M6) was the overwhelming species detected
during the monitoring process, small amounts of free phosphine
and phosphine oxide, indicative of a small degree of complex
decomposition, were observed. Minute quantities of the carbonyl
5
complex (h -C5H5)Ru(PPh3)(Ph)(CO) (M7) and an unknown
species were also formed. Formation of the phenyl carbonyl com-
plexes M5 and M7 is probably due to benzaldehyde decarbonyla-
tion at the metal center. We have recently reported the synthesis
of M7;8 the aminocyclopentadienyl analogue M5 is independently
prepared in this study. Decarbonylation of aldehydes by transition
metal complexes to form carbonyl complexes is well-established.22
Aldehyde decarbonylation forming metal carbonyl species often
causes catalyst deactivation.22f,23
Scheme 4
a C6D5Cl solution of M4 in the presence of 50 equiv of benzyl
alcohol at 120 ◦C for 48 h only resulted in the formation of
1
0.2 equiv of benzaldehyde. 31P{ H}NMR spectroscopy showed
that M4 basically remained unchanged although minute amounts
of free phosphine, phosphine oxide and a couple of unknown
species, probably due to M4 decomposition, were observed. In
the case of M6, under identical conditions, only a trace amount of
benzaldehyde was generated; M6 was recovered unchanged. These
experiments seem to indicate that the reaction shown in Scheme 4
occurred to a negligibly small extent for M4 and M6.
Reactions of (g5-C5H4NMe2)Ru(PPh3)2H (M4) and
(g5-C5H5)Ru(PPh3)2H (M6) with DBUH+ and with benzyl alcohol
We studied reactions of the hydride species M4 and M6 with
DBUH+BF4 in THF in the presence of excess CH3CN. It was
-
◦
found that at 60 C M4 in a THF solution containing 50 equiv
Theoretical calculations on the protonation of hydride complexes
with DBUH+ in CH3CN
each of DBUH+BF4- and CH3CN was converted to M1 (70%) after
30 min; the pseudo first order rate constant k was determined to
be 0.0589 min-1. On the other hand, under identical conditions,
only a trace amount of M3 was generated from M6. Reaction of
To gain support for the much more facile regeneration of
the solvent complex from the hydride species in the case of
the aminocyclopentadienyl complex in comparison to the non-
substituted Cp-Ru system, theoretical calculations, performed at
the Becke3LYP level of theory, on the displacement of H2 from the
dihydrogen complexes by CH3CN (Schemes 2 and 5) were carried
out.
In the presence of DBUH+, both M4 and M6 could be reversibly
converted to the corresponding dihydrogen complexes and their
dihydride tautomers; the equilibria would probably lie to the sides
of the hydrides because after all DBUH+ is a weak acid. We focus
on the processes of the H2/CH3CN exchange of the two dihydro-
gen species. In the DFT calculations, PMe3 is used as a model for
PPh3. Both dihydrogen complexes are 18-electron species and each
2
M4 with DBUH+ first generated the h -dihydrogen intermediate
M-(H2), it then came into equilibrium with the more stable and
dominating dihydride tautomer. Formation of M1 resulted via
extrusion of H2 from M-(H2) and coordination of CH3CN to the
metal center (Scheme 2). Protonation of CpRu complexes to yield
2
h -dihydrogen complexes as kinetic products which then come
into equilibrium with the more stable metal dihydride is well-
documented.24
It is known that transition-metal hydrides might be proto-
2
nated by acidic alcohols to form h -dihydrogen complexes;25
the dihydrogen ligand might then be displaced by the alkoxide.
(Scheme 3). We looked into the possibility that the metal hydrides
M4 and M6 would be protonated by benzyl alcohol to form the
metal alkoxide which then via b-elimination generate the metal
hydride and benzaldehyde (Scheme 4). It was found that heating
5
contain an h -cyclopentadienyl ligand. Therefore, the H2/CH3CN
exchange is expected to occur via either a dissociative mechanism
or an associative mechanism involving a h →h ring slippage of
5
3
Scheme 2
Scheme 3
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Scheme 5
5
the cyclopentadienyl ligand, a well-known phenomenon for h -Cp
deficient 16-e- metal fragment A¢. For complex B, in the absence
of an NMe2 substituent, H2 dissociation is much less favorable.
Brookhart and co-workers carried out a detailed mechanistic
study on the [CpFe(CO)(PPh3)]+-catalyzed silane alcoholysis and
transition metal complexes.26 Our calculations show that the Cp
ring slippage for the dihydrogen complexes is much less favorable
than H2 dissociation. The results are understandable because the
dihydrogen is a weakly-coordinated ligand.
Fig. 2 shows the energy required to dissociate H2 from A
to give (A¢ + H2) is 17.3 kcal mol-1. After H2 dissociation, A¢
takes in CH3CN to form M1’, which is more stable than A by
7 kcal mol-1. The activation barrier for the dissociative mechanism
was estimated by calculating the H2 dissociation energy, which
is the upper limit for a dissociative ligand substitution reaction.
Fig. 3 shows the corresponding energy profile calculated for the
dihydrogen complex B, the H2 dissociation energy was found to
be 21.3 kcal mol-1.
2
revealed that displacement of the H2 ligand of the h -dihydrogen
intermediate by silane is probably the rate-determining step.27
A
density functional study showed that introduction of an amino
substituent at the Cp ring of the catalyst lowers the barrier of
the H2/silane exchange step; it is rationalized by the p-donating
capability of the amine group which makes the Cp ring more
electron-rich, resulting in better stabilization of the electron-
deficient iron center upon H2 dissociation.28
Proposed mechanism for the catalytic a-alkylation of
arylacetonitrile with primary alcohol
The metal fragment A¢ derived from H2 dissociation from A
is relatively more stable than the metal fragment B¢ generated by
H2 dissociation from B. Therefore, the H2 dissociation process
from the complex A is more favorable. The electron donating
NMe2 moiety on the Cp ring contributes to stabilizing the electron
Taken together, a mechanism is proposed for the M1- or M2-
catalyzed a-alkylation of arylacetonitriles with primary alcohols
(Scheme 6). In the presence of a base (DBU) and the metal
Fig. 2 Energy profile calculated for the H2/CH3CN ligand exchange in the complex A. The calculated relative electronic energies are given in kcal mol-1.
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Fig. 3 Energy profile calculated for the H2/CH3CN ligand exchange in the complex B. The calculated relative electronic energies are given in kcal mol-1.
complex, the alcohol is oxidized to aldehyde while the metal com-
hydrogenation and via reversible insertion into the Ru–H bond
of M4 or M8 and subsequent protonation by the alcohol. The
reduction of 3 is not complete, probably due to the very low
concentration of H2 in the system and the sluggishness of the
alcohol protonation step.
Benzyl alcohols with electron-withdrawing and electron-
donating substituents giving higher and lower conversions, re-
spectively, is in consonance with the fact that the aldehyde with
a more electrophilic carbon center undergoes condensation with
arylacetonitrile more readily than the ones with less elec-
trophilic carbon centers. Lower conversion with 4-methoxy-
phenylacetonitrile is probably attributable to its less readiness to
be deprotonated by the base to generate the a-cyano carbanion,
which is the nucleophile attacking the aldehyde carbon in the
condensation reaction.
5
plex is converted to a hydride species (h -C5H4NMe2)Ru(PPh3)2H
5
(M4) or (h -C5H4NEt2)Ru(PPh3)2H (M8). The aldehyde, by the
action of the base, undergoes Knoevenagel condensation with the
arylacetonitrile to afford the unsaturated nitrile 3. The crucial
and slow step is the regeneration of the solvent complex M1
or M2 via protonation of the hydride species M4 or M8 (with
DHUH+) and subsequent H2/CH3CN exchange. This process is
very slow for the analogous Cp hydride species M6, and this is
probably the major reason for the unsubstituted Cp system M3
being a poor catalyst for the a-alkylation reactions. In the M1-
or M2-catalyzed reaction, the barrier of the ligand exchange step
is lowered with increased electron density at the metal center;
this is probably the reason for M2, which contains the more
electron-donating diethylamino group, being more active than
M1 bearing the dimethylamino substituent on the Cp ring. The
electron-donating nature of the amino group is also illustrated by
the nitrogen atom being coplanar with its substituents, as shown
in the X-ray structure of M1; the sp2-hybridized nitrogen atom
can better donate its lone pair into the ring. The initially formed
unsaturated nitrile 3 could be reduced via M4- or M8-catalyzed
Conclusion
Few examples of transition metal-catalyzed a-alkylation of nitriles
with primary alcohols, which is a green and atom-economical
reaction to yield a-alkylated nitriles, are known to date. Success of
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Scheme 6
this reaction lies in the ability of the metal to catalyze oxidation of
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