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Cyanosilylation of aldehydes catalyzed by mixed ligand copper(II) complexes
Inorganica Chimica Acta 471 (2018) 130–136
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Research paper
Cyanosilylation of aldehydes catalyzed by mixed ligand copper(II) complexes Atash V. Gurbanov a,b, Ghodrat Mahmoudi c, M. Fátima C. Guedes da Silva a, Fedor I. Zubkov d, Kamran T. Mahmudov a,b,⇑, Armando J.L. Pombeiro a,⇑ a
Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049–001 Lisbon, Portugal Department of Chemistry, Baku State University, Z. Xalilov Str. 23, Az 1148 Baku, Azerbaijan Department of Chemistry, Faculty of Science, University of Maragheh, P.O. Box 55181-83111, Maragheh, Iran d Organic Chemistry Department, RUDN University, 6 Miklukho-Maklaya str., Moscow 117198, Russian Federation b c
a r t i c l e
i n f o
Article history: Received 21 September 2017 Received in revised form 29 October 2017 Accepted 31 October 2017
Keywords: Arylhydrazones of acetoacetanilide Mixed ligand CuIIcomplexes Cyanosilylation reaction of aldehydes with trimethylsilyl cyanide
a b s t r a c t The new mixed ligand copper(II) complexes [Cu(HL)(H2O)(A)]2H2O (1, NaH2L = sodium (Z)-2-(2-(1,3dioxo-1-(phenylamino)butan-2-ylidene)hydrazinyl)benzenesulfonate, A = dimethylsulfoxide, [Cu(HL) (H2O)(B)] (2, B = 1,3,4-thiadiazol-2-amine), [Cu(HL)(H2O)(Y)]1/2CH3OH (3, C = hexamethylenetetramine) and [Cu(HR)2(H2O)2](H2L)2 (4, HR = methyl picolinimidate derived from 2-cyanopyridine) were synthesized and characterized by IR and ESI-MS spectroscopies, elemental and X-ray crystal structural analyses. These compounds act as homogenous catalysts for the cyanosilylation reaction of a variety of both aromatic and aliphatic aldehydes with trimethylsilyl cyanide affording the corresponding cyanohydrin trimethylsilyl ethers in high yields (up to 85–99 %) in methanol and at room temperature. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Catalytic hydrocyanation and cyanosilylation of aldehydes or ketones are among the most important strategies for synthesis of cyanohydrin trimethylsilyl ethers in synthetic chemistry (Scheme 1) [1]. The cyanohydrin trimethylsilyl ethers are industrially valuable and important intermediates for the synthesis of many valuable molecules such as a-hydroxy aldehydes, a-hydroxy acids and b-amino alcohols and other biologically active compounds [1]. Hence, a number of catalysts such as Lewis acids [2–4], Lewis bases [5–7], N-heterocyclic carbenes [8,9], aminothiourea [10,11], organic–inorganic salts [12–16], nonionic bases [17,18], oxazaborolidinium ion [19,20], V-, Mn-, Al-, and Ti-salen complexes [21–24], chiral Ti-a,a,a,a-1,3-dioxolane-tetraaryl-4,5dimethanols [25,26], Ti,Al-phosphine oxide bifunctional catalysts with carbohydrate or binaphthol scaffolds [27–29], Ti,Al-N-oxide bifunctional catalysts with proline, pyrrolidine and 1,2-diamino ligands [30–32], metal organic frameworks [33–36], Cu(II) and Co (II/III) hydrazone complexes [37–40], etc. have been utilized. Based on the existing methods, it is clear that most of these protocols have many disadvantages, such as low yield, long reaction time,
⇑ Corresponding authors. E-mail addresses: [email protected], [email protected] (K.T. Mahmudov), [email protected] (A.J.L. Pombeiro). https://doi.org/10.1016/j.ica.2017.10.042 0020-1693/Ó 2017 Elsevier B.V. All rights reserved.
harmful solvents, etc. Therefore, the development of a new homogeneous catalytic system, which is inexpensive, commonly available, easy to handle and time resolved, is highly desirable. On the other hand, the design and synthesis of mixed ligand complexes have received considerable attention due to their structural diversity as well as their potential applications as functional materials in various fields such as catalysis, pharmacology, molecular recognition, molecular switches, etc. [41–44]. Up to now, a great number of mixed ligand copper(II) complexes with interesting structures and properties have been obtained by selecting the appropriate hydrazone (main) and auxiliary ligands [45]. However, the controllable synthesis of mixed ligand copper(II) complexes with desired catalytic properties is still a great challenge because there are many chemical and physical factors playing important roles in the synthetic operation, such as the molecular structures of the main ligands, acid-base properties of auxiliary ligands, reaction condition, etc. Therefore, the two main objectives of the current work are as follows: i) to synthesize of mixed ligand copper(II) complexes by using known sodium (Z)-2-(2-(1,3-dioxo-1-(phenylamino)butan2-ylidene)hydrazinyl)benzenesulfonate (NaH2L) [46] and axulliary components such as dimethylsulfoxide, 1,3,4-thiadiazol-2-amine, hexamethylenetetramine and 2-cyanopyridine; ii) to apply the derived mixed ligand copper(II) complexes as the homogeneous
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MS (ESI, positive ion mode), m/z: 520.06 [Mr 2H2O+H]+. IR (KBr): 3480 (s, br) m(OH), 3005 m(NH), 1610 (s) m(C = O), 1594 m (C = N) cm–1. Scheme 1. Cyanosilylation of aldehydes.
catalysts for the cyanosilylation reaction of aldehydes with trimethylsilyl cyanide. 2. Experimental 2.1. Materials and instrumentation All the chemicals were obtained from commercial sources (Aldrich) and used as received. The NaH2L was synthesized according to the reported procedure [46]. Infrared spectra (4000–400 cm 1) were recorded on a Vertex 70 (Bruker) instrument in KBr pellets. Carbon, hydrogen, and nitrogen elemental analyses were done using a ‘‘2400 CHN Elemental Analyzer” (Perkin Elmer). The 1 H and 13C NMR spectra were recorded at room temperature on a Bruker Avance II + 300 (UltraShieldTM Magnet) spectrometer operating at 300.130 and 75.468 MHz for proton and carbon-13, respectively. The chemical shifts are reported in ppm using tetramethylsilane as the internal reference. Electrospray mass spectra (ESI-MS) were run with an ion-trap instrument (Varian 500-MS LC Ion Trap Mass Spectrometer) equipped with an electrospray ion source. For electrospray ionization, the drying gas and flow rate were optimized according to the particular sample with 35 p.s.i. nebulizer pressure. Scanning was performed from m/z 0 to 1100 in methanol solution. The compounds were observed in the positive mode (capillary voltage = 80–105 V). 2.2. Synthesis 2.2.1. Synthesis of 1 0.1 mmol (37 mg) of NaH2L were dissolved in 25 mL of methanol, then 0.1 mmol (23 mg) of Cu(NO3)22.5H2O and 0.1 mL of dimethylsulfoxide were added and the system was stirred for 10 min. After ca. 3 d at room temperature, greenish crystals precipitated which were then filtered off and dried in air. 1: Yield, 53 % (based on Cu). Calcd. for C18H25CuN3O9S2 (Mr = 555.08): C 38.95, H 4.54, N 7.57; found C 38.82, H 4.38, N 7.49.
2.2.2. Synthesis of 2 0.1 mmol (37 mg) of NaH2L were dissolved in 25 mL of methanol, then 0.1 mmol (23 mg) of Cu(NO3)22.5H2O and 0.1 mmol (10 mg) of 1,3,4-thiadiazol-2-amine were added and the system was stirred for 10 min. After ca. 2 d at room temperature, greenish crystals precipitated which were then filtered off and dried in air. 2: Yield, 53 % (based on Cu). Calcd. for C18H18CuN6O6S2 (Mr = 542.05): C 39.88, H 3.35, N 15.50; found C 39.47, H 3.43, N 15.23. MS (ESI, positive ion mode), m/z: 543.01 [Mr + H]+. IR (KBr): 3333 (s, br) m(OH), 3078 m(NH), 1600 (s) m(C = O), 1556 m (C = N) cm–1. 2.2.3. Synthesis of 3 0.1 mmol (37 mg) of NaH2L were dissolved in 25 mL of methanol, then 0.1 mmol (23 mg) of Cu(NO3)22.5H2O and 0.1 mmol (14 mg) of hexamethylenetetramine were added and the system was stirred for 10 min. After ca. 3 d at room temperature, greenish crystals precipitated which were then filtered off and dried in air. 3: Yield, 53 % (based on Cu). Calcd. for C45H58Cu2N14O13S2 (Mr = 1194.25): C 45.26, H 4.90, N 16.42; found C 45.15, H 4.76, N 16.37. MS (ESI, positive ion mode), m/z: 582.1 [C22H27CuN7O6S + H]+. IR (KBr): 3410 (s, br) m(OH), 3134 m(NH), 1665 (s) m(C = O), 1600 m (C = N) cm–1. 2.2.4. Synthesis of 4 0.1 mmol (37 mg) of NaH2L were dissolved in 25 mL of methanol, then 0.05 mmol (12 mg) of Cu(NO3)22.5H2O and 0.1 mmol (10 mg) of 2-cyanopyridine were added and the system was stirred for 10 min. After ca. 3 d at room temperature, greenish crystals precipitated which were then filtered off and dried in air. 4: Yield, 53 % (based on Cu). Calcd. for C46H48CuN10O14S2 (Mr = 1092.61): C 50.57, H 4.43, N 12.82; found C 50.49, H 4.25, N 12.69. MS (ESI, positive ion mode), m/z: 185.8 [C14H20CuN4O4]2+ and 362.4 [C16H14N3O5S + 2H]+. IR (KBr): 3427 (s, br) m(OH), 3158 and 2986 m(NH), 1633(s) m(C = O), 1587 m(C = N) cm–1.
Table 1 Crystallographic data and structure refinement details for 1–4.
Empirical formula fw Temperature (K) Cryst. Syst. Space group a (Å) b (Å) c (Å) a, ° b, ° c, ° V (Å3) Z qcalc (g cm 3) l(Mo Ka) (mm 1) F (0 0 0) R1a (I 2r) wR2b (I 2r) GOOF a b
1
2
3
4
C18H25CuN3O9S2 555.07 293(2) triclinic P-1 8.5077(9) 11.4878(12) 13.0362(13) 71.172(4) 82.268(4) 76.584(4) 1170.5(2) 2 1.575 1.165 574 0.0289 0.0729 1.025
C18H18CuN6O6S2 542.04 296(2) monoclinic P21/c 9.5281(4) 9.2619(4) 25.1485(9) 90 94.0610(10) 90 2213.74(16) 4 1.626 1.224 1108 0.0430 0.1142 1.052
C45H58Cu2N14O13S2 1194.25 293(2) triclinic P-1 11.633(3) 14.183(3) 15.958(4) 87.861(9) 82.562(9) 89.164(9) 2608.8(11) 2 1.520 0.972 1240 0.0310 0.0753 1.032
C46H48CuN10O14S2 1092.60 293(2) triclinic P-1 8.2542(4) 10.7845(5) 14.1552(6) 78. 288(2) 81.389(2) 77.816(2) 1198.49(10) 1 1.514 0.622 567 0.0317 0.0808 1.009
R1 = R||Fo| – |Fc||/R|Fo|. wR2 = [R[w(F2o – F2c )2]/ R[w(F2o)2]]1/2.
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Table 2 Selected bond distances (Å) and angles (°) for 1–4. 1
2
3
4
Cu(1)-O(1)
1.9435(11)
Cu(1)-O(2)
1.908(2)
N(3A)-Cu(1A)
1.9618(18)
Cu(1)-N(5)#1
1.9733(12)
Cu(1)-N(3) Cu(1)-O(5) Cu(1)-O(2) Cu(1)-O(7) O(1)-Cu(1)-N(3) O(1)-Cu(1)-O(5) N(3)-Cu(1)-O(5) O(1)-Cu(1)-O(2) N(3)-Cu(1)-O(2) O(5)-Cu(1)-O(2) O(1)-Cu(1)-O(7) N(3)-Cu(1)-O(7) O(5)-Cu(1)-O(7) O(2)-Cu(1)-O(7)
1.9526(12) 1.9707(11) 1.9781(12) 2.2753(13) 90.05(5) 89.65(5) 171.89(6) 162.98(6) 93.75(5) 84.26(5) 105.83(5) 96.87(5) 91.01(5) 90.20(5)
Cu(1)-O(1) Cu(1)-N(3) Cu(1)-N(4) Cu(1)-O(5) O(1)-Cu(1)-O(2) O(1)-Cu(1)-N(3) O(2)-Cu(1)-N(3) O(1)-Cu(1)-N(4) O(2)-Cu(1)-N(4) N(3)-Cu(1)-N(4) O(1)-Cu(1)-O(5) O(2)-Cu(1)-O(5) N(3)-Cu(1)-O(5) N(4)-Cu(1)-O(5)
1.964(2) 1.973(2) 2.007(3) 2.356(3) 169.17(10) 89.55(10) 94.68(9) 88.11(11) 86.29(11) 171.69(12) 87.04(10) 102.72(10) 92.21(10) 95.64(12)
N(3B)-Cu(1B) N(4A)-Cu(1A) N(4B)-Cu(1B) O(1A)-Cu(1A) O(1B)-Cu(1B) O(2A)-Cu(1A) O(2B)-Cu(1B) O(3A)-Cu(1A) O(3B)-Cu(1B) O(1A)-Cu(1A)-N(3A) O(1A)-Cu(1A)-O(2A) N(3A)-Cu(1A)-O(2A) O(1A)-Cu(1A)-O(3A) N(3A)-Cu(1A)-O(3A)
1.9652(18) 2.3277(18) 2.3308(18) 1.9348(16) 1.9405(16) 1.9673(16) 1.9872(16) 1.9936(16) 1.9798(17) 90.38(7) 87.00(7) 168.59(8) 162.87(7) 94.45(7)
Cu(1)-N(5) Cu(1)-N(4)#1 Cu(1)-N(4) N(5)#1-Cu(1)-N(5) N(5)#1-Cu(1)-N(4)#1 N(5)-Cu(1)-N(4)#1 N(5)#1-Cu(1)-N(4) N(5)-Cu(1)-N(4) N(4)#1-Cu(1)-N(4)
1.9733(12) 2.0342(12) 2.0342(12) 180.0 81.38(5) 98.62(5) 98.62(5) 81.38(5) 180.0
1
2
3
4 Scheme 2. Schematic representation of 1–4.
2.3. Crystal structure determination X-ray diffraction patterns of 1–4 were collected using a Bruker SMART APEX-II CCD area detector equipped with graphitemonochromated Mo-Ka radiation (k = 0.71073 Å) at room temperature. Absorption correction was applied by SADABS [47,48]. The structure was solved by direct methods and refined on F2 by fullmatrix least-squares using Bruker’s SHELXTL-97 [49]. All nonhydrogen atoms were refined anisotropically. The details of the crystallographic data, selected bond distances and angles for 1–4 are summarized in Tables 1 and 2. Crystallographic data for the structural analysis have been deposited to the Cambridge Crystallographic Data Center (CCDC 1556148 for 1, 1556147 for 2, 1556145 for 3 and 1556146 for 4). Copy of this information can
be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Fax: (+44) 1223-336033; E-mail: [email protected] or www.ccdc.cam.ac.uk/data_request/cif). 2.4. General procedure for the study of catalytic activity In a typical cyanosilylation procedure, to a stirred solution of benzaldehyde (0.4 mmol), catalyst (1 4) (1–8 mol%) in dry solvent [THF, CH2Cl2 or MeOH; 2 mL] trimethylsilyl cyanide (TMSCN) (0.6 mmol) was added dropwise. The resulting solution was stirred continuously and the progress of the reaction was followed by TLC. After a certain time, the solvent was evaporated, and the residue was diluted with 3 mL water and extracted with diethyl ether (3 10 mL). After removal of
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Fig. 1. Crystal structures of 1–4 with partial atom numbering scheme. H-bond interactions are shown as dashed blue lines.
diethyl ether under vacuum, the extract was taken for analysis by 1H NMR spectroscopy in CDCl3, to evaluate the yield of the cyanohydrin trimethylsilyl ethers [37–40]. The adequacy of the procedure was confirmed by blank 1H NMR analyses with 1,2dimethoxyethane as an internal reference. The internal standard method proved that no side products were formed.
3. Results and discussion 3.1. Synthesis and characterization of 1–4 Sodium (Z)-2-(2-(1,3-dioxo-1-(phenylamino)butan-2-ylidene) hydrazinyl)benzenesulfonate (NaH2L) was reported earlier by us
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Table 3 Selection of catalyst for the cyanosilylation of benzaldehyde with TMSCN.a.
a b
Entry
Catalyst
Amount of catalyst (mol%)
Solvent
Yieldb (%)
1 2 3 4 5 6 7 8 9 10 11 12 13c 14 15 16 17 18 19 20
1
5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 – – – –
THF CH2Cl2 MeOH THF CH2Cl2 MeOH THF CH2Cl2 MeOH THF CH2Cl2 MeOH THF CH2Cl2 MeOH MeOH – THF CH2Cl2 MeOH
23.6 25.0 76.9 25.4 26.9 78.3 24.7 27.4 77.8 27.3 29.1 83.6 20.4 22.3 27.1 32.9 20.0 14.0 14.4 24.7
2
3
4
NaH2L
Cu(NO3)22.5H2O – – – –
Reaction conditions: solvent (2 mL), TMSCN (0.6 mmol) and aldehyde (0.4 mmol), in air at room temperature, 24 h reaction time. Determined by 1H NMR analysis of crude products. c Entries 13–20 were adapted from [39].
Table 4 Optimization of the parameters of the cyanosilylation reaction between benzaldehyde and TMSCN with catalyst 4.a.
a b
Entry
Time (h)
Amount of catalyst (mol%)
T (°C)
Yieldb (%)
1 2 3 4 5
1 4 8 12 24
5 5 5 5 5
25 25 25 25 25
43.7 66.7 83.6 83.6 83.6
6 7 8 9 10
8 8 8 8 8
1 3 5 7 9
25 25 25 25 25
47.4 73.3 83.6 86.1 86.2
11 12 13 14
8 8 8 8
7 7 7 7
15 35 45 55
79.8 86.8 87.4 88.1
Reaction conditions: MeOH (2 mL), TMSCN (0.6 mmol) and benzaldehyde (0.4 mmol), in air. Determined by 1H NMR analysis of crude products (see Experimental part).
[46], and hence will not be discussed herein. Reaction of Cu(NO3)22.5H2O with NaH2L in methanol in the presence of dimethylsulfoxide (A), 1,3,4-thiadiazol-2-amine (B), hexamethylenetetramine (C) and 2-cyanopyridine (D) led to the mixed ligand copper(II) complexes [Cu(HL)(H2O)(A)]2H2O (1), [Cu(HL)(H2O)(B)] (2), [Cu (HL)(H2O)(C)]1/2CH3OH (3) and [Cu(HR)2(H2O)2](H2L)2 (4) (HR = methyl picolinimidate derived from D), respectively (Scheme 2). The compounds 1–4 are characterized by ESI-MS, IR spectroscopy, elemental analysis and single crystal X-ray diffraction (see Tables 1 and 2). The IR spectra of 1–4 (see experimental section) shows significantly m shift in relation to the corresponding signals of NaH2L [3532 m(OH), 3098 m(NH), 1669 m(C = O) and 1563 m(C = N) cm 1)] [46]. Fragmentation peaks in mass spectra of the compounds can be related as follows: 520.06 [Mr 2H2O+H]+ (for 1), 543.01 [Mr + H]+ (for 2), 582.1 [C22H27CuN7O6S + H]+ (for 3), 185.8 [C14H20CuN4O4]2+ and 362.4 [C16H14N3O5S + 2H]+ (for 4), accounting for the existence of the monomeric mixed ligand units in solution. Elemental analyses and X-ray crystallography are also in agreement with the proposed formulations. In 1–3, the copper ion has a distorted square-pyramidal coordination sphere with the chelating HL2 and water (1,3,4-thiadiazol-2-amine in the case of 2) ligands
occupying the four equatorial positions and the apical site being engaged with an axulliary (dimethylsulfoxide and hexamethylenetetramine) ligand (Fig. 1). Whereas, in 4, the copper ion has a distorted octahedral geometry, where the basal plane is made of four N-atoms from two bidentate methyl picolinimidate ligands and the axial positions are occupied with two O-atoms from water molecules (Fig. 1). In all complexes, the carbonyl group acts as Hbond acceptor towards the amide group leading to a strong intramolecular hydrogen bonding with N O distance of ranging from 2.606 to 2.645 Å (Fig. 1). 3.2. Catalytic activity of 1–4 in cyanosilylation reaction Complexes 1–4 have been tested as homogeneous catalysts for the cyanosilylation reaction of benzaldehyde with cyanide (TMSCN), as the representative substrates, in different solvents (tetrahydrofurane, dichloromethane or methanol), at room temperature (Scheme 1, Table 3). As can be seen in Table 3 the reaction is promoted by 1–4 in methanol (entries 3, 6, 9 and 12, Table 3), the yield are much higher than those achieved with NaH2L and Cu (NO3)22.5H2O as well as in the absence of any catalyst or solvent
A.V. Gurbanov et al. / Inorganica Chimica Acta 471 (2018) 130–136 Table 5 Cyanosilylation of various aldehydes with TMSCN using catalyst 4a. Entry
Substrate
4. Conclusions Yield, %b
1
89.8
2
87.6
3
86.9
4
86.1
5
85.7
6
85.1
7 8 9
CH3CHO CH3CH2CHO CH3(CH2)4CHO
135
99.1 98.5 98.0
a Reaction conditions: 7 mol% of catalyst 3, MeOH (2 mL), TMSCN (0.6 mmol) and aldehyde (0.4 mmol). Reaction time: 8 h. b Determined by 1H NMR analysis of crude products (see Experimental part).
We have prepared four mixed ligand copper(II) complexes and applied for the cyanosilylation of aromatic and aliphatic aldehydes with TMSCN. Complex 4 with ionic character was found to be the more efficient catalyst for the cyanosilylation reaction in methanol, leading cyanohydrin trimethylsilyl ethers in high yields (85–99%) at room temperature in 8 h. Electron-donation substituent in the para position of the aromatic aldehydes inhibits the reaction yield, while an opposite effect is observed with the electron-withdrawing groups. Compared to other related homogeneous catalytic systems, our mixed ligand copper(II) complex exhibits higher catalytic activity at room temperature. Acknowledgements Authors are grateful to the Fundação para a Ciência e a Tecnologia: (project UID/QUI/00100/2013), Portugal, for financial support. GM acknowledges to the University of Maragheh for the financial support of this research. This work also was supported by the ’’RUDN University Program 5-100’’. Authors are thankful to the Portuguese NMR Network (IST-UL Centre) for access to the NMR facility and the IST Node of the Portuguese Network of mass-spectrometry for the ESI-MS measurements. References
free conditions (entries 13–20, Table 3). The reaction in MeOH leads to higher yields in comparison to THF and CH2Cl2, and in general the yield decreases in the order of polarity MeOH > CH2Cl2 > THF, for the studied solvents (entries 1–12, Table 3). The catalytic activities of all complexes are comparable in all studied solvents, 4 (ionic) being significantly more active than other ones. The higher activity of 4 may be due to the involvement of its cationic and anionic moieties in noncovalent interaction with TMSCN and benzaldehyde, respectively, which promotes reaction. Thus, we furthermore studied different effects using 4 as the catalyst and methanol as the solvent (Table 4). Increasing the reaction time up to 8 h led to a remarkable increase in yield (Table 4, entries 1–5), which did not increase considerable for longer times. The increase of the catalyst amount up to 7 mol % loading resulted also in a marked yield improvement (entries 6–10, Table 4), whereas a higher catalyst amount had a much lower effect. The temperature (in the 15–55 °C range) had not a crucial effect on the yield of cyanohydrin trimethylsilyl ether (entries 11–14, Table 4). Having the reaction conditions optimized, we investigated the reactivity of a variety of aldehydes, and the appropriate cyanohydrin trimethylsilyl ethers were generally obtained in good yields (Table 5). Aromatic aldehydes bearing electron-withdrawing groups (bromo, chloro or nitro) exhibit relatively higher reactivities (Table 5, entries 1–3) as compared to those having electrondonating substituents (methoxy or methyl) (Table 5, entries 5 and 6), conceivably resulting from an increase of the substrate electrophilicity in the former case. Aliphatic aldehydes, such as acetaldehyde, propionaldehyde and hexanal (Table 5, entries 7– 9), have also been used as substrates for this reaction. The aldehyde conversion slightly decreases with the increase of the size of the aliphatic fragment (99% for acetaldehyde relatively to 98% for hexanal). The obtained yield in our system are usually higher than the other reported Cu(II) (78%) [39] and (27%) [50], Co(II) (78%) [40], Zn(II) (30%) [51], etc. complexes. Our system have several advantages such as a short reaction time (8 h) and room temperature in comparison some systems, which a longer reaction time (96 h) [52], higher (40 °C) [50] or lower (even negative) (-50 °C) [53] temperatures are required.
[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] [31] [32] [33] [34] [35] [36] [37] [38] [39]
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Research paper
Cyanosilylation of aldehydes catalyzed by mixed ligand copper(II) complexes Atash V. Gurbanov a,b, Ghodrat Mahmoudi c, M. Fátima C. Guedes da Silva a, Fedor I. Zubkov d, Kamran T. Mahmudov a,b,⇑, Armando J.L. Pombeiro a,⇑ a
Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049–001 Lisbon, Portugal Department of Chemistry, Baku State University, Z. Xalilov Str. 23, Az 1148 Baku, Azerbaijan Department of Chemistry, Faculty of Science, University of Maragheh, P.O. Box 55181-83111, Maragheh, Iran d Organic Chemistry Department, RUDN University, 6 Miklukho-Maklaya str., Moscow 117198, Russian Federation b c
a r t i c l e
i n f o
Article history: Received 21 September 2017 Received in revised form 29 October 2017 Accepted 31 October 2017
Keywords: Arylhydrazones of acetoacetanilide Mixed ligand CuIIcomplexes Cyanosilylation reaction of aldehydes with trimethylsilyl cyanide
a b s t r a c t The new mixed ligand copper(II) complexes [Cu(HL)(H2O)(A)]2H2O (1, NaH2L = sodium (Z)-2-(2-(1,3dioxo-1-(phenylamino)butan-2-ylidene)hydrazinyl)benzenesulfonate, A = dimethylsulfoxide, [Cu(HL) (H2O)(B)] (2, B = 1,3,4-thiadiazol-2-amine), [Cu(HL)(H2O)(Y)]1/2CH3OH (3, C = hexamethylenetetramine) and [Cu(HR)2(H2O)2](H2L)2 (4, HR = methyl picolinimidate derived from 2-cyanopyridine) were synthesized and characterized by IR and ESI-MS spectroscopies, elemental and X-ray crystal structural analyses. These compounds act as homogenous catalysts for the cyanosilylation reaction of a variety of both aromatic and aliphatic aldehydes with trimethylsilyl cyanide affording the corresponding cyanohydrin trimethylsilyl ethers in high yields (up to 85–99 %) in methanol and at room temperature. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Catalytic hydrocyanation and cyanosilylation of aldehydes or ketones are among the most important strategies for synthesis of cyanohydrin trimethylsilyl ethers in synthetic chemistry (Scheme 1) [1]. The cyanohydrin trimethylsilyl ethers are industrially valuable and important intermediates for the synthesis of many valuable molecules such as a-hydroxy aldehydes, a-hydroxy acids and b-amino alcohols and other biologically active compounds [1]. Hence, a number of catalysts such as Lewis acids [2–4], Lewis bases [5–7], N-heterocyclic carbenes [8,9], aminothiourea [10,11], organic–inorganic salts [12–16], nonionic bases [17,18], oxazaborolidinium ion [19,20], V-, Mn-, Al-, and Ti-salen complexes [21–24], chiral Ti-a,a,a,a-1,3-dioxolane-tetraaryl-4,5dimethanols [25,26], Ti,Al-phosphine oxide bifunctional catalysts with carbohydrate or binaphthol scaffolds [27–29], Ti,Al-N-oxide bifunctional catalysts with proline, pyrrolidine and 1,2-diamino ligands [30–32], metal organic frameworks [33–36], Cu(II) and Co (II/III) hydrazone complexes [37–40], etc. have been utilized. Based on the existing methods, it is clear that most of these protocols have many disadvantages, such as low yield, long reaction time,
⇑ Corresponding authors. E-mail addresses: [email protected], [email protected] (K.T. Mahmudov), [email protected] (A.J.L. Pombeiro). https://doi.org/10.1016/j.ica.2017.10.042 0020-1693/Ó 2017 Elsevier B.V. All rights reserved.
harmful solvents, etc. Therefore, the development of a new homogeneous catalytic system, which is inexpensive, commonly available, easy to handle and time resolved, is highly desirable. On the other hand, the design and synthesis of mixed ligand complexes have received considerable attention due to their structural diversity as well as their potential applications as functional materials in various fields such as catalysis, pharmacology, molecular recognition, molecular switches, etc. [41–44]. Up to now, a great number of mixed ligand copper(II) complexes with interesting structures and properties have been obtained by selecting the appropriate hydrazone (main) and auxiliary ligands [45]. However, the controllable synthesis of mixed ligand copper(II) complexes with desired catalytic properties is still a great challenge because there are many chemical and physical factors playing important roles in the synthetic operation, such as the molecular structures of the main ligands, acid-base properties of auxiliary ligands, reaction condition, etc. Therefore, the two main objectives of the current work are as follows: i) to synthesize of mixed ligand copper(II) complexes by using known sodium (Z)-2-(2-(1,3-dioxo-1-(phenylamino)butan2-ylidene)hydrazinyl)benzenesulfonate (NaH2L) [46] and axulliary components such as dimethylsulfoxide, 1,3,4-thiadiazol-2-amine, hexamethylenetetramine and 2-cyanopyridine; ii) to apply the derived mixed ligand copper(II) complexes as the homogeneous
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MS (ESI, positive ion mode), m/z: 520.06 [Mr 2H2O+H]+. IR (KBr): 3480 (s, br) m(OH), 3005 m(NH), 1610 (s) m(C = O), 1594 m (C = N) cm–1. Scheme 1. Cyanosilylation of aldehydes.
catalysts for the cyanosilylation reaction of aldehydes with trimethylsilyl cyanide. 2. Experimental 2.1. Materials and instrumentation All the chemicals were obtained from commercial sources (Aldrich) and used as received. The NaH2L was synthesized according to the reported procedure [46]. Infrared spectra (4000–400 cm 1) were recorded on a Vertex 70 (Bruker) instrument in KBr pellets. Carbon, hydrogen, and nitrogen elemental analyses were done using a ‘‘2400 CHN Elemental Analyzer” (Perkin Elmer). The 1 H and 13C NMR spectra were recorded at room temperature on a Bruker Avance II + 300 (UltraShieldTM Magnet) spectrometer operating at 300.130 and 75.468 MHz for proton and carbon-13, respectively. The chemical shifts are reported in ppm using tetramethylsilane as the internal reference. Electrospray mass spectra (ESI-MS) were run with an ion-trap instrument (Varian 500-MS LC Ion Trap Mass Spectrometer) equipped with an electrospray ion source. For electrospray ionization, the drying gas and flow rate were optimized according to the particular sample with 35 p.s.i. nebulizer pressure. Scanning was performed from m/z 0 to 1100 in methanol solution. The compounds were observed in the positive mode (capillary voltage = 80–105 V). 2.2. Synthesis 2.2.1. Synthesis of 1 0.1 mmol (37 mg) of NaH2L were dissolved in 25 mL of methanol, then 0.1 mmol (23 mg) of Cu(NO3)22.5H2O and 0.1 mL of dimethylsulfoxide were added and the system was stirred for 10 min. After ca. 3 d at room temperature, greenish crystals precipitated which were then filtered off and dried in air. 1: Yield, 53 % (based on Cu). Calcd. for C18H25CuN3O9S2 (Mr = 555.08): C 38.95, H 4.54, N 7.57; found C 38.82, H 4.38, N 7.49.
2.2.2. Synthesis of 2 0.1 mmol (37 mg) of NaH2L were dissolved in 25 mL of methanol, then 0.1 mmol (23 mg) of Cu(NO3)22.5H2O and 0.1 mmol (10 mg) of 1,3,4-thiadiazol-2-amine were added and the system was stirred for 10 min. After ca. 2 d at room temperature, greenish crystals precipitated which were then filtered off and dried in air. 2: Yield, 53 % (based on Cu). Calcd. for C18H18CuN6O6S2 (Mr = 542.05): C 39.88, H 3.35, N 15.50; found C 39.47, H 3.43, N 15.23. MS (ESI, positive ion mode), m/z: 543.01 [Mr + H]+. IR (KBr): 3333 (s, br) m(OH), 3078 m(NH), 1600 (s) m(C = O), 1556 m (C = N) cm–1. 2.2.3. Synthesis of 3 0.1 mmol (37 mg) of NaH2L were dissolved in 25 mL of methanol, then 0.1 mmol (23 mg) of Cu(NO3)22.5H2O and 0.1 mmol (14 mg) of hexamethylenetetramine were added and the system was stirred for 10 min. After ca. 3 d at room temperature, greenish crystals precipitated which were then filtered off and dried in air. 3: Yield, 53 % (based on Cu). Calcd. for C45H58Cu2N14O13S2 (Mr = 1194.25): C 45.26, H 4.90, N 16.42; found C 45.15, H 4.76, N 16.37. MS (ESI, positive ion mode), m/z: 582.1 [C22H27CuN7O6S + H]+. IR (KBr): 3410 (s, br) m(OH), 3134 m(NH), 1665 (s) m(C = O), 1600 m (C = N) cm–1. 2.2.4. Synthesis of 4 0.1 mmol (37 mg) of NaH2L were dissolved in 25 mL of methanol, then 0.05 mmol (12 mg) of Cu(NO3)22.5H2O and 0.1 mmol (10 mg) of 2-cyanopyridine were added and the system was stirred for 10 min. After ca. 3 d at room temperature, greenish crystals precipitated which were then filtered off and dried in air. 4: Yield, 53 % (based on Cu). Calcd. for C46H48CuN10O14S2 (Mr = 1092.61): C 50.57, H 4.43, N 12.82; found C 50.49, H 4.25, N 12.69. MS (ESI, positive ion mode), m/z: 185.8 [C14H20CuN4O4]2+ and 362.4 [C16H14N3O5S + 2H]+. IR (KBr): 3427 (s, br) m(OH), 3158 and 2986 m(NH), 1633(s) m(C = O), 1587 m(C = N) cm–1.
Table 1 Crystallographic data and structure refinement details for 1–4.
Empirical formula fw Temperature (K) Cryst. Syst. Space group a (Å) b (Å) c (Å) a, ° b, ° c, ° V (Å3) Z qcalc (g cm 3) l(Mo Ka) (mm 1) F (0 0 0) R1a (I 2r) wR2b (I 2r) GOOF a b
1
2
3
4
C18H25CuN3O9S2 555.07 293(2) triclinic P-1 8.5077(9) 11.4878(12) 13.0362(13) 71.172(4) 82.268(4) 76.584(4) 1170.5(2) 2 1.575 1.165 574 0.0289 0.0729 1.025
C18H18CuN6O6S2 542.04 296(2) monoclinic P21/c 9.5281(4) 9.2619(4) 25.1485(9) 90 94.0610(10) 90 2213.74(16) 4 1.626 1.224 1108 0.0430 0.1142 1.052
C45H58Cu2N14O13S2 1194.25 293(2) triclinic P-1 11.633(3) 14.183(3) 15.958(4) 87.861(9) 82.562(9) 89.164(9) 2608.8(11) 2 1.520 0.972 1240 0.0310 0.0753 1.032
C46H48CuN10O14S2 1092.60 293(2) triclinic P-1 8.2542(4) 10.7845(5) 14.1552(6) 78. 288(2) 81.389(2) 77.816(2) 1198.49(10) 1 1.514 0.622 567 0.0317 0.0808 1.009
R1 = R||Fo| – |Fc||/R|Fo|. wR2 = [R[w(F2o – F2c )2]/ R[w(F2o)2]]1/2.
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Table 2 Selected bond distances (Å) and angles (°) for 1–4. 1
2
3
4
Cu(1)-O(1)
1.9435(11)
Cu(1)-O(2)
1.908(2)
N(3A)-Cu(1A)
1.9618(18)
Cu(1)-N(5)#1
1.9733(12)
Cu(1)-N(3) Cu(1)-O(5) Cu(1)-O(2) Cu(1)-O(7) O(1)-Cu(1)-N(3) O(1)-Cu(1)-O(5) N(3)-Cu(1)-O(5) O(1)-Cu(1)-O(2) N(3)-Cu(1)-O(2) O(5)-Cu(1)-O(2) O(1)-Cu(1)-O(7) N(3)-Cu(1)-O(7) O(5)-Cu(1)-O(7) O(2)-Cu(1)-O(7)
1.9526(12) 1.9707(11) 1.9781(12) 2.2753(13) 90.05(5) 89.65(5) 171.89(6) 162.98(6) 93.75(5) 84.26(5) 105.83(5) 96.87(5) 91.01(5) 90.20(5)
Cu(1)-O(1) Cu(1)-N(3) Cu(1)-N(4) Cu(1)-O(5) O(1)-Cu(1)-O(2) O(1)-Cu(1)-N(3) O(2)-Cu(1)-N(3) O(1)-Cu(1)-N(4) O(2)-Cu(1)-N(4) N(3)-Cu(1)-N(4) O(1)-Cu(1)-O(5) O(2)-Cu(1)-O(5) N(3)-Cu(1)-O(5) N(4)-Cu(1)-O(5)
1.964(2) 1.973(2) 2.007(3) 2.356(3) 169.17(10) 89.55(10) 94.68(9) 88.11(11) 86.29(11) 171.69(12) 87.04(10) 102.72(10) 92.21(10) 95.64(12)
N(3B)-Cu(1B) N(4A)-Cu(1A) N(4B)-Cu(1B) O(1A)-Cu(1A) O(1B)-Cu(1B) O(2A)-Cu(1A) O(2B)-Cu(1B) O(3A)-Cu(1A) O(3B)-Cu(1B) O(1A)-Cu(1A)-N(3A) O(1A)-Cu(1A)-O(2A) N(3A)-Cu(1A)-O(2A) O(1A)-Cu(1A)-O(3A) N(3A)-Cu(1A)-O(3A)
1.9652(18) 2.3277(18) 2.3308(18) 1.9348(16) 1.9405(16) 1.9673(16) 1.9872(16) 1.9936(16) 1.9798(17) 90.38(7) 87.00(7) 168.59(8) 162.87(7) 94.45(7)
Cu(1)-N(5) Cu(1)-N(4)#1 Cu(1)-N(4) N(5)#1-Cu(1)-N(5) N(5)#1-Cu(1)-N(4)#1 N(5)-Cu(1)-N(4)#1 N(5)#1-Cu(1)-N(4) N(5)-Cu(1)-N(4) N(4)#1-Cu(1)-N(4)
1.9733(12) 2.0342(12) 2.0342(12) 180.0 81.38(5) 98.62(5) 98.62(5) 81.38(5) 180.0
1
2
3
4 Scheme 2. Schematic representation of 1–4.
2.3. Crystal structure determination X-ray diffraction patterns of 1–4 were collected using a Bruker SMART APEX-II CCD area detector equipped with graphitemonochromated Mo-Ka radiation (k = 0.71073 Å) at room temperature. Absorption correction was applied by SADABS [47,48]. The structure was solved by direct methods and refined on F2 by fullmatrix least-squares using Bruker’s SHELXTL-97 [49]. All nonhydrogen atoms were refined anisotropically. The details of the crystallographic data, selected bond distances and angles for 1–4 are summarized in Tables 1 and 2. Crystallographic data for the structural analysis have been deposited to the Cambridge Crystallographic Data Center (CCDC 1556148 for 1, 1556147 for 2, 1556145 for 3 and 1556146 for 4). Copy of this information can
be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Fax: (+44) 1223-336033; E-mail: [email protected] or www.ccdc.cam.ac.uk/data_request/cif). 2.4. General procedure for the study of catalytic activity In a typical cyanosilylation procedure, to a stirred solution of benzaldehyde (0.4 mmol), catalyst (1 4) (1–8 mol%) in dry solvent [THF, CH2Cl2 or MeOH; 2 mL] trimethylsilyl cyanide (TMSCN) (0.6 mmol) was added dropwise. The resulting solution was stirred continuously and the progress of the reaction was followed by TLC. After a certain time, the solvent was evaporated, and the residue was diluted with 3 mL water and extracted with diethyl ether (3 10 mL). After removal of
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133
Fig. 1. Crystal structures of 1–4 with partial atom numbering scheme. H-bond interactions are shown as dashed blue lines.
diethyl ether under vacuum, the extract was taken for analysis by 1H NMR spectroscopy in CDCl3, to evaluate the yield of the cyanohydrin trimethylsilyl ethers [37–40]. The adequacy of the procedure was confirmed by blank 1H NMR analyses with 1,2dimethoxyethane as an internal reference. The internal standard method proved that no side products were formed.
3. Results and discussion 3.1. Synthesis and characterization of 1–4 Sodium (Z)-2-(2-(1,3-dioxo-1-(phenylamino)butan-2-ylidene) hydrazinyl)benzenesulfonate (NaH2L) was reported earlier by us
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Table 3 Selection of catalyst for the cyanosilylation of benzaldehyde with TMSCN.a.
a b
Entry
Catalyst
Amount of catalyst (mol%)
Solvent
Yieldb (%)
1 2 3 4 5 6 7 8 9 10 11 12 13c 14 15 16 17 18 19 20
1
5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 – – – –
THF CH2Cl2 MeOH THF CH2Cl2 MeOH THF CH2Cl2 MeOH THF CH2Cl2 MeOH THF CH2Cl2 MeOH MeOH – THF CH2Cl2 MeOH
23.6 25.0 76.9 25.4 26.9 78.3 24.7 27.4 77.8 27.3 29.1 83.6 20.4 22.3 27.1 32.9 20.0 14.0 14.4 24.7
2
3
4
NaH2L
Cu(NO3)22.5H2O – – – –
Reaction conditions: solvent (2 mL), TMSCN (0.6 mmol) and aldehyde (0.4 mmol), in air at room temperature, 24 h reaction time. Determined by 1H NMR analysis of crude products. c Entries 13–20 were adapted from [39].
Table 4 Optimization of the parameters of the cyanosilylation reaction between benzaldehyde and TMSCN with catalyst 4.a.
a b
Entry
Time (h)
Amount of catalyst (mol%)
T (°C)
Yieldb (%)
1 2 3 4 5
1 4 8 12 24
5 5 5 5 5
25 25 25 25 25
43.7 66.7 83.6 83.6 83.6
6 7 8 9 10
8 8 8 8 8
1 3 5 7 9
25 25 25 25 25
47.4 73.3 83.6 86.1 86.2
11 12 13 14
8 8 8 8
7 7 7 7
15 35 45 55
79.8 86.8 87.4 88.1
Reaction conditions: MeOH (2 mL), TMSCN (0.6 mmol) and benzaldehyde (0.4 mmol), in air. Determined by 1H NMR analysis of crude products (see Experimental part).
[46], and hence will not be discussed herein. Reaction of Cu(NO3)22.5H2O with NaH2L in methanol in the presence of dimethylsulfoxide (A), 1,3,4-thiadiazol-2-amine (B), hexamethylenetetramine (C) and 2-cyanopyridine (D) led to the mixed ligand copper(II) complexes [Cu(HL)(H2O)(A)]2H2O (1), [Cu(HL)(H2O)(B)] (2), [Cu (HL)(H2O)(C)]1/2CH3OH (3) and [Cu(HR)2(H2O)2](H2L)2 (4) (HR = methyl picolinimidate derived from D), respectively (Scheme 2). The compounds 1–4 are characterized by ESI-MS, IR spectroscopy, elemental analysis and single crystal X-ray diffraction (see Tables 1 and 2). The IR spectra of 1–4 (see experimental section) shows significantly m shift in relation to the corresponding signals of NaH2L [3532 m(OH), 3098 m(NH), 1669 m(C = O) and 1563 m(C = N) cm 1)] [46]. Fragmentation peaks in mass spectra of the compounds can be related as follows: 520.06 [Mr 2H2O+H]+ (for 1), 543.01 [Mr + H]+ (for 2), 582.1 [C22H27CuN7O6S + H]+ (for 3), 185.8 [C14H20CuN4O4]2+ and 362.4 [C16H14N3O5S + 2H]+ (for 4), accounting for the existence of the monomeric mixed ligand units in solution. Elemental analyses and X-ray crystallography are also in agreement with the proposed formulations. In 1–3, the copper ion has a distorted square-pyramidal coordination sphere with the chelating HL2 and water (1,3,4-thiadiazol-2-amine in the case of 2) ligands
occupying the four equatorial positions and the apical site being engaged with an axulliary (dimethylsulfoxide and hexamethylenetetramine) ligand (Fig. 1). Whereas, in 4, the copper ion has a distorted octahedral geometry, where the basal plane is made of four N-atoms from two bidentate methyl picolinimidate ligands and the axial positions are occupied with two O-atoms from water molecules (Fig. 1). In all complexes, the carbonyl group acts as Hbond acceptor towards the amide group leading to a strong intramolecular hydrogen bonding with N O distance of ranging from 2.606 to 2.645 Å (Fig. 1). 3.2. Catalytic activity of 1–4 in cyanosilylation reaction Complexes 1–4 have been tested as homogeneous catalysts for the cyanosilylation reaction of benzaldehyde with cyanide (TMSCN), as the representative substrates, in different solvents (tetrahydrofurane, dichloromethane or methanol), at room temperature (Scheme 1, Table 3). As can be seen in Table 3 the reaction is promoted by 1–4 in methanol (entries 3, 6, 9 and 12, Table 3), the yield are much higher than those achieved with NaH2L and Cu (NO3)22.5H2O as well as in the absence of any catalyst or solvent
A.V. Gurbanov et al. / Inorganica Chimica Acta 471 (2018) 130–136 Table 5 Cyanosilylation of various aldehydes with TMSCN using catalyst 4a. Entry
Substrate
4. Conclusions Yield, %b
1
89.8
2
87.6
3
86.9
4
86.1
5
85.7
6
85.1
7 8 9
CH3CHO CH3CH2CHO CH3(CH2)4CHO
135
99.1 98.5 98.0
a Reaction conditions: 7 mol% of catalyst 3, MeOH (2 mL), TMSCN (0.6 mmol) and aldehyde (0.4 mmol). Reaction time: 8 h. b Determined by 1H NMR analysis of crude products (see Experimental part).
We have prepared four mixed ligand copper(II) complexes and applied for the cyanosilylation of aromatic and aliphatic aldehydes with TMSCN. Complex 4 with ionic character was found to be the more efficient catalyst for the cyanosilylation reaction in methanol, leading cyanohydrin trimethylsilyl ethers in high yields (85–99%) at room temperature in 8 h. Electron-donation substituent in the para position of the aromatic aldehydes inhibits the reaction yield, while an opposite effect is observed with the electron-withdrawing groups. Compared to other related homogeneous catalytic systems, our mixed ligand copper(II) complex exhibits higher catalytic activity at room temperature. Acknowledgements Authors are grateful to the Fundação para a Ciência e a Tecnologia: (project UID/QUI/00100/2013), Portugal, for financial support. GM acknowledges to the University of Maragheh for the financial support of this research. This work also was supported by the ’’RUDN University Program 5-100’’. Authors are thankful to the Portuguese NMR Network (IST-UL Centre) for access to the NMR facility and the IST Node of the Portuguese Network of mass-spectrometry for the ESI-MS measurements. References
free conditions (entries 13–20, Table 3). The reaction in MeOH leads to higher yields in comparison to THF and CH2Cl2, and in general the yield decreases in the order of polarity MeOH > CH2Cl2 > THF, for the studied solvents (entries 1–12, Table 3). The catalytic activities of all complexes are comparable in all studied solvents, 4 (ionic) being significantly more active than other ones. The higher activity of 4 may be due to the involvement of its cationic and anionic moieties in noncovalent interaction with TMSCN and benzaldehyde, respectively, which promotes reaction. Thus, we furthermore studied different effects using 4 as the catalyst and methanol as the solvent (Table 4). Increasing the reaction time up to 8 h led to a remarkable increase in yield (Table 4, entries 1–5), which did not increase considerable for longer times. The increase of the catalyst amount up to 7 mol % loading resulted also in a marked yield improvement (entries 6–10, Table 4), whereas a higher catalyst amount had a much lower effect. The temperature (in the 15–55 °C range) had not a crucial effect on the yield of cyanohydrin trimethylsilyl ether (entries 11–14, Table 4). Having the reaction conditions optimized, we investigated the reactivity of a variety of aldehydes, and the appropriate cyanohydrin trimethylsilyl ethers were generally obtained in good yields (Table 5). Aromatic aldehydes bearing electron-withdrawing groups (bromo, chloro or nitro) exhibit relatively higher reactivities (Table 5, entries 1–3) as compared to those having electrondonating substituents (methoxy or methyl) (Table 5, entries 5 and 6), conceivably resulting from an increase of the substrate electrophilicity in the former case. Aliphatic aldehydes, such as acetaldehyde, propionaldehyde and hexanal (Table 5, entries 7– 9), have also been used as substrates for this reaction. The aldehyde conversion slightly decreases with the increase of the size of the aliphatic fragment (99% for acetaldehyde relatively to 98% for hexanal). The obtained yield in our system are usually higher than the other reported Cu(II) (78%) [39] and (27%) [50], Co(II) (78%) [40], Zn(II) (30%) [51], etc. complexes. Our system have several advantages such as a short reaction time (8 h) and room temperature in comparison some systems, which a longer reaction time (96 h) [52], higher (40 °C) [50] or lower (even negative) (-50 °C) [53] temperatures are required.
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