Chemistry and catalysis advances in organometallic chemistry and catalysis
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227 228 VANADIUM(IV) COMPLEXES DERIVED FROM AROMATIC O-HYDROXYALDEHYDES AND TYROSINE DERIVATIVES N N
N N O V O HO OH NH N O N H N O V O HO OH V IV O(pyren) V IV O(pyran) Scheme 17.1 Molecular formulae of the water-soluble V IV O(pyren) and V IV O(pyran) [12]. NH O
HO OH NH O OH OH OH NH O O N HO OH HO NH O OH N OH HO OH sal- L -Tyr 1 sal-o-Tyr 2 pyr-
L -Tyr 3 pyr-o-Tyr 4 NH O OH OH O OH NH O O BnO
OH NH O O BnO
OH O NH O O OH O V O Solvent o -Van- O -Tyr 5 sal-O-Bn- L -Tyr 6 o-van-O-Bn- L -Tyr 7 VO(sal-o-Tyr) 8 NH O O O V O HO Solvent NH O O O V O HO O Solvent NH O O O V O BnO Solvent
V IV O(sal- L -Tyr) 9 V IV
L -Tyr) 10 V IV
L -Tyr) 11 NH O
O V O BnO O Solvent NH O O O V O O Solvent
PS NH O O O V O O Solvent PS O
IV O(o-van-O-Bn- L -Tyr)
13 PS-[V IV O(sal- L -Tyr)]
14 PS-[V IV O(o-van- L -Tyr)]
Scheme 17.2 Molecular formulae of the ligand precursor and V IV O-compounds prepared. Note that Tyr corresponds to l-Tyr and o-Tyr to the racemic mixture d,l-o-Tyr. The polystyrene polymer is represented by the circle with PS written inside. RESULTS AND DISCUSSION 229 17.2 RESULTS AND DISCUSSION 17.2.1 Synthesis and Characterization Globally following the method described in [17], the reduced SBs 1–7 depicted in Scheme 17.2 were prepared in alcoholic medium by the condensation of 1 equiv of the appropriate aldehyde with 1 equiv of amino acid in the presence of an equivalent of KOH. Treatment of these condensation products with NaBH 4 resulted in the reduction of the imine bonds, yielding the reduced SBs. Five neat V IV O-complexes: 8–12 (Scheme 17.2) were also prepared. The IR spectra of the solid V IV O-complexes present the characteristic ν(V=O) bands at 950–1000 cm −1 . The electron paramagnetic resonance (EPR) parameters for the five V IV O-compounds are listed in Table 17.1. For 9, 10, 11, and 12, the parameters are consistent with either a (COO − , N amine , O
− phenolato , O ROH
) eq or a (COO – , N
amine , O
− phenolato , O DMF
) eq donor set in methanol and dimethyl formamide (DMF) solutions, respectively. The PS-supported versions of compounds 9 and 10 (PS-[V IV O(sal-l-Tyr)] 13 and PS-[V IV O(o-van-l-Tyr)] 14, Scheme 17.2) were prepared according to the synthetic procedure shown in Scheme 17.3. The chloromethylated PS is firstly treated with the K + salt of l-Tyr to yield a tyrosine-modified PS-resin. The free amine groups therein are then condensed with the appropriate aldehyde and the respective imine is reduced with NaBH 4 . The resulting resin is finally reacted with V IV O(acac) 2 to form the desired heterogeneous V IV O-compound. The PS-supported compounds were characterized mainly by room temperature EPR. The obtained EPR powder spectra of 13 and 14 are characteristic of magnetically diluted V IV O-complexes and the well-resolved EPR pattern indicates that the vanadium centers are well dispersed in the polymer matrix. The spectra were simulated [18] and the spin Hamiltonian parameters obtained (Table 17.1) TABLE 17.1 Experimental Spin Hamiltonian Parameters for the Prepared Homogeneous (Measured at 77 K) and Heterogeneous V IV O-Compounds at Room Temperature) Compound
g x , g y A x , A y , ×10 4 cm –1 g z
z ,
4 cm –1 Predicted Equatorial Donor Set 9 a 1.978, 1.981 51, 60 1.951
162 O COO − , N
amine , O
ArO − , O ROH 10 a 1.980, 1.978 62, 59 1.948
169 O COO − , N
amine , O
ArO − , O ROH N amine , O ArO
− , O
ROH , O
ROH 11 b 1.979, 1.979 53, 58 1.950
163 O COO − , N
amine , O
ArO − , O DMF 12 b 1.979, 1.978 58, 56 1.949
164 O COO − , N
amine , O
ArO − , O DMF 13 c 1.983, 1.978 66, 60 1.945
170 N amine , O ArO
− , O
water , O
water 14 c 1.983, 1.976 64, 62 1.947
168 O COO − , N
amine , O
ArO − , O water a EPR spectrum measured in methanol. b EPR spectrum measured in DMF. c EPR spectrum measured with the neat solid at room temperature. Cl PS O OK H 2 N KO O OK H 2 N O PS O O NH O PS O V O Solvent O OK NH KO OH OH O 1. 2. NaBH 4 3. V
IV O(acac)
2 THF/MeOH, reflux 72 h 1. THF/MeOH, reflux 72 h 2. V
IV O(acac)
2 Scheme 17.3 General synthetic procedures used to obtain polystyrene-anchored compound 13 (and 14). The circle represents the polystyrene matrix.
230 VANADIUM(IV) COMPLEXES DERIVED FROM AROMATIC O-HYDROXYALDEHYDES AND TYROSINE DERIVATIVES were compared with those for 9–12. The hyperfine coupling constants A z for the PS-supported complexes are slightly higher than those found for their homogeneous counterparts. Nevertheless, it is possible to identify the donor set of existing V IV O- species bound within the polymer resin and, by extension, to confirm the successful immobilization of the tyrosine-based ligand. Additionally thermogravimetric analysis of 13 and 14 gave the amount of V in the resin, the obtained V-loadings being 1.1 mmol V/g for 13; 0.9 mmol V/g for 14.
While the side-chain phenolic moiety of l-Tyr is not in a coordinative position relative to the metal center, deprotonation and coordination of the side-chain phenol to another metal center is possible, especially if the pH is within the range 9–10. The preparation of V IV O(sal-O-Bn-l-Tyr) 11 gave no problems, but with V IV O(o-van-O-Bn-l-Tyr) 12, the final product was unexpectedly water soluble. The solid nature of the PS-supported compounds 13 and 14 does not allow their adequate characterization by the usual spectroscopic methods. Nevertheless, EPR characterization alone provided valuable information regarding the entire preparation procedure. With A z values of 170 and 168 × 10 −4 cm −1 for 13 and 14, respectively, there is strong indication that the sole existing V IV O-species within the resin are those with the expected (O COO − , N amine , O
ArO − , O water
) eq donor group set. In the IR of 13 and 14 the ν(V=O) are detected at 955 and 995 cm −1 , respectively, and medium- to-strong bands are detected at 1625 and 1610–1700 cm −1 , assignable to ν(C–O phenolato ) and ν
(COO), respectively. These compounds were tested as catalysts in the asymmetric sulfoxidation of thioanisole, using H 2 O
as oxidant. The results obtained using both the homogeneous and heterogenized compounds are listed in Tables 17.2 and 17.3. Control reactions made to test the oxidation of thioanisole in the absence of catalyst gave very low conversions to sulfoxide even after a 24 h period at room temperature: 9% in 1,2-dichloroethane (DCE), 3% in acetone, 2% in ethyl acetate, 2% in acetonitrile, 0% of sulfone in all control reactions. In general, the obtained enantiomeric excesses were very low. Nearly complete conversions were obtained with 11 in acetone and ethyl acetate, but no enantioselectivity was observed (entries 1 and 2, Table 17.2). TABLE 17.2 Sulfoxidation of Thioanisole With the Homogenous V IV O Catalyst Precursors a Entry Catalyst Solvent
b t, h Conversion, % ee, % b
1 11 (CH
3 ) 2 CO 24 99 0 5
11 AcOEt
24 >99
0 6
12 DCE
24 0 0 0 4 12 (CH
3 ) 2 CO 24 41 26 2
12 (CH
3 ) 2 CO 48 63 26 4
12 EtOH
24 8 0 0 7 12 AcOEt
24 96 3 11 a Conditions: 4 ml of solvent; nS = 1 mmol; nH 2 O 2 : nS = 1.2; 1 mol% of catalyst; T = 0 ◦ C; DCE, 1,2-dichloroethane. b AcOEt, ethyl acetate; DCE, 1,2-dichloroethane; EtOH, ethanol; S, substrate; ee, enantiomeric excess. TABLE 17.3 Sulfoxidation of Thioanisole With V IV O Catalyst Precursors 13 and 14 a Entry Catalyst Solvent
mol% Catalyst Cycle
Conversion, % ee, %
Sulfone, % 1 13 AcOEt
3.6 1 38 0 5
13 (CH
3 ) 2 CO 1.4
1 0 0 0 3 13 DCE
1.4 1 37 0 5
b
H 2 O 3.3
1 37 0 0 5 14 AcOEt
5.5 1 99 0 12 2 99 0 13 3 99 0 11 4 99 0 10
14 AcOEt
1.7 1 59 0 8
14 (CH
3 ) 2 CO 3.3
1 91 0 8 8 14 DCE
1.7 1 36 0 5 a 4 ml of solvent; nS = 1 mmol, nH 2 O
: nS = 1.2, 4 ml of solvent. Reaction time = 24 h. b Reaction was run at 5 ◦ C.
REFERENCES 231 Only 12 exhibited appreciable enantioselectivity in acetone (entries 4 and 5, Table 17.2), although the reaction was not complete even after a 48 h period. This catalyst showed no activity in DCE (entry 3, Table 17.2), very low activity in ethanol (entry 6, Table 17.2) but demonstrated good activity in ethyl acetate (entry 7, Table 17.2). None of the tested PS- supported V IV O-compounds exhibited enantioselectivity. Compound 13 exhibited lower activity in ethyl acetate and acetone when compared to its homogeneous variant 11, while exhibiting some activity in DCE (entries 1, 2 and 3, Table 17.3). Compound 14 exhibited better catalytic characteristics. For instance, the sulfoxidation could be carried out in water (entry 4, Table 17.3). High conversions and relatively low sulfone percentages were obtained in ethyl acetate and these results were achieved even after five consecutive recycling procedures (entry 5, Table 17.3). Lowering the catalyst loading from 5.5 to 1.7 mol% resulted in lower conversions under the same conditions (entry 6, Table 17.3). Replacing ethyl acetate by acetone and lowering the catalyst loading from 5.5 to 3.3 mol% resulted in a high conversion and a slightly lower sulfone formation (entry 7, Table 17.3). In DCE, 14 performed comparably to 13 although the catalyst loading was slightly higher in the former case (entry 8, Table 17.3). Globally, the performance of the PS-supported V IV O-complexes is good and at least equivalent to their homogeneous counterparts, with the advantage of being easily recyclable. However, no enantioselectivity was obtained. The reason for this is not clear. It is know that vanadium may be very active in the racemization of amino acids in the presence of aromatic aldehydes [19 and refs. therein], so it is possible that during the step of addition of V IV O(acac) 2 to the PS-supported ligand, which takes place at 50 ◦ C for 24 h, the l-Tyr moiety racemizes, the actual compounds being then better designated as PS-[V IV O(sal-d,l-Tyr)] (13) and PS-[V IV O(o-van-d,l-Tyr)] (14). Studies will be carried out in the future to evaluate the existence and extent of vanadium-mediated racemization of homogenous analogues, 11 and 12.
The PS-supported catalysts are stable, active, and easily recyclable, not losing activity upon use at least after four catalytic cycles. In this respect, they are quite promising products, representing positive steps toward achieving more sustainable sulfoxidation procedures, despite the disappointing enantioselectivities. Globally, the reported catalysts are quite stable and showed better performance in acetone and/or ethyl acetate relative to DCE in all aspects. Water could be used as solvent, moderate conversions being obtained. Complex 12 exhibits better activity and enantioselectivity in sulfoxidation compared to earlier l-alanine- and l-lysine-derived V IV O(SB) complexes [20], while requiring less toxic solvents such as acetone and environmentally benign oxidants such as hydrogen peroxide. ACKNOWLEDGMENTS The authors thank Fundac¸˜ao para a Ciˆencia e Tecnologia (FCT), PEst-OE/QUI/UI0100/2013; SFRH/BPD/79778/2013, and the Portuguese NMR and MS Networks (IST-UTL Centers).
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3. Jakusch, T.; Hollender, D.; Enyedy, E. A.; Gonzalez, C. S.; Montes-Bayon, M.; Sanz-Medel, A.; Costa Pessoa, J.; Tomaz, I.; Kiss, T. Dalton Trans. 2009, 2428. 4. Sanna, D.; Micera, G.; Garribba, E. Inorg. Chem. 2009, 48 , 5747. 5. Davies, D. R.; Interthal, H.; Champoux, J. J.; Wim, G. J.; Hol, W. G. J. Chem. Biol. 2003, 10 , 139. 6. Rehder, D.; Hoist, H.; Priebsch, W.; Vute, H. J. Inorg. Biochem. 1991, 41 , 171. 7. Cavaco, I.; Costa Pessoa, J.; Costa, D.; Duarte, M. T.; Gillard, R. D.; Matias, P. M. J. Chem. Soc., Dalton Trans. 1994, 149. 8. Costa Pessoa, J.; Cavaco, I.; Correia, I.; Duarte, M. T.; Gillard R. D.; Henriques, R. T.; Higes, F. J.; Madeira, C.; Tomaz, I. Inorg. Chim. Acta 1999, 293 , 1. 9. Dutta, S.; Mondal, S.; Chakravorty, A. Polyhedron 1995, 14 , 1163. 10. Cavaco, I.; Costa Pessoa, J.; Duarte, M. T.; Henriques, R. T.; Matias, P. M.; Gillard, R. D. J. Chem. Soc., Dalton Trans. 1996, 1989. 11. Fulwood, R.; Schmidt, H.; Rehder, D. J. Chem. Soc., Chem. Commun. 1995, 1443. 232 VANADIUM(IV) COMPLEXES DERIVED FROM AROMATIC O-HYDROXYALDEHYDES AND TYROSINE DERIVATIVES 12. Correia, I.; Costa Pessoa, J.; Duarte, M. T.; Henriques, R. T.; Piedade, M. F. M.; Veiros, L. F.; Jakusch, T.; Dornyei, A.; Kiss, T.; Castro, M. M. C. A.; Geraldes, C. F. G. C.; Avecilla, F. Chem. -Eur. J. 2004, 10 , 2301. 13. Koh, L. P.; Ranford, J. O.; Robinson, W. T.; Svensson, J. O.; Tan, A. L. C.; Wu, D. Inorg. Chem. 1996, 35 , 6466. 14. Maurya, M. R.; Khurana, S.; Rehder, D. Trans. Met. Chem. 2003, 28 , 511. 15. Costa Pessoa, J.; Correia, I.; Kiss, T.; Jakusch, T.; Castro, M. M. C. A.; Geraldes C. F. G. C.; J. Chem. Soc., Dalton Trans. 2002, 4440.
16. Costa Pessoa, J.; Marc˜ao, S.; Correia, I.; Goncalves, G.; Dornyei, A.; Kiss, T.; Jakusch, T.; Tomaz, I.; Castro, M. M. C. A.; Geraldes, C. F. G. C.; Avecilla, F. Eur. J. Inorg. Chem. 2006, 3595. 17. Ad˜ao, P.; Costa Pessoa, J.; Henriques, R. T.; Kuznetsov, M. L.; Avecilla, F.; Maurya, M. R.; Kumar, U.; Correia, I. Inorg. Chem.,
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18 MICROWAVE-ASSISTED CATALYTIC OXIDATION OF ALCOHOLS TO CARBONYL COMPOUNDS Yauhen Yu. Karabach, Maximilian N. Kopylovich*, Kamran T. Mahmudov, and Armando J. L. Pombeiro*
Aldehydes and ketones, which are produced by different methods and starting materials, are important carbonyl compounds that are widely used as such and for further processing [1]. For example, cyclohexanone can be prepared upon cyclohexane oxidation, dehydrogenation of cyclohexanol at high temperature, or via catalytic oxidation of cyclohexanol under milder conditions. Acetophenone can be produced by the Hock process or be obtained via ethylbenzene oxidation with dioxygen, whereas benzaldehyde is produced industrially by toluene oxidation or by hydrolysis of benzal chloride in the presence of different acids and/or metal salts (i.e., tin(II), tin(IV), iron, or zinc chlorides) [1]. Although the methods of production of bulk chemicals often suffer from low selectivity and/or productivity per pass, they continue to be the major methods for the synthesis of carbonyl compounds as the formed by-products are also demanded chemicals. However, such protocols are not economically attractive for the production of specific carbonyl compounds in the fine chemicals industry, where the selective and effective oxidation of valuable starting materials (e.g., alcohols) to the corresponding carbonyl compounds is required. Hence, the development of alcohol oxidation methods in fine chemicals industry directs toward achieving highly selective, efficient, and environmentally friendly catalytic systems [2]. Thus, the oxidation of alcohols with stoichiometric amounts of inorganic oxidants, such as chromium(VI) or manganese compounds, is being substituted for the TEMPO-mediated aerobic (TEMPO–2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl) [3], metal-catalyzed oxidations with hydrogen- or organoperoxides [4], or hydrogen-transfer reactions catalyzed by rhodium or ruthenium complexes [5]. Nevertheless, the search for new, environmentally friendly, and efficient alcohol oxidation protocols, as well as the improvement of the known ones, is of current interest. In this respect, leverage points for research concern the application of green oxidants, green solvents, or solvent-free processes, development of new catalytic systems, use of energy efficient heating methods, etc. Microwave (MW) heating is one of the simplest but effective ways to enhance energy efficiency and productivity in small-scale chemical production [6]. In contrast to conventional heating (CH), MW irradiation interacts directly with the polar molecules (which can be of substrates, intermediates, or catalysts) and, if nonpolar solvents or solvent-free processes are used, it can provide an efficient energy transfer directly to the reactants [7]. Generally, it is assumed that MW irradiation itself can accelerate the reaction rate, on account of (i) thermal and (ii) nonthermal effects, namely, (i) local overheating (“hot spot” effect, etc.); (ii) influence on the preexponential factor A in the Arrhenius equation k = A exp(− G = /RT), and on the −T S = part in the G = = H = − T S = equation for the activation energy [7, 8]. Nevertheless, these effects, especially the nonthermal ones, are still disputed and it has been claimed [9] that there is no direct evidence yet for nonthermal effects. Recent studies of MW-assisted homogeneous reactions have shown that external infrared (IR) temperature controllers that Advances in Organometallic Chemistry and Catalysis: The Silver/Gold Jubilee International Conference on Organometallic Chemistry Celebratory Book, First Edition. Edited by Armando J. L. Pombeiro. © 2014 John Wiley & Sons, Inc. Published 2014 by John Wiley & Sons, Inc.
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