New catalysts for the conversion of urea into carbamates and carbonates with C1 and C2 alcohols

Studies in Surface Science and Catalysis 153 S.-E. Park, J.-S. Chang and K.-W. Lee (Editors) ©2004 Published by ElsevierB.V.

213

New catalysts for the conversion of urea into carbamates and carbonates with Cl and C2 alcohols Michele Aresta*a, Angela Dibenedetto", Cira Devitaa, Olga A. Bourovab, Oleg N. Chupakhinb a

Department of Chemistry and METEA Research Center, Universita degli Studi di Bari, via Celso Ulpiani 27, 70126 Bari, Italy; b

Ural State Technical University, 19, Mira St., 620002, Ekaterinburg, Russia

The use of urea as an active form of carbon dioxide is a way to substitute phosgene in the chemical industry. This approach is particularly useful when the synthesis of urethanes or organic carbonates is considered. So far, only long chain alcohols have been successfully used in such reactions. In this paper, new catalysts are described that are able to convert urea into carbamates and carbonates, using either methanol or ethanol to afford dimethyl- and diethylcarbonate, respectively. In the best conditions, 92% conversion of urea into carbamate has been observed, using either methanol or ethanol. The resulting urethanes can be further converted into the relevant carbonates in a second step. The two step reaction reveals to be particularly useful as it makes easier the separation process. 1. INTRODUCTION Nowadays, many research efforts are directed towards developing synthetic technologies that may replace toxic chemicals such as phosgene and dimethyl sulfate (dms) with environmentally friendly reagents in the chemical industry. Alkylcarbonates are good candidates for a number of applications and, among them, dimethyl and diethyl carbonates are receiving the greatest attention. Dimethylcarbonate (DMC) is a versatile reagent that can replace dms in methylation [1] and phosgene in carboxymethylation reactions [2], for the synthesis of Pharmaceuticals, agrochemicals, dyes and polymers. It can also function as a polar solvent or additive for fuels [3]. Diethyl carbonate (DEC) is an excellent solvent in the mid-boiling range and an intermediate for the synthesis of various pharmaceuticals. Also, both carbonates are used in transesterification reactions for the production of aromatic-carbamates and -carbonic acid esters such as diphenyl carbonate, or cyclic carbonates [4]. Carbamates are important final products and intermediates in the synthesis of fertilisers, pesticides and isocyanates, that are usually prepared by reacting alcohols with phosgene [5]. Recently, new methods for the production of DMC have been developed that do not employ phosgene and are based on the oxidative carbonylation of methanol with either copper catalysts [6], or also using palladium(II) as catalyst and NO as a promoter [7]. These methodologies suffer of some drawbacks essentially linked to the use of either chlorides, that may attack the metal parts of the reactors, or NO, that may form explosive mixtures with air. Therefore, alternative technologies are sought that may use safer operative conditions, such as those based on the use of CO2 for the direct carboxylation of alcohols [8-10]. This approach

214

appears quite interesting, but suffers thermodynamic limitations [10]. Among the innovative methodologies that have a low environmental impact, the use of urea appears quite interesting. As a matter of fact, urea can be considered an active form of carbon dioxide and has been shown to react with alcohols via a urethane intermediate [Eq. (1)] [11] ROH O=C(NH2)2

ROH • H2NCOOR

-NH3

• O=C(OR)2

(1)

-NH3

Essentially, primary or secondary long-chain alcohols have been used so far to form in a first step a urethane using metal salts like zinc acetate and lead acetate [12] as catalysts. The prolongation of the reaction or the addition of triphenylphosphine as cocatalyst leads to the formation of carbonates. [Eq. (2)] The second step may have both a low yield and selectivity because of a concurrent reaction in which the urethane is implied. The formation of isocyanuric acid is a major drawback.

g f ' O=C(OR)2 + NH3 H2NCOOR—

(2) R0H

> HNCO

->1/3(HNCO)3

As Eq. 2 shows, the urethane may decompose before reacting with the alcohol to form isocyanic and isocyanuric acid [12], which is observed using catalytic systems such as zinc acetate/triphenylphosphine. According to the literature data such reaction occurs at a much less extent when aluminium or cobalt species are used in combination with the suitable cocatalysts [11]. As reported above, the majority of papers describes the use of long chain alcohols such as 2ethyl-hexanol, 1-pentanol, cyclohexanol, 3-methylhexanol, isononyl alcohol, 6-methyl-2heptyl alcohol and recently the use of glycols has been demonstrated to afford cyclic carbonates [13]. Dibutyltin oxide, dibutyltin dimethoxide, diisobutylaluminum hydride/tributyl phosphite and triphenyltin chloride have been used as catalysts with an appreciable yield of carbonates [4]. So far, only very little information has been made available in the literature about the alcoholysis of urea with methanol or ethanol. Such reactions are of great importance in view of the large industrial use that DMC and DEC may have or considering their use as fuel additives. The objective of this work was to develop and to explore the role of new catalysts for the conversion of urea into DMC and DEC by reaction with methanol or ethanol, respectively. For comparison, we have used either Zn(acetate)2 or other catalysts analogous of those described in the literature to be active with long-chain alcohols. The new metal complexes used as catalysts in this work bear the ligand 3-(pyridyl-2)-5-cyano-6-phenyl-l,2,4 triazine or its derivatives. 1,2,4-triazines are the analogous of bipyridines that find an extended use in catalysis [14]. Triazines easily form complexes with transition metals. The synthesis and characterization of new complexes of 3-(pyridyl-2)-6-(4-tolyl)-1,2,4 triazine and 6-cyano-5phenyl-2(2'-pyridyl)-3,4-cyclo-pentenopyridine with zinc (II) chloride, of 3-(pyridyl-2)-5cyano-6-phenyl-1,2,4 triazine with copper (I), iron (II), zinc (II) chlorides, and of 3-(pyridyl-

215 2)-5-(2,2,2-tri-fluoroethoxy)-6-phenyl-l,2,4,-triazine and 3-(pyridyl-2)-5-(2,2,3,3-tetrafluoropropoxy)-6-phenyl-l,2,4-triazine with nickel(II) chloride are also reported. 2. EXPERIMENTAL PART 2.1. Materials All air sensitive compounds were manipulated under an atmosphere of dinitrogen. The catalytic reactions under pressure of CO2 were run in a stainless-steel autoclave of 100 mL. The gases N 2 (99.99 %) and CO2 (99.99 %) were purchased from Rivoira. Vacuum-inert-gas lines and syringe-techniques were utilised for handling air-sensitive compounds. The heterocyclic ligands of 1,2,4-triazine were synthesised according to the literature [15]. All other chemicals were commercial products. Solvents were purified following the standard literature procedure [16] and stored under nitrogen. IR spectra in the range 4000-200 cm"1 in Nujol with Csl or KBr plates were recorded on a Perkin-Elmer Model 883 spectrometer. GC analyses of carbamates and carbonates were performed using a Hewlett Packard HP 6850 (capillary column: 30 m, MDN-5S, layer film 0.25|j.m, FID) instrument and GC-MS analyses were carried out using a Shimadzu GCMS-QP 5050 (capillary column: 60 m, MDN-5S, layer film 0.25um) apparatus. The C, H, N elemental analyses were obtained using an elemental analyser mod. 1106 Carlo Erba. Metals were determined using the Perkin Elmer Atomic Absorption Spectrometer 3110. The chlorine analysis was carried out by means of combustion of a weighted sample followed by titration of the chloride with AgNC-3 10"2 M using a E436 Metrohm Herisau Potentiograph. *H NMR spectra of ligands were recorded on a Bruker 250 spectrometer and those of the complexes on a Varian XL 200 MHz instrument. 2.2. General method for the synthesis of complexes with the ligands: 6-cyano-5-phenyl-2(pyridyl-2')-3,4-cyclopentenopyridine (cppc), 3-(pyridyl-2')-6-(4-tolyl)-l,2,4-triazine (ptt) and 5-cyano-6-phenyl-3-(pyridyl-2')-l,2,4 triazine (cppt) To a solution of the ligand (1 mmol) in boiling dry acetonitrile under N2, the solution of ZnCl2 or CuCl (1 mmol) in dry acetonitrile was added dropwise. The mixture was refluxed for one hour under stirring. After cooling, a white solid was obtained, that was separated and washed with cold acetonitrile. The isolated complex was characterized by IR, 'H NMR, and C, H, Cl, N, Zn or Cu elemental analysis as reported below. 2.2.1. Characterization of ZnCl2(cppc) White crystals, (yield: 0.13g, 32.59%). IR (Nujol, KBr) 2240 (u, O N ) , 1535 (u, C=N) cm"'. 'H NMR (DMSO-d6,): 5 2.02-2.2 (m, 2H, Hcydopenten), 2.82-2.9 (m, 2H, Hcyciopenten), 3.53-3.6 (m, 2H, Hcyciopen,en), 7.46-7.53 (m, 6H), 7.95-8.03 (m, 1H, Hheteroaromatic), 8.22-8.26 (d, 1H, Hheteroaromatic), 8.71-8.73 (m, 1H, Hheteroaromatic). Anal. Calc. for CzoH^ClzNjZn: C=55.39 %; H=3.48%, Cl=16.35%, N=9.69%, Zn=15.07. Found: C=54.98%, H=3.58%, Cl=16.83%, N=9.47,Zn=15.0%. 2.2.2. Characterization ofZnCl2(ptt) Yellow crystals, (yield: 0.3087g, 68.26%). IR (Nujol, Csl) 1580 (u, C=N), 350, 332 (u, ZnCl) cm'1. 'H NMR (DMSO-d6): 8 2.41-2.46 (s, 3H, Hmethyi), 7.42-7.46 (d, 2H, Haromatlc), 7.637.66 (m, 1H), 8.03-8.21 (m, 3H), 8.48-8.52 (m, 1H, Hheteroaromatic), 8.82-8.84 (m, 1H, Hheteroaromatic), 9.52 (s, 1H, Hheteroaromatic). Anal. Calc. for C15H12Cl2N4Zn: C=46.85; H=3.14; Cl=18.44, N=14.57, Zn=17.0. Found: C=47.45, H=3.31, Cl=18.15; N=14.11, Zn=17.13 %.

216

2.2.3. Characterization ofCuCl(cppt) Violet crystals, (yield: 0.3525g; 51.02%). IR (Nujol, KBr) 2228 (u, O N ) , 1591 (u, C=C), 1526 (o, C=N) cm"1. Anal. Calc. for C15H9ClCuN5: C=50.28, H=2.53, Cl=9.9, N=19.54, Cu=17.73. Found: C=49.92, H=2.81, Cl=9.7, N=18.9, Cu=17.2 %. 2.3. Synthesis and characterization of FeCl2(cppt) 5-Cyano-3-(pyridyl-2)-1,2,4 triazine (0.5 g, 2 mmol) was dissolved in boiling dry acetonitrile under N2, the solution of FeCl2*1.5 THF (0.453 g, 2 mmol) in dry acetonitrile was added dropwise. The solution was refluxed under stirring for twenty minutes. The solvent was evaporated using a vacuum pump and the residual solid was washed with toluene and filtered to afford of 0.3958 g (yield=53.14 %) of the FeCl2(cppt) complex. Dark blue crystals. IR (Nujol, KBr) 2234 (u, O N ) , 1606 (u, C=C), 1578 (u, C=N), 1532 (o, C=N) , 1498 (u, C=N) cm"1. Anal. Calc. for C15H9C12FeN5: C=46.67, H=2.35, Cl=18.37, Fe=14.47, N=18.14. Found: C=46.08, H=2.63, Cl=17.9, Fe=14.5, N=18.05 %. 2.4. Synthesis and characterization of ZnCl2(cppt)2 To the boiling solution of 5-cyano-6-phenyl-3-(pyridyl-2')-l,2,4 triazine (0.5 g, 2 mmol) in dry acetonitrile under an atmosphere of nitrogen, the solution of ZnCl2 (0.263 g, 2 mmol) in dry acetonitrile was added dropwise. The reaction mixture was refluxed under stirring for 1.5 hours. The obtained solution was concentrated and kept at a low temperature (243 K) for one night. The formed precipitate was washed with cold acetonitrile (253 K) and isolated by filtration. After drying, 0.3187 g (yield=25.24 %) of the ZnCl2(cppt)2 were obtained. Brown crystals: IR (Nujol, KBr) 2249(u, O N ) , 1599(u, C=C), 1577(u, C=N), 1509(u, C=N), 1451(u, C=N) cm"1. Anal. Calc. for C3oHi8Cl2NioZn: C=55.03, H=2.77, Cl=10.8, N=21.39, Zn=21.39. Found: C=55.39 , H=2.55, Cl=11.4, N=15.52, Zn=20.8 %. 2.5. General method for the synthesis of complexes of 6-phenyl-3-(pyridyl-2)- 5-(2,2,2trifluoroethoxy)-l,2,4-triazine (pptt) and 6-phenyl-3-(pyridyl-2)-5-(2,2,3,3tetrafluoropropoxy)-l,2,4,-triazine (pptot) with nickel (II) chloride NiCl2 (0.26 g, 2 mmol) was dissolved in boiling absolute methanol (50 mL) under an atmosphere of nitrogen. The solution of the ligand (pptt or pptot) (2 mmol) in absolute methanol (20 mL) was added dropwise. The reaction mixture was refluxed under stirring for 45 minutes. The obtained solution was concentrated and kept at 243 K for one night. The unreacted nickel (II) chloride was removed by filtration. The solvent was evaporated under vacuum, and the residual solid was separated and washed with toluene to afford the relevant complex. 2.5.1. Characterization of NiCl2(pptt) Dark green crystals. Yield: 0.6892 g (74.23%) IR (Nujol, KBr) 1604(u, O C ) , 1577(u, O N ) , 1531(u, O N ) , 1495(u, O N ) , 1278(u, Cheteroaromatic-O), 1164(u, Caliphatic-O) cm"1. Anal. Calc. for Ci6H11Cl2F3N4Ni0: O41.61, H=2.4, N=12.13, C1=15.35, Ni=12.70. Found: 0 4 1 . 0 2 , H=2.39, N=12.21, Cl=15.68, Ni=12.87 %. 2.5.2 Characterization ofMCl2(pptot) NiCl2 (pptot) was obtained in a similar way from NiCl2 (2 mmol) and pptot (2 mmol). Green crystals. Yield: 0.48135 g (59.42%). IR (Nujol, KBr) 1630(u, O N ) , 1600(u, O N ) , 1531(u, O N ) , 1263(u, Cheteroaromatic-O), 1104(u, Caiiphatio-0) cm"1. Anal. Calc. for C17Hi2Cl2F4N4Ni0:

217

C=41.34, H=2.45, N=11.34, Cl=14.35, Ni=l 1.88. Found: C=41.2, H=2.38, N=11.42, Cl=13.9, Ni=11.5%. 2.6. General procedure for the catalytic reaction of conversion of urea into carbamates and carbonates in a single step. Urea (0.5g, 8.33 mmol) in EtOH (10 mL) and the catalyst (see Table 1) were heated in a stainless-steel autoclave (V = 100 mL) under a CO2 pressure as indicated in Table 1. After cooling, the amount of ethyl carbamate and diethyl carbonate were determined by GC-MS. Table 1. Catalyst, mg Pressure, Time, T, Yield, % Yield, % N Alcohol K urethanes DEC,DMC h Cocatalyst, mg 10 mL CO2, atm 393 trace Trace 8 20 1 MeOH Zn (OAc)2; 25 mg trace 22 393 20 60.8 2 EtOH Zn (OAc)2; 50 mg trace 83.12 17 433 20 Zn (OAc)2; 56 mg 3 EtOH 8.37 33.32 17 483 20 4 EtOH Zn (OAc)2; 56.05 mg 2.9 10.39 65 483 20 Zn (OAc)2; 55.55 mg 5 EtOH 1.4 91.39 21 483 20 ZnCl2(cppt)2; 50.45 mg 6 EtOH 0.85 77.53 17 483 20 ZnCl2(cppc), 49.1 mg 7 EtOH trace 85.2 17 483 20 ZnCl2(ptt), 50 mg 8 EtOH trace 62 393 FeCl 2 *1.5THF;31.8mg Trace 20 9 MeOH 22 393 trace 29.52 20 FeCl2(cppt); 48.35 mg 10 EtOH 2.68 42.13 21 483 20 FeCl2(cppt); 55.3 mg 11 EtOH trace 51.04 18 413 CuCl(cppt);53.1mg 20 12 EtOH trace 21 483 CuCl(cppt); 54 mg 76.61 20 13 EtOH 0.74 69.23 17 483 20 NiCl 2 ,52mg 14 EtOH 1.3 69.8 17 483 20 15 EtOH NiCl 2 ,55.5mg;NaH,23mg 5.1 84.43 20 21 483 (PEt2Ph)2NiCl2; 58.3 mg 16 EtOH 4.9 45.14 17 483 20 17 EtOH NiCl 2 , 49.05 mg; LiAlH4, 9.95 mg;PPh 3 , 103.4 mg (1:1:1) 5.1 20 17 483 20 18 EtOH NiCl 2 , 51.25 mg;LiAlH4, 37 mg;PPh 3 , 200.45 mg (1:2:2) 6.49 80.37 17 483 1 L1AIH4, 17.4 mg 19 EtOH 9.86 17 483 50 20 NiCl2(pptt) 52.55 mg 20 EtOH 0.96 1 89.23 17 483 51.55 mg 1 0.95 85.1 17 483 NiCl2(pptot), 50.55 mg 21 EtOH trace 74.4 413 65 20 EtOH 22 Sc(CF3SO3)3 67.5 mg trace 75.4 18 483 20 Sc(CF3SO3)3 33.75 mg; 23 EtOH PyridineO.OlmL 2.7. Isolation of the intermediate urethane from the reaction of urea with methanol or ethanol Urea (0.5g, 8.33 mmol) in EtOH (30 mL) or MeOH was heated in a stainless-steel autoclave (V = 100 mL) at 423 K under 1 atm of CO2 for 17 h. After cooling, the reaction mixture was dried under vacuum and the residue was treated with anhydrous diethyl ether to extract the urethane that was isolated and further reacted using the appropriate catalyst to afford the carbonate.

218

2.8. Conversion of the urethane into the relevant carbonate by reaction with the parent alcohol 2.5 g of the urethane isolated as reported in § 2.7 was reacted with the parent alcohol in a stainless steel autoclave in the conditions reported in Table 2 to afford the relevant carbonate that was isolated from the reaction mixture (the isolated yield is given). Table 2. N Alcohol,

mL 1 2 3 4

10 10 10 10

5

10

6

10

7

10

8 9 10 11 12

10 10 10 10 10

Catalyst, mg Cocatalyst, mg Zn(OAc)2, 52.05 mg LiAlH4, 21.25 mg L1AIH4, 14.5mg LiAlH4, 17.35 mg; PPh3, 123 mg LiAlH4, 14.8 mg; PPh3, 103.6 mg (C2H5CH(CH3)O)3A1, 432.2 mg (C2H5CH(CH3)O)3A1, 374.25 mg A1(OC2H5)3, 50 mg NiCl2(pptt), 51.5 mg ZnCl2(ptt), 16.25 mg ZnCl2(cppc), 49.87 mg ZnCl2(cppt)2, 65.3 mg

Pressure Time, T, K h CO2, atm 483 17 20 1 483 17 483 17 20 1 483 17

Yield, % Yield, % DMC DEC 2.1 2.2 18 6.4 19.6

20

17

483

6.3

1

65

423

Traces

1

17

483

18.5

1 1 1 1 1

17 17 17 17 17

483 483 483 483 483

11.5 29.3 9.7 14.0 25.3

15.5 30.4

27.2

3. RESULTS AND DISCUSSION The triazine derivative ligands easily react with the anhydrous metal halides FeCl2, CuCl, NiCl2, ZnCl2 to afford 1:1 adducts [or 1:2 in the case of ZnCl2(cppt)2] that have been fully characterized in the solid state and solution. The ! H NMR spectra of the free ligand and the relevant complex show that the bonding of the ligand to the metal occurs through the interaction of two nitrogen atoms, namely N-2 of the triazine system and N of the pyridyl moiety, that give rise to a penta-atomic ring upon reaction with the metal. (Fig. 1)

Figure 1. This is true also when the ligand bears a potential bonding site as the CN-group. In fact the IR spectrum of the Zn(cppc)Cl2 and Cu(cppt)Cl complexes do not show any sensible shift of the CN-vibration, showing that there is not co-ordination to the metal centre (See the

219

Experimental Part). All synthesized complexes have been checked for their catalytic activity in the alcoholysis of urea. We have used Zn(0Ac)2 as a test catalyst, as it has already been described to be an active promoter of the reactions discussed in this paper. Entries 1-4 (Table 1) show the effect of time and temperature on the reaction. It is quite evident that a temperature of at least 433 K is necessary in order the first step of the reaction may take place, and the conversion of the urethane into carbonate requires at least 483 K, confirming the findings of other authors [2]. These reaction conditions have been used in the rest of the experiments. Interestingly, when ZnCl2-complexes with the ligands cppt, cppc, and ptt (Entries 6-8) are used, the yield of urethane is greatly increased with respect to Zn(0Ac)2. That the ligands have a positive effect on the metal catalytic activity if much more evident with Fe (Entries 9-11). Cu(I) complex with the cppt ligand confirms the data observed with Zn and Fe. We have used anhydrous NiCk and found that it is active in the alcoholysis of urea. The addition of NaH does not improve the yield. Hydrides have been used by other authors to assist the proton-elimination from urea and form a M-NHCONH2 moiety that has been suggested to be the active species in the alcoholysis. We have evidence of the fact that the hydride in the reaction conditions is completely converted by the solvent alcohol into the alkoxo compound NaOR with formation of H2. As a matter of fact, a positive influence of the hydride has not been observed in our studies (Entries 14-15, and 16-18). Conversely, the Nicatalytic properties are improved when the diethylphenylphosphine complex is used, that seems to be more active than the triphenylphosphine, also in presence of L1AIH4 (Entries 1618). The latter per se shows a catalytic activity (Entry 19). It should be emphasized that in alcohol it is converted into Al(0R)3 and LiOR, the former being the real catalytic species, as shown by the use of Al(0R)3 alone. The Ni-complexes with the pptt and pptot fluorurated ligands (Entries 20-21) are also good catalysts for the first step of the alcoholysis of urea to afford the urethane, a reaction that is not affected by the presence of carbon dioxide. We have tried to carry out the reactions under carbon dioxide pressure in order to verify if it were possible to recycle ammonia that is released in the alcoholysis. So far we have not observed any beneficial effect of the presence of carbon dioxide. Naked ions like Sc3+ or its complexes with pyridine (Entries 22-23) do not represent a better catalytic system. Therefore, the complexes Ni(pptt)Cl2 (Entry 20b) and Zn(cppf)2Cl2 (Entry 6) represent the best options as they can afford yields of the order of 90% in urethane. Considered all the results, we have adopted a two step procedure for the synthesis of the carbonate, with a first step carried out at 433 K with isolation of the urethane and its further conversion into the carbonate at 483 K. The results reported in Table 2 show some interesting features. Al-trialkoxo species are able to catalyse the alcoholysis of urethanes (Entries 2-8). It is worth to emphasize that whatever alkoxo complex is used, it reacts with the excess methanol or ethanol to afford Al(0Me) 3 and Al(OEt)3, respectively. This has been proved with Al(iso-butoxo)3 that is converted into the above mentioned species in the relevant alcohol with release of iso-butanol measured by GCMS. Similarly, LiAlH4 converts into LiOR and A1(OR)3 as reported above. The effective catalyst is Al(OEt)3 in EtOH and Al(0Me) 3 in MeOH. CO2 has a negative effect on the reaction yield (Entries 2, 3 and 4, 5). Positive results are obtained using Zn and Ni complexes with the ligands pptt, ptt, cppc and cppt. In particular, Ni(pptt)Cl2 and Zn(cppt)2Cl2 show the most interesting results (Entries 9 and 12, Table 2). The reaction mechanism is not yet clear, but one can assume that the alcohol reacts in a concerted way with the co-ordinated urea to afford NH3 and the urethane (first step) or the carbonate (second step). (Fig. 2)

220

Figure 2. Then role of the ligands on the metal may be relevant to the charge delocalisation that may make the urea carbonyl carbon more positive and prone to the nucleophilic attack by the alcohol. In a concerted way, the proton may be transferred to the -NH2 group causing the release of ammonia. Studies are still in progress for the elucidation of the mechanism. REFERENCES 1. U. Romano, R. Tesei, M.M. Mauri, P. Rebora, Ind. Eng. Chem. Prod. Res. Dev. 9 (1980) 396. 2. M. Aresta, A. Dibenedetto, E. Quaranta, Green Chemistry 1 (1999), 237. 3. M. Pacheco, C.L. Marshall, Energy & Fuels, 11 (1997) 2. 4. E.N. Suciu, B.Kuhlmann, G.A. Knudsen, R.C. Michaelson, J. Organomet Chem., 556 (1998)41. 5. M. Aresta, E. Quaranta, CHEMTECH 27 (1997) 32. 6. U. Romano, F. Rivetti, D. Delle Donne, Appl. Catal. A: Gen. 221 (2001) 241 and references therein. 7. K. Nishihira, S. Tanaka, Y. Nishida, N. Manada, T. Karafuji, M. Marukami, Ube Industries, US Patent 5 869 729 (1999). 8. a) J.C. Choi, T. Sakakura, T. Sako, J. Am. Chem. Soc, 121 (1999) 3793; b) D. BallivetTkatchenko, H. Chermette, T. Jerphagnon in Environmental Challenges and Greenhouse Gas Control for Fossil Fuel Utilization in the 21 st Century, M.M. Maroto-Valer et al. Eds, Kluwer Academic/Plenum Publishers, New York (2002) 371; c) D. Ballivet-Tkatchenko, O. Douteau, S. Stutzmann, Organometallics 19 (2000) 4563. 9. K. Tomishige, T. Sakaihori, Y. Ikeda, K. Fujimoto, Catal. Lett. 58 (1999) 225. 10. M. Aresta, A. Dibenedetto and C. Pastore, Inorg. Chem., 42 (2003) 3256. 11. P. Ball, H. Fuellmann and W. Heitz, Angew. Chem. In. Ed. Eng 19 (1980) 718. 12. M. Paquin, Z. Naturforschg, 1 (1946) 518. 13. W.Y. Sao, G.P. Speranza US Patent 5349077 (1994). 14. G. Chelucci, R.P. Thummel, Chem. Rev. 102 (2002) 3129. 15. V.N. Kozhevnikov, D.N. Kozhevnikov, T.V. Nikitina, V.L. Rusinov, O.N. Chupakhin, M. Zabel, and B. Koenig., J. Org. Chem. 68 (2003) 2882. 16. D.D. Perrin, W.L.F. Armarego, D.R. Perrin, Purification of Laboratory Chemicals, 2nd Edition Pergamon Press: Oxford England, 1986.