Title of Invention

A PROCESS FOR THE PREPARETION OF A DIARYLALKANE

Abstract

A process for the preparation of diarylalkanes, a class of aromatic compounds having proven industrial importance. The invention particularly relates to a catalytic process of preparing diarylalkanes in general, and benzyltoluene, in particular, from the reaction of aromatic hydrocarbon and an alcohol in the presence of a selective combination of a transition metal compound as catalyst, and tin (IV) chloride as co-catalyst, without using high dilution condition and avoiding unusually stringent process parameters which would be industrially viable and with possible high yield of the desired product.The catalytic process for the preparation of diarylalkanes, in general, and dibenzyltoluene in particular can be advantageously be carried out with very low catalyst loading, high turnover number and which results in very high yield of the desired product.

Full Text

FIELD OF THE INVENTION The present invention relates to a process for the preparation of diarylalkanes, a class of aromatic compounds having proven industrial importance. The invention particularly relates to a catalytic process of preparing diarylalkanes in general, and benzyltoluene, in particular, from the reaction of ar.o.matic hydrocarbon. and an alcohol in the presence of a transition metal compound as catalyst, and tin (IV) chloride as co-catalyst, without using high dilution condition and avoiding unusually stringent process parameters which would be industrially viable and with possible high yield of the desired product. BACKGROUND OF THE INVENTION Diarylalkanes, for example benzylbenzene, benzyltoluene and related aromatic hydrocarbons are constituents of crude oils and have been shown to occur in crude oils from various source types and geological ages (Polycyclic Aromatic Compounds (1996), 9, 137-142). They are also of industrial interest due to their application as components of high-octane fuels for aircraft engines (Journal of Chemical and Engineering Data (1999), 44, 175). Diarylalkanes for example benzyltoluenes are essential ingredients of electrical insulating oils useful for high-voltage electrical devices (IEEE Trans. Elect, Insul. EI-21 Feb, 1986, 59). These oils have excellent low-temperature properties, and are particularly suited as impregnant for high voltage power capacitors (JP 01228924, Eur. Pat. Appl. (1988), EP 262456,_262454, 259798, JP 621.85907). By virtue of their improved pour point characteristics, increased stability and reduced oxidation rate, formulations containing diarylalkanes are used as dielectric coolant in electrical transformers (PCT Int. Appl. (1997), WO 9722572). Benzyltoluenes has been also used in formulating emulsions of termiticides, which show good penetration, and emulsion stability. For example, emulsions prepared by mixing Chlorpyrifos 40, benzyltoluene 55, water 150, and emulsifier 7 parts showed stability for greater than 1 month (JP 95-197975). Diarylalkanes of the type mentioned above has been prepared by several known methods that includes reaction of suitable aromatics with either formaldehyde, benzyl chloride or benzyl alcohol. Due to serious environmental hazards 2 associated with formaldehyde or benzyl chloride, any processes based on these chemicals are no more industrially viable. Corrosion of reactor while using benzyl chloride is also noteworthy. In view of the above, several attempts have been made to synthesize diarylalkanes from suitable aromatics and benzyl alcohol in presence of a stoichiometric or catalytic reagent. These reagents are categorized as Bronsted acids, Lewis acids, solid acids, solid superacids, ion-exchange resins, ion-exchanged clays, heterogeneous transition metal oxides or phosphates. Each of the above processes are limited by one or more of the following factors: (a) high arene to alcohol ratio, (b) poor selectivity and by-product formation, (c) poor conversion, (d) use of very costly additives, (e) high catalyst loading, (f) high temperature of reaction, (g) unusually stringent process parameters. Thus, DE-PS 638.756 discloses the reaction of benzyl alcohol with benzene (molar ratio 1:2.77) at 230 degree C in an autoclave using 50% by weight of Tonsil as catalyst. The yield of the desired product diphenylmethane was about 40% of the theoretical yield and byproduct dibenzylbenzenes in about 25% yield. In DE 3836780, the yield is improved by using low catalyst concentration, very high arene:alcohol ratio, and by stringently metering the introduction of benzyl alcohol (under nitrogen flow of 100 L/h) and continuously redistilling water and alcohol. Processes based on metal halide catalysts such as aluminum trichloride, tellurium tetrachloride, and titanium tetrachloride suffer from very high catalyst loading (50 to 120 mol%), high arene:alcohol (nearly 30:1) ratio and the requirement to use reflux temperature (Kogyo Kagaku Zasshi 1967, 70, 2287-91, Kogyo Kagaku Zasshi 1968, 71, 229, Bulletin of the Chemical Society of Japan, 1986, 59, 3617-20; Nippon Kagaku Kaishi, 1981, 12, 1911-15). Silver perchlorate or silver triflate were used as additive to silicon tetrachloride or stannic chloride to lower the reaction temperature to below 50-degree C, however the silver salts are expensive and/or explosive [Chemistry Letters 1972, 6, 443-4, Tetrahedron Letters 2002, 43, 6391-6394], Scandium triflate (10 mol%) had been used as effective catalyst at 120 degree C at arene:alcohol ratio of 30-45:1 (Synlett, 1996, 557-559, J. Org. Chem., 1997, 62, 6997). Cation-exchanged K10-clay, Filtrol 24 and sulfated zirconia were used as heterogeneous catalysts under reflux condition in the reaction of benzyl alcohol with benzene or toluene (Applied 3 Catalysis A: 2001, 209, 229-235, Tetrahedron Letters, 1991, 32, 2903-2904, Tetrahedron Letters, 1991, 32, 1423-1424, Tetrahedron Letters, 1996, 37, 5405-5408). Low turnover frequency or poor conversion and by-product formation had been observed in certain cases. Mineral acid treated resins, and sofid perfluorinated resin sulfonic acids are capable of providing good yield of diarylalkanes at high arene:alcohol ratio (Reactive Polymers, 1995, 25, 55, J. Org. Chem. 1991, 56, 2089-2091). For example, benzyltoluene was obtained in 86 % yield after 2 h, using 10 wt% of Nafion-H, at toluene:benzyl alcohol ratio of 30:1 at 90-95 deg C (J, Org. Chem. 1991, 56, 2089-2091). Although phosphoric acid treated niobium oxide catalyst worked at arene:aicohoi ratio of 10:1, the yield of benzyl toluene was only 32%, along with 57% of dibenzylether after a reaction time of 9h (Catalysis today 1996, 28, 17-21, Applied Catalysis A: 1996, 138, L7-L12, Applied Catalysis A: 2003, 245, 377-382). OBJECTS OF THE INVENTION The main object of the present invention, therefore is to provide an industrially favourable and free of environmental hazards catalytic process for the preparation of diaryialkanes, in general, and benzyltoluene in particular from the reaction of benzyl alcohol ,and arene without using high-dilution condition. It is another object of the invention to provide a catalytic process for the preparation of diarylalkanes, in general, and dibenzyltoluene in particular with very low catalyst loading, high turnover number and which results in very high yield of the desired product. SUMMARY OF THE INVENTION Thus according to the basic aspect of the present invention there is provided a process for the preparation of a diarySalkane, comprising reacting an arene and an alcohol using a selective combination of transition metal compound as catalyst and tin halide as co-catalyst in the temperature range of 30-100 degree C. and isolating the desired diarylalkane. The arene used in the above process can preferably be selected from the group consisting of benzene, toluene, xylene, naphthalene and biphenyl. 4 The benzyl alcohol used in the process can preferably be selected from the group consisting of benzyl alcohol and substituted benzyl alcohols, where the substitution is made in the ring by functional groups such as C1-C6 alkyls, halo, nitro, alkoxy, acyl and ester. The transition metal catalyst used in the process can preferably be selected from the group consisting of tris(triphenylphosphine)rhodium(l) halide, bis(i,5-cyclooctadiene)dirhodium(i) dihalide, bis(1,5-cyclooctadiene)diiridium(l) dihalide, and carbonylbis(triphenylphosphine)rhodium(l) chloride. According to an aspect of the invention, the weight proportion of catalytic transition metal compound used is preferably in the range of 0.1-2,0 mol % with respect to benzyl alcohol. The tin halide co-catalyst used in the process can preferably be selected from anhydrous tin (II) chloride, tin (II) chloride dihydrate, and tin(lV) chloride. According to another aspect of the invention, the weight proportion of tin halide is in the range of 1-10 mol% with respect to benzyl alcohol. In accordance with yet another aspect of the invention the molar proportion of arene and benzyl alcohol used is preferably in a ratio from 10:1 to 1:1. DETAILED DESCRIPTION OF THE INVENTION In the process of the invention, Diarylalkanes are prepared from the reaction of arene and benzyl alcohol with a transition metal catalyst and tin halide as co- catalyst. Arene refers to benzene, toluene, naphthalene and alkyl or aryl derivatives thereof. Benzyl alcohol refers to benzyl alcohol and substituted benzyl alcohols, where the substitution is made in the ring by functional groups such as C1-C6 alkyls, halo, nitro, alkoxy, acyl and ester. The transition metal catalyst can be tris(triphenylphosphine)rhodium(l) chloride, tris(triphenylphosph!ne)rhodium(l) bromide, tris(tripheny!phosphine)rhodium(l) iodide, bis(1,5- cyclooctadiene)dirhodium(l) dichloride, bis(1,5-cyclooctadiene)dirhodium(l) 5 dibromide, bis(1,5-cyclooctadiene)dirhodium(l) diiodide, bis(1,5- cyclooctadiene)diiridium(l) dichloride, bis(1,5-cyclooctadiene)diiridium(l) dibromide, bis(1,5-cyclooctadiene)diiridium(l) diiodide, and carbonylbis(triphenylphosphine) rhodium(l) chloride. Tin halide co-catalyst can be anhydrous tin(ll) chloride, tin(ll) chloride dihydrate, and tin(IV) chloride. The arene:alcohol can be in a mole ratio from 1:1 to 10:1. Following the above process of the invention Diarylalkanes can be obtained by way of a simple, cost-effective and environment friendly process which can be carried out in the temperature range of 30-100°C. The product diarylalkane is isolated by direct distillation from reaction mixture. The transition metal catalyst can be extracted from the residue with hot petroleum ether and reused for further reaction. The process of the invention basically involved providing a reactor equipped with stirring assembly and charging the same successively with the transition metal catalyst, tin halide cocatalyst and the selected arene. The reactor was immersed in a constant temperature bath maintained at 90±0.1 degree C. Alcohol was added to the above solution during 2 hours, and product profile was monitored by Gas Chromatography. Following full conversion the product was isolated from the reaction mixture by distillation under reduced pressure or column chromatography. The ratio of isomers, wherever applicable determined. It is found that the catalytic reaction proceeds equally well with other selective catalysts, co-catalyst to yield variety of products as indicated under Table-I hereunder. 6 Table I: List of Catalysts, Co-catalysts, Products & their No. Catalyst Catalyst No. [lr(COD)(CI)]2 la [Rh(COD)(CI)]2 Ib RhCI(PPh3)3 Ic CoCI(PPh3)3 Id RhCI(CO)(PPh3)2 le lrCI(CO)(PPh3)2 If RuCI2(PPh3)3 Ig NiCI2(PPh3)2 Ih PdCI2(PPh3)2 li PtCI2(PPh3)2 Ij Co-catalyst Co-catalyst No. SnCl4 lla SnCI2 llb SnCI2.2H2O llc Product Product No. Benzyltoluene 1 Diphenylmethane 2 2,5- 3 Dimethyldiphenylmethane Benzylnaphthalene 4 Benzylbiphenyl 5 (4-nitrophenyl)-tolylmethane 6 It is further observed that reactions also proceed at room temperature in presence of varying amount of co-catalyst. Importantly, it was found that the selective combination effect of both the transition metal catalyst and tin halide co- 7 catalyst in the transformation was essential as no reaction occurred in absence of either of the catalyst and co-catalyst under the same reaction condition. The details of the invention its objects and advantages are explained hereunder in greater detail in relation to non-limiting exemplary illustrations as discussed hereunder: EXAMPLE 1: Preparation of benzyltoluene according to the invention using co-catalyst SnCI4 at 90 degree C. A solution containing toluene (4 ml, 37.6 mmol), bis(1,5-cyclooctadiene)diiridium(l) dichloride (7 mg, 0.01 mmol) and anhydrous tin(IV) chloride (0.012 ml, 0.1 mmol) was stirred for 15 minutes in a constant temperature bath at 90 degree C under an argon atmosphere. Benzyl alcohol (1.241 ml, 12 mmol) was added to the above solution at a rate of 0.01ml/min during 2 hours. After a further stirring for 20 min the reaction was stopped. GC analysis showed full conversion of benzyl alcohol to benzyltoluene. The product benzyltoluene (2.075 gm, 95%) was isolated from the reaction mixture by distillation under reduced pressure (bp, 114-115 degree C at 3 torr). The ratio of ortho:para isomers determined by GC and NMR analysis is found to be 35:65. 1H NMR (CDCI3). delta (ppm): 2.24 (3H,s), 2.31 (3H,s), 3.94 (2H,s), 3.98 (2H,s), 7.01-7.30 (18H,m). EXAMPLE 2: Preparation of benzyltoluene according to the invention using co-catalyst SnCI4 at 30 degree C. A solution containing toluene (4 ml, 37.6 mmol), tris(triphenylphosphine)rhodium(l) chloride (18 mg, 0.02 mmol) and anhydrous tin(IV) chloride (0.024 ml, 0.2 mmol) was stirred for 15 minutes at room temperature under an argon atmosphere. Benzyl alcohol (0.207 ml, 2 mmol) was added to the above solution at a rate of 0.01 ml/min. After a further stirring for 5 hours at 30 degree C the reaction was stopped. GC analysis showed full conversion of benzyl alcohol to benzyltoluene. The product benzyltoluene (327 mg, 90%) was isolated from the reaction mixture by distillation under reduced 8 pressure (bp, 114-115 degree C at 3 torr). The ratio of ortho:para isomers determined by GC and NMR analysis is found to be 35:65. EXAMPLE 3: Preparation of benzyltoluene according to the invention using co-catalyst SnCI2 at 100 degree C. A solution containing toluene (4 ml, 37.6 mmol), tris(thphenylphosphine)rhodium(l) chloride (9 mg, 0.01 mmol) and anhydrous tin(ll) chloride (38 mg, 0.2 mmol) was stirred for 15 minutes at 100 degree C under an argon atmosphere. Benzyl alcohol (0.207 ml, 2 mmol) was added to the above solution at a rate of 0.01 ml/min. After a further stirring for 2 hours at 100 degree C the reaction was stopped. GC analysis showed full conversion of benzyl alcohol . The product benzyltoluene (182 mg, 50%) was isolated from the reaction mixture by column chromatography (silica gel 60-120 mesh, petroleum ether eluent) after addition of 5 ml of 2% aqueous ammonium fluoride solution, extracting the organic layer with diethylether (5 times 20 ml), washing with water (3 times 10 ml) and brine (2 times 10 ml) and drying with anhydrous magnesium sulfate. The ratio of ortho:para isomers determined by GC and NMR analysis is found to be 35:65. EXAMPLE 4: Preparation of Diphenylmethane according to the invention using co-catalyst SnCl4 at 90 degree C. [0021] A solution containing benzene (3 ml, 33.8 mmol), bis(1,5-cyclooctadiene)diiridium(l) dichloride (7 mg, 0.01 mmol) and anhydrous tin(IV) chloride (0.012 mL, 0.1 mmol) was stirred for 15 minutes in a constant temperature bath at 90.0±0.1 degree C under an argon atmosphere. Benzyl alcohol (1.241 ml,.12 mmol) was added to the above solution at a rate of 0.01 ml/min during 2 hours. The reaction was continuously monitored by Gas Chromatographic Analyzer and showed completion after a further stirring for 30 min. The product diphenylmethane (1.512 gm, 75%) was isolated from the reaction mixture by distillation under reduced pressure (bp, 120-122 degree C at 9 torr). 1H NMR (CDCI3). delta (ppm): 3.95 (2H,s), 6.78-7.31 (10H,m). 9 EXAMPLE 5: Preparation of 2,5-dimethyldiphenylmethane using co-catalyst SnCl4 at 90 degree C. A solution containing p-xylene (4.5 ml, 38.7 mmol), bis(1,5-cyclooctadiene)diiridium(l) dichioride (7 mg, 0.01 mmol) and anhydrous tin(IV) chloride (0.012 ml, 0.1 mmol) was stirred for 15 minutes in a constant temperature bath at 90 degree C under an argon atmosphere. Benzyl alcohol (1.241 ml, 12 mmol) was added to the above solution at a rate of 0.Q1mS/min during 2 hours. After a further stirring for 15 min the reaction was stopped. GC anaiysis showed full conversion of benzyl alcohol. The product 2,5-dimethyldiphenylmethane (1.746 gm, 75%) was isolated from the reaction mixture by distillation under reduced pressure (bp, 120-122 degree C at 3 torr). EXAMPLE 8: Preparation of benzyfnaphthafene using co-catalyst SnCI4 at 90 degree C. A solution of naphthalene (256 mg, 2 mmol), bis(1,5-cyclooctadiene)diihdium(l) dichloride (7 mg, 0.01 mmol) and anhydrous tin(IV) chloride (0.012 ml, 0.1 mmol) in 5 ml n-nonane was stirred for 15 minutes in a constant temperature bath at 90 degree C under an argon atmosphere). Benzyl alcohol (0.207 ml, 2 mmo!) was added dropwise to the above solution. After a further stirring for 4 hours the reaction was stopped. An aqueous solution of ammonium fluoride (2%, 5 ml) was added to the reaction mixture, organic layer was extracted with diethyl ether (4 times 25 ml), washed with water (2 times 25 ml), brine (2 times 10 ml) and dried over magnesium sulfate. Solvent removal followed by column chromatography (Silica gel 100-200 mesh) afforded benzyinaphthaiene 4 (314mg, 72%, ??? = 85:15). 1H NMR (CDCl3). delta (ppm): 7.89-7,95 (m), 7.71-7.81 (m), 7.61 (d), 7.35-7.39 (m), 7.1-7.3 (6H,m), 4.36 (s, a), 4.07 (s, ?). EXAMPLE 7; Preparation of benzylbiphenyl at 90 degree C A solution of biphenyl (138 mg, 0.9 mmol), bis(1,5-cyclooctadiene)diiridium(i) dichloride (3.5 mg, 0.005 mmol) and anhydrous tin(IV) chloride (0.006 ml, 0.05 10 mmol) in 5 ml n-nonane was stirred for 15 minutes in a constant temperature bath at 90 degree C under an argon atmosphere). Benzyl alcohol (0.093 ml, 0.9 mmol) was added dropwise to the above solution. After a further stirring for 4 hours the reaction was stopped. An aqueous solution of ammonium fluoride (2%, 5 ml) was added to the reaction mixture, organic layer was extracted with diethyl ether (4 times 25 ml), washed with water (2 times 25 ml), brine (2 times 10 ml) and dried over magnesium sulfate. Solvent removal followed by column chromatography (Silica gel 100-200 mesh) afforded benzylbiphenyl 5 (119 mg, 54%, ortho;para = 62:38). 1H NMR (CDCI3). delta (ppm): 3.89 (s, para), 3.96 (s, ortho), 6.89-7.52 (m). at 90 degree C A solution containing toluene (4 ml, 37.6 mmol), bis(1,5-cyclooctadiene)diiridium(l) dichloride (7 mg, 0.01 mmol) and anhydrous tin(IV) chloride (0.012 ml_, 0.1 mmol) was stirred for 15 minutes in a constant temperature bath at 90.0±0.1 degree C under an argon atmosphere. 4-Nitrobenzyl alcohol (0.306 gm, 2 mmol) was added to the above solution at 90 degree C. The reaction was continuously monitored by Gas Chromatographic Analyzer and showed complete conversion of the benzyl alcohol in 180 minutes. The product (4-nitrophenyl)-tolylmethane (0.236 gm, 52%, ortho:para = 38:62) was isolated from the reaction mixture by column chromatography (silica gel 100-200 mesh, petroleum ether eluent) after addition of 5 ml of 2% aqueous ammonium fluoride solution, extracting the organic layer with diethylether (5 times 20 ml), washing with water (3 times 10 ml) and brine (2 times 10 ml) and drying with anhydrous magnesium sulfate. 1H NMR (CDCl3). delta (ppm): 2.13 (s, ortho), 2.25 (s, para), 3.95 (s, para), 4.00 (s, ortho), 7.01-13 (m), 8.04 (d). The yield of the Diaryalkanes from Arenes and Alcohols following the processes as per the examples 1,4 to 8 are noted hereunder in Table II: 11 Table II: Preparation of Diarylalkanes from Arenes and Alcohols using Catalyst la (0.5 mol%), Co-Catalyst lla (5 mol%) at 90°C Example. Arene Alcohol Time Isolated Yield (%) (mmol) (mmol) (min) 1 Toluene (37.6) Benzyl alcohol (12) 140 95 (o:p=35:65) 4 Benzene Benzyl alcohol (12) 150 75 (33.8) 5 p-Xylene Benzyl alcohol (12) 135 75 (36.7) 6 Naphthalene Benzyl alcohol (2) 240 72 (?:?=85:15) (2) 7 Biphenyl (0.9) Benzyl alcohol (0.9) 240 54 8 Toluene (37.6) 4-Nitrobenzyl alcohol 180 52 (2) As would be apparent from the results above the process provided for good yield of the product and being environment friendly is necessarily advantageous. Further studies were carried out to identify the effect of the process in use of only the catalyst/co-catalyst vis-a-vis the selective combination of both the catalyst and the co-catalyst as proposed under the invention as detailed in Table III hereunder: Table III: Preparation of Benzyltoluene from Benzyl Alcohol (2 mmol) and Toluene (28 mmol): Effect of Catalyst, Co-catalyst and Temperature SI. Catalyst Cat. Co-cat. Co- Temp Time Yield of No (mol%) Cat. (° C) (min) BZT (%) (mol%) 1 NIL NIL NIL NIL 90 360 0 2 - - SnCI4 5 90 420 8 3 RhCI(PPh3)3 0.5 - - 90 360 5 4 RhCI(PPh3)3 0.5 SnCI4 5 90 240 95 12 As would be clearly apparent from the results in Table III, the above go to confirm the selective advantages in the combination of the catalyst and the co-catalyst in the process of the invention. In the absence of both the catalyst and the co-catalyst, the invention does not proceed as desired and it is only in the presence of both the catalyst and the co-catalyst that the process leads to significantly higher yield in the given operating conditions. Some additional exemplary processes in accordance with the invention were carried out by way of preparation of Benzyltoluene from Benzyl Alcohol (2 mmol) and Toluene (28 mmol) and the process conditions and yield are as detailed hereunder in Table IV: Table - IV : Preparation of Benzyltoluene from Benzyl Alcohol (2 mmol) and Toluene (28 mmol): Effect of Catalyst, Co-catalyst and Temperature SI. Catalyst Cat. Co-cat. Co- Temp Time Yield of No. (mol%) Cat. (° C) (min) BZT (%) (mol%) 1 RhCI(PPh3)3 0.5 SnCU 5 90 240 95 2 RhCI(PPh3)3 0.5 SnCI2 10 100 120 50 3 RhCI(PPh3)3 0.5 SnCU 10 30 300 90 4 [lr(COD)(CI)]2 0.5 SnCI4 5 90 20 95 5 RhCI(CO)(PPh3)2 0.5 SnCU 5 90 180 96 As would be apparent from the results in Table IV above, the process which does not involve high dilution conditions even with low catalyst loading achieves high yield following an environment friendly process. 13 WE CLAIM: 1. A process for the preparation of a diarylalkane, comprising reacting an aromatic hydrocarbon and an alcohol using selective combination of catalytic transition metal complex and catalytic amount of tin halide in the temperature range of 30-100 degree C. and isolating the desired diarylalkane. 2. A process as claimed in claim 1, wherein the aromatic hydrocarbon is selected from the group consisting of benzene, ring substituted benzene, toluene, ring substituted toluene, xylene, ring substituted xylene, naphthalene, ring substituted naphthalene, biphenyl and substituted biphenyl. 3. A process as claimed in claim 2, wherein the ring substitution is selected from the group consisting of alkyl, aryl, acyl, alkoxy, halo, nitro and ester. 4. A process as claimed in anyone of claims 1 to 3, wherein the alcohol used is selected from the group consisting of benzyl alcohol and substituted benzyl alcohols, where the substitution is made in the ring by functional groups preferably C1-C6 alkyls, halo, nitro, alkoxy, acyl and ester. 5. A process as claimed in anyone of claims 1 to 4, wherein the transition metal catalyst used is selected from the group consisting of tris(triphenylphosphine)rhodium(l) chloride, tris(triphenylphosphine)rhodium(l) bromide, and tris(triphenylphosphine)rh'odium(i) iodide. 6. A process as claimed in anyone of claims 1 to 5, wherein the transition metal catalyst used is selected from the group consisting of bis(1,5- cyclooctadiene)dirhodium(l) dichloride, bis(1,5-cyclooctadiene)dirhodium(l) dibromide, and bis(1,5-cyclooctadiene)dirhodium(l) diiodide. 7. A process as claimed in anyone of claims 1 to 6, wherein the transition metal catalyst used is selected from the group consisting of bis(1,5- cyclooctadiene)diiridium(l) dichloride, bis(1,5-cyclooctadiene)diiridium(l) dibromide, and bis(1,5-cyclooctadierie)diiridium(l) diiodide. 14 8. A process as claimed in claim 1, wherein the transition metal catalyst used is carbonylbis(triphenylphosphine) rhodium(l) chloride. 9. A process as claimed in anyone of claims 1 to 8, wherein the weight proportion of transition metal catalyst used is in the range of 0.1-2.0 mol % with respect to benzyl alcohol. 10. A process as claimed in anyone of claims 1 to 9, wherein the tin halide co- catalyst is selected from the group consisting of tin(IV) chloride, anhydrous tin (II) chloride and tin (M) chloride dihydrate. 11. A process as claimed in anyone of claim s 1 to 10, wherein the weight proportion of tin halide is in the range of 1-10 mol% with respect to benzyl alcohol. 12. A process as claimed in anyone of claims 1 to 11, wherein the weight proportion of aromatic hydrocarbon and alcohol taken is in ratio from 10:1 to 1:1. 13. A process as claimed in claim 1 wherein benzyl alcohol and toluene are reacted in the presence of catalytic bis(1,5-cyclooctadiene)diindium(l) dichloride (0.1 mol%) and tin (IV) chloride (1 mol%) at 85-100 degree C. to obtain benzyltoluene (BZT). 14. A process for the preparation of a diarylalkane substantially as herein described and illustrated with reference to the accompanying examples. A process for the preparation of diarylalkanes, a class of aromatic compounds having proven industrial importance. The invention particularly relates to a catalytic process of preparing diarylalkanes in general, and benzyltoluene, in particular, from the reaction of aromatic hydrocarbon and an alcohol in the presence of a selective combination of a transition metal compound as catalyst, and tin (IV) chloride as co-catalyst, without using high dilution condition and avoiding unusually stringent process parameters which would be industrially viable and with possible high yield of the desired product.The catalytic process for the preparation of diarylalkanes, in general, and dibenzyltoluene in particular can be advantageously be carried out with very low catalyst loading, high turnover number and which results in very high yield of the desired product.

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