the process of the present invention is preferably carried out using neat compound ii or , optionally , a suitable solvent can be used . suitable solvents include diphenyl ether , dimethyl phthalate , biphenyl , and the like . in carrying out the process of the present invention , an appropriate bibenzyl derivative is typically formed ( as used herein &# 34 ; bibenzyl derivative &# 34 ; or &# 34 ; bibenzyl &# 34 ; refers to a dimethyl bibenzyldicarboxylate derivative and / or a diphenyl bibenzyldicarboxylate derivative ). it is desirable to increase the rate of compound i relative to the amount of bibenzyl derivative formed . even though the bibenzyl derivative can be relatively easily converted to the desired compound i by dehydrogention with sulfur ( further explained hereinafter ), such additional conversion adds to the cost and time requirements . it has been found that the ratio of compound i relative to bibenzyl derivative is substantially increased when the hydrogen sulfide ( also produced during the reaction ) is removed from the reaction zone . the more rapid the removal of hydrogen sulfide from the reaction zone , the higher the ratio of compound i to bibenzyl derivative which is produced . therefore , removal of the hydrogen sulfide at substantially the same rate that it is produced is most desirable . such a rate of removal can be accomplished by continually sweeping the reaction zone with an inert gas such as nitrogen or argon . the process of the present invention is typically carried out at a temperature of 230 ° c . to 320 ° c . ; preferably 245 ° c . to 285 ° c . at temperatures below 230 ° c ., the reaction is slower and may not be commercially feasible , and above 320 ° c . some decomposition may occur which results in lower yield and poor color . the reaction time for the process of the present invention is typically from about 2 to about 6 hours , preferably from about 3 to about 4 hours . the pressure required for the process of the invention can vary from about atmospheric to about 250 pounds per square inch gauge ( psig ) depending upon the particular reactants and other reaction conditions . when using methyl p - toluate to form dimethyl trans 4 , 4 &# 39 ;- stilbenedicarboxylate ( dmsdc ), at least about 50 psig is required . when r &# 39 ; of compound i and compound ii is phenyl , the reaction can be carried out at atmospheric pressure thereby eliminating the need for costly pressurized equipment . the initial molar ratios of reactants for the process of the invention are critical . an initial molar excess of compound ii of at least about 5 : 1 relative to sulfur is required to obtain satisfactory yield ; preferred is at least about a 10 : 1 molar excess . the upper limit of the excess of compound ii is not critical and the only constraint on the upper limit is that of praticality and economics . a most preferred initial molar ratio of compound ii : sulfur is between about 5 : 1 and about 20 : 1 . after the initial ratios of reactants are allowed to react , the excess of compound ii becomes greater and greater as the sulfur is being consumed . the reaction typically proceeds until the sulfur is exhausted . therefore an initial ratio of compound ii : sulfur of 5 : 1 results in about a 20 % conversion of compound ii . since the bibenzyl derivative can be easily converted to the desired compound i , the combined yields of bibenzyl derivatives plus compound i are important ( as well as the yield of compound i alone ). reactant ratios of compound ii to sulfur less than about 5 : 1 result in increased formation of other by - products that cannot as easily be converted to the desired compound i , and therefore such ratios are undesirable . in another embodiment of the present invention the appropriate bibenzyl derivative ( formed with compound i ) is reacted with elemental sulfur in the presence of a suitable solvent such as diphenyl ether to form compound i . suitable conditions for this reaction are a temperature of from about 240 ° c . to about 300 ° c ., a pressure of from about 10 to about 100 psig , a reaction period of from about 1 to about 24 hours , and with an inert gas sparge such as with nitrogen or argon . in a preferred process of the invention methyl p - toluate is placed in a steel reactor with a magnetic stirrer and fittings so configured that nitrogen can be sparged into a reaction mixture of the bottom , passed through a condenser at the top of the reactor and allowed to escape through a pressure - dropping orifice so that a pressure of 50 to 250 psig can be maintained inside the reactor . co - product hydrogen sulfide is thus continuously removed from the reaction zone . the progress of the reaction can be followed by monitoring the concentration of hydrogen sulfide in the purge gas thus indicating when to discontinue the heating and sparging . secondary destruction of products by unnecessarily long reaction times is thus avoided . it is highly desired to work with the ester derivatives , particularly the methyl ester derivatives , of stilbenedicarboxylic acids as compared with the free acid derivatives . for example , when trans 4 , 4 &# 39 ;- stilbenedicarboxylic acid is formed using para - toluic acid , an elaborate isolation procedure is required to isolate the free acid derivative . also a mixture of para - toluic acid and trans 4 , 4 &# 39 ;- stilbenedicarboxylic acid sets up to a hard residue requiring xylene and koh extraction to obtain the free acid derivative . in contrast , a mixture of methyl para - toluate and dmsdc is a slurry , requiring only simple filtration for isolation of dmsdc . in addition , for most applications , trans 4 , 4 &# 39 ;- stilbenedicarboxylic acid must be purified via the acid chloride ( which is an additional step ). the acid chloride must then be converted to an ester derivative such as msdc ( still another step ). in contrast , when working with an ester derivative of toluic acid as a starting material , such as methyl para - toluate , the corresponding stilbene ester deriviative , such as dnsdc , is formed directly which can be purified , if necessary , by simple recrystallization . the novel compounds of the present invention ( i . e ., compound i wherein r &# 39 ; is phenyl ) can alternatively be prepared by ester interchange of the appropriate methyl ester stilbene derivatives ( i . e ., compund i wherein r &# 39 ; is methyl ) with phenyl acetate using a suitable catalyst such as dibutyltin oxide . after the stilbene phenyl ester derivative has been formed by this reaction , the reaction product typically contains the desired product , unreacted starting materials , excess catalyst and reaction by - products . it has been found that the desired product can be purified from such a reaction product by recrystallization using an appropriate solvent . it has been discovered that a particularly good solvent for this purpose is one which contains dimethylformamide ( dmf ). the following examples are to illustrate the invention , but should not be interpreted as a limitation thereon . about 600 grams ( 4 mols ) of methyl p - toluate ( mpt ) and 12 . 8 grams ( 0 . 4 mol ) of sulfur ( elemental ) was added to a stirred , 1 - liter autoclave designed to allow nitrogen flow through the reactor sufficient to remove the by - product h 2 s essentially as fast as it is formed . exit of gases from the autoclave is through a condenser kept at 35 ° c . the autoclave was pressured with nitrogen to about 75 psig and the nitrogen flow was adjusted so that a steady flow passes through the liquid content of the autoclave at a rate of about 300 cc / min . the autoclave was then heated to a temperature of 280 ° c . and heating , stirring and nitrogen sparging were maintained until the hydrogen sulfide content of the off gas , as measured by an ultraviolet gas cell meter , was less than 0 . 1 vol . %. about 2 hours at 280 ° c . were required for this process . the reactor was then cooled and unloaded . on filtration at 45 ° c . and washing with methanol , 34 . 8 g ( 59 . 5 % on mpt consumed ) of dimethyl trans 4 , 4 &# 39 ;- stilbenedicarboxylate ( dmsdc ) was obtained . the crude material assayed & gt ; 98 mol % pure by nmr and contained 244 ppm sulfur ( total ) by x - ray fluorescence analysis . additionally , the filtrate contained 9 . 12 g of dimethyl 4 , 4 &# 39 ;- bibenzyldicarboxylate which may be isolated by high vacuum distillation and further converted to dmsdc by sulfur - promoted dehydrogenation for an efficient process . the dmsdc was recrystallized from methyl benzoate and melted at 230 ° c . to 233 ° c . a solution of the bibenzyl ( dimethyl 4 , 4 &# 39 ;- bibenzyldicarboxylate ) in diphenyl ether was made up to give a near 1 m concentration of the bibenzyl . enough sulfur was added to make the solution initially 0 . 1 m in sulfur and the solution was brought to 260 ° c . and sparged with inert gas at the rate of 1 scfh per 150 ml of reactant solution . sampling of the reaction mixture and gas chromotagraphic analysis showed that the concentration of dmsdc had reached 0 . 094 m by 480 minutes . the reaction mixture was cooled to 35 ° c . and the crystallized dmsdc product was isolated by filtration . the filtrate can be made up with additional bibenzyl and sulfur and recycled . the effect of not removing the h 2 s properly from the mpt / s reaction mixture can be seen by comparison of the dmsdc and corresponding bibenzyl yields in the following tabulated runs . table 1__________________________________________________________________________effect of h . sub . 2 s retention on yields ofdimethyl trans - 4 , 4 &# 39 ;- stilbenedicarboxylate at 280 ° c . experi - reactants , products , dmsdc / ment mols n . sub . 2 moles bibenzylno . mpt . sup . a s . sup . b ph . sub . 2 o . sup . c sparge dmsdc . sup . d bibenzyl . sup . e ( mol ratio ) __________________________________________________________________________071 2 . 0 0 . 2 2 . 0 yes 0 . 0680 0 . 0110 6 . 18134 2 . 0 0 . 2 2 . 0 no 0 . 0137 0 . 0905 0 . 151079 4 . 0 0 . 2 0 . 0 yes 0 . 068l 0 . 0117 5 . 82198 4 . 0 0 . 2 0 . 0 no 0 . 0238 0 . 0819 0 . 291__________________________________________________________________________ . sup . a mpt = methyl ptoluate . sup . b s = elemental sulfur . sup . c ph . sub . 2 o = diphenyl ester , an inert diluent and solvent . . sup . d dmsdc = dimethyl trans4 , 4stilbenedicarboxylate . . sup . e bibenzyl = dimethyl 4 , 4bibenzyldicarboxylate . other products besides the dmsdc and associated bibenzyl are formed in this reaction . table 2 illustrates the effect of varying molar ratios of reactants on combined yields of dmsdc and associated bibenzyl . all experiments in table 2 were performed under n 2 sparge of varying rates . table 2__________________________________________________________________________effect of conversion on yields ofdimethyl trans - 4 , 4 &# 39 ;- stilbenedicarboxylate ( dmsdc ) at 280 ° c . ex - peri - product , % yield . sup . gment reactants , moles millimoles ( dmsdc + no . mpt . sup . a s . sup . b ph . sub . 2 o . sup . c mpt : s dmsdc . sup . d bibenzyl . sup . e other . sup . f bibenzyl ) __________________________________________________________________________079 4 . 00 0 . 200 0 . 00 20 : 1 68 . 1 11 . 7 9 . 3 84 . 6073 4 . 00 0 . 400 0 . 00 10 : 1 116 . 9 30 . 6 30 . 9 75 . 1037 4 . 00 0 . 800 0 . 00 5 : 1 179 . 8 89 . 0 43 . 7 78 . 8087 . sup . i 4 . 00 1 . 000 0 . 00 4 : 1 123 . 0 53 . 5 -- 37 . 3110 . sup . h , i 1 . 04 0 . 258 1 . 04 4 : 1 88 . 3 23 . 6 104 . 2 47 . 3126 . sup . h , i 1 . 04 0 . 258 1 . 04 4 : 1 121 . 1 42 . 1 87 . 8 63 . 0071 2 . 00 0 . 200 2 . 00 10 : 1 68 . 0 11 . 0 9 . 9 83 . 3119 2 . 00 0 . 1 2 . 00 20 : 1 39 . 8 6 . 1 -- 81 . 1__________________________________________________________________________ . sup . a mpt = methyl ptoluate . sup . b s = elemental sulfur . sup . c ph . sub . 2 o = dipehnyl ester , an inert diluent and solvent . . sup . d dmsdc = dimethyl trans4 , 4stilbenedicarboxylate . . sup . e bibenzyl = dimethyl 4 , 4bibenzyldicarboxylate . . sup . f side products include a benzothiophene , tetrakis ( 4carbomethoxyphenyl ) thiophene , 1 , 2 , 3tris ( 4 - carbomethoxyphenyl ) propane and 1 , 2 , 3tris ( 4 - carbomethoxyphenyl ) propene . . sup . g yield based on mpt consumed . . sup . h reaction temperature = 300 ° c . . sup . i not an example of the present invention . as can be seen from table 2 , low values of mpt / s also tend to promote formation of the bibenzyl over the stilbene product . two hundred twelve g ( 1 . 00 mole ) of phenyl p - toluate and 340 g of diphenyl ether were heated to 280 ° c . with 3 . 20 g ( 0 . 10 mole ) of sulfur in an autoclave at a maintained pressure of 100 psig with a nitrogen purge of about 2 standard cubic feet per hour . after about 2 hours the evolution of hydrogen sulfide , as measured in the nitrogen purge gas stream , had ceased and the reactor was allowed to cool . the reaction product mixture was filtered at 45 ° c . to yield 420 . 6 g of filtrate ( mixture of diphenyl ether and reacted phenyl p - toluate ) and 110 . 8 g of solid ( product and unreacted phenyl p - toluate ). the solid on the filter funnel was washed with several 100 ml portions of acetone to yield , after drying , 14 . 0 g of diphenyl 4 , 4 &# 39 ;- stilbenedicarboxylate . an additional 0 . 46 g of product was obtained by acetone rinse of the reactor vessel ; the total yield , 14 . 46 g , is 68 . 8 % of the theoretical based on the sulfur consumed . the combined products were recrystallized from cyclohexanone to give 13 . 2 g of light yellow product , mp . 227 °- 229 ° c . the nmr spectrum ( 70 % cdc13 , 30 % trifluoroacetic acid ) of the product was in accord with the proposed structure : δ 8 . 28 ( d , 4h ), 7 . 75 ( d , 4h ), 7 . 48 ( t , 4h ), 7 . 35 ( m , 4h ), 7 . 20 ( d , 4h ). phenyl acetate ( 204 . 3 g , 1 . 5 mole ) was charged to a 1 - liter , 3 - neck flask equipped with a magnetic stirrer , a distillation column , and a distillation head . the phenyl acetate was heated to 200 ° c . and dibutyltin oxide ( 1 . 0 g ) was added while stirring . dmsdc ( 148 . 1 g , 0 . 50 mole ) was then added portionwise to the mixture . all materials were in solution as the temperature approached 200 ° c . the solution was stirred for about 6 hours at 205 °- 208 ° c . while methyl acetate was collected overhead as it was formed . after standing overnight at 180 ° c ., the reaction mixture was reheated to 224 ° c . and held there for an additional 8 . 5 - hour period during which time methyl acetate was collected ; a total of 68 . 9 g of low boilers were collected . the reaction product was poured into a baking dish wherein the liquid solidified almost immediately . the solidified material was then ground with a mortar and pestle and slurried with cold dmf . the material was collected by filtration , washed with acetone and air dried ( yield : 182 g ). the crude product was recrystallized from hot dmf with a solvent / solute ratio of about 10 : 1 . crystals were precipitated by the cooling bright yellow dmf solution and were collected by filtration , washed with cold dmf than with acetone , and air dried . the solid was pale yellow ( yield : 177 . 4 g , m . p . 221 °- 223 ° c .). gas chromatography confirmed the purity of the desired product as greater than 99 %. sixty five g ( 0 . 306 mole ) of neat phenyl p - toluate were maintained in the temperature range of 280 °- 310 ° c . ( boiling point ) while a slow stream of nitrogen was bubbled in from below the surface ( sparged ). sulfur ( 1 . 92 g , 0 . 060 mole ) dissolved in more phenyl p - toluate ( 20 g , 0 . 099 mole ) at 100 ° c . was added slowly with stirring over a period of 2 - 3 hours . the heating , stirring and nitrogen sparging were continued an additional four hour and then the mixture was allowed to cool near 80 ° c . and 100 ml of boiling acetone was added . the suspension was transferred to a larger flask and an additional 400 ml of acetone was added . the suspension was filtered at the boil to yield 3 . 8 g of diphenyl 4 , 4 &# 39 ;- stilbenedicarboxylate .