the solvents employed in the practice of this invention are n - heterocyclic compounds including pyridine , α - picoline , β - picoline , γ - picoline , 2 , 3 - lutidine , 2 , 4 - lutidine , 2 , 5 - lutidine , 2 , 6 - lutidine , 3 , 4 - lutidine , 3 , 5 - lutidine , 4 - benzyl pyridine , 4 - vinyl pyridine , quinoline and isoquinoline . of these compounds , pyridine , β - picoline , γ - picoline , 3 , 4 - lutidine , 3 , 5 - lutidine and isoquinoline are preferred . it is critical that at least two solvents as mentioned above be used . in general , any one of at least two solvents is preferably used in an amount of more than 2 moles %, more preferably 5 moles %, most preferably 10 moles % on the basis of total mole of the solvents . the use of mixture of the solvents in which any one of the solvents is present in an amount of more than 2 moles % gives results superior to those obtained with the use of one solvent . the amount of mixture of the solvents employed is not critical . in general , the mixture of the solvents is used in an amount of from 0 . 05 to 10 % by weight on the basis of weight of butadiene , preferably from 0 . 2 to 3 % by weight . use of the mixed solvents in an amount less than 0 . 05 % by weight is likely to cause side - reaction . the mixed solvents in an amount of more than 10 % by weight lowers the reaction speed and increases the cost of recovering the solvents . the cobalt carbonyl catalyst employed in the practice of this invention include cobalt carbonyl and cobalt carbonyl complex . the cobalt carbonyl catalyst may be the synthetic solution obtained by reacting synthetic gas ( co and h 2 ) with cobalt compound ( s ) selected from the group consisting of inorganic cobalt compounds , such as cobalt hydroxide , cobalt carbonate and basic cobalt carbonate or organic cobalt compounds , such as cobalt organic acid salts , cobaltocene and cobalt acetylacetonate in the alcohol employed as a starting material , and the synthetic solution obtained by reacting the cobalt compound ( s ), said n - heterocyclic compound or other compound having ligand , and co gas . in the prior method for producing a 3 - pentenoic ester by hydroesterificating butadiene by using a large amount of cobalt carbonyl or cobalt carbonyl complex , the cobalt carbonyl or the cobalt carbonyl complex must be prepared by a complicated and costly process . on the other hand , since a 3 - pentenoic ester can be prepared by using a small amount of catalyst according to the present invention , the synthetic solution containing cobalt carbonyl catalyst can be prepared easily from an inorganic or organic cobalt compound . the amount of cobalt carbonyl catalyst employed is not critical . when dicobalt octacarbonyl is employed , preferably , dicobalt octacarbonyl in an amount of 0 . 001 to 0 . 05 moles per 1 mole of butadiene , more preferably dicobalt octacarbonyl in an amount of 0 . 005 to 0 . 03 moles may be industrially practiced . the use of catalyst in an amount of less than the lower limit as mentioned above lowers the reaction speed too much . the use of catalyst in an amount of more than the upper limit merely adds to production cost , since the cost of recovering the catalyst increases . in the present invention , a specified polymerization inhibitor may be added to the reaction system . addition of the polymerization inhibitor to the reaction system increases yield of the object product and selectivity to the object product without retarding the reaction speed . the polymerization inhibitors employed in the practice of this invention are inhibitors for vinyl polymerization . the compounds which are known as an inhibitor for vinyl polymerization are amino compounds , quinone compounds , hydroxy compounds , nitro compounds , nitroso compounds , sulfur compounds , organic sulfur compounds and inorganic salts . however , of these inhibitors , only aromatic amino compounds , quinone compounds and aromatic hydroxy compounds ( except hydroquinone ) can be employed as a polymerization inhibitor of this invention . examples of the aromatic amino compounds include n - methylaniline , diphenylamine , n , n &# 39 ;- tetraethyl - p - phenylenediamine , n , n &# 39 ;- di - β - naphthyl - p - phenylenediamine . examples of quinone compounds include p - benzoquinone , chloranil , anthraquinone and phenanthraquinone . examples of the hydroxy compounds include catechol and its derivatives , such as p - tert .- butylcatechol . one of the polymerization inhibitors or mixtures thereof may be used in the present invention . the amount of the polymerization inhibitor employed may be more than 0 . 0001 mole per 1 mole of butadiene . the upper limit of the amount if unnecessary to be specified . in general , the use of the polymerization inhibitor in an amount of less than 0 . 1 mole % is sufficiently effective . the use of the inhibitor in an amount of more than 0 . 1 mole merely adds to production cost . however , compounds which are liquid at room temperature and have relatively low melting point , for example , n - methyl aniline and diphenylamine in an amount of less than 10 moles , preferably less than 5 mole per 1 mole of butadiene may be used as a solvent in combination with the reaction medium compound of at least two solvents as mentioned above . the use of the compound , such as n - methyl aniline or diphenylamine can reduce the amount of an expensive tertiary amine employed . alcohols employed in the practice of this invention include alcohols having 1 - 10 carbon atoms , such as methanol , ethanol , propanol and butanol . methanol and ethanol are important industrially . one of these alcohols or mixture thereof may be used . the amount of the alcohol employed is not critical . preferably , the alcohol in an amount of more than equivalent mole to butadiene , more preferably the alcohol in an amount of 1 to 5 moles per 1 mole of butadiene may be used . when the alcohol in an amount of less than equivalent mole to butadiene is used , expensive butadiene is consumed for undesirable side reaction . the use of the alcohol in an amount of more than 5 moles per mole of butadiene lowers the hydroesterification reaction speed . the reaction temperature is not critical . profitably , the reaction temperature may be in the range of 80 ° to 200 ° c ., preferably 100 ° to 140 ° c . partial pressure of carbon monoxide is not critical . preferably , partial pressure of carbon monoxide may be more than 50 kg / cm 2 , and the partial pressure in the range of 100 to 400 kg / cm 2 . when the hydroesterification reaction of butadiene is carried out in a reaction medium composed of at least two n - heterocyclic compounds , the reaction speed is sufficiently fast , even if only a small amount of cobalt carbonyl catalyst is used , and the undesirable side reaction is sufficiently suppressed . according to the present invention , hydroesterification reaction of butadiene can be effected with little expenditure for catalyst . the present invention can be carried out either as batch process or as a continuous process . into a 200 ml stainless steel autoclave equipped with magnet stirrer were charged 15 grs . ( 0 . 277 moles ) of butadiene , 11 grs . ( 0 . 343 moles ) of methanol and dicobalt octacarbonyl catalyst and pyridine - isoquinoline mixture in an amount as given in table 1 . the reaction was effected at 122 ° c . under carbon monoxide partial pressure of 300 kg / cm 2 . the results are shown in table 1 . control tests 1 - 3 correspond to example 1 and control tests 4 - 6 correspond to example 2 . the reaction conditions are shown in table 1 . the results are also shown in table 1 . when the reaction was effected in pyridine alone or isoquinoline alone , the reaction speed is slow and yield of methyl 3 - pentenoate and selectivity to methyl 3 - pentenoate are very low . table 1__________________________________________________________________________ control control control control control controlexamples and control tests example 1 test 1 test 2 test 3 example 2 test 4 test test__________________________________________________________________________ 6components butadiene g ( mole ) 15 ( 0 . 277 ) 15 ( 0 . 277 ) methanol g ( mole ) 11 ( 0 . 343 ) same as same as same as 11 ( 0 . 343 ) same as same same as co . sub . 2 ( co ). sub . 8 g ( mole ) 2 ( 0 . 0058 ) the left the left the left 0 . 9 ( 0 . 0026 ) the left the the left pyridine g ( mole ) 10 ( 0 . 126 ) 10 ( 0 . 126 ) -- -- 10 ( 0 . 126 ) 10 ( 0 . 126 ) 20 ( 0 . 253 ) -- isoquinoline g ( mole ) 10 ( 0 . 077 ) -- 10 ( 0 . 077 ) 20 ( 0 . 155 ) 10 ( 0 . 077 ) -- -- 20 ( 0 . 155 ) conditions reaction temp . (° c .) 122 same as same as same as 125 same as same same as reaction pressure (° c .) 300 the left the left the left 300 the left the the left reaction time ( hr ) 1 . 8 2 . 5 2 . 5 3 . 5 3 . 0 3 . 5 5 . 0 3 . 5reactivity of butadiene 93 . 5 79 . 0 80 . 8 94 . 0 92 . 6 62 . 1 91 . 0 57 . 4mole % yield of methyl 84 . 7 48 . 2 61 . 5 58 . 9 83 . 3 28 . 9 37 . 2 30 . 43 - pentenoate mole % selectivity to methyl 90 . 6 61 . 0 76 . 1 62 . 7 90 . 0 46 . 5 40 . 9 53 . 03 - pentenoate mole % selectivity to methyl 3 . 9 1 . 1 1 . 5 1 . 2 3 . 1 0 . 2 0 . 3 1 . 12 or 4 pentenoate mole % selectivity to 4 - vinyl - 1 - 3 . 8 7 . 9 5 . 2 5 . 0 3 . 7 10 . 6 14 . 5 11 . 8cyclohexene mole % selectivity to dibutyl 1 . 4 2 . 6 2 . 3 1 . 3 1 . 1 1 . 3 1 . 2 1 . 1ketone mole % selectivity to dimethyl 0 . 2 2 . 0 1 . 2 1 . 0 0 . 2 0 . 6 0 . 4 0 . 7methylglutarate mole % selectivity to other un - 0 . 2 25 . 4 13 . 6 28 . 8 2 . 0 40 . 8 42 . 7 32 . 4known components contain - ing polymers mole % __________________________________________________________________________ note : yield and selectivity were calculated on the basis of butadiene in table 1 - 5 . the procedures of example 1 were repeated except that the proportion of the mixed solvents as given in table 2 was used . the results are shown in table 2 . table 2______________________________________example no . example 3 example 4______________________________________component butadiene g ( mole ) 15 ( 0 . 277 ) 15 ( 0 . 277 ) methanol g ( mole ) 11 ( 0 . 343 ) 11 ( 0 . 343 ) co . sub . 2 ( co ). sub . 8 g ( mole ) 2 ( 0 . 0058 ) 2 ( 0 . 0058 ) pyridine g ( mole ) 10 ( 0 . 126 ) 0 . 6 ( 0 . 008 ) isoquinoline g ( mole ) 1 . 5 ( 0 . 012 ) 10 ( 0 . 077 ) conditions reaction temperature (° c .) 122 122 reaction pressure of 300 300 co ( kg / cm . sup . 2 ) reaction time ( hr ) 2 . 0 2 . 0reactivity of 93 . 9 92 . 0butadiene mole % yield of methyl 82 . 5 79 . 73 - pentenoate mole % selectivity to methyl 87 . 9 86 . 63 - pentenoate mole % selectivity to methyl 2 . 2 1 . 62 or 4 pentenoate mole % selectivity to 4 - vinyl - 1 - 4 . 1 4 . 7cyclohexene mole % selectivity to dibutyl 1 . 8 1 . 6ketone mole % selectivity to dimethyl 1 . 6 1 . 0methylglutarate mole % selectivity to other unknown 2 . 4 4 . 6components containing polymersmole % ______________________________________ the reactions were effected under the conditions as given in table 3 . the results are shown in table 3 . table 3__________________________________________________________________________ control control control controlexamples and control tests exampl 5 test 7 exampl 6 test 8 exampl 7 test 9 exampl 8 exampl test__________________________________________________________________________ 10com - butadiene g ( mole ) 15 ( 0 . 277 ) 15 ( 0 . 277 ) ponentsmethanol g ( mole ) 11 ( 0 . 343 ) same as same as same as same as same as same as 11 ( 0 . 343 ) same asco . sub . 2 ( co ). sub . 8 g ( mole ) 2 ( 0 . 0058 ) the left the left the left the left the left the left 2 ( 0 . 0058 ) the leftsolvent i kind pyridine -- pyridine -- pyridine -- γ - picoline pyridine -- g 10 ( 0 . 126 ) -- 10 ( 0 . 126 ) -- 10 ( 0 . 126 ) -- 10 ( 0 . 107 ) 10 ( 0 . 126 ) -- solvent ii kind γ - picoline same as β - picoline same as 3 , 5 - same as β - picoline 3 , 4 - same as the the lutidine the lutidine the g 10 ( 0 . 107 ) left 10 ( 0 . 107 ) left 10 ( 0 . 093 ) left 10 ( 0 . 107 ) 10 ( 0 . 093 ) leftcon - reaction temp . ° c . 122 122 122 122 122 122 122 122 122ditionreaction pressure 300 300 300 300 300 300 300 300 300 ( co ) kg / cm . sup . 2reaction time hr 1 . 5 2 . 5 1 . 5 2 . 5 1 . 5 2 . 5 1 . 5 1 . 5 2 . 5reactivity of butadienemole % 99 . 3 97 . 3 99 . 0 96 . 7 95 . 2 80 . 5 98 . 1 96 . 9 85 . 4yield of methyl3 - pentenoate mole % 90 . 8 75 . 3 92 . 6 76 . 3 85 . 1 52 . 6 89 . 1 87 . 7 56 . 7selectivity to methyl3 - pentenoate mole % 91 . 0 77 . 4 93 . 5 78 . 9 89 . 4 65 . 3 90 . 8 90 . 5 66 . 4selectivity to methyl2 or 4 pentenoate mole % 2 . 0 0 . 5 1 . 3 0 . 4 2 . 1 1 . 2 1 . 5 2 . 0 1 . 3selectivity to 4 - vinyl - 1 - cyclohexene mole % 3 . 4 5 . 7 3 . 5 6 . 2 4 . 0 7 . 4 3 . 7 3 . 9 6 . 7selectivity to dibutylketone mole % 0 . 7 1 . 1 0 . 8 2 . 0 1 . 2 2 . 2 1 . 0 1 . 0 2 . 4selectivity to dimethylmethylglutarate mole % 0 . 3 1 . 3 0 . 4 1 . 5 0 . 5 1 . 3 0 . 5 0 . 4 1 . 8selectivity to other unknowncomponents containing 2 . 6 14 . 0 0 . 5 11 . 0 2 . 8 22 . 6 2 . 5 2 . 2 21 . 4polymers mole % __________________________________________________________________________ into a 500 ml stainless steel autoclave equipped with magnet stirrer were charged 56 grs . ( 1 . 035 mole ) of butadiene , 63 grs . ( 1 . 367 mole ) of ethanol and dicobalt octacarbonyl catalyst and pyridine - isoquinoline in an amount as given in table 4 . the reaction was effected at 123 ° c . under carbon monoxide partial pressure of 300 kg / cm 2 . the conditions are shown in table 4 . control tests 11 and 12 corresponding to example 10 are shown in table 4 . when the reaction was effected in pyridine alone or isoquinoline alone , the reaction speed was slow and yield of ethyl 3 - pentenoate and selectivity to ethyl 3 - pentenoate were very low . table 4__________________________________________________________________________ control controlexample no . or control test no . example 10 test 11 test 12__________________________________________________________________________compo - butadiene g ( mole ) 56 ( 1 . 035 ) same as same asnentsethanol g ( mole ) 63 ( 1 . 367 ) the left the leftco . sub . 2 ( co ). sub . 8 g ( mole ) 7 ( 0 . 0205 ) pyridine g ( mole ) 40 ( 0 . 506 ) 40 ( 0 . 506 ) isoquinoline g ( mole ) 40 ( 0 . 294 ) 40 ( 0 . 506 ) reactionreaction temperature ° c . 123 same as same ascondi - reaction pressure ( co ) kg / cm . sup . 2 300 the left the lefttion reaction time hr 2 . 5 4 . 0 4 . 0reactivity of butadiene mole % 96 . 4 78 . 6 82 . 0yield of ethyl 3 - pentenoate mole % 88 . 2 42 . 4 48 . 0selectivity of ethyl 3 - pentenoatemole % 91 . 5 54 . 0 58 . 5selectivity to ethyl 2 - or 4 - pentenoate mole % 2 . 0 1 . 5 1 . 8selectivity to 4 - vinyl - 1 - cyclohexenemole % 0 . 4 6 . 3 5 . 9selectivity to dibutyl ketone mole % 0 . 5 3 . 0 2 . 6selectivity to diethyl methyl - glutarate mole % 0 . 1 2 . 7 2 . 5selectivity to other unknowncomponents containing polymers mole % 5 . 2 32 . 5 28 . 7__________________________________________________________________________ into a 200 ml stainless steel autoclave equipped with magnet stirrer 15 grs . ( 0 . 277 mole ) of butadiene , 11 grs . ( 0 . 343 mole ) of methanol and 2 grs . of dicobalt octacarbonyl and solvent and polymerization inhibitor in an amount as given in table 5 were charged . the reactions were effected at 120 ° c . under 300 kg / cm 2 g . table 5__________________________________________________________________________examples and control tests example 11 example 12 example 13 example 14 example 15 example example__________________________________________________________________________ 17componentbutadiene g ( mole ) 15 ( 0 . 277 ) same as same as same as 15 ( 0 . 277 ) same 15 ( 0 . 277 ) methanol g ( mole ) 11 ( 0 . 343 ) the left the left the left 11 ( 0 . 343 ) the left 11 ( 0 . 343 ) co . sub . 2 ( co ). sub . 8 g ( mole ) 2 ( 0 . 0058 ) 2 ( 0 . 0058 ) 2 ( 0 . 0058 ) solvent i kind pyridine pyridine pyridine g ( mole ) 10 ( 0 . 126 ) 10 ( 0 . 126 ) 10 ( 0 . 126 ) solvent ii kind isoquinoline β - picoline γ - picoline 1 g ( mole ) 10 ( 0 . 077 ) 10 ( 0 . 107 ) 10 ( 0 . 107 ) polymeriza - kind diphenyl - n , n &# 39 ;- di - β - t - butyl p - benzo - same as same diphenyltion amine naphthyl p - catechol quinone ex . 11 ex . 12 amineinhibitor phenylenediamine g ( mole ) 1 . 0 ( 0 . 006 ) 1 . 0 ( 0 . 003 ) 1 . 0 ( 0 . 006 ) 1 ( 0 . 010 ) 1 . 0 ( 0 . 006 ) 1 . 0 ( 0 . 003 ) 1 . 0 ( 0 . 006 ) conditionreaction temperature ° c . 122 same as same as same as same as same 122reaction pressure ( co ) kg / cm . sup . 2 300 the left the left the left the left the left 300reaction time hr 2 . 0 2 . 0 2 . 0 2 . 0 1 . 5 1 . 5 1 . 5reactivity of butadiene mole % 93 . 4 94 . 7 94 . 3 92 . 1 99 . 3 99 . 5 98 . 4yield of methyl 3 - pentenoatemole % 91 . 2 92 . 8 91 . 5 89 . 1 97 . 5 98 . 0 96 . 4selectivity to methyl 3 - pentenoate mole % 97 . 6 98 . 0 97 . 1 96 . 7 98 . 2 98 . 5 98 . 0selectivity to methyl 2 or 4pentenoate mole % 0 . 3 0 . 2 0 . 4 0 . 3 0 . 2 0 . 2 0 . 2selectivity to 4 - vinyl - 1 - cyclohexene mole % 1 . 4 1 . 3 1 . 4 1 . 5 1 . 2 1 . 0 1 . 1selectivity to dibutyl ketonemole % 0 . 2 0 . 1 0 . 2 0 . 3 0 . 1 0 . 1 0 . 1selectivity to dimethylmethylgultarate mole % & lt ; 0 . 1 0 . 1 0 . 1 & lt ; 0 . 1 & lt ; 0 . 1 & lt ; 0 . 1 & lt ; 0 . 1selectivity to other unknown com - ponents containing polymers mole % 0 . 4 0 . 3 0 . 8 1 . 1 0 . 2 0 . 2 0 . 5__________________________________________________________________________examples and control tests example 18 example 19 example 20 example 21 example 22 example example__________________________________________________________________________ 24componentbutadiene g ( mole ) 15 ( 0 . 277 ) 15 ( 0 . 277 ) same as 15 ( 0 . 277 ) same as 15 ( 0 . 277 ) same asmethanol g ( mole ) 11 ( 0 . 343 ) 11 ( 0 . 343 ) the left 11 ( 0 . 343 ) the left 11 ( 0 . 343 ) the leftco . sub . 2 ( co ). sub . 8 g ( mole ) 2 ( 0 . 0058 ) 2 ( 0 . 0058 ) 2 ( 0 . 0058 ) 2 ( 0 . 0058 ) solvent i kind pyridine pyridine pyridine β - picoline g ( mole ) 10 ( 0 . 126 ) 10 ( 0 . 126 ) 10 ( 0 . 126 ) 10 ( 0 . 107 ) solvent ii kind γ - picoline 3 , 4 - lutidine 3 , 5 - lutidine γ - picoline g ( mole ) 10 ( 0 . 107 ) 10 ( 0 . 093 ) 10 ( 0 . 093 ) 10 ( 0 . 107 ) polymeriza - kind t - butyl diphenyl n , n &# 39 ;- di - β - diphenyl p - benzo - diphenyl n , n &# 39 ;- di - β - tion catechol amine naphthyl p - amine quinone amine naphthyl p - inhibitor phenylene phenylene diamine diamine g ( mole ) 1 . 0 ( 0 . 006 ) 1 . 0 ( 0 . 006 ) 1 . 0 ( 0 . 003 ) 1 . 0 ( 0 . 006 ) 1 . 0 ( 0 . 010 ) 1 . 0 ( 0 . 006 ) 1 . 0 ( 0 . 003 ) conditionreaction temperature ° c . 122 same as same as same as same as same as same asreaction pressure ( co ) kg / cm . sup . 2 300 the left the left the left the left the left the leftreaction time hr 1 . 5 1 . 5 1 . 5 1 . 5 1 . 5 1 . 5 1 . 5reactivity of butadiene mole % 97 . 7 97 . 8 97 . 3 98 . 0 96 . 8 98 . 5 99 . 1yield of methyl 3 - pentenoatemole % 95 . 0 95 . 5 94 . 2 95 . 4 93 . 9 96 . 5 97 . 4selectivity to methyl 3 - pentenoate mole % 97 . 2 97 . 6 96 . 9 97 . 4 97 . 0 98 . 0 98 . 3selectivity to methyl 2 or 4pentenoate mole % 0 . 3 0 . 3 0 . 3 0 . 3 0 . 3 0 . 2 0 . 2selectivity to 4 - vinyl - 1 - cyclohexene mole % 1 . 4 1 . 1 1 . 3 1 . 2 1 . 4 0 . 9 1 . 0selectivity to dibutyl ketonemole % 0 . 1 0 . 1 0 . 2 0 . 1 0 . 2 0 . 1 0 . 1selectivity to dimethylmethylgultarate mole % & lt ; 0 . 1 & lt ; 0 . 1 & lt ; 0 . 1 & lt ; 0 . 1 & lt ; 0 . 1 & lt ; 0 . 1 & lt ; 0 . 1selectivity to other unknown com - ponents containing polymers mole % 0 . 9 0 . 8 1 . 2 0 . 9 1 . 0 0 . 7 0 . 3__________________________________________________________________________