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Mechanism of the photoaddition of maleic anhydride to benzene
Tetrahedron Lettera Ro. 19, pp 1627 - 1630, 1975.
Pargcuon Freer.
Printed In &ant Britain.
MECBANIEM OF THE PHCTGADDITION OF MALEIC ANHYDRIDE TO BENZENE By Derek Bryce-iSmith*, Bama Bao Deshpandeand Andrew Gilbert ChemiBtry Department, Univereity of Reading. whiteknlghte park, ReadiDg (Rooeived in RE 6 -oh
1975; aooepted for publloation 3 April1975)
Althoughthe 2:l photoadditionof maleio anhydrideto benzene was fir& reported in 1959,la* nature of the 1:l intermediate(s) has until recently been in question: cf. refs. 1 and 2. The bicyclola octadiene (I) 0riginaBy proposed - bud subsequentlyappeared unlikely in view.of the failure by Beveral workers 2a - to intercept it with tetracyanoethylene, and an alternative awitter-ionic intermediate (IRJ 2b ha6 been suggested: - thie latter WBBconsistent with the charge-tramvfer nature of the excitation la 2c prooeee. - Bartmanu et al. have recently revived interest in the original proposal - by their discovery tlmt the trapping produot (m) of (9 with tetracyanoethylene can be obtained. albeit iu modest yield unleee dichloromethane ie used a8 a co-solvent. 3
The previoue failure6 to obtain (III)
resulted from an umupectedly low dienophilic reactivity of tetracyanoethylene towards (9 in the presence of benaene.3
4 Tetracyanoethylene is apparently completely complexed in benzene solution.
Although Hartmann*sfindings effectively reinstate the bicycle-octadiene (IQ aa an intermediate, high trapping efficiency with tetracyenoethylene requires the presence of dichloromethfme - which ie evidently affecting the oouree of the reaotion. Moreover, thie intermediate could not readily give rise 2b to the results reported from trapping experiments with a&de. 2b We now report evidence which, in coqjunotionwith our previoue findings, - euggeet that bicyclooctadiene (9 is the immediate precuroeor of the 2:l adduct (IV) in uneensitised and eensitised reactions in the absence of dichloromethane, and that zwitter-ion (Daa is the precursor of (I) in uneeneitised l-WCtiOIEk
We aleo find that dikromomethane. but not dichloromethane, a8 a solvent can effectively
promote a triplet procese. Tbue the relative quantumyields of (IV) from 0.2%&Mmaleic anhydride in benzene + diluent (2 :4 by vol. ) were 1.0 : 1.1 : 1.05 : 1. i for cyclobaxane, acetonitrfle, dichloromethane, and dibromomethanerespectively. x
It is noteworthy that dibromomethaneenhanceethe adduct formation
to a degree similar to that which results from incorporation of 0.06-M benxophenoneinto the cyclohexane solution. Thie effect is attributed to S, ‘Ti
intersyetem crossing in the excited benxene-
maleic anhydride complex under the influence of the heavy atom eolvent. As the benxophenonesensitized triplet pathwayis lees sensitive than the unsensitieed route to diversion by proton donors (giving phenylsucclnic anhydride), 2k the observation that the unsensitized reaction is eimilarly lese I Footnote.
All irradiations were carried out in Pyrex apparatus at 20. under argon using light
from a medium pressure mercury arc lamp.
1627
No. 19
1628
a
'a a
‘a
&
0
\
0
0
/: lc 0
0
0"
\S
a
\
tx
Gw”
Iloo. 19
1629
sensitive in dibromomethane further mggeste that dibromomethane act8 by imlucing a triplet pathway, and that the unsensitfsed process occurs by a singlet mechanism in the absence of heavy atom solvents. chemical, as distinct from physical, effects of heavy atom solvents have hitherto only been 5 observed in the photoreactions of acenapM.hyltmee. It is possible to reconcile the evidence of (Da, B on the one hand, with that for (9 on the other by the reasonable postulate that the former can cyclise into (I) unless intercepted by a proton donor, and that (Baais more readily protonated than the inductively polarised (l&J. We find thst N-phenylmaleimide is much more effective thsn tetracyanoethylene as a trap for (I). Irradiation of a benzene solution 0.17M in maleic anhydriae and 0.0846 M in N-phenylmaleimide in the presence and absence of dibromometbane gave the 1:l:l adduct (V) as the only detectable product. The yields of (V) were comparable with those of (IV) obtained in the absence of Kphenylmaleimtde. As expected, in the presence of 0.825M trifluoroacetic acid, the rate of formation of (V) was reduced drastically Q-fold), much less (2.2-fold).
but when dfbromomethane (19&Qwas also present, the reduction was
since efficient formation of (V) by addition of N-phenylmaleimide to (I@ and &
seems highly improbable, these results strongly support the proposal that (I@ and B are produced in the singlet and triplet processes respectively, and that each transforms into (I). We agree titb Hartmann that (I) is the immediate precursor of (IV) in the eeneitised and unsensitieed photoadditions of maleic anhydride to benzene.
The zwttterionic precursor of (9, namely (I&A, is evidently
involved in the unseneitised singlet reaction, but to a smaller extent, if at all, in the triplet process induced by bensophenone or dibromometbane. References 1.
(a)
IL J. F. Angus and D. Bryce-Smith, Proc. Ghem. 80~. , 1959, 326 ;
H. J. F. Angus and D. Bryce-Smith J. Ghem. &c.,
2.
1960, 4791.
(b)
G.Ck Bchench and R &einmetz, Tet.Lett.,
1960, No. 21. 1.
(c)
E. Grovenstein, D. V. Bao and J. W. Taylor, J.Am. Ghem. 8oc.)
(a)
W. M. Hardham snd G. 8. Hammoml, J.Am. Chem. kc.,
1961, 83_ 1705.
1967, 8& 3200 ;
G. L Fray and A. Gilbert quoted by D.Bryce-Smith, Fure Anal. them., (b)
D. Bryce+mith,
(c)
D. Bryce-smith and J. E. Lodge, J. Ghem. Sec., 1962, 2675.
R Deshpande, A. Gilbert and J. Grzonka,
3.
W. Hartmann, IL G. Heme, and L. tlchrader, Tet. Lett.,
4.
D. Bryce-Emith, B. E. Gannett and k Gilbert., J. Ghem. &c.(ag,
1968, 3
47.
Ghem. Corn... 1970, 561.
1974, 883, ibid. 3101.
1968, 816.
Ho. 19
1630
5.
D. 0. Cowan and R. L:Drisko, J.&uem. Chem. kc.,
T@.. L&t.,
1967, 83 3068 ;
1967, 1255 ; ibid.,
1970, 92, 6281 ;
B. F. Plummer and R A. Hall, Chem. Corn., 1970, 44
;
J. Meinwald, G. E. Samueleon and M. lkeda, J. Amer. Chem. Sot., 1970, g RF. Plummer and D. M. Chihal, J.Amer. Chem. Sot., W. L Ferrea and RF. Plummer, ibid.,
19’71, 93, 2071
Chem. Corn.., 1972, 306.
J. Amer. Chem. Sot., 1973,s
6709.
;
7604
;
Pargcuon Freer.
Printed In &ant Britain.
MECBANIEM OF THE PHCTGADDITION OF MALEIC ANHYDRIDE TO BENZENE By Derek Bryce-iSmith*, Bama Bao Deshpandeand Andrew Gilbert ChemiBtry Department, Univereity of Reading. whiteknlghte park, ReadiDg (Rooeived in RE 6 -oh
1975; aooepted for publloation 3 April1975)
Althoughthe 2:l photoadditionof maleio anhydrideto benzene was fir& reported in 1959,la* nature of the 1:l intermediate(s) has until recently been in question: cf. refs. 1 and 2. The bicyclola octadiene (I) 0riginaBy proposed - bud subsequentlyappeared unlikely in view.of the failure by Beveral workers 2a - to intercept it with tetracyanoethylene, and an alternative awitter-ionic intermediate (IRJ 2b ha6 been suggested: - thie latter WBBconsistent with the charge-tramvfer nature of the excitation la 2c prooeee. - Bartmanu et al. have recently revived interest in the original proposal - by their discovery tlmt the trapping produot (m) of (9 with tetracyanoethylene can be obtained. albeit iu modest yield unleee dichloromethane ie used a8 a co-solvent. 3
The previoue failure6 to obtain (III)
resulted from an umupectedly low dienophilic reactivity of tetracyanoethylene towards (9 in the presence of benaene.3
4 Tetracyanoethylene is apparently completely complexed in benzene solution.
Although Hartmann*sfindings effectively reinstate the bicycle-octadiene (IQ aa an intermediate, high trapping efficiency with tetracyenoethylene requires the presence of dichloromethfme - which ie evidently affecting the oouree of the reaotion. Moreover, thie intermediate could not readily give rise 2b to the results reported from trapping experiments with a&de. 2b We now report evidence which, in coqjunotionwith our previoue findings, - euggeet that bicyclooctadiene (9 is the immediate precuroeor of the 2:l adduct (IV) in uneensitised and eensitised reactions in the absence of dichloromethane, and that zwitter-ion (Daa is the precursor of (I) in uneeneitised l-WCtiOIEk
We aleo find that dikromomethane. but not dichloromethane, a8 a solvent can effectively
promote a triplet procese. Tbue the relative quantumyields of (IV) from 0.2%&Mmaleic anhydride in benzene + diluent (2 :4 by vol. ) were 1.0 : 1.1 : 1.05 : 1. i for cyclobaxane, acetonitrfle, dichloromethane, and dibromomethanerespectively. x
It is noteworthy that dibromomethaneenhanceethe adduct formation
to a degree similar to that which results from incorporation of 0.06-M benxophenoneinto the cyclohexane solution. Thie effect is attributed to S, ‘Ti
intersyetem crossing in the excited benxene-
maleic anhydride complex under the influence of the heavy atom eolvent. As the benxophenonesensitized triplet pathwayis lees sensitive than the unsensitieed route to diversion by proton donors (giving phenylsucclnic anhydride), 2k the observation that the unsensitized reaction is eimilarly lese I Footnote.
All irradiations were carried out in Pyrex apparatus at 20. under argon using light
from a medium pressure mercury arc lamp.
1627
No. 19
1628
a
'a a
‘a
&
0
\
0
0
/: lc 0
0
0"
\S
a
\
tx
Gw”
Iloo. 19
1629
sensitive in dibromomethane further mggeste that dibromomethane act8 by imlucing a triplet pathway, and that the unsensitfsed process occurs by a singlet mechanism in the absence of heavy atom solvents. chemical, as distinct from physical, effects of heavy atom solvents have hitherto only been 5 observed in the photoreactions of acenapM.hyltmee. It is possible to reconcile the evidence of (Da, B on the one hand, with that for (9 on the other by the reasonable postulate that the former can cyclise into (I) unless intercepted by a proton donor, and that (Baais more readily protonated than the inductively polarised (l&J. We find thst N-phenylmaleimide is much more effective thsn tetracyanoethylene as a trap for (I). Irradiation of a benzene solution 0.17M in maleic anhydriae and 0.0846 M in N-phenylmaleimide in the presence and absence of dibromometbane gave the 1:l:l adduct (V) as the only detectable product. The yields of (V) were comparable with those of (IV) obtained in the absence of Kphenylmaleimtde. As expected, in the presence of 0.825M trifluoroacetic acid, the rate of formation of (V) was reduced drastically Q-fold), much less (2.2-fold).
but when dfbromomethane (19&Qwas also present, the reduction was
since efficient formation of (V) by addition of N-phenylmaleimide to (I@ and &
seems highly improbable, these results strongly support the proposal that (I@ and B are produced in the singlet and triplet processes respectively, and that each transforms into (I). We agree titb Hartmann that (I) is the immediate precursor of (IV) in the eeneitised and unsensitieed photoadditions of maleic anhydride to benzene.
The zwttterionic precursor of (9, namely (I&A, is evidently
involved in the unseneitised singlet reaction, but to a smaller extent, if at all, in the triplet process induced by bensophenone or dibromometbane. References 1.
(a)
IL J. F. Angus and D. Bryce-Smith, Proc. Ghem. 80~. , 1959, 326 ;
H. J. F. Angus and D. Bryce-Smith J. Ghem. &c.,
2.
1960, 4791.
(b)
G.Ck Bchench and R &einmetz, Tet.Lett.,
1960, No. 21. 1.
(c)
E. Grovenstein, D. V. Bao and J. W. Taylor, J.Am. Ghem. 8oc.)
(a)
W. M. Hardham snd G. 8. Hammoml, J.Am. Chem. kc.,
1961, 83_ 1705.
1967, 8& 3200 ;
G. L Fray and A. Gilbert quoted by D.Bryce-Smith, Fure Anal. them., (b)
D. Bryce+mith,
(c)
D. Bryce-smith and J. E. Lodge, J. Ghem. Sec., 1962, 2675.
R Deshpande, A. Gilbert and J. Grzonka,
3.
W. Hartmann, IL G. Heme, and L. tlchrader, Tet. Lett.,
4.
D. Bryce-Emith, B. E. Gannett and k Gilbert., J. Ghem. &c.(ag,
1968, 3
47.
Ghem. Corn... 1970, 561.
1974, 883, ibid. 3101.
1968, 816.
Ho. 19
1630
5.
D. 0. Cowan and R. L:Drisko, J.&uem. Chem. kc.,
T@.. L&t.,
1967, 83 3068 ;
1967, 1255 ; ibid.,
1970, 92, 6281 ;
B. F. Plummer and R A. Hall, Chem. Corn., 1970, 44
;
J. Meinwald, G. E. Samueleon and M. lkeda, J. Amer. Chem. Sot., 1970, g RF. Plummer and D. M. Chihal, J.Amer. Chem. Sot., W. L Ferrea and RF. Plummer, ibid.,
19’71, 93, 2071
Chem. Corn.., 1972, 306.
J. Amer. Chem. Sot., 1973,s
6709.
;
7604
;