Abstract
The 'ene' reaction is a pericyclic reaction related to the Diels-Alder reaction. It can be used to synthesise products of commercial importance. An example is its use to attach a 'succinic acid functionality' (via maleic anhydride -MA-), to the ene-terminated polyisobutylene (PIB) to yield PIBSA. While there have been a limited number of studies of the ene reaction, none have concentrated on the reactivity of the vinylidene fragment (CH2-C(=CH2)-CH3) which is found in PIB. In addition there are difficulties in the determination of the kinetics of the PIBSA reaction, because of several competitive processes, which may occur. Therefore the overall aim of the project is to investigate the kinetics and mechanism of the ene reaction between a range of simple vinylidene alkenes (including PIB) and maleic anhydride. A small range of alkenes, 2-methyl-4-phenylbut-1-ene, 2-methyl-4-phenylpent-1-ene, 2.4-dimethyl-4-phenylpent-1-ene and 2,4-dimethyl-4-(-4-fluorophenyl)pent-1-ene, were synthesised, in overall yields of 10 to 35 %. Syntheses were achieved using 1,4-conjugate addition, Friedel-Crafts and Wittig methodologies, and purification involved column chromatography and distillation. Product identification was by 1H NMR and 13C NMR spectroscopy. NMR monitoring of the reaction of 4-phenylbut-1-ene under conditions of equimolar MA in 1,2,4-trichlorobenzene solvent allowed the identification of the adduct 3-(4- phenyl-2-butenyl)dihydro-2,5-furandione as a 9:1 mixture of E and Z isomers. No evidence for further reaction to yield a di-adduct. Similar monitoring of the reactions of the other enes yielded no clear evidence of the adduct. However, under conditions of excess MA, 2,4-dimethyl-4-phenylpent-1-ene, 2.4-dimethyl-4-(-4-fluorophenyl)pent-l-ene yielded the expected adduct. PIB showed evidence of the adduct. A comparison of the 1H-NMR spectra for 2,4-dimethyl-4-phenylpent-1-ene, 2,4-dimethyl-4-(-4-fluorophenyl)pent-1-ene, PIB showed that, in all cases, only product derived from migration of the 1° H was found. Furthermore, no di-adduct was formed even for 2,4-dimethyl-4-phenylpent-1-ene, 2,4-dimethyl-4-(-4-fluorophenyl)pent-1-ene, with large excess of MA. The rate constants in the range 160 °C to 190 °C were determined for all alkenes mostly under conditions of excess MA, but some under conditions of excess alkene. Second order rate constants at 180°C varied from 4*10-5 to 100*10-5 with 4-phenylbut-1-ene being significantly slower than the others. A small medium effect of ca. 2 was identified as resulting from the change from excess ene to excess maleic anhydride. The activation parameters were in the range DeltaH+ 48 to 120 kJ mol-1 and DeltaS+ -53 to -229 J mol K-1. A computational calculation using Hartree-Fock and Semi-Empirical methods of the FMO, HOMOene-LUMOenophile gap allowed ranking of the alkenes. In general the gross trend in reactivity found experimentally was reflected in the computational work. The findings are compared to others and rationalised in terms of a pericyclic, exo Transition State, with the incipient C=C preferring an E configuration.