This thesis deals the with steric and electronic effects inthe manganese(salen) catalyzed epoxidation of alkenes.Different methods were employed in the mechanisticinvestigations; molecular modelling, radical trapping, Hammettstudies on linear free-energy relationships and densityfunctional methods. The selectivity effects of phenol additiveswere also studied.
A new stereoselectivity rationale was introduced bymolecular modelling studies. A manganoxetane intermediate wasfound to provide an explanation to the steric effects on theenantioselectivity in the manganese(salen) catalyzedepoxidation of phenyl substituted alkenes. The conformationalpreferences of the diimine bridge were found to provide arationale for the enantioselectivity of the reaction.
The consistency of a radical intermediate on the reactionpathway was tested in the epoxidation of phenyl substitutedradical probes. No product epoxides resulting from CCbond rotation were found indicating that the cis-transisomerization process might involve a radical intermediate butthat syn O-addition proceeds in a non-radical manner. Theepoxidation of the radical probes proceeded under optimizedconditions without any radical intermediates being trapped.
A Hammett study was performed usingpara-substitutedcis-stilbenes in order to investigate electronic effectson the enantio- and diastereoselectivity of the reaction. Theresults indicate that epoxidation of aryl-alkenes proceed viatwo separate routes; one concerted but asynchronous giving cisepoxides, and one stepwise with the first step rate determininggiving trans epoxides, with a competing pathway in the secondstep leading to non-epoxide products for electron-richsubstrates.
A theoretical study using density functional methodssupports the experimental observations from the Hammett study.The calculations predict that the reaction involves differentspin states and that the timing of the spin change from thetriplet to the quintet state determines the cis-trans productdistribution in the reaction.
The addition of phenolates were found to influence thediastereoselectivity in the epoxidation ofcis-stilbene. 2,4,6-Tri-t-butyl phenol gave highly selective formation of thetrans epoxide with high enantioselectivity under optimizedconditions.
Keywords:manganese, salen, epoxidation, mechanism,oxetane, radical, intermediate, steric effect, electroniceffect, spin, selectivity.
Institutionen för organisk kemi , 1998. , 65 p.