Abstract
The importance of epoxides as synthetic intermediates in a number of highly added-value chemicals, as well as the search for novel and more sustainable chemical processes have brought considerable attention to the catalytic activity of manganese and iron complexes towards the epoxidation of alkenes using non-toxic terminal oxidants. Particular attention has been given to Mn(salen) and Fe(porphyrin) catalysts. While the former attain remarkable enantioselectivity towards the epoxidation of cis-alkenes, the latter also serve as an important model for the behavior of cytochrome P450, thus allowing the exploration of complex biological processes. In this review, a systematic survey of the bibliographical data for the theoretical studies on Mn- and Fe-catalyzed epoxidations is presented. The most interesting patterns and trends are reported and finally analyzed using an evaluation framework similar to the SWOT (Strengths, Weaknesses, Opportunities and Threats) analysis performed in enterprise media, with the ultimate aim to provide an overview of current trends and areas for future exploration.
Highlights
Introduction and ContextEpoxides are cyclic ethers in which one oxygen atom and two carbon atoms are arranged approximately as an equilateral triangle
CASSCF, they were unable to conclude on the overall quality of Density Functional Theory (DFT) calculations. Both works rely on SV basis sets with minimal polarization; Ivanic further resorted to Effective Core Potentials (ECP) in order to allow more polarization to be used in the description of the manganese and chlorine atoms
According to the model proposed by Cavallo and Jacobsen [17,75,76,81], the stereospecificity observed in this reaction derives from the relative stability of the radical intermediate: an unstable radical intermediate would promptly collapse in order to form the epoxide, whereas more stable intermediates would allow rotation of the substituents in one of the olefinic carbon atoms before the formation of the second C−O
Summary
Epoxides are cyclic ethers in which one oxygen atom and two carbon atoms are arranged approximately as an equilateral triangle. Transition Metal (TM) complexes are very important catalysts in the epoxidation of simple alkenes and other olefinic materials, such as allylic alcohols using H2 O2 or TBHP as primary oxygen sources. Their use allows the synthesis of more complex epoxides, as well as the regioselective epoxidation of poly-unsaturated compounds or the stereoselective formation of chiral epoxides. According to McGarrigle and Gilheany [11], the three main themes in theoretical studies are: the geometry, spin state and radical nature of the active catalyst; the conformation of both the catalyst in both active and inactive forms; and the reaction mechanism This latter category can be further sub-divided into studying the formation of the active specie(s) responsible for the epoxidation of the substrate and the oxidation of the C−C bond per se. It is justified that, over a decade later, the current state of the art in the theoretical modeling of Mn- and Fe-catalyzed epoxidations is addressed
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