Titanium Salan Catalysts for the Asymmetric Epoxidation of Alkenes: Steric and Electronic Factors Governing the Activity and Enantioselectivity

Abstract

A new insight into the highly enantioselective (up to >99.5 % ee) epoxidation of olefins in the presence of chiral titanium(IV) salan complexes is reported. A series of 14 chiral ligands with varying steric and electronic properties have been designed, and it was found that electronic effects modulate the catalytic activity (without affecting the enantioselectivity), whereas the steric properties account for the enantioselectivity of the epoxidation. Competitive oxidations of p-substituted styrenes reveal the electrophilic nature of the oxygen-transferring active species, with a Hammett ρ value of -0.51; the enantioselectivity is unaffected by the electron-donating (or withdrawing) ability of the p-substituents. Mechanistic studies provide evidence in favor of a stepwise reaction mechanism: in the first (rate-determining) stage, olefin most probably coordinates to the active species, followed by intramolecular enantioselective oxygen transfer. The enantioselectivity increases with decreasing temperature. The modified Eyring plots for the epoxidation of styrene and (Z)-β-methylstyrene are linear, indicating a single, enthalpy-controlled mechanism of stereoselectivity, with ΔΔH(≠) =-6.6 kJ mol(-1) and -5.4 kJ mol(-1) , respectively.

Heterodinuclear titanium/zinc catalysis: synthesis, characterization and activity for CO2/epoxide copolymerization and cyclic ester polymerization

“…Titanium catalysts are useful in various transformations including hydroaminoalkylation reactions, 18 aldol and allylic additions to ketones and aldehydes, 19,20 and the epoxidation of alkenes. 21 In polymerization catalysis, titanium complexes are particularly effective single site catalysts for olefin polymerization, [22][23][24] and active titanium catalysts have been reported for oxygenated monomers including ε-caprolactone, 15,[25][26][27][28] rac-lactide and, more recently, CO 2 /epoxide ring opening co-polymerization (ROCOP). 27,[50][51][52][53][54][55][56][57] Heterodinuclear complexes have shown great promise in polymerization catalysis and have allowed greater activities and selectivities to be achieved.…”

Synthetic utility of metal catalyzed hydrogen peroxide oxidation of C-H, C-C and C=C bonds in alkanes, arenes and alkenes: Recent advances

“…The results revealed, that the electronic effect controls the catalytic activity, whereas the steric effect accounts for the facial selectivity of the epoxidation. 123 Recently, it was reported that Ti-salan complex is able to catalyze distal selective epoxidation of conjugated dienes, even in the presence of other olefins and adjacent stereocenters. In this context, a variety of wellestablished methods for asymmetric epoxidation were surveyed for the regioselective epoxidation of conjugated dienes.…”

Asymmetric Epoxidation Using Hydrogen Peroxide as Oxidant

“…Alongside titanium–salalen complexes, more synthetically accessible titanium—salan complexes have also been investigated as catalysts for asymmetric epoxidation with hydrogen peroxide . Bryliakov and co‐workers systematically studied the influence of the ligand structure on the catalytic activity and selectivity by surveying the performance of a series of chiral salan ligands with varying steric and electronic properties in the asymmetric epoxidation of aromatic epoxides (Scheme ) . The results revealed that the electronic effect controls the catalytic activity, whereas the steric effect accounts for the facial selectivity of the epoxidation.…”