Mild Epoxidation of Allylic Alcohols Catalyzed by Titanium(III) Complexes: Selectivity and Mechanism

Abstract

A novel methodology for the epoxidation of a broad range of primary, secondary, and tertiary allylic alcohols is described using tert -butyl hydroperoxide as oxidant and Ti(III) species generated by reduction of Ti(IV) complexes, with manganese (0) in 1,4-dioxane under mild reaction conditions. The reaction proceeded with wide substrate scope and high chemo- and diastereoselectivity. A mechanistic pathway for the reaction is also discussed.

Oxide-Supported Titanium Catalysts: Structure–Activity Relationship in Heterogeneous Catalysis, with the Choice of Support as a Key Step

“…This catalytic reaction should first involve a protonolysis of Ti–H moieties by TBHP along with the formation of hydrogen and a Ti–OO t Bu active surface species (Scheme S2). The formation of the epoxide consists of an oxygen transfer to the olefin, as already reported in the literature. − Ti­(III) (species C ) may also catalyze an epoxidation with TBHP, but the reaction rate is reported to be rather low, even for an allylic alcohol.…”

“…The formation of the epoxide consists in an oxygen transfer to the olefin, as already reported in the literature. [54][55][56] Ti(III) (species C) may also catalyze an epoxidation with TBHP, 57 but the reaction rate is reported to be rather low, even for an allylic alcohol.…”

The Proven Versatility of Cp2TiCl

“…As mentioned before, the presence of a vacant site in the inner sphere of Cp 2 TiCl allows the coordination of functional groups having heteroatoms with free valence electrons. The target functional groups of Cp 2 TiCl are listed in Scheme : epoxides ( 22 ), ,,, oxetanes ( 23 ), ozonides ( 24 ), peroxides ( 37 ), aziridines ( 25 ), carbonyl groups ( 26 ), , α-haloketones ( 27 ), alcohols ( 38 ), , water ( 39 ), ,, halides ( 28 ), allyl ( 29 ) and propargyl ( 30 ) halides and carboxylates, amides ( 31 ), imines ( 32 ), nitriles ( 33 ), geminal dinitriles ( 34 ), α-cyanosulfones ( 35 ), Michael acceptors, ,, and hemiaminals ( 36 ) …”

Cp2Ti(III)Cl and Analogues as Sustainable Templates in Organic Synthesis

“…The reaction was also mediated by chiral (R,R)-(ebthi)Ti(III)Cl 2 catalyst 5, but it did not proceed with any enantioselectivity. 35…”

“…It is worth noting that the opening of the epoxide was not detected due to the low concentration of Cp 2 Ti(III)Cl which was not co-ordinated to the allylic alcohol. 35…”