Systematic Development of Vanadium Catalysts for Sustainable Epoxidation of Small Alkenes and Allylic Alcohols

Abstract

The catalytic epoxidation of small alkenes and allylic alcohols includes a wide range of valuable chemical applications, with many works describing vanadium complexes as suitable catalysts towards sustainable process chemistry. But, given the complexity of these mechanisms, it is not always easy to sort out efficient examples for streamlining sustainable processes and tuning product optimization. In this review, we provide an update on major works of tunable vanadium-catalyzed epoxidations, with a focus on sustainable optimization routes. After presenting the current mechanistic view on vanadium catalysts for small alkenes and allylic alcohols' epoxidation, we argue the key challenges in green process development by highlighting the value of updated kinetic and mechanistic studies, along with essential computational studies.

Keywords: alkene epoxidation; allylic alcohol epoxidation; sustainable chemistry; vanadium catalysts.

Figure 1

Common commercial epoxides (from left to right): glycidol (oxiran-2-yl)methanol)—(a); ethylene oxide (oxirane)—(b); propylene oxide (2-methyloxirane)—(c).

Figure 2

Schematic representation of allylic alcohol epoxidation.

Figure 3

Graphical distribution of the possible epoxidation routes for small alkenes and allylic alcohol by oxidizing agents and catalysts.

Figure 4

Some examples of vanadium-based catalyst complexes [VO(acac)₂] (a); vanadium(IV) complexes with Schiff base ligands (b); VO(porphyrin)-complexes (c); and [VO(OiPr)₃] (d).

Figure 5

Schematic overview of the oxovanadium catalytic system for the epoxidation of olefins and allylic alcohols. Side reactions represent the inception of active complexes.

Figure 6

Proposed mechanism for the irreversible formation of an active epoxidation catalyst from [VO(acac)₂] complex in the presence of acetic acid.

Figure 7

Mechanism outline for epoxidation Sharpless route (‡ = transition state) [34].

Figure 8

Mechanism outline for epoxidation Mimoun-type route (‡ = transition state) [61].

Figure 9

Mechanism outline for epoxidation Radical pathway route (‡ = transition state; · = radical) [57].

Figure 10

Schematic representation of the mechanism of epoxidation of alkenes with H₂O₂ catalyzed by a divanadium-substituted phosphotungstate [65].

Figure 11

Schematic representation of [VO(acac)₂] catalyst inserted through the oxygen subunits of CAT1 [95].

Figure 12

Schematic representation of enantioselective asymmetric epoxidation of a meso secondary homoallylic alcohol with [VO(OiPr)₃] as catalyst [55].

Figure 13

Epoxidation reaction scheme for oxovanadium-based complexes with olefins [29].

Figure 14

Structure of oxovanadium(IV) complexes used by Mohebbi [48].

Figure 15

Representation of oxo(peroxo)(corrolato)vanadium(V) for epoxidation of cycloalkenes using KHCO₃ as a promoter [117].

Figure 16

Schematic representation of (pre-reaction) synthesis example of [VO(acac)₂]-based active catalysts [29].