Vanadium complexes alkyl peroxides

There are also several situations where the metal can act as both a homolytic and heterolytic catalyst. For example, vanadium complexes catalyze the epoxidation of allylic alcohols by alkyl hydroperoxides stereoselectively,57 and they involve vanadium(V) alkyl peroxides as reactive intermediates. However, vanadium(V)-alkyl peroxide complexes such as (dipic)VO(OOR)L, having no available coordination site for the complexation of alkenes to occur, react homolyti-cally.46 On the other hand, Group VIII dioxygen complexes generally oxidize alkenes homolytically under forced conditions, while some rhodium-dioxygen complexes oxidize terminal alkenes to methyl ketones at room temperature. 

Novel vanadium(V)-alkyl peroxide complexes with the general formula VO(OOBu )(ROPh-salR ) (197 ROPhsalR = tridendate N-(2-oxidophenyl)salicylidenaminato Schiff base ligand) were found to epoxidize alkenes stoichiometrically with high selectivity.620... 

Alkyl hydroperoxides, including ethyl hydroperoxide, cuminyl hydroperoxide, and tert-butyl hydroperoxide, are not used by V-BrPO to catalyze bromination reactions [29], These alkyl hydroperoxides have the thermodynamic driving force to oxidize bromide however, they are kinetically slow. Several examples of vanadium(V) alkyl peroxide complexes have been well characterized [63], including [V(v)0(OOR)(oxo-2-oxidophenyl) salicylidenaminato] (R = i-Bu, CMe2Ph), which has been used in the selective oxidation of olefins to epoxides. The synthesis of these compounds seems to require elevated temperatures, and their oxidation under catalytic conditions has not been reported. We have found that alkyl hydroperoxides do not coordinate to vanadate in aqueous solution at neutral pH, conditions under which dihydrogen peroxide readily coordinates to vanadate and vanadium( V) complexes (de la Rosa and Butler, unpublished observations). Thus, the lack of bromoperoxidase reactivity with the alkyl hydroperoxides may arise from slow binding of the alkyl hydroperoxides to V-BrPO. 

Vanadium complexes, 3,453-569 acetylacetone exchange reactions, 2,380 alkyl peroxides... 

If we consider the d0 metal-N,JV-dialkylhydroxylamino complexes (79), (80) and (81) as valid models for the reactive but unstable alkyl peroxide species Mo02(OOR)2, VO(OOR)3 or V203(00R)4, and Ti(OOR)4 presumably involved in catalytic oxidations, the low activity of vanadium and titanium for the epoxidation of simple alkenes could be interpreted by the fact that these alkenes cannot displace the O.O-bonded alkyl peroxide groups in the coordinatively saturated Vv- and Tiiv-alkyl peroxide species, whereas allylic alcohols can displace the alkyl peroxide groups by forming bidentate allylic alkoxides as in equation (75).162... 

The heterolytic reactivity of complex (197) contrasts with the homolytic reactivity of the vanadium(V) (dipicolinato)( alkyl peroxide) (22), and is presumably due to the lower binding ability of the OBu oxygen atom in (197), shown by NMR, which allows the coordination of alkene to the metal. 

Labeling studies have clearly shown that the active catalyst is not a peroxo compound but an alkyl peroxide in which the alkyl peroxide group is probably 0,0-triangularly bonded to the metal,as illustrated by the X-ray crystal structure of the vanadium complex (22). The well-characterized d" metal 0,N-bonded N,JV-dialkylhydroxylamino complexes (79), (80) and (81) might be reminiscent of the alkyl peroxide complexes of Mo " , V " and Ti ", which are difiBcult to isolate and involved as reactive intermediates in reactions (64) or (65). 

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