Vanadyl alcoholate

Similar complexes are formed between poly(viayl alcohol) and titanium lactate (96,101), titanyl sulfate (102), or vanadyl compounds (103). 

Oxovanadium(IV) complexes with (97) and (98) derived from alkyl aminoalcohols and Hsal or o-hydroxyacetophenone (and substituted derivatives), respectively, have been prepared as well as VOz+ complexes with (99) derived from 2-hydrazinoethanol and Hsal and substituted Hsal. Most of these and some properties are summarized in Table 42. Generally the complexes were prepared by adding an alcoholic solution of the SB, either previously obtained as a solid or prepared in situ, to alcoholic vanadyl acetate. The mixture is normally refluxed for several hours and the precipitates are collected by filtration as they are not soluble they are not recrystallized. 

Vanadium pentoxide dissolves in acids, both organic and inorganic, to form vanadyl or unstable vanadic salts,7 and in alkalis to produce ortho-, pyro-, meta-, and poly-vanadates. The physico-chemical changes involved when vanadium pentoxide is heated with various basic oxides in the powder state have been investigated by Tammann.8 On being digested with liquid ammonia slow absorption of ammonia takes place the composition of the product has not been definitely established.9 The oxide also dissolves in alcohols to produce esters,10 and combines with methylamine and ethylamine to form compounds of the type 2(R.NHB).V205, where R represents the alkyl radical.11... 

Chiral A-salicylidene vanadyl carboxylates are efficient catalysts for asymmetric aerobic oxidation of a-hydoxy esters and amides with divergent substituents. These catalysts have been explored for the kinetic resolution of secondary alcohols also. The stereochemical origin of the almost total asymmetric control has been probed.262... 

If the OH group is not blocked at all but left free, and the epoxidation reagent is the vanadium complex VO(acac)2 combined with f-BuOOH, the syn epoxide is formed instead. The vanadyl group chelates reagent and alcohol and delivers the reactive oxygen atom to the same face of the alkene. 

The delivery of an oxygen atom through a cyclic transition state by vanadyl complexes is also particularly effective with allylic alcohols. Here is a simple example—the green arrow shows merely the directing effect and is not a mechanism. 

VPD route two-step preparation of V0EIP04 iiH20 from the reduction of vanadyl phosphate dihydrate (V0P04-2H20) with alcohol... 

The readily available catalyst vanadyl bisacelylacetonate when used with r-bulyl hydroperoxide in benzene will oxidize secondary alcohols (Scheme 20) much mote rapidly than primary ones (rate ratio > 100 1), but the other oxidizing prqieities of this system, in particular the epoxidation of allylic alcohols and the cleavage of 1,2-diols might well limit its uses somewhat. ... 

Very interesting results were obtained during studies on the epoxidation of /f-hydroxy enones 8. Attempts at selective epoxidation under typical basic conditions failed and 2 3 mixtures of syn- and anf/-epoxides 9 were obtained. Surprisingly, epoxidation using Sharpless conditions, titanium tetraisopropoxide/tm-butyl hydroperoxide or vanadyl acetylacetonate/rm-butyl hydroperoxide, which are normally unreactive towards simple enones, proceeded smoothly, yielding exclusively syn-epoxy alcohols syn-9. No epoxidation took place for the corresponding nitrile or when the hydroxy function was blocked as a silyl ether32. 

Hydrogen peroxide or t-butyl hydroperoxide may be used in the presence of a catalyst such as sodium tungstate(VI) or vanadyl acetylaceto-nate [ MeC0CH=C(0 )Me 2V0] for the epoxidation of allylic alcohols. The stereochemistry of the hydroxyl group has a profound effect on the stereochemistry of epoxidation. A system which has been applied to allylic alcohols, to make optically active epoxides, utilizes titanium(rV) isopropoxide, t-butyl hydroperoxide and either of the enantiomeric forms of diethyl tartrate. This system forms chiral epoxides of predictable stereochemistry. When the reactivity of epoxides is combined with the... 

Vanadyl acetylacetonate, VO(acac)2, catalyzes stereoselective epoxi-dations of unsaturated alcohols [216, 328]. 

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