Alkenes vanadium complexes

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. 

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. 

Early work [5,6] showed that stereoselective direct air-oxidation of alkenes to epoxyalcohols was possible in the presence of vanadium complexes (Fig. 8). Reaction proceeded via autoxidation of the alkene to an allylic hydroperoxide and its decomposition to an allylic alcohol. This initial reaction could be accelerated by a first-row... 

Other transition-metal oxidants can convert alkenes to epoxides. The most useful procedures involve /-butyl hydroperoxide as the stoichiometric oxidant in combination with vanadium, molybdenum, or titanium compounds. The most reliable substrates for oxidation are allylic alcohols. The hydroxyl group of the alcohol plays both an activating and a stereodirecting role in these reactions. /-Butyl hydroperoxide and a catalytic amount of VO(acac)2 convert allylic alcohols to the corresponding epoxides in good yields.44 The reaction proceeds through a complex in which the allylic alcohol is coordinated to... 

The quasi living polymerization of ethene and norbornene has been reviewed, among other topics in living polymerization of alkenes (19). Specifically, arylimido-aryloxo-vanadium(V) complexes with methylaluminoxane or Et2AlCl as co-catalyst have been used as catalyst systems. The polymers exhibit a low polydispersity and a high molecular weight (20). 

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... 

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... 

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. 

The peroxo complexes of vanadium have not, by comparison with the other three elements (Ti, W, Mo) cited, been extensively employed for oxygen transfer reactions. The ease of the redox step vanadium(V) to vanadium(IV) introduces a mixture of two-electron and one-electron character into vanadium peroxo chemistry, which in the case of alkene epoxidation leads to side reactions of the substrate and products.81... 

Group 5 complexes tend to be less active in alkene polymerizations, although vanadium-based Ziegler catalysts have long been known. The compounds Cp Nb(i74-diene)X2 (X = CH3 or Cl), activated by MAO, catalyze the living polymerization of ethylene at -20°C.117... 

The chemistry of Lewis acids is quite varied, and equilibria such as those shown in Eqs. (28) and (29) should often be supplemented with additional possibilities. Some Lewis acids form dimers that have very different reactivities than those of the monomeric acids. For example, the dimer of titanium chloride is much more reactive than monomeric TiCL (cf., Chapter 2). Alkyl aluminum halides also dimerize in solution, whereas boron and tin halides are monomeric. Tin tetrachloride can complex up to two chloride ligands to form SnCL2-. Therefore, SnCl5 can also act as a Lewis acid, although it is weaker than SnCl4 [148]. Transition metal halides based on tungsten, vanadium, iron, and titanium may coordinate alkenes, and therefore initiate polymerization by either a coordinative or cationic mechanism. Other Lewis acids add to alkenes this may be slow as in haloboration and iodine addition, or faster as with antimony penta-chloride. 

Epoxidation of allyhc and homoallylic alcohols not part of the diene complex can be achieved using the Sharpless r-butyl hydroperoxide/vanadium acetylacetonate protocol. Dihydroxylation of alkenes adjacent to the diene complex using osmium tetraoxide-r-butyl peroxide has been reported... 

Hydride derivatives are of interest in coimection with the catalytic properties of vanadium, for example, in alkene dehydrogenation or polymerization. Complexes with sulfhr-donor and possibly also nitrogen-donor atoms are relevant to the role played by vanadium in the poisoning of the Co Mo catalyst used for the hydrodesulfiirization of crude oU. ... 

Since the vanadium center of vanadocenes is electron-deficient, various vanadium derivatives are formed by the attack with PR3, CO, or unsaturated tt-donating hgands/ This reaction provides a useful synthetic method for the synthesis of jj -alkene complexes as described later. Thus formed Cp 2V(CO) (22) is converted to the cyano derivative (23, see Cyanide Complexes of the Transition Metals) on treatment with the isocyanide (Scheme 12). ( -C5Me5)2V is incubated with O2 at -78 °C to yield [(/u.-)] -C5Me503)V(0)]2 via oxygen insertion into the carbon-vanadium bond." ... 

Side-on coordination of an unsaturated ligand, alkyne or alkene, provides the corresponding complexes in the ) -mode. Vanadium-alkyne complexes like Cp2V(RC=CR) (R = COOMe, CF3) are formed by treatment of vanadocene with alkynes as shown in Scheme 32. (jj -Alkyne)(j) -cyclopentadienyl)bis(trimethylphosphine)vanadium(I) (59) is obtained by reductive coordination of CpVCl2(PMe3)2 (58) with alkyne (Scheme 33). ... 

Cp2V()] -alkene) is synthesized by treatment of vanadocene with aUcene bearing an electron-withdrawing group. The reaction of vanadocene with dimethyl frunarate or dimethyl maleate leads to the formation of the same complex, bis() -cyclopentadienyl)() -dimethyl fumarate)vanadium (72), via isomerization (Scheme 40). 

---