Vanadium catalysts, alkyl

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. 

Alkyl hydroperoxides can oxidize a variety of other nucleophilic substrates in the presence of d° metal catalysts. Thus molybdenum and vanadium catalysts have been used for the selective oxidation of tertiary amines to the corresponding JV-oxides (equations 79 and 80).225,254... 

Rouchaud and co-workers492 494 studied the liquid phase oxidation of propylene in the presence of insoluble silver, molybdenum, tungsten, and vanadium catalysts. Moderate yields of propylene oxide were obtained in the presence of molybdenum catalysts. These reactions almost certainly proceed via the initial formation of alkyl hydroperoxides, followed by epoxidation of the propylene by a Mo(VI)-hydroperoxide complex (see preceding section). 

Epoxidation of allylic alcohols (5, 75-76). Corrected values for the erythro threo ratios obtained on epoxidation of allylic alcohols with TBHP catalyzed with VO(acac)2 or Mo(CO)6 have been reported. In general greater selectivity is observed with the vanadium catalyst. In simple systems formation of erythro-epoxy alcohols is markedly favored over the threoAsomers, but substitution of a c -alkyl group on the double bond favors formation of threo-epoxy alcohols. 

In addition to titanium-based Ziegler-Natta catalysts, vanadium-based systems have also been developed for PE and ethylene-based co-polymers, particularly ethylene-propylene-diene rubbers (EPDM). Homogeneous (soluble) vanadium catalysts produce relatively narrow molecular mass distribution PE, whereas supported V catalysts give broad molecular mass distribution.422 Polymerization activity is strongly enhanced by the use of a halogenated hydrocarbon as promoter in combination with a vanadium catalyst and aluminum alkyl co-catalyst.422,423... 

Inoue et al. ( ) found that a porphyrin-Zn alkyl catalyst polymerized methyloxirane to form a polymer having syndio-rich tacticity. The relative population of the triad tacticities suggests that the stereochemistry of the placement of incoming monomer is controlled by the chirality of the terminal and penultimate units in the growing chain. There is no chirality around the Zn-porphyrin complex. Achiral zinc complex forms syndio-rich poly(methyloxirane), while chiral zinc complex, as stated above, forms isotactic-rich poly(methyloxirane). The situation is just the same as that for propylene polymerizations. Achiral vanadium catalyst produces syndiotactic polypropylene, while chiral titanium catalyst produces isotactic polypropylene. 

A similar reaction system was used for determining alkyl groups bonded to a silicon atom [214, 215]. In this instance reaction with concentrated sulphuric acid was carried out in the presence of a vanadium catalyst, as a result of which alkyl groups were split off in the form of the corresponding hydrocarbons. Quantitative determination of ethyl and phenyl groups is also possible by their thermal cleavage from a silicon atom with the formation of ethane and benzene [216]. 

In the absence of a metal catalyst, alkyl hydroperoxides react with primary amines to give either ketimines or aldimines [188]. In the presence of molybdenum-, vanadium-, or titanium naphthenate, however, the oxidation of cyclohexylamine with cumene hydroperoxide [189] is reported to give cyclohexanone oxime in good yield, equation (136). 

Normally, the most electron-rich, and therefore the most highly alkyl-substituted, alkene reacts first, but the vanadium catalyst shows strong di-... 

Reardon D, Conan F, Gambarotta S, Yap G, Wang Q (1999) Life and death of an active ethylene polymerization catalyst. Ligand involvement in catalyst activation and deactivation, isolation and characterization of two unprecedented neutral and anionic vanadium(I) alkyls. J Am Chem Soc 121 9318-9325. doi 10.1021/jll90263x... 

Normally, the most basic, and therefore the most highly alkyl-substituled alkene reacts first, but the vanadium catalyst shows strong directing effects that allow the catalyst to overcome the usual selectivity order if an allylic or homoallylic -OH group is present (e.g., Eq. 14.47). In cyclic compounds the stereochemistry of the final epoxide is determined by the directing effect of the -OH group to which the catalyst binds (Eq. 14.48). Peracids tend to give the other isomer of the product, by a simple steric effect. 

Natta and Ziegler found early on that vanadium halides could be treated with aluminum alkyls to form catalysts competent for alkene polymerizations. A variety of simple precursors in various oxidation states, for example, VCI3, VCI4, and VOCI3. can be treated with aluminum alkyl halides to produce active catalysts. Unlike the titanium and chromium systems, the vanadium catalysts can be single sited with narrow MWDs and more importantly narrow composition distributions. Also unlike the other systems, vanadium incorporates a-olefms at rates somewhat slower but comparable to ethylene. For this reason, vanadium catalysts are used commercially to make EPMs and EPDMs that can have at least 50 mol.% ethylene with the balance being predominantly other olefins and much smaller amounts of diolefins (Figure 9). 

Al—Ti Catalyst for cis-l,4-PoIyisoprene. Of the many catalysts that polymerize isoprene, four have attained commercial importance. One is a coordination catalyst based on an aluminum alkyl and a vanadium salt which produces /n j -l,4-polyisoprene. A second is a lithium alkyl which produces 90% i7j -l,4-polyisoprene. Very high (99%) i7j -l,4-polyisoprene is produced with coordination catalysts consisting of a combination of titanium tetrachloride, TiCl, plus a trialkyl aluminum, R Al, or a combination of TiCl with an alane (aluminum hydride derivative) (86—88). 

The mechanism for such a process was explained in terms of a structure as depicted in Figure 6.5. The allylic alcohol and the alkyl hydroperoxide are incorporated into the vanadium coordination sphere and the oxygen transfer from the peroxide to the olefin takes place in an intramolecular fashion (as described above for titanium tartrate catalyst) [30, 32]. 

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