Vanadium complexes olefin

Vanadium precursors tend to promote addition polymerization of olefins on activation with aluminum co-catalysts. Nomura and coworkers recently reported the first example of catalytic olefin metathesis by a vanadium alkylidene complex.The well-defined, thermally stable V(v) alkylidene complex 7 effected ROMP of NBE at 80 °C. Cettain alkylvanadium complexes, including V(CH2Ph)2(=NAr )(OAr ) (Ar = 2,6-Pr 2C6H3 Ar = 2,6-Me2C6H3) and... 

The first metal-olefin complex was reported in 1827 by Zeise, but, until a few years ago, only palladium(II), platinum(Il), copper(I), silver(I), and mercury(II) were known to form such complexes (67, 188) and the nature of the bonding was not satisfactorily explained until 1951. However, recent work has shown that complexes of unsaturated hydrocarbons with metals of the vanadium, chromium, manganese, iron, and cobalt subgroups can be prepared when the metals are stabilized in a low-valent state by ligands such as carbon monoxide and the cyclopentadienyl anion. The wide variety of hydrocarbons which form complexes includes olefins, conjugated and nonconjugated polyolefins, cyclic polyolefins, and acetylenes. 

Oxovanadium(V) and oxomolybdenum(VI) were incorporated into crosslinked polystyrene resins functionalized with iminodiacetic acid or diethylenetriamine derivatives 921 The polymer complexes were used as catalysts in the oxidation of olefins with f-butylhydroperoxide. Vanadium(V) complexes promote the epoxidation of allylic alcohols in a highly regioselective manner, e.g., 2,3-epoxide was obtained in 98 % selectivity from e-geraniol at 80 °C. The catalytic activity of the vanadium(V) complexes is generally higher than that of the molybdenium(VI) complexes in the oxidation of allylic alcohols, whereas an opposed trend holds for the epoxidation of cyclohexene. 

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. 

Consequently, the elements to the left of the noble metals show strongest (ft)-character in their zero-valent oxidation state. Thus iron(O), cobalt(O) and nickel(O) are typically (b), forming inter alia strong carbonyl complexes, while the higher oxidation states of these elements have no marked ( )-character at all. Elements in zero-valent state in fact display (b) -character as far left in the periodic system as chromium, or even vanadium, which in higher oxidation states behave as very typical (a)-acceptors. To the right of the noble metals, on the other hand, the metals in their zero-valent states do not show any marked (6)-character they do not form e.g. carbonyl or olefin complexes. 

Olefin Polymerization with Homogeneous Vanadium(III) Complexes - Cocatalyst Systems... 

Sometimes at low temperatures, it is possible to isolate alkene complexes. At 195 K, VCI4 forms complexes of the type [VCl3(alkene) ] ( = 1,2, alkene = 1-heptene, 1-decene = 2, alkene = acrylonitrile, 1-heptene, 4-methyl-1-pentene) in pentane solution. The formation of colored complexes is also observed in solution of VOCI3 in carbon tetrachloride containing olefins. These complexes decompose immediately after evaporation of the solvent. Stable vanadium(O) complexes containing 6 -cw-pro-penylphenyldiphenylphosphine and orM -allylphenyldiphenylphosphine are known. These complexes are formed from stoichiometric amounts of hexacarbonylvanadium(O). 

The epoxidation of olefmic hydrocarbons without other coordinating groups is 10 times slower in the presence of vanadium complexes than with molybdenum catalysts. Nonetheless, the reaction of tert-bnXyX hydroperoxide with an olefin such as cyclohexene in the presence of [VO(acac)2], [V(acac)3], [V(oct)3] or [VO( -BuO)3], is nearly quantitative at 84 C [408, 386]. Rate laws are consistent with reaction via rate determining attack of olefin on a vanadium (V)-hydroperoxide complex. Epoxidations were first order each in olefin and in catalyst but exhibited a Michaelis-like dependency on hydroperoxide, equation (258), where is a limiting specific rate (at very high ratios of hydroperoxide to catalyst), [Vq] is the total concentration of added vanadium, and Kp is the association constant for the vanadium(V) complex presumed to be the active intermediate. 

The reaction rates are greater in sulphate media, possibly owing to the reactions of complexes of the type V(0H)3HS04+, and it is considered that the mechanism involves free radical formation since polymerization of an added olefinic derivative such as acrylamide has been observed. Vanadium(v) complexes have also been described in reactions with p-phenetidine and whereas the rates for V02 are slow, those for the higher polymeric aggregates are faster. 

The tert-huty hydroperoxide is then mixed with a catalyst solution to react with propylene. Some TBHP decomposes to TBA during this process step. The catalyst is typically an organometaHic that is soluble in the reaction mixture. The metal can be tungsten, vanadium, or molybdenum. Molybdenum complexes with naphthenates or carboxylates provide the best combination of selectivity and reactivity. Catalyst concentrations of 200—500 ppm in a solution of 55% TBHP and 45% TBA are typically used when water content is less than 0.5 wt %. The homogeneous metal catalyst must be removed from solution for disposal or recycle (137,157). Although heterogeneous catalysts can be employed, elution of some of the metal, particularly molybdenum, from the support surface occurs (158). References 159 and 160 discuss possible mechanisms for the catalytic epoxidation of olefins by hydroperoxides. 

Figure 6.5 Proposed structure for the vanadium complex prior to the oxygen transfer from peroxide to the allylic olefin.

The catalyst is preliminarily oxidized to the state of the highest valence (vanadium to V5+ molybdenum to Mo6+). Only the complex of hydroperoxide with the metal in its highest valence state is catalytically active. Alcohol formed upon epoxidation is complexed with the catalyst. As a result, competitive inhibition appears, and the effective reaction rate constant, i.e., v/[olefin][ROOH], decreases in the course of the process due to the accumulation of alcohol. Water, which acts by the same mechanism, is still more efficient inhibitor. Several hypothetical variants were proposed for the detailed mechanism of epoxidation. 

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