Molybdenum complexes reaction with alcohols

The carbanions take up 02 and these take up protons to give hydroperoxides in good yields. But because they are explosive in nature, they are not usually isolated and on reduction with sodium sulphite on trialkyl phosphite give alcohols. Alcohols can also be prepared via hydroperoxy molybdenum complexes and alkyl boranes. These reactions are summarized as follows ... 

To probe hydroperoxide reactivity in these systems we studied the reaction of tert-butyl hydroperoxide in the presence of [C5H5V(CO)4]. In contrast to the rhodium(I) and molybdenum complexes, [C5H5V-(CO)4] catalyzed the rapid decomposition of tert-butyl hydroperoxide to oxygen and tert-butyl alcohol in both toluene and TME (Table II). When reaction was done by adding the hydroperoxide rapidly to the vanadium complex in TME, no epoxide (I) was produced. However, when the TME solution of [C5H5V(CO)4] was treated with a small amount (2-3 times the molar quantity of vanadium complex) of tert-butyl hydroperoxide at room temperature, a species was formed in situ which could catalyze the epoxidation of TME. Subsequent addition of tert-butyl hydroperoxide gave I in 13% yield (Table II). This vanadium complex also could catalyze the epoxidation of the allylic alcohol (II) to give tert-butyl alcohol and IV (Reaction 14). Reaction 14 was nearly quantitative, and the reaction rate was considerably faster than with TME. 

Alcohols may be oxidized in a similar way. However, these reactions strongly resemble those reported for Cr molecular sieves, and a small concentration of Cr in solution may well account for most of the observations of catalysis. Binary molybdenum-chromium oxides supported on alumina have been used in the autoxidation of cyclohexene with 02 and r-BuOOH as an initiator (62). This is a complex reaction in which uncatalyzed and Cr-catalyzed oxidation combine to yield 2-cyclohexen-l-one, 2-cyclohexen-l-ol, and 2-cyclohexenyl hydroperoxide the Mo compound can use the hydroperoxide formed in situ as an oxidant for the epoxidation of cyclohexene. Although much lower oxygen consumption was observed for the reaction filtrate than for the suspension, it is unclear how the Cr is held by the oxide. 

Epoxidation of olefins (2, 287). The procedure for epoxidation of olefins with t-butyl hydroperoxide catalyzed by molybdenum hexacarbonyl has been published. Kinetic data have been obtained from the reaction. The mechanism is believed to involve I) reversible complex formation between the catalyst and the hydroperoxide, 2) reversible inhibition by the coproduct alcohol, and 3) reaction of the hydroperoxide-molybdenum complex with the olefin to form the epoxide and by-product alcohol. 

The. V-alkylation of ephedrine is a convenient method for obtaining tertiary amines which are useful as catalysts, e.g., for enantioselective addition of zinc alkyls to carbonyl compounds (Section D. 1.3.1.4.), and as molybdenum complexes for enantioselective epoxidation of allylic alcohols (Section D.4.5.2.2.). As the lithium salts, they are used as chiral bases, and in the free form for the enantioselective protonation of enolates (Section D.2.I.). As auxiliaries, such tertiary amines were used for electrophilic amination (Section D.7.I.), and carbanionic reactions, e.g., Michael additions (Sections D. 1.5.2.1. and D.1.5.2.4.). For the introduction of simple jV-substituents (CH3, F.t, I-Pr, Pretc.), reductive amination of the corresponding carbonyl compounds with Raney nickel is the method of choice13. For /V-substituents containing further functional groups, e.g., 6 and 7, direct alkylations of ephedrine and pseudoephedrine have both been applied14,15. 

Asymmetric epoxidation of allylic alcohols. This reaction has been accomplished by use of the ligand 1 with VO(acac)2 as catalyst and -butyl hydroperoxide as the epoxidation reagent. Optical yields as high as 50% have been obtained substantially lower inductions were obtained with cumene hydroperoxide. Molybdenum complexes with 1 give low asymmetric inductions. ... 

Finally, the use of stoichiometric amounts of transition metal complexes can play an important role in the synthesis of functionalized piperidines. <01H14.39> Liebeskind and coworkers have developed a chiral transition metal complex and have used it in the synthesis of (-)-indolizidine 209B <01JA12477>. A lipase mediated allylic alcohol resolution provides access to both antipodes of enantiomerically pure allyl acetates (115) which can be used to form an ri -allyl molybdenum complex (116), Hydride abstraction followed by methoxide quench yields a reactive species 117 which may be further functionalized through reactions with Grignard reagents. The eventual products 119 arc 2,3,6-trisubstituted piperidines in enantiomerically pure form. 

Sheldon has considered the competing process of homolytic decomposition of hydroperoxides during the epoxidation of olefins with tert-h xty hydroperoxide in the presence of molybdenum complexes. It was found that homolytic decomposition of the hydroperoxide is initiated by electron transfer reactions of Mo(V) and Mo(VI) complexes with the hydroperoxide giving rise to free radical species. Reaction rates and products of hydroperoxide decomposition were dependent on the solvent and on the presence or absence of an olefin. The rates and selectivities of epoxidation were highest in polychlorinated hydrocarbons and very poor in coordinating solvents such as alcohols or ethers [387]. 

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. 

The ruthenium carbene catalysts 1 developed by Grubbs are distinguished by an exceptional tolerance towards polar functional groups [3]. Although generalizations are difficult and further experimental data are necessary in order to obtain a fully comprehensive picture, some trends may be deduced from the literature reports. Thus, many examples indicate that ethers, silyl ethers, acetals, esters, amides, carbamates, sulfonamides, silanes and various heterocyclic entities do not disturb. Moreover, ketones and even aldehyde functions are compatible, in contrast to reactions catalyzed by the molybdenum alkylidene complex 24 which is known to react with these groups under certain conditions [26]. Even unprotected alcohols and free carboxylic acids seem to be tolerated by 1. It should also be emphasized that the sensitivity of 1 toward the substitution pattern of alkenes outlined above usually leaves pre-existing di-, tri- and tetrasubstituted double bonds in the substrates unaffected. A nice example that illustrates many of these features is the clean dimerization of FK-506 45 to compound 46 reported by Schreiber et al. (Scheme 12) [27]. 

Hydride transfer reactions from [Cp2MoH2] were discussed above in studies by Ito et al. [38], where this molybdenum dihydride was used in conjunction with acids for stoichiometric ionic hydrogenations of ketones. Tyler and coworkers have extensively developed the chemistry of related molybdenocene complexes in aqueous solution [52-54]. The dimeric bis-hydroxide bridged dication dissolves in water to produce the monomeric complex shown in Eq. (32) [53]. In D20 solution at 80 °C, this bimetallic complex catalyzes the H/D exchange of the a-protons of alcohols such as benzyl alcohol and ethanol [52, 54]. 

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... 

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