Epoxides vanadium

Allylic alcohols, for example geraniol, 2-methylallyl alcohol, 3,3-dimethylallyl alcohol, 3-buten-2-ol, l-octen-3-ol, and l-hexen-3-ol, are epoxidized with tert-butyl hydroperoxide in the presence of a vanadyl salen oxo-transfer catalyst in supercritical CO2. The metal catalyst was prepared in a simple two-step, Schiff base-type reaction to form the salen ligand, followed by complexation to the vanadyl group. The use of non-toxic supercritical CO2 in the presence of the new epoxidation vanadium catalyst led to yields and diastereoselectivities that were comparable to those resulting from the use of environmentally hazardous solvents such as CH2CI2 [59]. 

Epoxidations. Vanadium based catalysts are largely used for olefin epoxidation by hydroperoxides (such as butyl hydroperoxide). These catalysts are very regioselective for epoxidation of double bonds of allylic alcohols [56]. 

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. 

Titanium—Vanadium Mixed Metal Alkoxides. Titanium—vanadium mixed metal alkoxides, VO(OTi(OR)2)2, are prepared by reaction of titanates, eg, TYZOR TBT, with vanadium acetate ia a high boiling hydrocarbon solvent. The by-product butyl acetate is distilled off to yield a product useful as a catalyst for polymeri2iag olefins, dienes, styrenics, vinyl chloride, acrylate esters, and epoxides (159,160). 

When heated in the presence of a carboxyHc acid, cinnamyl alcohol is converted to the corresponding ester. Oxidation to cinnamaldehyde is readily accompHshed under Oppenauer conditions with furfural as a hydrogen acceptor in the presence of aluminum isopropoxide (44). Cinnamic acid is produced directly with strong oxidants such as chromic acid and nickel peroxide. The use of t-butyl hydroperoxide with vanadium pentoxide catalysis offers a selective method for epoxidation of the olefinic double bond of cinnamyl alcohol (45). 

Liquid-Phase Epoxidation with Hydroperoxides. Molybdenum, vanadium, and tungsten have been proposed as Hquid-phase catalysts for the oxidation of the ethylene by hydroperoxides to ethylene oxide (205). tert- uty hydroperoxide is the preferred oxidant. The process is similar to the arsenic-catalyzed route, and iacludes the use of organometaUic complexes. 

Recently (79MI50500) Sharpless and coworkers have shown that r-butyl hydroperoxide (TBHP) epoxidations, catalyzed by molybdenum or vanadium compounds, offer advantages over peroxy acids with regard to safety, cost and, sometimes, selectivity, e.g. Scheme 73, although this is not always the case (Scheme 74). The oxidation of propene by 1-phenylethyl hydroperoxide is an important industrial route to methyloxirane (propylene oxide) (79MI5501). 

Table 6.4 Comparison of the performances showed by the vanadium complexes with ligands 4—7 in the AE reaction generating the epoxides A and B, respectively.

Transition Metal-Catalyzed Epoxidation of Alkenes. Other transition metal oxidants can convert alkenes to epoxides. The most useful procedures involve f-butyl hydroperoxide as the stoichiometric oxidant in combination with vanadium or... 

Catalytic turn-over [59,60] in McMurry couplings [61], Nozaki-Hiyama reactions [62,63], and pinacol couplings [64,65] has been reported by Fiirst-ner and by Hirao by in situ silylation of titanium, chromium and vanadium oxo species with McaSiCl. In the epoxide-opening reactions, protonation can be employed for mediating catalytic turn-over instead of silylation because the intermediate radicals are stable toward protic conditions. The amount of Cp2TiCl needed for achieving isolated yields similar to the stoichiometric process can be reduced to 1-10 mol% by using 2,4,6-collidine hydrochloride or 2,6-lutidine hydrochloride as the acid and Zn or Mn dust as the reduc-tant (Scheme 9) [66,67]. 

The vanadium(IV) complex of salen in zeolite was found to be an effective catalyst for the room temperature epoxidation of cyclohexene using t-butyl hydroperoxide as oxidant.88 Well-characterized vanadyl bis-bipyridine complexes encapsulated in Y zeolite were used as oxidation catalysts.101 Ligation of manganese ions in zeolites with 1,4,7-triazacyclononanes gives rise to a binu-clear complex stabilized by the zeolites but allows oxidation with excellent selectivity (Scheme 7.4). 

Vanadium-catalyzed asymmetric epoxidation has recently been re-examined with a newly designed chiral hydroxamic acid (3).43-45 The hydroxamic acid (3) forms a 1 1 complex with vanadium ions and induces high enantioselectivity (Scheme 6). 

In acyclic secondary -allylic alcohols, epoxidation by the vanadium system shows opposite stereospecificity to that of peracid and molybdenum carbonyl-mediated epoxidation (see Scheme 6)22 The predominance of the erythro isomer in the former process is rationalized22 in terms of the energetically more favorable transition state (6, cf. 5) and in this context the mechanism has analogy in the epoxidation behavior of medium-ring cyclic allylic alcohols.23... 

The vanadium-promoted epoxidation method has proved to be valuable in synthetic routes leading to dl-Ci8 Cecropia juvenile hormone20 and lasalocid A.24 The mechanism of vanadium-mediated epoxidation has been elucidated25 but an evaluation of the use of other organometallic compounds as epoxidation reagents is required.26... 

The vanadium-catalyzed epoxidation of hindered homoallylic alcohols has been described by Prieto and coworkers [339]. Reaction times for the epoxidation in a series of cis- and trans-2-methyl-alkenols were significantly reduced from 6-10 days to... 

The reaction of olefin epoxidation by peracids was discovered by Prilezhaev [235]. The first observation concerning catalytic olefin epoxidation was made in 1950 by Hawkins [236]. He discovered oxide formation from cyclohexene and 1-octane during the decomposition of cumyl hydroperoxide in the medium of these hydrocarbons in the presence of vanadium pentaoxide. From 1963 to 1965, the Halcon Co. developed and patented the process of preparation of propylene oxide and styrene from propylene and ethylbenzene in which the key stage is the catalytic epoxidation of propylene by ethylbenzene hydroperoxide [237,238]. In 1965, Indictor and Brill [239] published studies on the epoxidation of several olefins by 1,1-dimethylethyl hydroperoxide catalyzed by acetylacetonates of several metals. They observed the high yield of oxide (close to 100% with respect to hydroperoxide) for catalysis by molybdenum, vanadium, and chromium acetylacetonates. The low yield of oxide (15-28%) was observed in the case of catalysis by manganese, cobalt, iron, and copper acetylacetonates. The further studies showed that molybdenum, vanadium, and... 

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. 

Another interesting asymmetric epoxidation technique using metal catalysis involves the vanadium complexes of A-hydroxy-[2.2]paracyclophane-4-carboxylic amides (e.g., 19), which serve as catalysts for the epoxidation of allylic alcohols with f-butyl hydroperoxide as... 

SMPO [styrene monomer propylene oxide] A process for making propylene oxide by the catalytic epoxidation of propylene. The catalyst contains a compound of vanadium, tungsten, molybdenum, or titanium on a silica support. Developed by Shell and operated in The Netherlands since 1978. 

The development of transition metal mediated asymmetric epoxidation started from the dioxomolybdcnum-/V-cthylcphcdrinc complex,4 progressed to a peroxomolybdenum complex,5 then vanadium complexes substituted with various hydroxamic acid ligands,6 and the most successful procedure may now prove to be the tetroisopropoxyltitanium-tartrate-mediated asymmetric epoxidation of allylic alcohols. 

Like the vanadium-based epoxidation reaction, the Sharpless reaction in-... 

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