Vanadium complexes allyl

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

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... 

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. 

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. 

CO)J. Furthermore, when V was contacted with the vanadium complex in the presence of the more reactive allylic alcohol, II, the predominant product was IV (Reaction 18). Results are summarized in Table VII. 

As with TME oxidation, the vanadium (IV) complex, [(CsH oVClo], did not readily initiate cyclohexene oxidation. This complex, however, is an efficient catalyst for allylic alcohol epoxidation. The ability of the vanadium complex to initiate oxidation seems to be a function of its... 

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. 

Heteroalkenes, with iron, 6, 132 Heteroannulation, allylic benzylamines, 10, 156 Heteroarene chromium carbonyls, preparation and characteristics, 5, 260 Heteroarenes borylation, 10, 242 C—H functionalizations, 10, 127 as metal vapor synthesis milestone, 1, 237 with titanium, 4, 246 vanadium complexes, 5, 48 7]6-Heteroarenes, with platinum, 8, 664 Heteroaromatic compounds... 

A vanadium complex with C2-symmetric bishydroxamic acids catalyses the epox-idation of allylic alcohols with ees up to 97% and excellent yields. The catalysts have been used for kinetic resolution of secondary allylic alcohols with high ee. The structure of a possible intermediate has been suggested.38... 

Epoxidation of allylic alcohols with peracids or hydroperoxide such as f-BuOaH in the presence of a transition metal catalyst is a useful procedure for the synthesis of epoxides, particularly stereoselective synthesis [587-590]. As the transition metal catalyst, molybdenum and vanadium complexes are well studied and, accordingly, are the most popular [587-590], (Achiral) titanium compounds are also known to effect this transformation, and result in stereoselectivity different from that of the aforementioned Mo- and V-derived catalysts. The stereochemistry of epoxidation by these methods has been compared for representative examples, including simple [591] and more complex trcMs-disubstituted, rrans-trisubstituted, and cis-trisubstituted allyl alcohols (Eqs (253) [592], (254) [592-594], and (255) [593]). In particular the epoxidation of trisubstituted allyl alcohols shown in Eqs (254) and (255) highlights the complementary use of the titanium-based method and other methods. More results from titanium-catalyzed diastereoselective epoxidation are summarized in Table 25. 

Epoxidations. Grafting tantalum onto silica to form a useful catalyst for the Sharpless asymmetric epoxidation of allyl alcohols is contrary to the ineffective titanium species on a similar support. Vanadium-complexed chiral hydroxamic... 

In contrast to the results obtained with simple olefins, olefins containing alcohol functionality were epoxidized much more rapidly in the presence of vanadium complexes than with molybdenum [409,410]. The efficiency of the vanadium catalyzed epoxidation of allyl alcohol has been rationalized on the basis of an intermediate complex having a geometry which places the electron-deficient oxygen of the hydroperoxide in the vicinity of the double bond, equation (265). 

Whereas the major products of the [CsH5Mo(CO)3]-catalyzed oxidation of substituted olefins are epoxides and allylic alcohols [390,392], oxidation of substituted olefins in the presence of vanadium complexes gives rise to epoxy alcohols as the major products [390, 392, 507-511]. When cyclohexene is the olefin used, reaction is observed to occur with a high degree of stereoselectivity [511], equation (309). 

It has been noted [390,392,512] that the intermediate allylic hydroperoxide is stereoselectively converted to the cis-epoxy alcohol in the presence of vanadium complexes. Cross-product experiments [390,392], equations (310) and (311), experiments measuring relative rates of epoxidation of cyclohexene and 2-cyclohexene-l-ol [390, 392], effects of added 2other data [390,392] indicate that in the case of vanadium-complex catalyzed oxidation of olefins, epoxy alcohols are formed via intermolecular epoxidation of allylic alcohols rather than by intramolecular rearrangement of allylic hydroperoxides, equation (312). 

Thus, vanadium complexes preferentially epoxidize small amounts of allylic oxygen species formed in situ to give epoxy alcohols whereas molybdenum complexes catalyze epoxidation of the excess of unreacted olefin to give epoxides. The mechanism of vanadium catalyzed epoxidation of allylic alcohols has been discussed in an earlier section. 

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... 

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