Vanadium enolate

Recently, Trost et al. reported the vanadium-catalyzed addition reaction of 2,3-allenols [180], Here the oxygen in 401 served as an intramolecular nucleophile to attack the center carbon atom of allene to form a vanadium enolate 402. Aldol condensation of 402 with an aldehyde afforded (2-hydroxy)alkyl vinylic ketones 403. 

Samarium has two common oxidation states +2 and -h3. Upon solution in toluene under nitrogen, an anionic Sm(II) species, [(—CH2—)5]4-calix-tetrapyrrole Sm(THF)[Li (THF)]2[Li(THF)2]Cl, forms, in part, the compound [(—CH2—)5]4-calix-tetrapyrrole Sm(THF)Li2[Li(THF)](/r -OCH=CH2) . However, this compound is a lithium enolate derived by elimination of THF. In that the metalloorganic reagent is rather similar to what will be discussed in Section XI as part of vanadium enolate chemistry, we fail to understand why in the former case with Sm a lithium enolate is formed but in the latter with V it is an ynolate that is produced. Almost nothing is known to allow comparing the energetics of metal enolates and related ynolates. We note from the enthalpies of... 

An example of fortuitous vanadium enolate chemistry is the CO addition reaction to a silylamido vanadium species in which the dimeric metallocycle 32 is transformed into the corresponding cyclic enolate 33, as shown in equation 12. Given silicon s profound oxophilicity, the absence of the Si-O moieties in 33 is surprising. For example, the liquid phase reaction shown in equation 13 is exothermic by ca 420 kJmol , as determined from the enthalpies of formation of tetramethoxymethane and the silicon compounds . 

DFT study of interception of the allenyl enolate intermediate of Meyer-Schuster rearrangement using aldehydes and imines has shown that the active form of the vanadium catalyst bears two triphenyl siloxy ligands and that vanadium enolate is directly involved in the C-C bond formation." DFT calculations have elucidated the mechanisms and diastereoselectivities of phosphine-catalysed [4-1-2] annulations between allenoates and ketones or aldimines (Scheme 44). 

The transient allenyl enolate intermediate of the vanadium-catalysed Meyer-Schuster rearrangement with aldehydes and imines has been studied computationally and the active form of the catalyst is considered to contain two triphenylsiloxy ligands. The transesterification of vanadate occurs via tr-bond metathesis and the vanadium enolate is directly involved in the key C-C bond formation (Scheme 175). ... 

If the alkene is an alkyne instead, we are dealing with a propargylic alcohol and now the thermodynamics are more favourable and the product is an enone. Commercial application is found in the production of citral from dehydrolinalool via vanadium-catalysed isomerisation (Figure 5.9). Note that the last step involves a transfer of hydrogen as well when the enol rearranges to the aldehyde ... 

Selective oxidation of methyl ethyl ketone to diacetyl has been studied by passing a mixture of the ketone in artificial air over vanadium phosphorus oxide catalysts in the temperature range 200-350 C. Products observed included diacetyl, methyl vinyl ketone, acetaldehyde, acetic acid and carbon dioxide. C4 products were favoured at low temperatures and at low or zero oxygen partial pressures. These results are rationalised in terms of two pathways for C2 products, namely oxidation of the double bond in the enol form of methyl ethyl ketone to yield acetic acid and acetaldehyde, and acid catalysed hydration of the keto form to yield acetaldehyde only. The C4 products are envisaged to go through a common intermediate, namely, CH3COCHOHCH3, formed by interaction between methyl ethyl ketone and lattice oxygen. 

Hydroxamic acid exists in two tautomeric forms, (1) and (2), and such keto-enol tautomerism provides a number of sites for coordination and chelation. The keto form (1) predominates in acid media and the enol form (2) in alkaline media 25 this has been corroborated by the extraction of vanadium benzohydroxamic acid complexes in organic solvents.26... 

This vanadium method enables the cross-coupling only in combinations of silyl enol ethers having a large difference in reactivity toward radicals and in their reducing ability. To accomplish the crosscoupling reaction of two carbonyl compounds, we tried the reaction of silyl enol ethers and a-stannyl esters based on the following consideration. a-Stannyl esters (keto form) are known to be in equilibrium with the enol form such as stannyl enol ethers, but the equilibrium is mostly shifted toward the keto form. When a mixture of an a-stannyl ester such as 45 and a silyl enol ether is oxidized, it is very likely that the stannyl enol ether will be oxidized preferentially to the silyl enol ether. The cation radical of 45 apparently cleaves immediately giving an a-keto radical, which reacts with the silyl enol ether selectively because of the low concentration of the stannyl enol... 

Salts of other transition metals including vanadium, cerium, chromium and manganese have been used for a-oxygenation, although rarely applied in synthesis. Manganese triacetate has been used for the efficient a -oxidation of enones (Section 2.3.2.2.1.i), but appears not to have been used for the a-hydrox-ylation of saturated ketones des]Hte its known ability to form the corresponding a-keto radicals. Similarly the use of Lewis acid assisted enolization in the oxidative process appears to have been limited to the LTA-mediated examples. 

The dinuclear enolate complex (105) is then formed by treatment of (104) with carbon monoxide, which inserts into the vanadium-carbon bonds. 

Symmetrical (equation 22a) and unsymmetrical 1,4-diketones (equations 22b and 22c) are obtained in good yield with the vanadium(V) alkoxo derivatives VO(OR)Cl2 (R = Et,, -Pr)65 xfjg same reagent induces cross coupling between allyl silanes and silyl enolates to y,5-unsaturated ketones. Noteworthy is the fact that only traces of the homocoupled 1,4-diketones and 1,5-hexadienes are produced. 

Compared with metal enolates, there have been very few reports on the direct structural analysis and theoretical studies of ynolates. An X-ray crystal structure of a vanadium complex of lithium ynolate with a porphyrinogen ligand (56) is reported. This metal complex was incidentally formed from VCl3(THF)3 with tetralithium salt of the octaethyl-porphyrinogen ligand. In this complex, the lithium cation seems to interact with the 7T-electrons of the ynolate. The four atoms of the ynolate group in 56 are not collinear due to a partial sp character of the group in this complex. 

The V(acac)3-mediated hompolymerization of ethylene is not living and the polydisper-sity index is quite high (2.0). Nevertheless, ethylene can be successfully copolymerized with propylene while maintaining the livingness of the process. Moreover, the enolate ligated vanadium is a catalyst for the living polymerization of 1,5-hexadiene and copolymerization with propylene. It must be noted that polymerization of 1,5-hexadiene is a route to a polymer that combines constitutive 1,3-cyclopentylenemethylene units (2 ) and vinyltetramethylene units. Therefore, pendant unsaturations are available for further functionalization. 

Many other metal ions have been reported as catalysts for oxidations of paraffins or intermediates. Some of the more frequently mentioned ones include cerium, vanadium, molybdenum, nickel, titanium, and ruthenium [21, 77, 105, 106]. These are employed singly or in various combinations, including combinations with cobalt and/or manganese. Activators such as aldehydes or ketones are frequently used. The oxo forms of vanadium and molybdenum may very well have the heterolytic oxidation capability to catalyze the conversion of alcohols or hydroperoxides to carbonyl compounds (see the discussion of chromium, above). There is reported evidence that Ce can oxidize carbonyl compounds via an enol mechanism [107] (see discussion of manganese, above). Although little is reported about the effectiveness of these other catalysts for oxidation of paraffins to acetic acid, tests conducted by Hoechst Celanese have indicated that cerium salts are usable catalysts in liquid-phase oxidation of butane [108]. 

A successful example for this class of reaction was reported by Choudary et al. [53a]. With the combination of a dialkyl tartrate and titanium-pillared montmorillonite (Ti-PILC), excellent ee-values in the range of90-98% were achieved (Scheme 2.19). In contrast to the homogeneous conditions, this heterogeneous system was operational without the use of molecular sieves however, no recychng experiment was reported. Distinct from Ti-PILC, the use of vanadium-piUared montmorillonite catalyst for the AE of ( )-hex-2-enol, however, led to only 20% enantiomeric excess [53b]. 

Preliminary results of the reaction between vanadium(iii)-tetrasulpho-phthalocyanine complex with oxygen have been reported these data were compared with those obtained for the corresponding reaction of the hexa-aquo complex ion. The oxidation of methyl ethyl ketone by oxygen in the presence of Mn"-phenanthroline complexes has been studied Mn " complexes were detected as intermediates in the reaction and the enolic form of the ketone hydroperoxide decomposed in a free-radical mechanism. In the oxidation of 1,3,5-trimethylcyclohexane, transition-metal [Cu", Co", Ni", and Fe"] laurates act as catalysts and whereas in the absence of these complexes there is pronounced hydroperoxide formation, this falls to a low stationary concentration in the presence of these species, the assumption being made that a metal-hydroperoxide complex is the initiator in the radical reaction. In the case of nickel, the presence of such hydroperoxides is considered to stabilise the Ni"02 complex. Ruthenium(i) chloride complexes in dimethylacetamide are active hydrogenation catalysts for olefinic substrates but in the presence of oxygen, the metal ion is oxidised to ruthenium(m), the reaction proceeding stoicheiometrically. Rhodium(i) carbonyl halides have also been shown to catalyse the oxidation of carbon monoxide to carbon dioxide under acidic conditions ... 

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