Oxidation of vinyl group

Pellissier, H., Michellys, P.-Y., Santelli, M. Regiochemistry of Wacker-type oxidation of vinyl group in the presence of neighboring oxygen functions. Part 2. Tetrahedron 1997, 53,10733-10742. 

A different oxidative approach toward die preparation of aldehydes uses tire ozonolysis of vinyl groups. If a vinyl group is present in a molecule, it can be oxidatively cleaved to an aldehyde by ozonolysis. This process cleaves tire carbon-carbon double bond, but it is mild and very successful in many cases. 

Barba and coworkers reported that the anodic oxidation of vinyl acetates in DMF affords alpha-formyloxy ketones (20) (equation 10)24. A novel feature of this reaction is the fact that the formyloxy carbonyl group of 20 is derived from the DMF solvent24. 

This combination of reagents h s been used to oxidize terminal vinyl groups to methyl ketones and is known as the Wacker oxidation. The nucleophile is simply water, which attacks the activated alkene at the more substituted end in an oxypalladation step. (3-Hydride elimination from the resulting a-alkyl palladium complex releases the enol, which is rapidly converted into the more stable keto form. Overall, the reaction is a hydration of a terminal alkene that can tolerate a range of functional groups. 

An H-abstraction from the R group will lead to other species, however, the only products observed were formaldehyde and formates which indicates that the OH-radical oxidation of vinyl ethers proceeds essentially by OH addition to the carbon-carbon double bond. 

The quantum yields of the Norrish t5q>e I and type II reactions have been measured for a variety of aliphatic ketones (6, 87) the type I process seems to be far less important than type II. As already seen (Section II.2.2.2.), also in the photooxidation of polymer systems containing carbonyl groups the Norrish type II process is considered the main cause of chain scission (82). This assumption is further supported by several photolysis studies (80, 83, 84) of polyethylene, where the IR investigations indicate the formation of vinyl groups in the oxidized polymers. 

The 7, i5-unsaturated alcohol 99 is cyclized to 2-vinyl-5-phenyltetrahydro-furan (100) by exo cyclization in aqueous alcohol[124]. On the other hand, the dihydropyran 101 is formed by endo cyclization from a 7, (5-unsaturated alcohol substituted by two methyl groups at the i5-position. The direction of elimination of /3-hydrogen to give either enol ethers or allylic ethers can be controlled by using DMSO as a solvent and utilized in the synthesis of the tetronomycin precursor 102[125], The oxidation of the optically active 3-alkene-l,2-diol 103 affords the 2,5-dihydrofuran 104 in high ee. It should be noted that /3-OH is eliminated rather than /3-H at the end of the reac-tion[126]. 

Pd-cataly2ed reactions of butadiene are different from those catalyzed by other transition metal complexes. Unlike Ni(0) catalysts, neither the well known cyclodimerization nor cyclotrimerization to form COD or CDT[1,2] takes place with Pd(0) catalysts. Pd(0) complexes catalyze two important reactions of conjugated dienes[3,4]. The first type is linear dimerization. The most characteristic and useful reaction of butadiene catalyzed by Pd(0) is dimerization with incorporation of nucleophiles. The bis-rr-allylpalladium complex 3 is believed to be an intermediate of 1,3,7-octatriene (7j and telomers 5 and 6[5,6]. The complex 3 is the resonance form of 2,5-divinylpalladacyclopentane (1) and pallada-3,7-cyclononadiene (2) formed by the oxidative cyclization of butadiene. The second reaction characteristic of Pd is the co-cyclization of butadiene with C = 0 bonds of aldehydes[7-9] and CO jlO] and C = N bonds of Schiff bases[ll] and isocyanate[12] to form the six-membered heterocyclic compounds 9 with two vinyl groups. The cyclization is explained by the insertion of these unsaturated bonds into the complex 1 to generate 8 and its reductive elimination to give 9. 

Donor substituents on the vinyl group further enhance reactivity towards electrophilic dienophiles. Equations 8.6 and 8.7 illustrate the use of such functionalized vinylpyrroles in indole synthesis[2,3]. In both of these examples, the use of acetyleneic dienophiles leads to fully aromatic products. Evidently this must occur as the result of oxidation by atmospheric oxygen. With vinylpyrrole 8.6A, adducts were also isolated from dienophiles such as methyl acrylate, dimethyl maleate, dimethyl fumarate, acrolein, acrylonitrile, maleic anhydride, W-methylmaleimide and naphthoquinone. These tetrahydroindole adducts could be aromatized with DDQ, although the overall yields were modest[3]. 

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