Alkenes palladium-catalyzed oxidation

In most palladium-catalyzed oxidations of unsaturated hydrocarbons the reaction begins with a coordination of the double bond to palladium(II). In such palladium(II) olefin complexes (1), which are square planar d8 complexes, the double bond is activated towards further reactions, in particular towards nucleophilic attack. A fairly strong interaction between a vacant orbital on palladium and the filled --orbital on the alkene, together with only a weak interaction between a filled metal d-orbital and the olefin ji -orbital (back donation), leads to an electrophilic activation of the alkene9. 

Palladium-catalyzed oxidative couplings of aromatic compounds with alkenes in air lead to cinnamate products with TONs attaining 280 (Equations (66) and (67)).67,67a,67b... 

Stereo- and regioselective palladium-catalyzed oxidation of 1,3-dienes in acetic acid to give l,4-diacetoxy-2-alkenes has been accomplished using Mn02 and catalytic amounts of p-benzoquinone (BQ)11. The reaction can be made to take place with cis- or trans-1,4-diacetoxylation across the diene in cyclic systems as shown in equation 6. 

SCHEME 137. Proposed mechanism for the palladium-catalyzed oxidation of terminal alkenes to methyl ketones using TBHP oxidant... 

Scheme 14 Possible outcomes for the palladium-catalyzed oxidative acetoxylation of alkenes...

The synthetic applications of the palladium-catalyzed oxidation of alkenes to ketones have recently been reviewed.639 Improvements in the Wacker palladium-catalyzed ketonization of terminal alkenes have been obtained using phase-transfer catalysis,641 polyethylene glycol642 or phosphomolybdovanadic acids.643... 

Palladium-catalyzed oxidation of 1,3-dienes in the presence of LiCl and LiOAc produces l-acetoxy-4-chloro-2-alkenes with high selectivity. The reaction is stereospecific and cyclic dienes give an overall cis [1,4] addition (equation 299).644... 

Palladium-catalyzed oxidative cyclization of aryl homoallyl ethers affords 4-methyl-2//-chromenes in moderate yield. The reaction is proposed to proceed via activation of the alkene by coordination to Pd(ll) followed by intramolecular nucleophilic attack by the arene. Subsequent [1-hydride elimination and isomerization then affords 4-methyl-27/-chromenes (Scheme 13). Electron-rich aryl homoallyl ethers give the best yield and good regio-selectivity is observed for the reaction of unsymmetrical arenes <2005OL3355>. 

Oxidation-carbonylation of 1-alkenes. These alkenes can undergo an aerobic palladium-catalyzed oxidation-carbonylation in acetic acid-acetic anhydride (4 1) with CuCl2 as a reoxidant and a trace of NaCl, required for satisfactory yields.1... 

A simple synthetic method for 1,4-dicaibonyl compounds was introduced, based on the allylation of carbonyl compounds with allyl halide as a C3 component, followed by the palladium-catalyzed oxidation of the terminal alkenes (20) to methyl ketones (21). In this method, the allyl group is a synthetic equivalent of the 2-oxopropyl group (Scheme 5). This is a good anellation method for cyclopentenones. 

Palladium-catalyzed oxidative allylic C-H functionalization provides attractive methods for the transformations of olefins, and their utility can be further enhanced by the development of more effective ways to use molecular oxygen (or air) to promote the catalytic cycle. The results outlined in this chapter summarize significant progress in the coupling reaction between terminal alkene and various types of nucleophiles. Further studies will be directed to explorations of the scope of nucleophilic reagents and olefins, and elucidation of the mechanisms of those reactions. Such studies will play an important role in the ongoing development of Pd-catalyzed C-H bond activations. 

Although isomerization of alkenes occurs simultaneously with the oxidation, rhodium and ruthenium complexes can also be used instead of palladium for the oxidation of terminal alkene [15]. With these catalysts, symmetrical quaternary ammonium salts such as tetrabutylammonium hydrogensulfate are effective. Interestingly, the rate of palladium-catalyzed oxidation of terminal alkenes can be improved by using poly(ethylene glycol) (PEG) instead of quaternary ammonium salts [16]. Thus, the rates of PEG-400-induced oxidation of 1-decene are up three times faster than those observed with cetyltrimethylammonium bromide under the same conditions. Interestingly, internal alkenes can be efficiently oxidized in this polyethylene glycol/water mixture. 

Miura, M., Tsuda, T., Satah, T. and Nomura, M. (1997) Palladium-catalyzed oxidative crosscoupling of 2-phenylphenols with alkenes. Chem. Lett., 26, 1103. ... 

Scheme 1.13 Palladium-catalyzed oxidative arylation of alkenes with arenes (Fujiwara,...

Scheme 8.4 Palladium-catalyzed oxidative carbonylation of alkenes to branched products...

The oxidative carbonylation of hydroxy-substituted alkenes can lead to synthetically interesting tetrahydrofurans and pyrans." As early as 1984, Semmelhack and his group developed a methodology for the synthesis of furans and pyrans using a palladium-catalyzed oxidative carbonylation of hydroxyalkenes. In the presence of CUCI2 (3 equiv), PdCl2 (0.1 equiv) and CO (1.1 bar) at 25 °C, the heterocycles were isolated in good yields (Scheme 8.9) [50]. In another study, they performed the carbonylative synthesis of tetrahydrofurans [51, 52], and this procedure was applied to the synthesis of pyran-lactones [53], racemic frenolicin [54], and also plakortones [55-58]. 

Scheme 8.11 Palladium-catalyzed oxidative carbonylation of functionalized alkenes...

The reaction conditions are similar to those employed in the diacetoxylation reaction, the difference being that the halide concentration (usually Cl ) has been increased. Thus, palladium-catalyzed oxidation of 1,3-dienes with p-BQ in the presence of lithium chloride and lithium acetate gives l-acetoxy-4-chloro-2-alkenes [89]. For example, 1,3-cyclohexadiene and 1,3-cycloheptadiene afforded the corresponding chloroacetates 66a,b in good yields and with >98% cis selectivity (Eq. (11.39)). 1,3-Cyclooctadiene gave a 61% yield of acetoxychlorination product (>98% cis), but in this case a 3 1 mixture of 1,4- and 1,2-addition products was formed. A number of substituted cyclic conjugated dienes were found to work well, and in all cases tried, the reaction proceeded with >97-98% cis addition [58, 89-92]. 

Allylic C-H Bond Activation. Internal alkenes, in particular cyclic ones, can be transformed into allylic acetates in a palladium-catalyzed oxidation (eq 6). With benzoquinone as stoichiometric oxidant or electron transfer mediator, the allylic acetoxy-lation proceeds with high selectivity for the allylic product and usually in excellent yield. 

Mu X, Wu T, Wang H-Y, Guo Y-L, Liu G (2012) Palladium-catalyzed oxidative aryltrilluor-omethylation of activated alkenes at room temperature. J Am Chem Soc 134 878-881... 

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