Palladium alkene oxidation

Another important type of reactivity of palladium, namely oxidative addition to Pd(0), is the foundation for several methods of forming carbon-carbon bonds. Aryl126 and alkenyl127 halides react with alkenes in the presence of catalytic amounts of palladium to give net substitution of the halide by the alkenyl group. The reaction, known as the Heck reaction,128 is quite general and has been observed for simple alkenes, aryl-substituted alkenes, and substituted alkenes such as acrylate esters, vinyl ethers, and A-vinylamides.129... 

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...

Palladium-catalyzed allylic oxidations, in contrast, are synthetically useful reactions. Palladium compounds are known to give rise to carbonyl compounds or products of vinylic oxidation via nucleophilic attack on a palladium alkene complex followed by p-hydride elimination (Scheme 9.16, path a see also Section 9.2.4). Allylic oxidation, however, can be expected if C—H bond cleavage precedes nucleophilic attack 694 A poorly coordinating weak base, for instance, may remove a proton, allowing the formation of a palladium rr-allyl complex intermediate (89, path by694-696 Under such conditions, oxidative allylic substitution can compete... 

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... 

Terminal alkenes can be selectively oxidized to aldehydes by reaction with oxygen, using a palladium-copper catalyst in tertiary butanol (equation 35)160. This reaction is contrary to the normal oxidation process which yields a ketone as the major product. The palladium(II) oxidation of terminal alkenes to give methyl ketones is known as the Wacker process. It is a very well established reaction in both laboratory and industrial synthesis161162. The Wacker oxidation of alkenes has been used in the key step in the synthesis of the male sex pheromone of Hylotrupes bajulus (equation 36)163. 

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. 

The selectivity of palladium and gold for alkene oxidation to aldehydes 28,29,170) was attributed initially to adsorption strength. However, electrooxidation in the presence of palladium ions indicates possible homogeneous alkene insertion, similar to the Wacker process 304). Homogeneous reaction is also involved in redox oxidations of hydrocarbons. In this case, the nature of the metal ions is expected to control selectivity. Indeed, toluene yields 20% benzaldehyde in electrolytes containing Ce salts, while oxidation proceeds to benzoic acid with Cr redox catalysts 311). In addition, the concentration of redox catalysts appears to affect yields in nonelectrochemical oxidation of ethylene large amounts of palladium chloride promote butene formation at the expense of acetaldehyde 312). Finally, the role of the electrolyte and solvent should not be ignored. For instance, electrooxidation of ethylene on carbon, in aqueous solution of acetic acid yields acetaldehyde 313) in the... 

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. 

---