Butadienes oxidative coupling, alkenes

The oxidative coupling of alkenes which have two substituents at the 2 posi-tion, such as isobutylene, styrene, 2-phenylpropene, 1,1-diphenylethylene, and methyl methacrylate, takes place to give the 1,1,4.4-tetrasubstituted butadienes 285 by the action of Pd(OAc)2 or PdCF in the presence of sodium acetate[255-257]. Oxidation of styrene with Pd(OAc)2 produces 1.4-diphenylbutadiene (285, R = H) as a main product and a- and /3-acetoxystyrenes as minor pro-ducts[258]. Prolonged oxidation of the primary coupling product 285 (R = Me) of 2-phenylpropene with an excess of Pd(OAc)2 leads slowly to p-... 

Palladium catalysts are widely used in liquid phase aerobic oxidations, and numerous examples have been employed for large-scale chemical production (Scheme 8.1). Several industrially important examples are the focus ofdedicated chapters in this book Wacker and Wacker-type oxidation of alkenes into aldehydes, ketones, and acetals (Scheme 8.1a Chapters 9 and 11), 1,4-diacetoxylation of 1,3-butadiene (Scheme 8.1b Chapter 10), and oxidative esterification of methacrolein to methyl methacrylate (Scheme 8.1c Chapter 13). In this introductory chapter, we survey a number of other Pd-catalyzed oxidation reactions that have industrial significance, including acetoxylation of ethylene to vinyl acetate (Scheme 8. Id), oxidative carbonylation of alcohols to dialkyl oxalates and carbonates (Scheme 8.1e), and oxidative coupling of dimethyl phthalate to 3,3, 4,4 -tetramethyl biphenylcarboxy-late (Scheme 8.1f). 

The anodic behavior of A -substituted alkenes can be described as the oxidation of an electron-rich double bond. Tetraamino-substituted alkenes are extremely easily oxidized. Tetrakis(dimethylamino)ethylene exhibits two reversible one-electron processes at —0.75 and —0.61 V vs. SCE at a dropping mercury electrode in acetonitrile [140]. The anodic behavior of A, A -dimethylaminoalkenes has been studied intensively by cyclic voltammetry and electron spin resonance (ESR) spectroscopy [141]. The anodically E° = 0.48 V vs. SCE) generated cation radical of l,l-bis(iV,iV-dimethylamino)ethylene is shown to undergo C-C coupling, forming l,l,4,4-tetrakis(A, iV-dimethylamino)butadiene, which subsequently is further oxidized to its dication at —0.8 V [141,142]. With vicinal diamino ethylenes, usually two reversible one-electron oxidations are observed [143], while gem-inal diamino ethylenes exhibit an irreversible behavior [141]. Aryl-substituted vicinal diamino ethylenes (endiamines) can undergo a double cyclization to give an indolo-oxazoline when oxidized at 0.4 V vs. SCE in acetonitrile in the presence of 2,6-lutidine [144] ... 

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