Terminal alkenes oxidations, palladium® acetate

Backwall and coworkers have extensively studied the stereochemistry of nucleophilic additions on 7r-alkenic and ir-allylic palladium(II) complexes. They concluded that nucleophiles which preferentially undergo a trans external attack are hard bases such as amines, water, alcohols, acetate and stabilized carbanions such as /3-diketonates. In contrast, soft bases are nonstabilized carbanions such as methyl or phenyl groups and undergo a cis internal nucleophilic attack at the coordinated substrate.398,399 The pseudocyclic alkylperoxypalladation procedure occurring in the ketonization of terminal alkenes by [RCC PdOOBu1], complexes (see Section 61.3.2.2.2)42 belongs to internal cis addition processes, as well as the oxidation of complexed alkenes by coordinated nitro ligands (vide in/ra).396,397... 

In fact, the role of copper and oxygen in the Wacker Process is certainly more complicated than indicated in equations (151) and (152) and in Scheme 10, and could be similar to that previously discussed for the rhodium/copper-catalyzed ketonization of terminal alkenes. Hosokawa and coworkers have recently studied the Wacker-type asymmetric intramolecular oxidative cyclization of irons-2-(2-butenyl)phenol (132) by 02 in the presence of (+)-(3,2,10-i -pinene)palladium(II) acetate (133) and Cu(OAc)2 (equation 156).413 It has been shown that the chiral pinanyl ligand is retained by palladium throughout the reaction, and therefore it is suggested that the active catalyst consists of copper and palladium linked by an acetate bridge. The role of copper would be to act as an oxygen carrier capable of rapidly reoxidizing palladium hydride into a hydroperoxide species (equation 157).413 Such a process is also likely to occur in the palladium-catalyzed acetoxylation of alkenes (see Section 61.3.4.3). 

Carbonyl Compounds by Oxidation of Alcohols and Aldehydes. Salts of palladium, in particular PdCl2 in the presence of a base, catalyze the CCI4 oxidation of alcohols to aldehydes and ketones. Allylic alcohols carrying a terminal double bond are transformed to 4,4,4-trichloro ketones at 110 °C, but yield halo-hydrins at 40 °C. These can be transformed to the corresponding trichloro ketones under catalysis of palladium acetate (eq 56). The latter transformation could be useful for the formation of ketones from internal alkenes provided the halohydrin formation is regioselective. 

Other Methods.— The palladium-catalysed oxidation of terminal olefins to methyl ketones is very efficient using 30% hydrogen peroxide in acetic acid or t-butyl alcohol. The method offers advantages in that conversions are usually high, aldehyde production is very low, and the method requires only very low concentrations of palladium [20—40p.p.m, as palladium(li) acetate], fi-Hydroxy-o-nitrophenylselenides, or their O-acyl derivatives, on oxidation with hydrogen peroxide undergo elimination to form ketones or enol esters [equation (10)]. The starting materials can be prepared easily from alkenes via their epoxides. 

---