Internal alkenes, palladium® chloride

Alkenes can be converted to succinic esters by reaction with carbon monoxide, an alcohol, and palladium chloride in the presence of mercuric chloride.1,12 The addition is mostly syn. In similar reaction, both terminal and internal alkynes can be converted to esters of maleic acid. 

Terminal alkenes could be efficiently aminated by nonhindered secondary amines in a process requiring 1 equiv. of palladium(II) chloride, 3 equiv. of amine and a reduction at temperatures below -20 C (path a, Scheme 5) 21,22 however, primary amines and/or internal alkenes were less efficient, producing only 40-50% yields of amination product. Oxidative cleavage of the unstable o-alkylpalladium(II) in the presence of a nucleophile resulted in vicinal oxamination or diamination of the alkene (path b).23,24 Car-bonylation resulted in the isolation of stable o-acylpalladium(II) species (path c),23 which were oxidatively cleaved to give 3-amino esters (path d)26 or further carbonylated to give y-amino-a-ketoamidei (path e).27... 

Terminal monoalkenes were alkylated by stabilized carbanions (p a 10-18) in the presence of 1 equiv. of palladium chloride and 2 equiv. of triethylamine, at low temperatures (Scheme l).1 The resulting unstable hydride eliminate to give the alkene (path b), or treated with carbon monoxide and methanol to produce the ester (path c).2 As was the case with heteroatom nucleophiles, attack at the more substituted alkene position predominated, and internal alkenes underwent alkylation in much lower (=30%) yield. In the absence of triethylamine, the yields were very low (1-2%) and reduction of the metal by the carbanion became the major process. Presumably, the tertiary amine ligand prevented attack of the carbanion at the metal, directing it instead to the coordinated alkene. The regiochemistry (predominant attack at the more sub-... 

Although these catalytic partial hydrogenations of alkynes may well be regarded as the procedure of choice for (Z)-alkenes,25 other catalytic systems have been explored. These include a sodium hydride-sodium alkoxide-nickel(n) acetate reagent,26 and a sodium borohydride-palladium chloride-polyethylene glycol system.27 Diisobutylaluminium hydride (DIBAL) has also been used for the conversion of alkynes into (Z)-alkenes.28 ( )-Alkenes are formed when the internal triple bond is reduced with sodium in liquid ammonia.29... 

Branched acids and esters are obtained from the palladium-catalyzed reaction in the absence of phosphines, and in the presence of copper chloride and HC1.79 The mild reaction conditions and the regio-specificity make this a very attractive carboxylation procedure (entry 5, Table 5). Internal straight chain alkenes can be hydrocarboxylated, but the rates are slower and the reaction is not regiospecific. 

In addition to the most important 1,2-difunctionalization assisted or catalyzed by palladium(II) complexes, a catalytic 1,1-arylamination process of alkenes, applied to the construction of nitrogen heterocycles from 4-pentenylamides, was realized29,30. The mechanism involves the formation of arylpalladium chloride from alkyl(aryl)stannanes, the addition to the alkene, the isomerization of the adduct to the more stable benzylic palladium complex, and the displacement of palladium by an internal nitrogen nucleophile. In the presence of a substituent, mixtures of diastereomers were generally obtained. 

Halide anions affect the rate of hydroformylation of internal olefins as well as the regioselective properties of the catalyst [136]. The rate of hydroformylation of thermally equilibrated internal higher alkenes increased by a factor of 6-7 by adding substoichiometric amounts (with respect to palladium) of Cl" or Br" and by a factor of 3-4 with I" [137]. When a thermally equilibrated mixture of internal Cg-Cj g olefins was subjected to isomerization-hydroformylation, a reverse effect on regioselectivity was observed [136e]. Thus, the formation of the linear aldehyde increased in the following order iodide > bromide > chloride. 

The Wacker oxidation of ethene to ethanal is an important industrial process for the oxygenation of a hydrocarbon feedstock. Essentially, the same process may be used to convert 1-alkenes to methyl ketones. The stoichiometry of the process is shown in Figure 23.23. The reaction is catalytic in both palladium and copper the ultimate oxidant is (inexpensive) molecular oxygen. A proposed mechanism is shown in Figure 23.24 the main controversy has been as to whether the attack of water on the coordinated alkene is external or via prior coordination of the water to palladium. Current thinking is that external attack predominates in high concentrations of chloride ion and internal attack when [CT] is low. Different details of mechanisms under the two conditions are supported by different reaction kinetics. 

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