Palladium alkoxides

Pair-of-dimer effects, chromium, 43 287-289 Palladium alkoxides, 26 316 7t-allylic complexes of, 4 114-118 [9JaneS, complexes, 35 27-30 112-16]aneS4 complexes, 35 53-54 [l5]aneS, complexes, 35 59 (l8)aneS4 complexes, 35 66-68 associative ligand substitutions, 34 248 bimetallic tetrazadiene complexes, 30 57 binary carbide not reported, 11 209 bridging triazenide complex, structure, 30 10 carbonyl clusters, 30 133 carboxylates... 

Palladium alkoxide complexes are thought to be formed in the reactions of alcohols catalyzed by palladium(II) chloride. These reactions include the oxidation of alcohols, yielding acetals or ketones,137,138 and their carbonylation, yielding esters.139 Alkoxide intermediates are also thought to be involved in the reaction of sulfur dioxide with [PdCl2] suspended in alcohol (equation 15).140,141... 

The mechanism of this transformation presumably involves palladium alkoxide formation followed by jS-hydride elimination (equation 191). 

Mann, G. Hartwig, J. F. Palladium alkoxides potential intermediacy in catalytic animation, reductive elimination of ethers, and catalytic etheration. Comments on alcohol elimination from Ir(III)./. Am. Chem. Soc. 1996, 118, 13109-13110. 

The kinetics of the aerobic oxidation of alcohols catalysed by Pd(OAc)2-triethylamine have been studied experimentally and computationally. Measurement of various kinetic isotope effects and the activation parameters and also rate law derivation support a rate-limiting deprotonation of the palladium-coordinated alcohol, contrary to the previously proposed rate-limiting /3-hydride elimination.234 The catalytic efficiency of Pd(OAc)2-triethylamine and palladium alkoxides in the aerobic oxidation of alcohols has been evaluated. A new catalyst, Pd(IiPr)(OPiv)2, is found to operate efficiently at room temperature.235... 

Here the Pd(0) complex reacts with diallyl carbonate to form a n-allyl palladium alkoxide that ligates to 16. The resulting intermediate then undergoes P-hydride elimination to produce the lactone and propene (Scheme 6.10). 

The cleavage of allcylamine N-H bonds by late transition metals to form metal amido complexes is also rare [69, 70]. When the transition metal is a low valent, late metal, the resulting amido complexes are highly reactive [71, 72]. It appears that the amination of aryl halides can involve an unusual N-H activation process by a palladium alkoxide to form a highly reactive palladium amide [65, 73]. 

For the case of chelating phosphines, Hartwig noted that palladium alkoxide complexes can be formed by treatment of the corresponding halides with base, and that amido complexes can, in turn, be formed from the alkoxide. This observation opened the possibility that alkoxide intermediates lie on the catalytic cycle (lower pathway in Scheme 7). Van... 

Displacement reactions. Haloarenes in which the p-position cames an electron-withdrawing group (e.g., CHO, COPh, and CN) undergo displacement by alkoxide ions in the presence of (dbaljPd-dppf. Perhaps palladium alkoxides are involved. 

Allyl alkyl carbonates, prepared from various alcohols except simple primary ones, are converted into aldehydes or ketones in the presence of a phosphine-free palladium catalyst. Acetonitrile as coordinating solvent is necessary for the success of this reaction. A mechanism via palladium alkoxides was proposed (Scheme 8). Ruthenium hydride complexes work similarly. A similar mechanism operates for the palladium-catalyzed decomposition of allylic carbonates. The reaction can be utilized for the mild deprotection of amines, e.g., for peptide synthesis shown in equation (20). 

The success of the new method is due to selection of bases which are stable under the reaction conditions and also the use of bulky phosphine ligands. Later, reductive elimination was fovmd to be crucial in the C—N bond formation, which is accelerated by bulky ligands. As bases, MN(TMS)2 (M = Li, Na, K) and r-BuONa give good results. Alkoxides such as MeONa, EtONa, and n-BuONa seem to be unsuitable, because oxidation of alcohols and reduction of aryl halides occur via facile jS-H elimination of the palladium alkoxide 5. 

As another possibility, the reaction of 44 might be explained by the formation of palladium alkoxide 49, followed by jS-carbon elimination to afford 50. Carbopalladation (5-exo cyclization) of 50 gives 51, and reductive elimination produces 45 via 48. However, this route seems to be less likely, since more substituted carbon migrates in this type of reactions as demonstrated by the following examples [9]. 

Three general mechanisms can be envisioned for the formation of antido complexes from arylpalladium halides direct substitution of halide by alkali antide generated by simple deprotonation of free amine by free base, coordination of antine to the metal center followed by deprotonation of the more acidic coordinated amine, or formation of a palladium alkoxide... 

Scheme 11.10). Kinetic studies found that the hydrogenolysis reaction was first order in both [22] and [Hjj. A mechanism similar to that proposed for the hydrogenolysis of the hydroxide 3 was then invoked for the hydrogenolysis of the pincer palladium alkoxide complexes. 

The alkoxide pathway occurs by initial insertion of CO into a palladium alkoxide, followed by insertion of the alkene into the bond between the metal and the alkoxycarbonyl group to form a paUadium-alkyl complex (Scheme 17.18). Protonation of this metal alkyl by alcohol would form the free organic product and regenerate the paUadium alkoxide. This mechanism has now been ruled out for the reactions of ethylene to form methyl propanoate. Although each of these steps has precedent, the absence of reduction products from the alkoxide argues against this pathway. Moreover, the alkyl generated from insertion of ethylene into the palladium-alkoxycarbonyl complex (Scheme 17.18) is chelated to the metal, and metha-nolysis of this species is slower than the steps of the alternative hydride mechanism. ... 

Sulphur dioxide inserts into palladium-oxygen bonds. When a solution of palladium chloride in an alcohol is treated with sulphur dioxide, a sulphinato-complex is produced, presumably by insertion into a palladium-alkoxide bond. The reaction of sulphur dioxide with molecular oxygen complexes of platinum and of iridium, producing sulphato-complexes, involves insertion of sulphur dioxide into one of the metal-oxygen links, as shown in Scheme 5. ... 

Smooth oxidation of alcohols under mild, neutral conditions can be achieved via Tt-allylpalladium intermediates. The reaction of aliphatic allyl carbonate 98 with a palladium(O) catalyst generates palladium alkoxide species 99, which subsequently undergoes (3-hydride elimination to give carbonyl compound 100. This reaction is very clean since the by-products are CO2 and propylene (101). The reaction can readily be applied to secondary alcohols, allylic primary and secondary alcohols and benzylic alcohols. The oxidation of primary alcohols is prohibitively slow. 

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