Palladium alkylation

A plausible mechanism proposed for this reaction involves migratory insertion of an olefin into the Pd-Si bond of a paUadium-silyl intermediate I followed by migratory insertion of the pendant olefin into the resulting Pd-C bond of II forming palladium-alkyl intermediate III. Reaction of Iff with hydrosilane releases the carbocy-cle to regenerate the palladium-silyl complex I (Scheme 3-21) [61]. 

A similar involvement of palladium hydride, palladium alkyl, and palladium acyl complexes as intermediates in the catalytic cycle of the Pd-catalyzed hydroxycarbonylation of alkenes was reported for the aqueous-phase analogs. The cationic hydride PdH(TPPTS)3]+ was formed via the reduction of the Pd11 complex with CO and H20 to [Pd(TPPTS)3] and subsequent protonation in the acidic medium. The reaction of the hydride complex with ethene produced two new compounds, [Pd(Et)(TPPTS)3]+ and Pd(Et)(solvent)(TPPTS)2]+. The sample containing the mixture of palladium alkyl complexes reacted readily with CO to afford trans-[Pd(C(Q)Et)(TPPTS)2]+.665... 

The factors that control the strictly alternating copolymer chain with no detectable errors (e. g., microstructures involving double insertion of ethene) have been the object of detailed studies since the discovery of the first Pd" catalysts for the alternating alkene/CO copolymerisation [11]. Sen was the first to demonstrate that double carbonylation is thermodynamically unfavorable and to suggest that the higher binding affinity of Pd" for CO relative to ethene inhibits multiple ethene insertions, even in the presence of very low concentrations of CO [12]. Therefore, once a palladium alkyl is formed, CO coordination ensures that the next monomer will be a CO molecule to generate the acyl complex. 

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry has contributed remarkably to unravelling the termination and initiation steps of the styrene/CO copolymerisation catalysed by the highly active bis-chelated complex [Pd(bipy)2](Pp5)2 in TFE [40]. Chain-end group analysis of the material produced in the absence of BQ showed that the termination by P-H elimination is accompanied by three different initiators two palladium alkyls from Pd-H formed by reaction of the precursor with CO and water (a and b) and a palladium carboalkoxy species formed by reaction of the precursor with the fluorinated alcohol and CO (c) (Chart 7.4). The suppression of the chain-transfer by alcoholysis was proposed to be responsible for the enhanced stability of the palladium acyl intermediates and hence for the high molecular weight of the copolymers produced. 

Two metal halides have been found to react with olefins by what appears to be insertion reaction. Palladium chloride and mercury chloride both will add to olefins. The palladium alkyls canot be isolated, but they go on to products which can be accounted for by an initial addition. 

Similarly to the hydroformylation, under certain reaction conditions the formation of the intermediate palladium-alkyl complex can be practically irreversible as shown by the different prevailing chirality of the 2-methylbutanoic acid ester obtained from 1-butene and (Z)-2-butene, as well as from ( )- and (Z)-2-butene. Therefore, re-gioselection and enantioface selection must occur, as in hydroformylation, during or before the formation of the postulated palladium-alkyl intermediate (see Scheme IV). 

Both 1,4- and 1,5-dienes form stable complexes with Pd. For most 1,3-dienes, such as 1,3-butadiene, reaction with Pd° compounds leads to 7r-allyl formation. These reactions are described in Section 7. The coordinated double bonds in palladium diene complexes are reactive toward attack by many nucleophiles, and the resulting chelating alkene palladium alkyls are easily isolated. Many useful reactions of dienes were discovered by Jiro Tsuji in the 1960s and 1970s. These have been recently reviewed in a historical memoir. ... 

For ethylene/CO copolymerization, two relevant termination mechanisms have been proposed. One mechanism, protonolysis of the palladium-alkyl bond, produces a saturated ketone end-group and a palladium methoxide (eq. (6)). The latter can again be converted to a palladium carbomethoxide initiator by CO insertion into the palladium-methoxide bond. 

In aprotic solvents diketones can be produced exclusively. This indicates that palladium hydrides, generated via water-gas shift reaction (eq. (9)), or by hetero-lytic hydrogen splitting (eq. (10)), are indeed efficient initiators and it also shows that protonolysis and/or hydrogenolysis of palladium alkyls can be an efficient termination mechanism. 

Firstly, the stabilization by chelate formation should increase the exothermicity of olefin insertion. Since part of the stabilization will already be felt in the transition state, it will reduce the barrier for olefin insertion in a palladium-acyl bond. Secondly, chelate formation should oppose or slow down termination by ff-elimi-nation from palladium-alkyl. For -elimination to occur the P-R atom has to approach the palladium ion, but that is opposed by coordination of the carbonyl group (eq. (13)). Inhibition ofy9-H elimination in metallacycles is well known [27]. 

Carbon monoxide insertions into palladium-alkyl or palladium-aryl bonds were extensively studied in connection with the palladium-catalyzed CO-olefin copolymerization process . 

Polymerization of isocyanides is a thermodynamically feasible process, in agreement with the stoichiometric multiple insertion observed in reactions between metal-alkyl complexes and isocyanides. The entropy loss in the case of isocyanides is lower than for insertion of CO. Isocyanide insertions into palladium-alkyl a bonds are faster than those for the platinum(II) analogues. The latter, on the other hand, usually lead to more stable and better defined products. Insertion of isocyanides into platinum-carbon bonds has been studied extensively Reaction (j) is typical the ionic product was strongly suggested by observation that the compounds isolated under mild conditions are 1 1 electrolytes. 

Oxidative decomplexation of the above described palladium alkyl or allyl complexes with Collin s reagent gave norbornenones 20 and nortricyclenones 21 in varying ratios depending on the starting material. ... 

Uchiumi, S. Ateka, K. Matsuraki, T. Oxidative reactions by a palladium-alkyl nitrite system. J. Organomet. Chem. 1999, 576, 279-289. 

This yields products with extremely low glass temperatures. An evaluation of the distribution of the branches indicates that in the polymerization of ethene with the 2,3-bis(2,6-dibromophenylimine)butane palladium catalyst the ratio of chain walking and subsequent insertion into the secondary palladium alkyl bond is higher and olefin elimination and reinsertion is faster than in the diisopropyl h-gand-substituted palladium catalyst (Tab. 3.6). The latter thus results in the production of 1-olefins which - after inserting - give more even numbered branches. 

P-hydride elimination, which is the Ayn-elimination of a hydrogen atom and Pd(II) from a palladium alkyl complex with no change in oxidation state ... 

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