Olefins palladium

Chalk and Elarrod (11a) compared the above ethylene Pt(II) complex with chloroplatinic acid for hydrosilation, and found that each gave essentially the same results in terms of rate, yields, and products. Plati-num(II) complexes and rhodium(I) complexes were very much alike in their behavior. No system was found in which a palladium olefin complex brought about hydrosilation. In most systems the palladium complex was very rapidly reduced to the metal. 

The proposed mechanism is illustrated in Figure 8-5.60a Oxidative addition of the phenyl triflate to the palladium(0)-BINAP species A gives phenylpalla-dium triflate B. Cleavage of the triflate and coordination of 2,3-dihydrofuran on B yields cationic phenyl palladium olefin species C. This species C bears a 16-electron square-planar structure that is ready for the subsequent enantio-selective olefin insertion to complete the catalytic cycle (via D, E, F, and G). The base and catalyst precursor have profound effects on the regioselectivity and enantioselectivity. 

My last comment concerns the reaction of palladium olefin complexes with carbon monoxide discovered by Tsuji. I agree that this is most likely to proceed by an insertion rather than an ionic mechanism. Chloride attack on coordinated olefin is rare however. Chloride ion is an inhibitor, for example in the palladous chloride catalyzed hydration of ethylene (0). I, therefore, wondered whether carbon monoxide was affecting the ease with which chloride attacks olefin. One can postulate that carbon monoxide participates in this insertion either as a gas phase reactant or by first forming a carbonyl olefin complex. Such complexes of the noble metals were unknown, but examining the reaction between carbon monoxide and the halogen bridged olefin complexes of platinum revealed that they are formed very readily... 

Palladium chloride or the chloropalladite ion catalyze the oxidation of olefins to aldehydes or ketones, presumably by forming unstable palladium-olefin complex intermediates 196). A reaction of great industrial importance is the palladium chloride/cupric chloride catalyzed oxidation of ethylene to acetaldehyde 195). The first stage is presumably the oxidative hydrolysis of ethylene,... 

Comparison of these results with those presented for ethylene in Table 7 show that, for each catalyst, the two reactions show close similarities suggesting that the general features of the mechanisms are the same for both reactions. There are some important differences in detail between the two reactions, particularly with palladium and iridium. With palladium, olefin exchange occurs more readily with propene than with ethylene. This may be due to easier desorption of propene or possibly due to an alternative mechanism such as... 

The mechanism is typical of palladium olefin chemistry, and water serves as the oxygen source the reduced palladium is reoxidized by Cu(II) and ultimately by atmospheric oxygen. 

New MM3+-based force fields for (p.3-allyl)palladium and palladium olefin complexes with various co-ligands, which are based on the points-on-a-sphere approach and do not require dummy atoms to define the connectivity, have been developed and validated with experimentally observed and quantum-mechanically computed data[454l... 

R. M. Atkins, R. Mackenzie, P. L. Timms, and T. W. Turney, The Preparation of Palladium-Olefin Complexes from Palladium Vapour, J. Chem. Soc., Chem. Comm. 1975, 764. 

Palladium olefin complexes are considerably less stable than the platinum analogues. The dibenzylideneacetone complex Pd2(dba)3 (18-H-IV), however, is air-stable it is a convenient source of Pd° for catalytic applications. 

Japan. Review of author s work on palladium-olefin complexes and nucleophiles... 

Syntheses of heterocycles via palladium-olefin and -alkyne chemistry 04CRV2285. 

Nucleophilic attack of the arene on the palladium-olefin complex I with loss of HCI... 

Interestingly, Widenhoefer reported a similar palladium(II) catalyzed cycliza-tion of indoles onto alkenes (Scheme 58) [72]. This mild protocol for cyclization/ carboxylation of 2-alkenyl indoles makes possible catalytic addition of a carbon-nucleophile and carbonyl group across a C-C bond. The mechanism, however, is thought to involve outer-sphere attack of indole onto a palladium-olefin complex rather than the electrophilic C-H activation of the indole C(3)-H bond, exhibited by the Stoltz carbocyclization. 

The stereochemical outcome was in agreement with a mechanism for the palladium-catalyzed cyclization/carboalkoxylation of a substituted alkene (Scheme 47) that involves outer-sphere attack of the indole on the palladium-olefin complex I which, coupled with loss of HCI, would form the alkylpalladium intermediate II. 1,1-Migratory insertion of CO into the Pd-C bond of II with retention of stereochemistry would form the acyl-palladium complex III, which could undergo methanolysis to release c/.v-product and form a palladium(0) complex. Oxidation with Cu(II) would then regenerate the active Pd(II) catalyst. 

The crystal structures of Zeise s salt and of two analogous palladium-olefin complexes were published,shortly after the publication of the Chatt-Duncanson paper. They confirmed the structural proposals made by Chatt, but none of these papers cites Dewar,though metal-olefin bonding models were not discussed. Two short reviews on the history of Zeise s salt, (one part of a more general discussion of the history of organometallic chemistry) also only refer to Chatt s contribution, though neither specifically address questions of bonding. 

Kurosawa has shown that the related palladium(II) olefin complex [( ti -C5H5)Pd(Ph3P) (CHj=CHj)] undergoes clean trans attack by both methoxide and the anion of acety-lacetone. These examples of additions to iron- and palladium-olefin complexes occur by attack only at the ligand and lead to stable o-alkyl products because the metal center is coordinatively saturated. This coordinative saturation disfavors attack at the metal that would lead to either products from syn addition or displacement of the olefin. The cr-alkyl complexes that are products of these examples of nucleophilic attack are stable because the most common mode for decomposition of o-alkyl complexes, 3-hydrogen elimination, requires the presence of a vacant coordination site cis to the alkyl group, as noted in Chapters 8 and 9. Such a site is not present in these cr-alkyl products. 

Many reactions of amines with palladium- and platinum-olefin complexes have been reported. Akermark showed that nucleophiles add to palladium-olefin complexes to generate aminoalkyl complexes, as shown by the example in Equation 11.26. In this case, reactions of a bis-olefin dichloropalladium complex with amines occurs by splitting of the chloro-bridged dimer by the first equivalent of amine to give a neutral olefin-ligated palladium-amine complex that undergoes attack of the coordinated alkene by a second equivalent of amine. The stereochemistry of the amination is cleanly trans. Akermark and Zetterberg isolated and characterized by C NMR spectroscopy the a-alkyl complexes formed by the amination of both cis- and frans-2-butene, and the stereochemistry of the product alkyl complexes results from external attack by amines, as shown in Scheme 11.5. 

The palladium-catalyzed reaction of olefins with aryl or vinyl halides or pseudohalides in the presence of base (the Heck reaction) follows a different course from the other crosscoupling reactions after the oxidative addition step. As shown in Scheme 19.5, the olefin coordinates to the palladium after oxidative addition of the aryl or vinyl halide. Tliis coordination of olefin may occur by associative displacement of a monodentate ligand from the palladium, or it may occur by replacement of halide by the olefin to generate a cationic olefin complex. In some cases, these reactions are conducted with aryl or vinyl triflates. In this case, the olefin readily displaces the triflate to generate a cationic palladium-olefin... 

The proposed meehanism for this transformation is outlined in Scheme 39. An acyl-palladium complex adds to the alkynol following either path A or B, to give I or II after insertion of CO into the palladium-olefin bond. Formation of a /3-lactone could then occur by attack of the hydroxyl group in I onto the acylpalladium complex. 

The organometallic chemistry of pahadium(II) is similar to that of platinum(II) except that the palladium compounds are less stable. This lability permits a wide variety of useful catalytic reactions (e.g., palladium olefin complexes in the Wacker process). Prominent examples are the formation and reaction of r-allyl complexes. The r-allyl complexes can be formed from an olefin bound to palla-dium(II) on heating or by the reaction of an allyl halide... 

In the case of palladium(ll), the stabilization of the filled 4r/-orbitals is high, and the importance of backdonation to the olefin is very much diminished. It is the donation of the Tt-orbitals of the olefin to the metal, the chelate effect (for chelating ligands), and the strain release for strained alkenes, that are the factors mainly responsible for the stabilization of the palladium-olefin bond. In other words, the dominant character of the metallic center, when coordinated to olefins, changes from Tt-nucleophilic palladium(0) to electrophilic palladium(ll). This does not mean, however, that backdonation to the tt" -orbital of the olefin does not play any role. On the contrary, as discussed in Section 8.06.2.3, it is considered an important factor for the stabilization of five-coordinated palladium (and platinum) complexes. 

In fact, Wacker type oxidations (largely applied for aldehyde synthesis, acetoxylation reactions) can be considered as an intra or, more probably according to the recent literature, as an out-of-sphere nucleophilic attack on a palladium-olefin 7r-complex. 

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