Palladium complexes diolefins

The mode of interaction of the oxidant with the acetoxypalladation adduct is not certain. The oxidant could be removing electrons from Pd as the Pd(II)—C bond is broken and Pd(0) is never formed, or the Pd(II) could be oxidized to Pd(IV) which would leave much more easily than Pd(II). Another possibility is that the organic radical is transferred to the oxidant followed by decomposition. It would be difficult to distinguish between the various possibilities. Related reactions are the cleavage of a-bonded palladium complexes with Collins reagent (280), decomposition of rr-allyls with oxidants (164), and the decomposition of oxypalladation adducts of diolefins with oxidants (Section IV, B). 

Significant advances in organonickel chemistry followed the discovery of frtzws,fraws,fraws-(l,5,9-cyclododecatriene)nickel, Ni(cdt), and bis(l,5-cycloocta-diene)nickel Ni(cod)2 by Wilke et. al.1 In these and related compounds, in which only olefinic ligands are bonded to the nickel, the metal is especially reactive both in the synthesis of other compounds and in catalytic behavior. Extension of this chemistry to palladium and to platinum has hitherto been inhibited by the lack of convenient synthetic routes to zero-valent complexes of these metals in which mono- or diolefins are the only ligands. Here we described the synthesis of bis(l,5-cyclooctadiene)platinum, tris(ethylene)-platinum, and bis(ethylene)(tricyclohexylphosphine)platinum. The compound Pt(cod)2 (cod = 1,5-cyclooctadiene) was first reported by Muller and Goser,2 who prepared it by the following reaction sequence ... 

In the case of certain diolefins, the palladium-carbon sigma-bonded complexes can be isolated and the stereochemistry of the addition with a variety of nucleophiles is trans (4, 5, 6). The stereochemistry of the addition-elimination reactions in the case of the monoolefins, because of the instability of the intermediate sigma-bonded complex, is not clear. It has been argued (7, 8, 9) that the chelating diolefins are atypical, and the stereochemical results cannot be extended to monoolefins since approach of an external nucleophile from the cis side presents steric problems. The trans stereochemistry has also been attributed either to the inability of the chelating diolefins to rotate 90° from the position perpendicular to the square plane of the metal complex to a position which would favor cis addition by metal and a ligand attached to it (10), or to the fact that methanol (nucleophile) does not coordinate to the metal prior to addition (11). In the Wacker Process, the kinetics of oxidation of olefins suggest, but do not require, the cis hydroxypalladation of olefins (12,13,14). The acetoxypalladation of a simple monoolefin, cyclohexene, proceeds by trans addition (15, 16). 

The palladium(II) and platinum(II) ions form stable complexes with a variety of chelating diolefins.1-"5 These may be either neutral or cationic in character. The preparative routes to the former type are, in general, well-documented. The spedies (I) through (IV) include all the presently known cationic species, and the preparation of each type is discussed and exemplified. 

Usually, NHC palladium diolefin complexes, similar to (89), have been synthesized according to equation (11). The Pd(0) precursor bearing diolefin, for instance, nbd, COD, etc., reacted with NHC in the presence of an excess of a ligand L. 

In case (3) it cannot be excluded that the intermediate olefin complex would be a chelate with both olefmic bonds of the 1,4-cyclohexadiene coordinated to the palladium, and the oxypalladation product is in fact expected to be trans orientated as rotation of the double bond is blocked as with analogous complexes of cyclic diolefins such as 1,5-cyclooctadiene, dicyclopentadiene, norbomadiene, etc. From these, stable fran -oxymetallation products can be obtained [39 1] others are listed in [12, Table IV]. 

Fourth, the complexes of acetylenes and diolefins react chiefly in polymerization, this tendency being most marked with the complexes of nickel and palladium. Platinum complexes are not generally active in polymerization. In the catalytic hydropolymerization of acetylene, nickel displays the greatest activity, palladium takes second place and platinum third place. The remaining noble metals are less active than platinum. Thus, again, a correlation between the two fields is observed to hold. 

Stoichiometric and catalytic reactions that result from attack of nucleophiles onto square-planar palladium(II) and platinum(II) olefin complexes are legion. The o-alkyl products are stable in some cases, but p-hydrogen elimination to generate products from functionalization of the C-H bond of an olefin occurs in other cases. Nucleophihc attack on palladium(II) and platinum(II) complexes of diolefins often generate stable a-alkyl products because the resulting alkyl group is stabilized by chelation. 

Dichlorobis(benzonitrile)palladium(II) is known to react with butadiene to give a yellow complex, formulated by previous workers as a diolefin complex [Pdcyc Hg)] or shown this compound... 

Palladium compounds are liable to react with unsaturated hydrocarbons such as monoolefins, diolefins, acetylenes and allyl compounds to afford their Ti-complexes. For example, reactions with monoolefins and diolefins are shown in eqs. (20.1)-(20.3) [14-17]. 

The first case of a tetrahedral palladium(O) tetraolefin complex (more exactly, Pd(diolefin)2) has been isolated in the course of the Saegusa oxidation of a silyl enol ether, aimed at the synthesis of alkaloids. Palladium acetate was used as oxidant in this reaction, and a brown compound separated from the solution, which was characterized by X-ray diffraction as 16 (Equation (5)). It decomposed upon heating to give the expected product of oxidation. This supports the accepted mechanism of Saegusa oxidation. ... 

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