Cycloocta-1,5-diene complexes palladium

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 ... 

These have previously been obtained by electrophilic attack on ene-yl complexes [equation (a) Y = CH(C02Me)2, OMe ch = 1J-C5H5 diene = 1,5-cyclooctadiene]1 or by reaction of the compounds (diene)MBr2 with 57-C5H6Fe(CO)2Br (diene = 1,5-cyclooctadiene or 1,2,3,4-tetraphenyl-l,3-cyclobutadiene).2 An example of the former method is given in which the methoxy-cyclooctenyl derivative is used as the substrate and tetrafluoro-boric acid as the electrophile. The substrate is conveniently prepared and used without isolation, and in this way the reaction takes only a few hours, starting with dichloro(l,5-cycloocta-diene)palladium, prepared as described above. 

Nickel(O) or palladium(II) compounds in stoichiometric amounts promote the ring enlargement of simple alkyl-substituted 1,2-divinylcyclobutanes in benzene at room temperature to give 1 1 metal complexes of cycloocta-1,5-dienes.119 Destruction of the palladium complexes with potassium cyanide affords the free cycloocta-1,5-dienes. The stereochemistry observed is the same as in the thermal reaction at 150°C. 

Oxatrimethylenemethanepalladium complexes can also be generated by oxidative addition of palladium(O) to 5-methylene-l,3-dioxolan-2-ones and subsequent decarboxylation. Again, reaction with norbornene, norbornadiene and dicyclopentadiene yields polycyclic cyclopropyl ketones in medium to high yield (Table 19). In this case, tetrakis(triphenylphosphane)pal-ladium(O) was the best catalyst found, whereas tris(dibenzylideneacetone)palladium(0)-chloro-form/triphenylphosphane (see above) and bis(cycloocta-l,5-diene)nickel/triphenylphosphane (used in stoichiometric amounts) proved less efficient. 

The following reactions of norbornene and other nonfunctionalized alkenes with substituted methylenecyclopropanes illustrate these points. (1-Methylethylidene)- and (diphenyl-methylene)cyclopropane (1 R = Me, Ph) give rise to the same type of cycloadducts in the presence of either nickel(O) or palladium(O) catalysts. Even at temperatures as low as 40 "C with bis(> -cycloocta-l,5-diene)nickel(0) as catalyst, 2 may be isolated in 70% yield. The reaction can be extended to vinylbenzene and ethene at temperatures of between 40 and 60 °C, where it may be advantageous to use (cyclododeca-l,3,5-triene)nickel(0) ° as a source of the catalytically active nickel(O) species instead of bis(j7" -cycloocta-l,5-diene)nickel(0). ° This is because ligand dissociation from the former complex is more facile, especially at relatively low temperatures. 

When phosphane-free palladium catalysts, such as bis(dibenzylideneacetone)palladium, bis(jy4-cycloocta-l,5-diene)palladium, tris(norbornene)palladium or a catalyst generated in situ from bis(acetylacetonato)palladium and ethoxydiethylaluminum, are used with 3,3-dimethylcyclopropene, dimer 17 is obtained as the major product (76% when R1 = R2 = Me), along with three tetrameric products in a combined yield of 12.3%. Complexes such as bis(7r-allyl)pal-ladium and bis(acetonitrile)dichloropalladium also act as catalysts but mainly lead to higher oligomers.42 Other 3,3-dialkylcyclopropenes react in the same manner.36 ... 

Some of the chloro-bridged palladium 105a (R = C H2 +1, R = OC H2m+i), a mixed-bridged /r-acetato /r-thiolato 106b ( = ot = 6), and a mononuclear complex 111 (R R, R, R = OCioH2i) discussed above were also found to exhibit additional lyotropic mesophases in contact with apolar organic solvents such as linear alkanes (octane, decane, dodecane, and pentadecane), cycloocta-1,5-diene, and the chiral limonene. A lyotropic lamellar phase was induced for the dichloro complexes with symmetrical chain length (n = m = 6, 10) in linear alkanes the transition temperatures were found to decrease from pentadecane to octane. 

In metal complexes the ligands may occupy different positions around the central atom. Since the ligands in question are usually either next to one another (cis) or opposite each other (trans), this type of isomerism is often referred to as cis-trans isomerism. Cis-trans isomerism is very common for square planar and octahedral complexes. Consider the square planar complexes shown in structures [5-1H5-4]. Cis-trans isomerism arises from the relative position of the ethylene ligands. Therefore, [5-1] and [5-3] are cis forms and [5-2] and [5-4] are trans forms, respectively. Norbornadiene-palladium dichloride [5-5] and rhodium (Ti-cycloocta-1,5-diene) chloride [5-6] can exist only in the cis structure because of the chelate nature of the diene. 

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