Cyclooctadiene complexes with palladium

A novel chiral dissymmetric chelating Hgand, the non-stabiUzed phosphonium ylide of (R)-BINAP 44, allowed in presence of [Rh(cod)Cl]2 the synthesis of a new type of eight-membered metallacycle, the stable rhodium(I) complex 45, interesting for its potential catalytic properties (Scheme 19) [81]. In contrast to the reactions of stabihzed ylides with cyclooctadienyl palladium or platinum complexes (see Scheme 20), the cyclooctadiene is not attacked by the carbanionic center. Notice that the reactions of ester-stabilized phosphonium ylides of BINAP with rhodium(I) (and also with palladium(II)) complexes lead to the formation of the corresponding chelated compounds but this time with an equilibrium be-... 

Sodium arylsulfinates and palladium salts react to form arylpaUadium salts and sulfur dioxide. When carried out in the presence of some dienes, stable organopalladium complexes have been obtained. For example, 1,5-cyclooctadiene and sodium p-toluenesulfinate with palladium chloride yields the a,it-complex shown in equation (48).109... 

Aryl triphenylstannyl tellurium reacted with 1,5-cyclooctadienedichloropalladium in chloroform at 20°. With equimolar amounts of the reagents 1,5-cyclooctadienechloro (arenetellurolato)palladiums were isolated with 2 moles of tellurium compound per mole of palladium complex, 1,5-cyclooctadiene bis[arenetellurolato]palladiums were obtained2. 

The product of the palladation reaction exists as an active intermediate and cannot be isolated in general. However, the product of palladation was isolated as a stable compound in the reaction of cyclooctadiene palladium complex with carbanions such as malonate or alcoholate. Further reaction of the complex with base to give bicyclo (6,1,0) nonene and bicyclo (3,3,0) octane systems was reported by Takahashi and Tsuji 108>. The reactions are understood as intra-and intermolecular nucleophilic addition reactions. 

Similar studies have been reported for the complexes of norbornadiene with cuprous chloride (5) and palladium chloride 4). A complex of 1,3- and 1,4-cyclooctadiene with palladium chloride has been reported to be a useful catalyst for the polymerization of olefins (5). 

Table 14.4 Selective hydrogenation of 1,3-cyclooctadiene to cyclooctene with various palladium complexes.

The complexes (l,5-cyclooctadiene)(2,4-pentanedionato)-palladium(II) and platinum(II) tetrafluoroborate are air-stable solids, soluble in polar organic solvents such as chloroform, methylene chloride, acetonitrile, acetone, or methanol but insoluble in nonpolar solvents such as alkanes, benzene, or ether. Their solutions in acetone have conductivities typical of 1 1 electrolytes. Their proton magnetic resonance spectra (in CDC13 solutions, internal tetramethylsilane reference at 60 MHz.) show peaks due to coordinated cyclooctadiene at 3.78 and 6.7-7.4r (Pd) and at 4.25 and 6.9-7.6r (Pt) and due to the chelated /3-diketone at 4.39 and 7.88r (Pd) and at 4.15 and 7.81r (Pt) with the expected area ratios. In the spectrum of the platinum compound coupling with the 95Pt isotope (33 %... 

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. 

As with Pd bonds, Pd H will add to unsaturated functional groups see Hydride Complexes of the Transition Metals). In the case of alkynes, the Pd H addition is cis, to give vinyl complexes. The addition of a Pd H bond to an alkene will normally generate a Pd alkyl with jS-hydrogens, which can undergo further jS-elimination. This process can lead to alkene isomerization. However, palladium complexes have not proved as useM as those of other transition metals for alkene isomerization. While 1,5-cyclooctadiene can be isomerized to 1,3-cyclooctadiene with PdCl2(PhCN)2, palladium on charcoal is a more convenient and active catalyst for this isomerization. 

In this reaction the first addition product was isolated, in catalytic reactions this is not the case and in these reactions the first (cr-alkyl)palladium complex formed from the nucleophilic addition reacts further. For example, in the palladium-catalyzed oxidation of 1,5-cyclooctadiene with Pb(OAc)4 in acetic acid the corresponding diacetate 6 was obtained in 76% yield together with some chloroacetate (equation Adduct 7 is the... 

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