Diolefin complexes

The infrared and Raman spectra of a number of gold(I) and gold(III) diolefin complexes have been recorded (84), and on this basis the gold(I) complexes [Au2X2(cod)] (X = Cl or Br) were thought to be polymeric with halide and perhaps diolefin bridges, with the latter adopting the chair form. 

At this point mechanistic studies have reached an impasse. All of the observable intermediates have been characterized in solution, and enamide complexes derived from diphos and chiraphos have been defined by X-ray structure analysis. Based on limited NMR and X-ray evidence it appears that the preferred configuration of an enamide complex has the olefin face bonded to rhodium that is opposite to the one to which hydrogen is transferred. There are now four crystal structures of chiral biphosphine rhodium diolefin complexes, and consideration of these leads to a prediction of the direction of hydrogenation. The crux of the argument is that nonbonded interactions between pairs of prochiral phenyl rings and the substrate determine the optical yield and that X-ray structures reveal a systematic relationship between P-phenyl orientation and product configuration. 

B2. With Neutral Molecules Electron Transfer Addition to Olefins Cycloaddition to Diolefins Complex Formation B3. With Ions or Dipolar Substrates Nucleophilic Capture B4. With Radicals Spin Labeling Oxygenation (302)... 

Olefin Substitution. Coordinated olefins are generally labUe hgands and their substitution is easily achieved by a more donating hgand. Ni(0) and Pt(0) biscarbene complexes (87) were prepared from the corresponding [(C0D)2M] complex by simple substitution (Scheme 15). When the reaction with IMes or SIMes was carried out in the presence of two equivalents of DMFU, the monocarbene diolefin complexes [(NHC)M(DMFU)2] (88) were obtained. ... 

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. 

The diolefin complexes are much more stable than monoolefin complexes and can be prepared by the direct reaction of olefin with Na2PdCl4 in various solvents. The 1,5-cyclooctadiene (COD) complex is prepared by treating COD with Na2PdCl4 in acetone, acetic acid, water, or ethanol 34, 87) ... 

A new diolefin complex reported recently is the Pd(II) complex with cis, namely, a j-l,4-cyclononadiene (16). As with other diolefin complexes, it is quite stable, decomposing at 150°C. The compound is insoluble and very likely has a polymeric structure. 

Numbers in parentheses indicate yield for the respective reactions. Diolefin complexes of cobalt dimers are prepared from the direct reaction of Co2(CO)s with the diolefin ... 

It is therefore correct to say that 7r-olefin and 7r-diolefin complexes are known for many of those metals which can be employed as metal catalysts for the hydrogenation of olefins and diolefins. 

Ab initio calculations at the HF/6-31G level, performed for 2,6-pyridinedithiol in the gas phase and in solution, predict the predominance of the dithiol tautomer in the gas phase and in solvents of low polarity, whereas the thione tautomer is favored in polar solvents (98MI1813). In contradiction to the theoretical predictions and polarity-based expectations, the predominance of the 6-mercaptopyridine-2-thione in toluene-Js solution and the equilibrium shift toward the dithiol tautomer in more polar CDC13 is reported on the basis of H NMR data. This shift to the dithiol tautomer was also observed on complexation with rhodium diolefin complexes (96IC1782). 

Cationic Ir diolefin complexes, e.g., [Ir(COD)L2]Pp6 and [Ir(COD)L(pyr)]PF6 where COD = 1,5-cyclooctadiene and L = P MePh2, PPhj, Pi-Prj rapidly reduce even tri-and tetrasubstituted alkenes in CH2CI2. Some of those catalysts are not poisoned by... 

Oxidation of the Cp -diolefin complex [Pt (Cp )(pentamethylcyclo-pentadiene)] results in formation of [Pt (Cp )2] " via a net loss of a hydrogen atom from the proposed [Pt Cp )(pentamethylcyclopentadiene)] intermediate. Reduction of [Pt (Cp )2] restores the original [Pt (Cp )(pentamethyl-cyclopenta-diene)] complex, which was proposed to proceed via hydrogen atom abstraction from the solvent by the [Pt (Cp )2] intermediate (Fig. 28). 

Not all olefins react with Pt(II) complexes to form the expected product. Thus while allyl alcohol displaces chloride from K2[PtCl4] to form the expected olefin complex initially, this reacts further if held at 50°C for a couple of days to form a diolefin complex through a dehydration reaction ... 

In 5.8.2.3.1 it is pointed out that olefins are relatively weak ligands for transition metals. Diolefins that can act as bidentate ligands form stronger complexes than monoolefins owing to the chelate effect. As a consequence most of the methods described in 5.8.2.3.1 have been used to prepare diolefin complexes. Thus, for example, diolefins displace anionic ligands from Pt(II) , Pd(II) , Rh(I) and Ir(I) ... 

The formation of chelating diolefin complexes by displacement of olefin ligands is rare because complexes containing two monpolefin ligands in suitable mutual positions to be replaced by one diene ligand are themselves rare. Thus diolefins such as 1,3-cyclohexadiene, 1,4-cyclohexadiene and nbrbornadiene all displace isobutylene from [Fe( 7 -Cp)(CO)2(isobutylene)][BF4], but only one double-bond of the diolefin coordinates to the iron . Diolefins readily displace 1,5-cyclooctadiene from Ru(1,5-COD)(CO)3 to form new diolefin complexes ... 

A number of chelating diolefin complexes of group-VIA metal carbonyls have been prepared by refluxing the hexacarbonyl in a hydrocarbon solvent " ... 

Similarly, heating Rh6(CO)i6 with norbornadiene in methylcyclohexane yields a series of diolefin complexes... 

The most facile syntheses of diolefin complexes are those in which the diolefin reacts with a coordinatively unsaturated metal complex without ligand displacement. Thus a unidentate norbornadiene complex of iron, [Fe(f -Cp)(CO)2( 7 -NBD)][BF4], can be prepared by treating [Fe( j -Cp)(CO)2]2 with xs Ph3C[BFJ, followed by quenching with cycloheptatriene prior to addition of norbornadiene . Tetracoordinate complexes of Ir(I) undergo diolefin addition ... 

Diolefin complexes of Ag(I) salts with a range of anions, including nitrate perchlorate , tetrafluoroborate , hexafiuoroacetylacetonate, jS-diketonate and trifluoromethylsulfonate , have been prepared by adding the salt(I) salt to the liquid... 

---