Vinyl complexes isomerization

Wakatsuki et al. (4) proposed vinyl complex, 5, and presented DFT results supporting isomerization to a vinylidene hydride as the rate determining step. Our results indicate that the rate determining step involves H-OH bond breaking and that protonation of a bound alkyne is the rate determining step in this... 

In another study the kinetics and mechanism of an unprecedented T/2-vinyl isomerization of a highly fluorinated tungsten(II) metalla-cyclopropene complex was studied (92). Photolysis of a tungsten(II) tetrafluoroaryl metallacycle 1 and perfluoro-2-butyne results in the formation of the kinetic rf -vinyl complex 2 in which the fluoride is trans to the inserted acetylene and cis to both carbonyl ligands. Upon heating 2 is converted to the thermodynamic rf -vinyl complex 3 in which the fluoride ligand is now cis to the inserted alkyne and trans to one CO and cis to the second CO ligand as shown in Scheme 1. 

Alkynes coordinated to platinum(O) are susceptible to electrophilic attack. The reaction which has been most fully studied is the protonation of complexes Pt(alkyne)(PPh3)2 to give vinyl platinum(Il) complexes then alkenes. The reaction has been discussed in Section 52. The vinyl complexes formed undergo isomerization in the final step, since the cis vinyl complex yields some tracts-alkene. Carbene intermediates have been proposed in the pathway for this isomerization.848 Platinum(II) alkyne complexes can be converted into carbene complexes, and this reaction has been discussed in Section 52.4,6. This pattern of differential reactivity is apparent in the IR spectra of the two sets of complexes. For alkyne complexes of platinum(O) the C==C stretching frequency is lowered by some 450 cm-1 upon coordination, but with the platinum(II) analogs the difference is only in the region of 200 cm-1. 

Allylic double bonds can be isomerized by some transition metal complexes. Isomerization of alkyl allyl ethers 480 to vinyl ethers 481 is catalysed by Pd on carbon [205] and the Wilkinson complex [206], and the vinyl ethers are hydrolysed to aldehydes. Isomerization of the allylic amines to enamines is catalysed by Rh complexes [207]. The asymmetric isomerization of A jV-diethylgeranylamine (483), catalysed by Rh-(5)-BINAP (XXXI) complex to produce the (f )-enaminc 484 with high optical purity, has been achieved with a 300 000 turnover of the Rh catalyst, and citronellal (485) with nearly 100% ee is obtained by the hydrolysis of the enamine 484 [208]. Now optically pure /-menthol (486) is commerically produced in five steps from myrcene (482) via citronellal (485) by Takasago International Corporation. This is the largest industrial process of asymmetric synthesis in the world [209]. The following stereochemical corelation between the stereochemistries of the chiral Rh catalysts, diethylgeranylamine (483), diethylnerylamine (487) and the (R)- and (5)-enamines 484... 

There is nucleophihc attack by external water (reaction 3) at a neutral Pd(C2H4) species to give a palladium hydroxyethyl intermediate. The following displacement of Cl by water is rate-limiting. The Pd—CH2CH2OH complex decomposes by j8-H elimination the resulting vinyl alcohol isomerizes to acetaldehyde. 

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. 

This study opens up the possibility that hydroaluminations by LAH or LiAlBu 2MeH actually occur by the dissociation of such complex hydrides into AIH3 and R2AIH at higher temperatures. Steps of conventional syn hydroalumination, aluminum exchange and vinylic lithium isomerization could then lead to the anti adduct (Scheme 10). 

If the benzene is replaced by ethanol, however, the chloroolefin-platinum(O) complex isomerizes to a Pt(II)- y -vinyl product ... 

Aryl, vinyl, and acyl ligands have empty it orbitals that can accept electron density from the metal, and these also form strong M—C bonds. Pentahal-ophenyl ligands are exceptionally stable and strongly bound. Vinyls and acyls also have an alternative -q -bonding mode " shown as 3.22 and 3.23, when the electron count goes from le to 3e. The forms are probable intermediates in the isomerization of metal vinyl complexes (Eq. 3.28)." ... 

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