Trans insertion

Recently, trans insertion of hexafluorobutyne into one of the M—H bonds in some metallocene hydrides, Cp2MH , was studied in some detail (47). Experiments carried out in the presence of various radical-sensitive reagents such as TV-phenyl-a-naphthylamine suggested that a free radical mechanism was unlikely. A stepwise ionic mechanism, involving a zwitter-ionic intermediate, Cp2(H2)M+—C(CF3)==CCF3, is improbable, since (i) the stereochemistry and the apparent rate are not influenced by the polarity of the solvents, (ii) no deuterium is incorporated in the reaction in EtOD, and (iii) the trend in reactivity (Mo > W) does not reflect the trend in v-basicity or M—C bond stability (W > Mo). An essentially concerted trans-insertion mechanism is inferred, which is supported inter alia by the low kinetic deuterium isotope effect (kH/k0 = 1). 

The concerted trans insertion formally belongs to a thermally allowed [>2. + 7r2a] reaction utilizing the acetylene x.,. orbital. A nonpolar four-centered heteroatomic transition state with a skewed disposition of participating [Pg.255]

The major point of controversy was the mechanism of the addition of OH to the coordinated ethylene. The hydroxyethyl palladium intermediate has never been observed but is commonly assumed to occur during the reaction. Two mechanisms are possible (Figure 29), both consistent with the kinetic expression A, migration of a coordinated OH ligand from palladium to carbon, leading to a cis insertion product and, B, external nucleophilic attack by water, leading to a trans insertion product. 

Insertion stereochemistries for the alkyne insertion process were found to be variable. Sometimes cis addition of the Ni-CHs bond occurred (e.g., for PhCsCCHs), sometimes trans addition occurred (e.g., for PhC=CPh), and sometimes a mixture of isomers was obtained. Allowing the complexes to stand led to a thermodynamically controlled mixture of cis and trans insertion products. Equilibration of cis and trans vinylaluminum compounds is known. Initial cis insertion of the alkyne yields a coor-dinately unsaturated vinylnickel intermediate that accounts for the different stereochemistry of products formed under kinetic control. Isomerization of the double bond occurs for this intermediate in competition with product formation. Thus the stereochemistry of the kinetic product does not necessarily give the stereochemistry of a preceding insertion step. 

Insertion of perfluorobut-2-yne between the metal and one end of the allyl system in Ir(w-allyl)(CO)(PPh3)2 gives products of both cis- and trans-insertion [182] to [186], (91) the products with the trans-—C(CFj) C(CF3>- system showing only weak coupling (3-5-4-5 Hz)... 

Like the Mo compound, trans insertion to WH2(Cp->7 )2 is observed with the reaction with hexafluorobut-2-yne . ... 

Cis insertion of RC=CR into the Pt—H bond of PtHCl(PEt3)2 is observed when R = CFj or COjMe is used, but trans insertion is seen with CF3C=CH... 

Since this initial report, significant advances have been made with Ni [121, 152-159], Pd [160-189], Pt [187, 190-201], Rh [202-212] and Ir [204, 211]. In general, group 10 metals generate the branched isomer and proceed by syn-insertion of the alkyne into an M-S bond, followed by protonolysis of the resulting M-C bond, although this is dependent on the reaction conditions. Eor example, under photolytic conditions, certain Pt complexes react through a trans-insertion mechanism [192, 200]. Likewise, Pd-catalyzed reactions of thiols with 1-alkynyl-phosphines proceeds by anfi-hydrothiolation (20) [213]. 

The concentration of trans-1,2-dimethylcyclohexane in dichloromethane affected the stereospecificity of insertion of ethoxycarbonylnitrene. The proportion of tertiary product decreased with increasing dilution by CHjClj and the stereoselectivity of insertion is such that the yield of trans-insertion product always exceeded 90%. 

Pathways to form trans addition products by Qialk-Harrod and modified Qialk-Harrod mechanisms are also shown in Scheme 16.10. The formation of trans addition products is rationalized by the dynamics of ri -vinyl complexes or by a zwitterionic intermediate." AVinyl complexes are known to exchange stereochemistry and are thought to do so by one of the mechanisms shown at the bottom left of tire two cycles in Scheme 16.10. One pathway involves rotation of the C-C bond upon reopening of the -r)--vinyl complex and a second involves formation of a zwitterionic intermediate. The steric interactions in the initial cis T -vinyl intermediate make it less stable than the trans T -vinyl complex. Such trans insertions of alkynes were discussed in detail in Chapter 9 (migratory insertions). 

For stereoregular insertion there are two modes to consider—cis insertion and trans insertion. For both isotactic and syndiotactic production, the cis mechanism has been determined to be in operation. This was established by polymerizing with cis-, and trans-l-deuteriopropylene or related monomers. The expected stereochemistry was demonstrated when deuteriopropylene was polymerized. The cis monomers produce erythro monomer imits whereas the trans monomer yields the threo units when cis- and tra/is-l-d-propylene is polymerized. In some cases the nomenclature appearing in the literature can be confusing and contradictory, but all indicate cis insertion. To be specific, as defined below, stereochemical structures from cis and trans addition to the double bond of cis-(l-di) and trans-(l-di)-propylene to isotactic polypropylene are as follows (229) ... 

The influence of the ligand on cis or trans insertion is particularly obvious for y -allyl nickel systems. 

Amatore, C., Arbault, S., GuiUe, M., Lemaitie, R, Verchier, Y. 2006. Regulation of exocytosis in chromaffin cells by trans-insertion of lyso-phosphatidylcholine and arachidonic acid into the outer leaflet of the cell membrane. ChemBioChem 7 1998-2003. 

Ohtaka A, Kuniyasu H, Kinomoto M, Kurosawa H (2002) Photo-and-thiol-driven trans insertion of phenylacetylene into H-Pt bonds. J Am Chem Soc 124 14324—14325... 

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