Vinyl-containing complex, reductive

In subsequent years, electropolymerization was definitely the established procedure. Polymerization by cathodic reduction, even exploiting redox mediation at the electrode, has been carried out on a wide series of Fe(II), Ru(II), and Os(II) vinyl-containing complexes based on differently substituted pyridine, and the relevant polymerization mechanism was extensively discussed in the same article [13]. An element of complexity of the RPs electrochemical growth hes in the presence, in the monomer, of additional electroactive groups. 

Meyer and coworkers investigated the photophysical behavior of vinyl containing Ru(II) and Os(II) complexes electropolymerized into the channels of silica sol-gel modified ITO electrodes. The monomeric complexes, [Ru(vbpy)3]2+ and [Os(vbpy)3]2+ (vbpy = 4-methyl-4/-vinyl-2,2/-bipyridine), have excited state lifetimes of approximately 900 and 60 ns, respectively. Incorporation into the sol-gel pores and polymerization (reductive polymerization initiated at the ITO electrode) results in chromophores that exhibit a remarkably small amount of self-quenching and have domains that reflect relatively isolated chromophores with excited state lifetimes longer than the solution values [125]. 

The cyclization did not work well with 4- or 6-carbon terminal aUcynols or with compounds containing nonterminal alkynes. The proposed mechanism involved initial oxidative addition of the OH group to the rhodium center with loss of CO and coordination of the pendant acetylene. Migratory insertion in a 5-exo-dig mode produces the coordinated cyclic vinyl ether, which could add an alcohol to the vinyl group and reductive elimination of the organic product regenerates the reactive metal complex. Alternatively, reductive elimination from the metal vinyl ether would produce a vinyl ether, which would be trapped by the alcoholic solvent (Scheme 14). 

The reduction of a benzenoid ring, except in benzoic acid derivatives, occurs only in the presence of a proton donor having a pKa of 19 or less (pKa of ammonia is about 33). With the exception of the vinyl group, the other functional groups listed above do not require a proton donor of this acidity in order to be reduced, although the course of reduction may then be complex, e.g. as with esters. " Consequently, a variety of functional groups should be capable of selective reduction in the presence of a benzenoid ring if the reaction medium does not contain an acid of pKa <19. A few examples of such selective reductions have been reported in the steroid literature. 

The Ni complexes have been used to perform intramolecular cyclizations to five-membered rings with a variety of substrate types containing unactivated double bonds linked to various halides, such as alkyl [234-236], vinyl [237,238], and aryl halides [236,239]. Standard conditions are reduction at the potential of the Ni(II) complex (0.1-0.2 eq.), in DMF with tetraalkylammonium salts as supporting electrolyte. The appropriate Ni(II) complex can be chosen as the one most easily reduced for which catalysis takes place on a voltammetric time scales [235]. 

Electropolymerization of 4-Vinylpyridine Complexes. Investigations of Structural and Electronic Influences on Thin Film Formation. The recent discovery of the reductive polymerization of complexes containing vinylpyridyl ligands (lg), such as Ru -(bpy)2(vpy)22+ has led to the preparation of homogeneous thin layers of very stable electroactive polymers. This method has been extended to 4-vinyl-4 -methyl-2,2 -bipyridine (lg, 21a) and 4-vinyl-l,10-phenanthroline (21b) on both ruthenium and iron. In the following section we discuss our results on thin films derived from the polymerizable ligands BPE and the trans-4 -X-stilbazoles, (4 -X-stilb X - Cl, OMe, CN and H). 

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