Metal groups conjugate substitution

In this paper I describe the synthesis of a new Group 6 monomer in which a carbonyl group on the metal has been substituted by a tributylphosphine moiety. This new monomer is then utilized in the preparation of new class of conjugated polymers. The palladium-catalyzed... 

Polyacetylenes are the most typical and basic r-conjugated polymers, and can ideally take four geometrical structures (trans-transoid, trans-cisoid, cis-transoid, cis-cisoid). At present, not only early transition metals, but also many late transition metals are used as catalysts for the polymerization of substituted acetylenes. However, the effective catalysts are restricted to some extent, and Ta, Nd, Mo, and W of transition metal groups 5 and 6, and Fe and Rh of transition metal groups 8 and 9 are mainly used. The polymerization mechanism of Ta, Nd, W, and Mo based catalysts is a metathesis mechanism, and that of Ti, Fe, and Rh based catalysts is an insertion mechanism. Most of the substituted polyacetylenes prepared with W and Mo catalysts provide trans-rich and cis-rich geometries respectively. Polymers formed with Fe and Rh catalysts selectively possess stereoregular cis main chains. 

Brummond [28] was the first to illustrate that cross-conjugated trienes could be obtained via an allenic Alder-ene reaction catalyzed by [Rh(CO)2Cl]2 (Eq. 14). Selective formation of the cross-conjugated triene was enabled by a selective cycloisomerization reaction occurring with the distal double bond of the aUene. Typically directing groups on the allene, differential substitution of the aUene termini, or intramolecularization are required for constitutional group selectivity. However, rhodium(f), unlike other transition metals examined, facihtated selective cyclization with the distal double bond of the allene in nearly aU the cases examined. 

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