Cross-coupling reactions polymer synthesis

The hydrosi(ly)lations of alkenes and alkynes are very important catalytic processes for the synthesis of alkyl- and alkenyl-silanes, respectively, which can be further transformed into aldehydes, ketones or alcohols by estabhshed stoichiometric organic transformations, or used as nucleophiles in cross-coupling reactions. Hydrosilylation is also used for the derivatisation of Si containing polymers. The drawbacks of the most widespread hydrosilylation catalysts [the Speier s system, H PtCl/PrOH, and Karstedt s complex [Pt2(divinyl-disiloxane)3] include the formation of side-products, in addition to the desired anh-Markovnikov Si-H addition product. In the hydrosilylation of alkynes, formation of di-silanes (by competing further reaction of the product alkenyl-silane) and of geometrical isomers (a-isomer from the Markovnikov addition and Z-p and -P from the anh-Markovnikov addition. Scheme 2.6) are also possible. 

Much work has been directed towards the synthesis of thiophene oligomers and polymers. This is due to the current interest in research on conducting polymers and molecular electronics (92CRV711). Two main approaches have been used for making such polymers (i) chemical (e.g. FeCl3) or electrochemical oxidation of monomeric thiophenes and (ii) transition metal-catalyzed cross-coupling reactions. 

Lightowler, S. and Hird, M. (2004) Palladium-catalyzed cross-coupling reactions in the synthesis of novel aromatic polymers. Chemistry of Materials ... 

Scheme 1 General procedure of silicone polymer synthesis by cross-coupling reaction...

In the decade since then, the major focus has been twofold improving the scope of such reactions and application of the cross-coupling reactions for the synthesis of thiophene dendrimers, thiophene polymers, and thiophenes as optoelectronic materials. 

The cross-coupling reactions easily lend themselves to use in controlled synthesis of polymers containing thiophene nuclei <2002JOM(653)195, 1994MM6620, 1997T10357>. Equations (35)-(37) provide representative examples. 

Discoveries made in the late 1970s of electrical conductivity in conjugated polymers spawned a monumental interest in the study of these materials, which are ideally suited for synthesis via cross-coupling reactions. Yamamoto was the first to realize this possibility with a regiochemicahy defined synthesis of poly(/ -phenylene) formed via the Kumada-Corriu protocol. Since then, various cross-coupling reactions have been applied in condensation polymerization reactions,with the Suzuki and Sonogashira protocols gaining the most widespread use. 

An analogous polymer, 162, with diethynyldiphenylene spacers that possesses 100% complexation of arene groups to Cr(CO)3 moieties has been prepared.The solubility was low, although molecular weights M of ca. 7,800 were estimated. In this case, the synthesis involved a cross-coupling reaction of the />-dichloroarene complex with the bis(trimethylstannyl)dialkyne. Similar materials with diethynylthiophene spacers have also been described. 

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