Alkenyl siloxane

For other recent examples of Hiyama couplings of aryl/alkenyl siloxanes with aryl chlorides, see (a) Wolf, C.,... 

A methodology to access trisubstituted macrocyclic ( )- and (Z)-alkenes from ring-closing metathesis (RCM) has been described. The strategy employs a substrate controlled RCM reaction of linear vinylsiloxanes to generate macrocyclic ( )-alkenyl siloxanes selectively. Conditions were developed to convert the alkenyl siloxane products into alkenyl halides with retention or inversion of configuration of the alkene geometry. 

Regeneration of tin hydride from the corresponding halide in situ by poly-(methylhydro)siloxane enables alkenyl-alkenyl- or alkenyl-aryl coupling by the use of only catalytic amount of tin (Scheme 74) [279-287]. 

Linear LC siloxanes have been well known since the end of the seventies [4] and an enormous variety of compounds, including networks, have been synthesized in the meantime [5]. These side-chain polymers are usually synthesized by a well-established standard procedure, that is, the coupling of co-alkenyl substituted mesogenic groups to H-siloxanes with the aid of a Pt catalyst. This concept has been extended to various geometric and chemical classes of backbone siloxanes, linear [6], cyclic [7, 8] and cage-like siloxanes [9], or siloxanes with a great variety of substituents [10]. 

On the other hand, there is a need to use inhibitors of the platinum catalysts temporarily to reduce their catalytic activity in the presence of hydro- and vinyl-polysiloxanes in order to stop the curing process at room temperature, but to allow the platinum catalyst to be activated at elevated temperature. Among the principal types of compounds reported are alkenyl derivatives, esters of unsaturated acids, crown ethers, organic nitrogen compounds, phosphines, linear and cyclic vinyl-siloxanes, and poly(vinyl)siloxanes [2], and recently fumarate [44] and maleinate [33]. New co-activators of the catalysts (precursors) have been revealed in the 1990s to reduce to ppm the levels of platinum required to effect hydrosilylation curing [45, 46]. 

Several independent protocols using a combination of transition metal-catalyzed stereoselective hydrosilylation, such as palladium-catalyzed crosscoupling sequence leading to stereodefined r-conjugated alkene derivatives, have been successfully developed in the last decade (4). Alkenylsilanes or siloxanes, prepared via platinum or rhodium complex-catalyzed intermolecular hydrosilylation of terminal alkynes have been highly stereospecifically cross-coupled with aryl and alkenyl halides to give unsymmetrical stilbenes, alkenylbenzenes, and conjugated dienes (Scheme 24) (4). 

Formation of a siloxane network via hydrosilylation can also be initiated by a free-radical mechanism (300-302). A photochemical route makes use of photosensitizers such as peresters to generate radicals in the system. Unfor-timately, the reaction is quite sluggish. Several complexes of platinum such as (jj-cyclopentadienyl)trialkylplatinum(rV) compoimds have been found to be photoactive. The mixture of silicone polymer containing alkenyl functional groups with silicon hydride cross-linker materials and a catalytic amoimt of a cy-clopentadienylplatinum(IV) compound is stable in the dark. Under UV radiation, however, the platinum complex imdergoes rapid decomposition with release of platinum species that catalyze rapid hydrosilylation and network formation (303-308). Other UV-active hydrosilylation catalyst precursors include (acetylacetonate)Pt(CH3)3 (309), (acetylacetonate)2Pt (310-312), platinum tri-azene compounds (313,314), and other sytems (315,316). 

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