Hydrosilylation

Hydrosilylation of C═C bonds as a most versatile synthetic route to formation of Si—C bond initiated by free-radicals has been well-known for 60 years, while that catalysed by platinum (starting from Speier catalyst) and other TM complexes – for 50 years. Chapter 1 presents a comprehensive survey of literature on hydrosilylation of alkenes and their derivatives by molecular compounds containing Si—H bond, published in the last two decades. Dehydrogenative silylation, related to the above process, is also discussed. New hydrosilylation catalysts, predominantly homogeneous and immobilised late and early TM-complexes have been developed but also nucleophilic-electrophilic catalysts, metals and supported metals as well as radical initiators have been cited. These catalysts offer many convenient synthetic routes to molecular organosilicon reagents characterised by increased efficiency i.e. the yield and selectivity as well as turnover rate. The choice of catalyst determines the mechanism of catalysis, whose rational development for individual catalytic systems is extensively discussed.

Growing interest in the development of sequential processes, including regio- and stereoselective hydrosilylation of functionalised alkenes and alkynes as the key step, stems from the ability to assemble stereodefined complex molecules from simple starting materials through organosilicon intermediates in a convergent and flexible manner. This chapter describes some recent advances in the sequential synthetic strategies including both transition-metal catalysed intra- and intermolecular hydrosilylation of C═C and C≡C bonds followed by desilylative oxidation, cross-coupling, proto- and halodesilylation, cycloaddition, nucleophilic addition and other transformations, leading to stereodefined organic derivatives which are widely applied as fine chemicals, synthetic building blocks or are key intermediates in total synthesis of natural products.

Asymmetric hydrosilylation of prochiral alkenes followed by Tamao-Fleming oxidation is a convenient method of synthesis of enantioenriched secondary alcohols. In this chapter asymmetric hydrosilylation of unsaturated carbon—carbon bond(s) in various families of reagents such as terminal and internal olefins, cycloolefins, 1,3-dienes, enynes and diynes as well as asymmetric intramolecular hydrosilylation of alkenylsilanes and cyclisation/hydrosilylation of non-conjugated dienes are reviewed. All these transformations proceed efficiently in the presence of palladium and to a minor extent in the presence of rhodium based complexes. Selected processes catalysed by yttrium and lanthanide complexes are also discussed. The chapter ends with brief presentation of some examples of free radical initiated asymmetric hydrosilylation.

This chapter presents the progress made over the last two decades in the modification (functionalisation) of polysiloxanes as well as curing of polysiloxanes via catalytic hydrosilylation. Well-defined silicones have been prepared by hydrosilylation of functional olefins with polymethylhydrosiloxanes predominantly in the presence of Pt-based catalysts to make a whole branch of materials for adhesives binders, ceramic and dielectric coatings, encapsulates, membranes, sealants and a lot more. On the other hand, the addition of polyfunctional silicon hydrides to poly(vinyl)organosiloxane leads to effective cure of silicon rubber. Pt-complex or other TM-complexes but yet usually Karstedt’s catalyst with various inhibitors added to reduce or temporarily inhibit its catalytic activity in the cross-linking, reported since 1990 are compiled. Fast cure can be also effected by radiation methods. Most patents describe the procedures for preparing silicon rubber used for dental materials, improving adhesion to metal, glass and plastics, electric insulators, membranes and many others.

The catalytic hydrosilylation of difunctional organosilicon monomers containing alkenyl- or alkynyl-groups and/or Si–H bonds has become a versatile synthetic tool for obtaining saturated or unsaturated organosilicon polymers bearing the Si–C bonds in the backbone. These important polymeric products have been prepared through two main routes: the intermolecular hydrosilylation polymerisation of alkenyl(alkynyl)hydrosilanes and the polyhydrosilylation of α,ω-dienes or diynes with dihydro-substituted organosilicon compounds. The chapter covers recent achievements on the synthesis of saturated polycarbosilanes and polycarbosiloxanes as well as unsaturated polycarbosilanes or related σ-π conjugated polymers using transition metal catalysed hydrosilylation polymerisation.

Functionalised (poly)silsesquioxanes which can be considered as organicmodified silica and silicon-containing dendrimers with their highly branched, tree-like structure offer special properties. They have inspired many chemists to develop new materials and nanomaterials and their several applications have been explored.

The syntheses of such compounds via hydrosilylation processes are described in this chapter.

Functionalised (poly)silsesquioxanes and silicon-containing dendrimers are belong to the most rapidly developing branches of organosilicon chemistry.

Hydrosilylation reactions using hydridosilsesquioxanes as the starting precursors have been used to prepare a number of interesting new adducts, proving a particularly high versatility of this method of functionalisation of silsesquioxanes.

On the other hand, a combination of hydrosilylation with alkenylation (usually allylation and vinylation) or ethynylation and with alcoholysis has provided powerful protocols for the rapid and efficient synthesis of silicon-containing dendrimers, mostly carbosilanes, carbosiloxanes and carbosilazanes, also dendritic poly(silanes) and poly(siloxanes).

Catalytic hydrosilylation is one of the most important and convenient methods of reduction of C═O and C═N bonds. Hydrosilylation of ketones (imines) produces in single step silyl ethers (silylamines) which can be easily transformed into alcohols (amines) via an additional hydrolysis step. In this chapter, the hydro-silylation of carbonyl compounds, imines and nitriles with emphasis put on the synthetic, catalytic and mechanistic aspects is reviewed. Efficient catalysts including mainly transition metal complexes but also main group metal complexes, nucleophiles or electrophiles, free-radical initiating systems as well as metals (or supported metals) are described. In a separate section, the progress in hydrosilylation of CO₂ is summarised. Finally, the addition of Si—H to other unsaturated bonds such as C═S, S═O and N═N is shortly described.

Asymmetric, catalytic hydrosilylation of prochiral ketones and imines with substituted silanes or siloxanes followed by hydrolysis provides a convenient access to chiral alcohols and amines, respectively. In this chapter, the asymmetric hydrosilylation of carbonyl compounds and imines as well as the asymmetric conjugate reduction of α,β-unsaturated enones, nitriles, sulphones and nitroalkenes are reviewed. Synthetic, catalytic and mechanistic aspects of the reactions ocurring in the presence of complexes of such transition metals as zinc, copper, rhodium, ruthenium, iridium, titanium and others are discussed. Moreover, catalysis of asymmetric hydrosilylation complexes of the main group metals, nucleophiles (including organocatalysts) and electrophiles is described.