Hydrosilylation

Addition to terminal alkenes is usually anti-Markovnikov: silicon ends up on the terminal carbon.

Platinum catalysts

• Speier's catalyst: H_2PtCl_6 in propan-2-ol, reduced in situ to Pt(0). Extremely active but not very stable or selective; colloidal Pt may form.
• Karstedt's catalyst: a Pt(0) complex of 1,3-divinyltetramethyldisiloxane, soluble in silicones and better defined. NHC–Pt(0) variants improve stability and selectivity.

The Chalk–Harrod mechanism

1. Oxidative addition of Si–H to Pt(0) gives H–Pt(II)–SiR_3. 2. Alkene coordination. 3. Migratory insertion of the alkene into Pt–H, forming Pt–alkyl. 4. Reductive elimination of the C–Si bond releases the alkylsilane.

The modified Chalk–Harrod mechanism instead inserts the alkene into Pt–Si; the resulting β-silylalkyl can undergo β-hydride elimination to give a vinylsilane plus alkane (dehydrogenative silylation), a side reaction the original scheme cannot explain. Alkene isomerisation by reversible insertion/β-H elimination is another side reaction.

Ligand balance: after oxidative addition, the metal needs room for the alkene, so supporting ligands must not be too bulky; yet strongly binding unsaturated substrates (especially alkynes) can block Pt(0) before Si–H adds. Good catalysts sit between these extremes.

Alkynes

Pt-catalysed hydrosilylation of terminal alkynes is a syn addition giving mainly the (E)-β-vinylsilane, with some α-isomer. Bulky NHC ligands increase β-selectivity. Cationic Ru catalysts can give trans-addition products instead.

Carbonyls and imines

Copper hydrides, L–Cu–H, reduce C=O and C=N by a non-redox cycle: insertion of the carbonyl into Cu–H gives a copper alkoxide, which undergoes σ-bond metathesis with the silane to release a silyl ether and regenerate Cu–H. Bulky (often chiral) bisphosphines stop Cu–H aggregating into inactive clusters and give highly enantioselective reductions.

Silicone curing

Vinyl-terminated poly(dimethylsiloxane) + Si–H-functional siloxane crosslinker + ppm Pt → crosslinked elastomer. Inhibitors such as alkynols or maleates bind Pt reversibly to give a long pot life at room temperature and fast cure on heating. No small-molecule by-product forms, so there is no shrinkage.

The research agenda in hydrosilylation is dominated by replacing platinum and by controlling selectivity beyond the anti-Markovnikov default.

Earth-abundant metals. Iron and cobalt complexes of bis(imino)pyridine, terpyridine and pincer ligands catalyse anti-Markovnikov hydrosilylation of alkenes with tertiary silanes relevant to silicone curing, at low loadings. Redox-active ligands store electrons so that the metal can avoid unfavourable oxidation states; mechanisms often involve Si–H activation by σ-bond metathesis or modified Chalk–Harrod-type insertion into M–Si. Nickel catalysts give Markovnikov products with styrenes, and Co catalysts can switch regioselectivity with ligand choice, enabling branched alkylsilanes.

Stereoselective variants. Asymmetric hydrosilylation of 1,1-disubstituted alkenes, dienes and ketones with chiral Co, Fe and Cu catalysts, and trans-selective alkyne hydrosilylation with cationic Ru complexes giving (Z)-vinylsilanes, extend the synthetic reach. Vinyl- and allylsilanes then serve in Hiyama couplings or Tamao–Fleming oxidation to alcohols.

Mechanistic tools. Natural-abundance ^{13}C kinetic isotope effects, measured by NMR on recovered starting material, identify which carbon is involved in the rate-limiting step without synthesising labelled substrates. Combined with deuterium labelling, Hammett studies and DFT, they distinguish Chalk–Harrod from modified pathways. Questions remain over Pt speciation in industrial formulations: whether colloids or molecular species do most of the work, and how inhibitors and Pt aggregation affect cure profiles.

Practical constraints. Silicone producers need catalysts that tolerate impurities, avoid isomerisation and colour, and work with cheap Si–H sources such as polymethylhydrosiloxane. Catalysts derived from base metals must be air-stable or activated in situ to compete with Karstedt's catalyst in real formulations.