Synthesis Of Polydentate Cobalt Complexes For Alkyne Hydrosilylation

Alkenylsilanes are regarded as versatile building blocks in organic synthesis,in virtue of low toxicity,high stability,and substantial transformations.Alkenylsilanes have been widely used in organic synthesis and materials science.It makes synthesis of alkenylsilanes mining one of the research highlights in organic synthesis.Among various kinds of synthetic methods,alkyne hydrosilylation is one of the most straightforward and atom-economic strategies.However,highly selective alkyne hydrosilylation is extremely challenging because such reaction normally generates multiple isomers.Moreover,alkyne hydrosilylation often depends on noble metal catalysts and has poor substrate tolerance.In recent years,base metal complexes of cobalt are usually used for hydrosilylation of unsaturated bonds.However,there are few researches in alkyne hydrosilylation to producedα-and conjugated alkenylsilanes.Based on these problems,a series of base metal cobalt complexes have been designed and synthesized in this thesis,and applied in selective hydrosilylation of versatile alkynes.The strategy shows excellent catalytic activity and selectivity,resulting in valuable alkenylsilanes precisely and controllably.The main research contents of this thesis are as follows:A series of pyridine-methylene-bipyridine（N^CNN）pincer cobalt complexes with simple and stable structures were synthesized and these complexes were tested as catalysts for alkyne hydrosilylation.For terminal alkyne,complex II-C4showed the highest selectivity and activity,Markovnikov selective alkyne hydrosilylation can be completed in the presence of 2 mol%catalyst at room temperature for 5 min（selectivity ofα/E up to 99:1）.For internal diaryl-alkynes,complex II-C6 was the most efficient and provided several E-vinylsilanes in high efficiency（1 mol%,5 min,r.t.）,TOF（turnover frequency）is 1200 h^-1.The results suggested that suitable electron-donating groups（hydroxy and methoxy）in the 2-position of the N^CNN ring might improve the reaction rate and selectivity.In order to synthesize enynylsilanes efficiently,we optimized the structure of cobalt complexes,and these complexes were tested as catalysts for hydrosilylation of conjugated diyne.Complex III-C5 showed the highest selectivity and activity,reaction can be completed in the presence of 2 mol%catalyst at room temperature for 5 min.In comparsion with palladium,rhodium and platinum catalytic systems,this catalyst has greater substrate applicability（ether,halogen,amine,cyano,thienyl,acetyl,alkenyl are tolerated）.By changing the ligand structures,simple bis（phosphine）cobalt complexes were also efficient,and equivalent conversion of silane and conjugated diacetylene was achieved,improving the yields and atom-economy.Moreover,an[Dppp-Co]-H intermediate was detected by ¹H NMR,suggesting a process in keeping with Chalk-Harrod mechanism.Bis（phosphine）cobalt complexes catalyzed hydrosilylation of conjugated enynes was achieved,and a series of conjugated dienylsilanes were synthesized by one step methods.For aryl-substrates,complex Dppf-Co Cl₂（III-C10）was the most efficient,providing severalα-silyl substituted conjugated dienes in high selectivity（regioselectivity up to 98:2）with 2 mol%catalyst loading for 1 hour.For alkyl-substrates,in the presence of complex Xantphos-Co Cl₂（III-C12）,hydrosilylation of alkyl-enynes affordedβ-silyl substituted conjugated dienes in high selectivity（regioselectivity up to 99:1）at room temperature for 1 hour.Mechanism studies,including a series of control and deuterium-labelling experiments,showed cobalt-hydride was the intermediate,in keeping with Chalk-Harrod mechanism.