Mechanistic Insights Into The Dehydrogenative Silylation Of Alkenes Catalyzed By Bis(imino)pyridine Cobalt Complex

The hydrosilylation of terminal alkenes with precious metal catalysts is well-established and widely practiced for the commercial manufacture.An alternative and related catalytic process is dehydrogenative silylation which is a potentially attractive route for the preparation of allylsilanes and vinysilanes.Allylsilanes and vinysilanes have been extensively exploited as versatile reagents and intermediates in organic synthesis due to their numerous favorable properties like virtual nontoxicity and high stability.In the past few years,most catalysts for dehydrogenative silylation rely on precious transition metals such as Ru,Rh,Ir,Pt and Pd,and the base metal-catalyzed dehydrogenative silylation remains rare.Recently,high abundance and low cost coupled with environmentally benign nature of Fe,Co and Ni catalysts,forming a supplement or replacement of precious metals catalysts,have inspired enormous researches.Along with the progress of the Fe-based hydrosilylation catalysts,the considerable efforts and explorations on the Co-based hydrosilylation catalysts are fruitful.The Co-catalyzed hydrosilylation reactions have shown diversified substrate scope and high selectivity.Amilestoneinnon-preciousmetal-basedhydrosilylationcatalystsis2,6-iminopyridine-iron-dinitrogen complex,which exhibits high activity in alkene hydrosilylation and is comparable to that of traditional platinum catalysts.Based on the further study of 2,6-iminopyridine-cobalt complexes,the new aryl-substituted bis?imino?pyridine cobalt methyl complex,(^MesPDI)Co?CH₃?(^MesPDI=2,6-?2,4,6-Me₃C₆H₂-N=CMe?₂C₅H₃N),was discovered to catalyze the dehydrogenative silylation with a variety of terminal olefins using commercially relevant tertiary silanes.Moreover,other catalysts can be derived from(^MesPDI)Co?CH₃?.An in-depth theoretical study has been performed to investigate the mechanism of alkene dehydrogenative silylation catalyzed by the bis?imino?pyridine cobalt methyl complex(^MesPDI)Co?CH₃?at B3LYP level of density functional theory.Through theoretical calculations,the relative energies and specific structures of reaction intermediates and transition states have been obtained,even for transient intermediates,which cannot be monitored using experimental techniques.The kinetic and thermodynamic data of various possible reaction paths were compared and analyzed,and the most favorable reaction path was proposed.More importantly,the combination of theoretical calculations and experimental results provides more convincing and comprehensive explanations for reaction mechanisms.This reaction could offer an ideal model for the mechanistic studies with the respect of developing regioselective catalytic systems.The main contents of this paper are listed as follows:?1?The three states of precatalyst(^MesPDI)CoCH₃ was considered.The calculations show that the open-shell singlet(^osCat)can be considered as the ground state.The species ³Cat has the similar lower energy as ^osCat,so there is a fast equilibrium transformation.The true active catalyst,(^MesPDI)Co-[Si],is generated via a?-bond metathesis??-BM?between(^MesPDI)Co?CH₃?and HSi?OMe?₃.There exist the minimum energy crossing points in the silane attack on(^MesPDI)Co?CH₃?.?2?Subsequently,(^MesPDI)Co-[Si]undertakes dehydrogenative silylation of alkene via the C=C insertion into the Co–[Si]bond and?-H reduction elimination,forming product,allylsilane or vinysilane.The secondary bis?imino?pyridine cobalt alkyl complex is the key intermediate in the catalytic cycle.Next,the coordination of the other equivalent terminal alkene to the fragment(^MesPDI)Co-H leads to the side-product alkane and provides the driving force for the reaction due to a large exoergicity.?3?The alkene insertion step and the?-H reduction elimination step determine a pronounced substrate-dependent regioselectivity that arises from the steric repulsion between alkene and silyl group.The first regioselectivity is determined by the olefin insertion,in which 2,1-insertion manner is both kinetically and thermodynamically more favored over the1,2-insertion manner.Moreover,the steric hindrance of the tertiary silane is dominant.The greater the steric repulsion,the easier the 2,1-insertion.?4?The?-H reduction elimination process determines the type of product.The reason for the exclusive product allylsilane used linear alkene is that only the methylene H_C3 away from the large tertiary silane substituent can be eliminated with a reasonable barrier.Increasing the steric of alkene,the mian product allylsilane and co-product vinysilane are generated that attributes to the methylene H_C3 and H_C1 are all eliminated.These results rationalize the experimentally observations and provide insights into the design of new catalysts for the chemo-and regioselectively dehydrogenative silylation reactions.