A Computational Study On The Mechanism Of Rh(Ⅰ)-Catalyzed Intermolecular Cyclization And Ni(0)-Catalyzed Hydrogenation Of Styrenes

Cyclopropenes as high strained unsaturated cyclic molecules have very high activity and reactivity.Under the transition metal catalysis,they can generate the alkenylcarbene or metal cyclobutene intermediate through the sigma bond cleavage to synthesize a series of complex compounds,such as furan,pyrene,pyrrole,ketene and other derivatives.Therefore,this kind of reaction has received widespread attention in recent years.Particularly,the cyclization of cyclopropenes catalyzed by rhodium catalyst has made great progress.However,little attention has been paid to its reaction mechanism.In this dissertation,the mechanism of intramolecular cyclization of tethered ene-cyclopropenes catalyzed by Rh(CO)2Cl is studied by using density functional theory.Besides,the effects of CO gas on the ring opening mode and the enantioselectivity of the reaction were discussed.The calculations result proves that:(1)If the CO balloon is used in the reaction conditions,the vinylcarbene intermediate would be firstly generated by tethered ene-cyclopropenes with Rh(I)catalyst,then it would be transformed to rhodium-cyclobutene,which preferentially undergoes CO insertion followed by alkene insertion,ultimately resulting in trans-fused product.The olefin insertion of ketene is not only the decisive step of the whole reaction,but also the key step to determine the trans-configuration.(2)However,Rh(I)catalyst would preferentially coordinates with olefins,followed by an oxidative addition to the adjacent C-C single bond of ene-cyclopropene to provide the fluorene cyclobutene intermediate in the absence the CO.Finally,the corresponding products are obtained through the process of olefin insertion and reductive elimination orβ-H elimination/reductive elimination.The oxidation addition of the C-C single bond of cyclopropene is a key step to generate the cis configuration.Therefore,the CO pressure has a good control over the ring opening mode of cyclopropene and the stereochemistry of reaction,which is of great significance for the design and synthesis of new carbocyclication reactions.In recent years,great progress has been made in the hydrogenation reaction.To date,two strategies have been developed for achieving the hydrogenation of alkenes,and they are characterized by using different way to generate active catalyst species M-H.However,the mechanism of the Ni(0)-catalyzed hydrogenation of olefins in the presence of methanol is still not thorough.It is still to be explored whether or not the M-H active catalyst species is also involved.Therefore,we use density functional theory to study the mechanism and analyze the problem of regioselectivity.According to the calculation,the methanol as a new protons source proposed in the literature to construct the Ni-H bond is not the best way to initiate the reaction.We found the protons on the alcohol can be directly transferred to the olefin under the synergistic effect of the two molecules of methanol through theoretical studies.Subsequently,the hydrogenation product is obtained by metallization and reductive elimination in the presence of an organoboron compound.In addition,we also explored the regioselectivity of this reaction.The distal carbon of styrene is highly charged and more readily accepts the protons on methanol,so the reaction selectively generates branched products.