Theoretical Study Of Hyperconjugative Aromaticity In Cyclopentadiene And Pyrrolium

Aromaticity and hyperconjugation have attracted considerable attention from scientists as two important concepts in organic chemistry.The seminal work applying hyperconjugation to the concept of aromaticity by Mulliken is to explain the stability of cyclopentadiene in 1939.Later,Schleyer and co-workers proposed and extended the concept of hyperconjugative aromaticity.However,previous work on hyperconjugative aromaticity have been focused on di-substituents and main-group substituents.The transition metal substituents and mono-substituents are largely undeveloped,much less the application of the concept of hyperconjugative aromaticity to the reaction mechanism.Therefore,we focus on probing hyperconjugative aromaticity of mono-substituents,transition metal substituents and probing the reaction mechanism of hyperconjugative aromatic compounds.Here,we report a thorough density functional theory study to evaluate the aromaticity in various cyclopentadienes that containing mono-substituents,transition-metal substituents and main-group substituents.In addition,the structure and bonding of the pyrroliums and pyrroles are analyzed in detail to probing the effect of migratory groups on[1,5]migration,opening an avenue to design other novel aromatic organometallics.The specific contents are summarized as follows:1.A systematic study of the hyperconjugative aromaticity of various cyclopentadienes that contain both transition-metal and main-group substituents.Our results reveal that the hyperconjugative aromaticity of cyclopentadiene caused by Pt-containing substituents can perform better than main-group substituents and Au-containing substituents.Additionally,our calculations show that BMe2 ligand features best performance when the trans influence was taken into account.In summary,the most aromatic cyclopentadiene was achieved by hyperconjugation and synergistic effect of trans influence.2.We systematically studied the hyperconjugative aromaticity of cyclopentadienes containing mono-and di-substitutents.Our results reveal that one electropositive substituent is able to induce aromaticity,while one electronegative substituted cyclopentadiene is nonaromatic rather than antiaromatic.When considering simultaneously an electronegative substituent and a transition metal substituent,cyclopentadiene ring could be slightly aromatic,while the combination of the main group substituent and electronegative substituent will produce nonaromaticity.3.The reaction mechanism of[1,5]migration of pyrroliums and pyrroles were explored systematically.The unexpected lower activation barriers of electropositive substituted pyrroliums for[1,5]migration can be attributed to aromatic transition states.Further studies show that the frontier molecular orbitals and charge distribution also have an effect on the[1,5]migration of pyrroliums.