Contrasteric Stereochemical Dictation of the Cyclobutene Ring-Opening Reaction by a Vacant Boron p Orbital

摘要

The torquoselectivity of the thermal ring-opening reaction of cyclobutene has been a subject of fundamental importance in organic and theoretical chemistry.Houk's theory states that an electron-accepting substituent at the 3-position prefers inward rotation.Delocalization of electron density of the HOMO of the opening cyclobutene skeleton to the electron-accepting substituent stabilizes the inward transition state.The electronic stabilization can outweigh or counterbalance the steric congestion developing during inward rotation.For example,a formyl group can accept electron density into its antibonding pi* orbital and thus rotates with high inward selectivity.We recently discovered the interesting preference of silyl groups to rotate inward.