A Preliminary Computation Study of Counter Ion Effect on Transition State Models in Ethynyl Substitution of Fluorine in F-Bodipy

Abstract

Substitution of the fluorine atoms bonded to the boron center in 4,4-difluoro-4-bora-3a,4adiaza- s-indacene (F-BODIPY) compounds with ethynyl or aryl linkages can be achieved through either Grignard or lithium organometallic reagents. Use of Grignard reagents allows for the isolation of monosubstituted and disubstituted products, whereas use of organolithium reagents allows for the isolation of only the disubstituted product. Using SPARTAN modeling software and quantum chemical calculations, preliminary theoretical SN2 transition state energies were determined for both the mono- and disubstituted transition states using both Grignard and lithium trimethyl silyl protected ethynyl reagents. These models were used to calculate differences in theoretical activation energies between the mono- and disubstitution processes, ΔΔE 5 211.3 kJ/mol for the lithium reagent and DDE 5 35.9 kJ/mol for the Grignard reagent. The lower second substitution activation energy for our lithium reagent (negative ΔΔE) and the higher second step for our Grignard reagent (positive DDE) supports the experimental findings where organolithium reagents are only capable of producing the disubstituted product.