An ab initio and MNDO-d SCF-MO computational study of stereoelectronic control in extrusion reactions of R2I–F iodine(III) intermediates†

Abstract

MNDO-d and ab initio RHF, B3LYP and MP2 energies and geometries are reported for reactant ground and transition states for F–R′ and R–R′ extrusion and R/R′ interconversion reactions of substituted RR′I–F iodine(III) reactive intermediates. The RR′I–F reactant is predicted to form a stable asymmetric bridged dimer involving a square planar iodine centre, hitherto unconsidered as a factor in the chemistry of hypervalent iodine species. Evidence in support of this hypothesis obtained from previously reported crystal structures is discussed. The reactions of both monomer and bridged dimer are found to exhibit unusually large stereoelectronic effects at the iodine centre, deriving from electron donating and withdrawing substituents on the R groups. They are also unusual in showing transition state substituent effects which are opposite to those controlling the ground state stabilities, for which an NBO analysis is presented. Both these effects are manifest in the transition states for reaction of the dimeric species, which is stabilised by electron withdrawing groups present in the pseudo equatorial R′ group of the reacting centre and in the pseudo axial position of the unreacting R component of the dimer.