Molecular-orbital calculations 1,2-migrations

Ab initio molecular orbital calculations have been carried out by Ignacio and Schlegel on the thermal decomposition of disilane and the fluorinated disilanes Si2H F6 17. Both 1,1-elimination of H2 or HF and silylene extrusion by migration of H and F atoms concerted with Si—Si bond cleavage were considered. The transition states for the extrusion reactions all involved movement of the migrating atom toward the empty p-orbital of the extruded silylene in the insertion which is the retro-extrusion (equation 5). 

More recently, there have been several investigations of CO insertion using high-level molecular orbital calculations at the DFT level. These have confirmed that CO insertion is indeed alkyl migration, and two intermediates, structures 3 and 4, have been shown to exist along the reaction path.10... 

RB3LYP calculations indicate that the s-cis conformer of peroxy acids is more stable than the s-trans conformer. Calculations on the reaction of prop-2-enol with some peroxy acids showed that trans-transition states collapse to the epoxide via a 1,2-shift, whereas a 1,4-shift is operable for cis-transition states.195 Quantum mechanical calculations have been performed on the migration step of the Baeyer-Villiger rearrangements of some substituted acetophenones with m-chloroperbenzoic acid (m-CPBA). The energy barriers, charge distributions and frontier molecular orbitals, determined for the aryl migration step, have been used to explain the effects of substituents on the reactivity of the ketones.196... 

This theorem is recognized as an approximation as, apart from the inaccuracies inherent in the S.C.F. method (such as neglect of electron correlation and relativistic effects), it assumes that the molecular orbitals are the same for the molecule and the molecular ion. Many ASCF calculations have shown (see for example 14 16)) that if an electron is removed from a metal localized orbital, considerable charge migration towards the metal occurs this is termed relaxation. These relaxation effects give ionization energies smaller values than those expected on the basis of Koopmans theorem. For ionization from ligand based orbitals, relaxation effects are smaller and more constant. 

In no case is an exo-endo isomerization at C-5 observed. This finding rules out a migration of C-5 with retention ( suprafacial retention, sr) in the bicyclopen-tene systems, a process calculated by semiempirical molecular orbital (MO) methods to be the favored one (28). The observed stereochemistry corresponds to an inversion at C-5 ( suprafacial inversion, si) and is in accord with the stereochemical requirements for an orbital symmetry controlled process. 

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