1.3- dipolar cycloaddition reactions dominant frontier orbitals

Dipolar cycloaddition reactions are generally classified into three types, dipole HO controlled, dipole LU controlled or HO,LU controlled, depending upon the relative energies of the dipole and dipolarophile frontier molecular orbitals. If the energy gap separating the dipole HOMO from the dipolarophile LUMO is smaller than that between the dipole LUMO and the dipolarophile HOMO, then the reaction is said to be dipole HO controlled. If the dipole LUMO-dipolarophile HOMO energy gap is smaller, then dipole LU control prevails. If the energy difference between the dipole HOMO and the dipolarophile LUMO is about the same as that between the dipole LUMO and the dipolarophile HOMO, dien neither interaction dominates and HO,LU control is operable. 

Scheme 14). The regiochemical outcome of the 1,3-dipolar cycloaddition reactions of the cyclic five-membered ring carbonyl yUde 48 with a variety of acycUc and cycHc alkenes having activated or inactivated r-bonds can be ra-tionaUzed [78,79] on the basis of frontier molecular orbital considerations, with the HOMO and LUMO of the carbonyl ylides dominating the reactions with electron-deficient and electron-rich dipolarophiles, respectively (Scheme 14). 

The 1,3-dipolar cycloaddition of organic azides with nitriles could give rise to two regioisomers. Since organic azides are Type II 1,3-dipoles on the Sustmann classification (approximately equal HOMO-LUMO gaps between the interacting frontier orbital pairs) the reactions could be dipole HOMO or LUMO controled and the regioselectivity should be determined by the orbital coefficients for the dominant HOMO-LUMO interaction. Such systems show U-shaped kinetic curves in their... 

A cyclic transition state model, that differs from the Zimmerman-Traxler and the related cyclic models inasmuch as it does not incorporate the metal in a chelate, has been proposed by Mulzer and coworkers [78] It has been developed as a rationale for the observation that, in the aldol addition of the dianion of phenylacetic acid 152, the high ti-selectivity is reached with naked enolate anions (e.g., with the additive 18-crown-6). Thus, it was postulated that the approach of the enolate to the aldehyde is dominated by an interaction of the enolate HOMO and the n orbital of the aldehyde that functions as the LUMO (Scheme 4.31), the phenyl substituents of the enolate (phenyl) and the residue R of the aldehyde being oriented in anti position at the forming carbon bond, so that the steric repulsion in the transition state 153 is minimized. Mulzer s frontier molecular orbital-inspired approach reminds of a 1,3-dipolar cycloaddition. However, the corresponding cycloadduct 154 does not form, because of the weakness of the oxygen-oxygen bond. Instead, the doubly metallated aldol adduct 155 results. Anh and coworkers also emphasized the frontier orbital interactions as being essential for the stereochemical outcome of the aldol reaction [79]. 

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