Alkenes, cycloadditions HOMO energy

Interaction of the HOMO of ozone with the LUMO of the alkene, and of the LUMO of ozone with the HOMO of the alkene are symmetry allowed. Just as with [4+2]-cycloaddition, the energy difference of these orbitals will control the facility of the reaction. In general, the low-energy interaction is the one between HOMOdipoie and LUMOalkene-... 

The decarboxylative approach to the ylide formation generated cycloaddition products derived from cycloaddition of the ylide to the carbonyl moiety of the molecule, as opposed to the alkene as seen in previous examples. Kanemasa has reconciled this observation by consideration of the postulated transition state model of the reaction. It was assumed that the steric repulsion of the terminal olehnic substituent and the ylide would favor transition state 309 (Fig. 3.19). Additionally, nonstabilized azomethine ylides have a higher energy HOMO than stabilized ylides, and would therefore prefer the LUMO of the carbonyl than the lower lying alkene LUMO. Formation of fused hve-membered rings would also be kinetically favored over construction of six-membered ring (Scheme 3.103). 

Cycloadditions of betaines are not restricted to electron-deficient alkenes. Pyridinium-3-olates also react with conjugated olefins (e.g., styrenes) and with electron-rich olefins (e.g., ethyl vinyl ether). In the latter case, the betaine LUMO/alkene HOMO interaction becomes dominant and reaction is only observed with pyridinium-3-olates having a low-energy LUMO... 

Such cycloadditions involve the addition of a 2tt- electron system (alkene) to a 4ir- electron system (ylide) and have been predicted to occur in a concerted manner according to the Woodward-Hoffmann rules. The two most important factors involved in the cycloaddition reactions are (i) the energy and symmetry of the reacting orbitals and (ii) the thermodynamic stability of the cycloadduct. The reactivity of 1,3-dipolar systems has been successfully accounted for in terms of HOMO-LUMO interactions using frontier MO theory (71TL2717). This approach has been extended to explain the 1,3 reactivities of the nonclassical A,B-diheteropentalenes <77HC(30)317). 

Azomethine ylides are not typically isolable but must be used in situ. They undergo cycloaddition reactions that produce highly functionalized pyrrolidines, dihydropyiroles and pyrroles. The success of these reactions often depends on a judicious choice of dipole and dipolarophile. Azomethine ylides are reluctant to cycloadd to nonactivated alkenes, in large part owing to electronic considerations. The LUMO of most azomethine ylides is high in energy and there is a large gap with the HOMO of a nonactivated alkene. 

The regioselectivity of the 1,3-dipolar cycloadditions of azides to alkenes is usually difficult to predict due to the similar energies for the transition states which involve either the HOMO (dipole) or the LUMO (dipole). The results of a study which utilized 5-alkoxy-3-pyrrolin-2-ones as dipolar-ophiles in reactions with a variety of aryl azides seemed to reflect this problem the results suggested that the low regioselectivity observed was due to the frontier molecular orbital interactions between dipole and dipolarophile, and not any steric hindrance offered by the 5-alkoxy function <84H(22)2363>. 

They react easily with electrophiles and add nucleophiles at C-6. In cycloaddition reactions they may react as 2jt, 4n, or 6 i compounds. According to frontier orbital considerations they readily react with electron-deficient dienophiles (e.g., silenes) in Diels-Alder reactions this is due to the strong interaction between the fulvene HOMO and dienophile LUMO [9]. Although the n and n orbitals of silenes are generally 1-2.5 eV higher in energy than is the case for the alkene congeners [10] a normal [4+2] cycloaddition behaviour for 3 is observed in earlier works [3-5]. 

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