Alkenes frontier molecular orbital

In a, P-unsaturated carbonyl compounds and related electron-deficient alkenes and alkynes, there exist two electrophilic sites and both are prone to be attacked by nucleophiles. However, the conjugated site is considerably softer compared with the unconjugated site, based on the Frontier Molecular Orbital analysis.27 Consequently, softer nucleophiles predominantly react with a, (i-unsaturated carbonyl compounds through conjugate addition (or Michael addition). Water is a hard solvent. This property of water has two significant implications for conjugate addition reactions (1) Such reactions can tolerate water since the nucleophiles and the electrophiles are softer whereas water is hard and (2) water will not compete with nucleophiles significantly in such... 

A similar polar cyclization of an enamine and a thioketene derivative is shown in equation (87) (76TL4283), but electron-deficient alkenes and alkynes react in a concerted fashion. Concerted cyclizations may be subdivided into those in which the sulfur atom is part of the enophile or the dienophile. Into the first category fall the dimerizations of a,( - unsaturated thioaldehydes (equation 88) which may be shown to closely follow frontier molecular orbital predictions of regioselectivity (79JOC486). Related to this are the thiochalcones (equation... 

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>. 

In the beginning, Ken created a frontier molecular orbital (FMO) theory of regioselectivity in cycloadditions. In particular, his classic series of papers showed how FMO theory could be used to understand and predict the regioselectivity of 1,3-dipolar cycloadditions. Ken s generalizations about the shapes and energies of frontier molecular orbitals of alkenes, dienes, and 1,3-dipoles, are in common use today and they appear in many texts and research articles. 

The frontier molecular orbitals in Figure 29.6 show why this is so. Under thermal conditions, suprafacial overlap is not symmetry-allowed (the overlapping orbitals are out-of-phase). Antarafacial overlap is symmetry-allowed but is not possible because of the small size of the ring. Under photochemical conditions, however, the reaction can take place because the symmetry of the excited-state HOMO is opposite that of the ground-state HOMO. Therefore, overlap of the excited-state HOMO of one alkene with the LUMO of the second alkene involves symmetry-allowed suprafacial bond formation. 

There are several major reaction types available to radicals coupling, addition, substitution, fragmentation, rearrangement, and oxidation. The frontier molecular orbital treatment shown here for addition to alkenes can, with modification, be used to predict reactivity in virtually all radical reactions, if the molecular orbitals for the reactive species are known. The following sections will describe the various radical reaction types. 

The rates of 1,3-dipolar cycloadditions of diazoalkanes to alkenes and alkynes have been determined electron-attracting substituents in the latter increase the rate, in accordance with frontier molecular orbital theory, which predicts that these reactions are controlled by the interaction of the highest occupied molecular orbital of the diazo-compound with the lowest unoccupied molecular orbital of the dipolarophile " the kinetics of the reactions of methyl diazoacetate or phenyl diazomethanesulphonate, on the other hand, give rise to U-shaped activity functions, which is also explained by the theory. Diazomethane or... 

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