Enolene Rearrangements. Relationship to the “Abnormal Claisen Rearrangement” and Other 1,5-Hydrogen Shift Processes

The Conservation of Orbital Symmetry

Die Erhaltung der Orbitalsymmetrie

Interrupting the Nazarov reaction: domino and cascade processes utilizing cyclopentenyl cations

Chemistry remains an experimental science. The theory of chemical bonding leaves much to be desired. Yet, the past 20 years have been marked by a fruitful symbiosis of organic chemistry and molecular orbital theory. Of necessity this has been a marriage of poor theory with good experiment. Tentative conclusions have been arrived a t on the basis of theories which were such a patchwork on approximations that they appeared to have no right to work; yet, in the hands of clever experimentalists, these ideas were transformed into novel molecules with unusual properties. In the same way, by utilizing the most simple but fundamental concepts of molecular orbital theory we have in the past 3 years been able to rationalize and predict the stereochemical course of virtually every concerted organic reaction.' In our work we have relied on the most basic ideas of molecular orbital theory-the concepts of symmetry, overlap, interaction, bonding, and the nodal structure of wave functions. The lack of numbers in our discussion is not a weakness-it is its greatest strength. Precise numerical values would have to result from some specific sequence of approximations. But an argument from first principles or symmetry, of necessity qualitative, is in fact much stronger than the deceptively authoritative numerical result. For, if the simple argument is true, then any approximate method, as well as the now inaccessible exact solution, must obey it. The simplest description of the electronic structure of a stable molecule is that i t is characterized by a finite band of doubly occupied electronic levels, called bonding orbitals, separated by a gap from a corresponding band of unoccupied, antiboding levels as well as a continuum of higher levels. The magnitude of the gap may range from 40 kcal/mole for highly delocalized, large aromatic systems to 250 kcal/mole for saturated hydrocarbons. It should be noted in context that socalled nonbonding electrons of heteroatoms are in fact bonding. Consider a simple reaction of two molecules to give a third species, proceeding in a nonconcerted manner through a diradical intermediate I. A + B + [I] + C

The Conservation of Orbital Symmetry (福井謙一とフロンティア軌導理論) -- (参考論文)

Perhaps it is appropriate that this review be written some 100 years after the report of the first syntheses of cyclopropane derivatives by von Baeyer1 and Perkin2 and the formulation of the ‘theory of ring strain’ by von Baeyer.1 The chemistry of small ring compounds has risen to prominence in the last 30 years. The popularity especially of the three-membered rings as intermediates in synthetic transformations has been due primarily to their latent energy content and to the almost endless number of chemical transformations in which these compounds and their derivatives can participate. New applications and novel permutations of the basic systems continue to appear at a fast pace. The fascinating chemistry associated with the strain of these intermediates continues to find wide applicability in the field of synthetic methodology and in the total synthesis of complex molecules.

Enolene Rearrangements. Relationship to the “Abnormal Claisen Rearrangement” and Other 1,5-Hydrogen Shift Processes

Citations

The Conservation of Orbital Symmetry

Die Erhaltung der Orbitalsymmetrie

Interrupting the Nazarov reaction: domino and cascade processes utilizing cyclopentenyl cations

The Conservation of Orbital Symmetry (福井謙一とフロンティア軌導理論) -- (参考論文)

Rearrangements of Vinylcyclopropanes and Related Systems

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The overlap of two allyl radicals or a four-centered transition state in the cope rearrangement☆