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Thcrmischc pcricyclischc Secliselektronen-Reahtionen werden anhand cines Scchwcntrcnund cines Fiinfzentrcn-Modclls bcziiglich dcr 7Ail moglicher Reaktionstypen. die sich sowohl durch Variation dcr ,,o-Schalc" als aiich durch Kombination verschicdcncr Atome in allcn miiglichcn Anordnungen ableitcn lasscn. systcniatisch untcrsucht. Fur diesc Rcaktionen, dcrcn Anzahl in dic Tausende geht, wird cine cinfachc und cinheitlichc Nomcnklatur vorgcschlagcn. Sic werdcn dcs weitercn nach ihrcm potcnticllcn Nutzcn fur synthetischc Grundverfahren gruppicrt (Aufbau. Eliminicrung, Umfti1iktionalisierung etc.). Die Methodik i c l t darauf ah. Auswahlkritericn anzugeben, nach dcnen ncue Resktionen aufgcfundcn wcrden kiinnen. Die Diskussion von Kcaktionsprofilcn erlaubt die Ikmittlung jcncr Atomanordnungcn in Molckiilcn, wclchc cine Rcahtion erleichtern.
Thcrmischc pcricyclischc Secliselektronen-Reahtionen werden anhand cines Scchwcntrcnund cines Fiinfzentrcn-Modclls bcziiglich dcr 7Ail moglicher Reaktionstypen. die sich sowohl durch Variation dcr ,,o-Schalc" als aiich durch Kombination verschicdcncr Atome in allcn miiglichcn Anordnungen ableitcn lasscn. systcniatisch untcrsucht. Fur diesc Rcaktionen, dcrcn Anzahl in dic Tausende geht, wird cine cinfachc und cinheitlichc Nomcnklatur vorgcschlagcn. Sic werdcn dcs weitercn nach ihrcm potcnticllcn Nutzcn fur synthetischc Grundverfahren gruppicrt (Aufbau. Eliminicrung, Umfti1iktionalisierung etc.). Die Methodik i c l t darauf ah. Auswahlkritericn anzugeben, nach dcnen ncue Resktionen aufgcfundcn wcrden kiinnen. Die Diskussion von Kcaktionsprofilcn erlaubt die Ikmittlung jcncr Atomanordnungcn in Molckiilcn, wclchc cine Rcahtion erleichtern.
The six-electron thermal pericyclic reactions are examined systematically as to the number and kind which are possible by varying both the 0 shell and the combinations of different atoms in all orientations, on both a six-atom and a five-atom framework. A simple unifying nomenclature is offered for these reactions, which number in the thousands. Further, in order to comprehend this very large number of possible reactions, they are also organized systematically in terms of their value for basic synthesis operations: construction, elimination, refunctionalization, etc. The methodology is aimed at providing a basis of selection for the invention of useful new reactions. A discussion of reaction energetics leads also to an analysis of molecular features which can facilitate reaction IntroductionThe common thermal pericyclic reactions[" are characterized by a cyclic transition state of three pairs of electrons, stabilized by an optimal overlap of all involved orbitals and by the delocalization of six electrons in a cycle. This resonance stabilization of six electrons in a cyclic transition state was early recognized by Evansrz1, and D e~a r [~~l has recently discussed pericyclic reactions in terms of the analogy with aromatic systems. Any cycle of (4n+ 2) delocalized electrons[3b1 is the basis for a stabilized aromatic molecule as well as a set of pericyclic reactions, derived formally as shown below by removal of various of the o bonds in the underlying molecular frame. The reactions are equilibria and a rough calculation of bond energies usually identifies the more stable molecular species and preferred direction of reaction. As the generalized reaction is nominally unaffected by changes in the involved atoms or by other molecular species present (e. g., reagents, solvents) it is capable of considerable variety as well as synthetic utility. In the latter connection it is often an excellent means of generating carbon-carbon bonds or hindered, sensitive, or otherwise difficultly accessible molecular groups, generally with a high order of stereospecificity. The purpose of this article is to examine the number and form of all the theoretically possible variants of the reaction obtainable by varying The Six-Center ModelThe problem of molecular permutations may be approached by writing the three shifting electron pairs as alternating bonds around a six-atom cycle and then generating all combinations of 0 to 6 immobile 0 bonds (B) linking these atoms. These immobile o bonds constitute the "shell" or backbone of the reacting species. There are thirteen possible shells, shown in Table 1 with and without the three shifting electron pairs. The shells are defined specifically as o bonds; in some cases extra K bonds occur, as in Diels-Alder addition to acetylenic dienophiles, but these are involved neither ,in the skeletal shell nor in the shifting electron set and so are not separated in the definition. The reacting pairs of molecules are all oriented in Table 1 with the more stable form on the right and its energy preference ...
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