Hexatrienes interaction schemes

The electron delocalizations in the linear and cross-conjugated hexatrienes serve as good models to show cyclic orbital interaction in non-cyclic conjugation (Schemes 2 and 3), to derive the orbital phase continuity conditions (Scheme 4), and to understand the relative stabilities (Scheme 5) [15]. 

There are assnmed to be three n bonds. A, B, and C, in benzene. Here we consider the electron delocalization from A to C. The electron delocalization via B is the same as that in the linear conjngate hexatriene (Schemes 2 and 3) used as a model of non-cyclic conjngate systems. The cyclic orbital interaction has been shown to be favored by the phase continnity (Scheme 5a). There is an additional path for the delocalization in cyclic geometry, which is the direct path from A to C or from a to c. The path gives rise to the cyclic a-b-c and a-b -c interactions. The cyclic orbital interactions satisfy the orbital phase continnity conditions... 

Very recently, MacGillivray et al. succeeded in the supramolecular construction of molecular ladders in the solid state using a linear template approach [48]. They designed the cocrystals 1,3-benzenediol (resorcinol) or a derivative with an all-trans-bis(4-pyridyl)butadiene or hexatriene, in which two resorcinol molecules preorganize two polyene molecules through two hydrogen bond interactions, for [2-1-2] photoaddition (Scheme 5). In this design, two polyenes would... 

One contribution to this problem is the compariajn of the PE spectrum of bis-tr-allylnickel with that of 1,3,5-hexatriene. The upper part of Scheme 2.5-3 diows the interpretation of the PE spectrum of bis- r-allylnickel by C. D. Batich Important for our comparison is that the marked occupied orbitals of the unified ligand system (El) are ligand-centered and practically do not interact with metal orbitals. 

A donor substituent may be represented by a doubly occupied orbital D, at (a + 2/3) for O and at (a + 1.5/ ) for N. Hence D always lies lower than the HOMOs of the diene and the dienophile. Scheme c illustrates the tricky case involving a first-order D-rc interaction and a second-order D- 2 term. One may wonder if the HOMO of the substituted dienophile cannot be higher than that of the diene. In fact, we just need to take a double bond for D to see that the HOMO of hexatriene lies higher than the butadiene HOMO when n is raised to the level of 2, the latter rises further, so that the diene always has the higher energy. 

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