Antiaromaticity electron transfer

A CIEEL approach can also be used to explain chemiexcitation in luminol chemilumi-nescence . Two possibilities arise (i) an electron transfer from the amino group to the peroxidic moiety in the antiaromatic peroxide 33, resulting in bond cleavage followed by intramolecular back-electron transfer and formation of excited 3-aminophthalate (Scheme 24) (ii) the equilibrium between the peroxycarboxylic aldehyde 34, formed after elimination of nitrogen, and the cyclic peroxy semiacetal 35 is shifted in the direction of 35, as the result of an electron transfer from the amino group to the cyclic peroxide moiety, followed by 0—0 bond cleavage . Back-electron transfer would result in chemiexcitation (Scheme 25). 

An alternative theory associates electron transfer with transfer of a state of aromaticity from molecule to molecule within the stack (77AG(E)519). Efficient charge transport was identified with conversion of a neutral, antiaromatic system to a charged, aromatic radical by electron transfer. This interpretation has been eroded by the synthesis of conductors from aromatic systems such as perylene hexafluoroarsenate (81MI11301) or polypyrrole tetrafluoroborate (80CC397, 81MI11300) where an electron is transferred from a neutral, aromatic molecule to a non-aromatic charged radical. 

Borole (IV in Figure 14) has been used to synthesize a plethora of sandwich and half-sandwich stmctures it is the bora analog of cyclopentadiene. Borole is an antiaromatic system see Antiaromatic Compound), and only the blue pentaphenyl derivative has been prepared. The general route to borole complexes consists of complexation under dehydrogenation of 2- or 3-borolenes, which are prepared from [Mg(butadiene)]x and RBX2. Thus, the heating of either isomer of borolene with Co2(CO)g produces [(borole)Co(CO)2]2 (47) (equation 60) from which further cobalt borole complexes are prepared (equation 61). CpCo(borole) (48) can be made from (47) by Cp transfer from the labile 20-electron nickelocene (equation 61). Pyrolysis of (48) at 160-180 °C gives a triple-decker complex (49) in nearly quantitative yield, which in turn can be cleaved by Cp into (48) and the bis(borole)cobaltate anion (50 ) (Scheme 34). 

In keeping with the oft-cited thermodynamic criterion for aromaticity, we consider an aromatic species as one in which cyclic 7r-electron delocalization results in an enthalpy of formation more negative than expected based on a related reference species, that is, it is stabilized relative to the reference. An antiaromatic species is one that is destabilized from cyclic delocalization, that is, its enthalpy of formation is more positive than expected, and a nonaromatic species is neither stabilized nor destabilized. We will make considerable use of the notion of resonance energy , a concept comprising many different types of calculations with an unfortunate deficit of unique identifying technical names, that is, resonance energy is frequently defined differently by different investigators and so, unlike thermochemical entities, is neither additive nor transferable. 

The stepwise transfer of electrons from alkali metals to Ti-conjugated systems enables the study of the relationship between the number of Ti-electrons and the aromatic/antiaromatic properties of the systems vis d vis the Hiickel rule. The magnetic criterion for aromaticity serves as a probe for the aromatic nature of the systems under study. 

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