Trienes, conjugated, aromatic systems

The size of the K-system chosen has important implication on the structural and functional aspects of metal binding. To explore the size effect calculations were performed on the cation-ir complexes of Li+ and Mg + with the Jt-face of linear and cyclic unsaturated hydrocarbons [45]. In the case of the acyclic Jt-systems, we started with the simplest system, e.g. ethylene followed by buta-1,3-diene, hexa-l,3,5-triene, and octa-1,3,5, 7-tetraene with 2, 3 and 4, conjugated jt units, respectively. These linear systems with two and more number of jt units can have various conformations wherein the jt units can have cis, trans or a combination of both cis and trans orientations. Similarly for cyclic systems cyclobutadiene, benzene, cyclooctateraene, naphthalene, anthracene, phenanthrene and naphthacene have been included. Thus a wide range of sizes for aromatic systems have been covered. 

The carbocation is aromatic the hydrocarbon is not Although cycloheptatriene has six TT electrons m a conjugated system the ends of the triene system are separated by an sp hybridized carbon which prevents continuous tt electron delocalization... 

As we have already seen, delocalization of electrons by conjugation decreases the energy difference between the HOMO and LUMO energy levels, and this leads to a red shift. Alkyl substitution on a conjugated system also leads to a (smaller) red shift, due to the small interaction between the cr-bonded electrons of the alkyl group with the K-bond system. These effects are additive, and the empirical Woodward-Fieser rules were developed to predict the 2max values for dienes (and trienes). Similar sets of rules can be used to predict the A ax values for a,P-unsaturated aldehydes and ketones (enones) and the Amax values for aromatic carbonyl compounds. These rules are summarized in Table 2.4. 

A final example of a rDA cycloreversion of a cycloadduct that acts as a protected butadiene illustrates the generation of extended conjugation in a non-aromatic cyclic system. Equation (65) shows that the addition of cyclopropene (147) to (146) can yield either (148) or (149), and both adducts undergo rDA extrusion of carbon dioxide to produce a triene. Hydrolysis of the triene ketals gives access to substituted tropones (150). 

The photochemistry of linear conjugated trienes is closely connected to the isomeric 1,3-cyclohexadienes which are both their photochemical precursors and products. Such systems have been dealt with under cyclic 1,3-dienes in the previous section. The photochemical cyclization of m-stilbene to dihydrophenanthrene and ultimately to phenanthrene and hydrogen is formally analogous to reaction (13). This reaction (17) has been observed in a large class of aromatic compounds under a... 

A six-membered cyclic conjugated triene system is called an aromatic ring. The individual single and double bonds are shown in individual canonical structures, while the resonance hybrid indicates the delocalisation that takes place in reality. 

After understanding the usefulness of unsaturated compound, or conjugated system, we hope to explore the unique structure of aromatic compounds, including why benzene should not be called 1,3,5-hexatriene because it is more stable than a typical triene, and seemingly unreactive. Called aromatic initially because of its fragrance, aromaticity now refers to the stability of compounds that are considered aromatic, not only benzene. Any cyclic compound with 4n+2 pi electrons in the system is aromatic. The stability of aromatic compounds arises because all bonding orbitals are filled and low in energy. 

The A ring of the female sex hormone is aromatic thus a C-10 substituent is not possible. However, for purposes of IUPAC nomenclature, since the ring system is in the estrane group (Fig. 14-1), the A ring is named as having three conjugated double bonds (a triene). The third double bond is between carbons 5 and 10 (not 5 and 6) and so must be indicated by the number 10 in parentheses. 

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