Aromatic carbonyl rules

Outline the basis of El-Sayed s rules and be able to use these to explain the differences in luminescence properties between aliphatic and aromatic carbonyl compounds. 

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. 

Using the Woodward-Fieser rules, the of dienes and trienes, a,p-unsaturated aldehydes and ketones (enones), and for aromatic carbonyl compounds can be reliably calculated and predicted. 

Table 1.3 Scotts rules for calculation of %.max of the ET (electron transfer) band of aromatic carbonyl compounds...

In heterocyclic chemistry model compounds are essential for the interpretation of most spectra. No rules tue available for the predicition of the wavelength maxima of aromatic compounds except in the case of aromatic carbonyl compounds where acetophenone is taken as the parent chromophore, and increments allotted on the usual basis (Table 1.3)... 

The nature of conjugated system and substituents, from the empirical rules for calculation of of (a) polyenes, (b) enones and (c) aromatic carbonyl compounds. 

Additions include the attachment of two univalent atoms or groups (called addends) to an unsaturated system, e. g., to olefins, carbonyl groups, aromatic systems, carbenes, etc. (Rule 2.1). For example, the addition of hydrocyanic acid to the car-... 

The rule in carbon-13 NMR is that sp2-hybridized carbons (carbonyl, aromatic, olefinic) absorb at lowest field, followed by sp-hybridized (acetylenic, nitrile) and sp3 (aliphatic). A first glance leads us to believe we have seven signals, but we must remember that the methine carbon is directly bonded to phosphorus, so that we shall expect a relatively large C-P coupling. The other C-P couplings will probably be very much smaller. 

Rule of thumb The stability of molecular ions roughly decreases in the following order aromatic compounds > conjugated alkenes > alkenes > alicyclic compounds > carbonyl compounds > linear alkanes > ethers > esters > amines > carboxylic acids > alcohols > branched alkanes. [81]... 

Nucleophilic substitutions of simple aromatic compounds which formally involve a hydride displacement are difficult to achieve because of the poor leaving group and the high electron density of the aromatic nucleus which repels approach of a nucleophile. However, rc-electron deficient aromatic compounds such as metal carbonyl complexes are susceptible to attack by certain carbon nucleophiles. Studies of this chemistry have shown [16] an opposite jegioselectivity to the corresponding electrophilic substitutions, in agreement with the polarity alternation rule. 

To all rules, there are always exceptions. These have been made to allow unexpected natural isoquinolines that just happen to present unexpected substituents that nature for some reason chose to contribute to this collection. Mention has been made of an occasional carbonyl group disrupting the aromaticity of the benzene ring (this is the basis of the quinonic isoquinolines). The nitrogen atom (position 2) occasionally displays an amide group (these have been entered at the fourth letter of the structural alphabet). Several natural compounds demand a hydroxyl or methoxyl function at the isoquinoline 3- or 4-positions. When this occurs, the compound is listed as a footnote under the parent structure. 

There are some known unsuccessful attempts to carry out alkylation (Mel, Me2S04), halogenation (tert-butyl hypochloride) and nitration of aromatic dihydrobenzodiazepines [7, 105]. Such attempts only resulted in the destruction of the seven-membered heterocycle. As a rule, these destructive processes are typical of dihydrodiazepine systems and often manifest themselves during the synthesis and study of these compounds. Therefore, the results of the destruction of a seven-membered heterocycle are most widespread and include its decomposition into ortho-diamine and carbonyl compounds (Scheme 4.43, reactions A and B) [105, 106] and benzimidazole rearrangement accompanied by splitting out of a methyl aryl ketone molecule (Scheme 4.43, reaction C) [117]. 

These three conjugation types may correspond to aromaticity or antiaromaticity, in accordance with the Hiickel rule. They can be depicted using the partial resonance structures given in Fig. 4. An additional requirement is that each branch required to participate in cyclic conjugation not contain a formally two-electron heteroatom site (such as -NH-, -O-, etc.) or exocyclic double bonds (such as carbonyl groups). These structural elements are incompatible with Kekule-type canonical structures. Finally, it should be noted that in the case of type C structures macrocyclic conjugation will be preserved, even when one of the a and b branches is not fully conjugated. 

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