Resonance-stabilized system

Resonance-stabilized systems include car-boxylate groups, as in formate aliphatic hydrocarbons with conjugated double bonds, such as 1,3-butadiene and the systems known as aromatic ring systems. The best-known aromatic compound is benzene, which has six delocalized k electrons in its ring. Extended resonance systems with 10 or more 71 electrons absorb light within the visible spectrum and are therefore colored. This group includes the aliphatic carotenoids (see p.l32), for example, as well as the heme group, in which 18 k electrons occupy an extended molecular orbital (see p. 106). 

Resonance forms are classical structures used to describe a more complex system they do not actually exist. The species is more accurately described by a resonance hybrid which can be imagined as an average of the resonance forms. Resonance always stabilizes a system. Each atom in a resonance stabilized system has a p-orbital. Allylic carbocations are stabilized by delocalization of the positive charge. 

Both carbon atoms become sp hybridized and the double bond is lost. A similar second step in aromatic molecules would result In destruction of the resonance-stabilized system and therefore does not occur. 

Mierobieides disposing of sueh a toxophoric structural element - as already mentioned, acrolein and a-bromo-einnamaldehyde do that - may add to nucleophilic cell components (Figure 8.) and release their antimicrobial effect in this way. However, sueh substances are only suitable for practical applications, e.g. as non-persistent slimieides, if the aetivated vinyl group is part of a resonance-stabilized system (which has been mentioned already in eonneetion with the stability of BCA). Without stabilisation by a resonance system the substances are too reaetive and unstable, i.e. polymerisation and reactions with components in the surrounding medium, reactions with ammonia and amines taking preeedence (Paulus, 1976). 

However in connection with the application of active vinyl compounds as mierobieides one has in any case to regard their reactivity, even if they contain a resonance-stabilized system. N-(3,4-dichlorophenyl)-N -acryl-urea for instance is a reliable fungicide (Paulus et al., 1980) in neutral or acidic media only in alkaline solutions it reacts intramolecularly to the corresponding dihydrouracil derivative, which is ineffective (Figure 9.). Aroylethy-lene reacts in hydrous ammoniac solution intermolecularly to tris(aroylethyl)amine which precipitates 3 Ar-CO-CH = CH2 + 1 NH3 N(H2C-CH2-CO-Ar)3... 

With regard to a proposed mechanism of action of OPA Simons et al. (2000) interpret the observation that OPA is a less potent crosslinking agent than GA as an influence of steric restrictions of the aromatic dialdehyde in contrast to GA which is composed of a flexible aliphatic chain. In addition the two carbonyl groups of OPA are part of a resonance stabilized system and consequently are less reactive than aliphatic aldehydes such as GA or FA in nucleophilic addition reactions. The fact that OPA in contrast to other aldehydes reacts only with... 

For veral classes of nitrogen bases, the position of protonation is a controversial matter and it appears that the pjnroles, indoles, and possibly amides undergo protonation on all possible basic sites depending on the circumstances. These same compounds are some of the womt offenders for non-adherence to the activity coefficient postulate, and furthermore have in common a resonance-stabilized system in which two of the basic sites are in a position 1, 3 to each other. Nitrobenzene and benzoic acid which exhibit abnormal activity be-... 

The results of the derivation (which is reproduced in Appendix A) are summarized in Figure 7. This figure applies to both reactive and resonance stabilized (such as benzene) systems. The compounds A and B are the reactant and product in a pericyclic reaction, or the two equivalent Kekule structures in an aromatic system. The parameter t, is the reaction coordinate in a pericyclic reaction or the coordinate interchanging two Kekule structures in aromatic (and antiaromatic) systems. The avoided crossing model [26-28] predicts that the two eigenfunctions of the two-state system may be fomred by in-phase and out-of-phase combinations of the noninteracting basic states A) and B). State A) differs from B) by the spin-pairing scheme. 

In principle, the direct hydride addition or catalytic hydrogenation, which did not give chlorins, was replaced by an electrocyclic intramolecular addition which is much easier with the above system. Complete regioselectivity was also achieved since electrocyclization did not occur with the resonance-stabilized ring C. 

We might be hard pressed to estimate the individual resonance stabilization energies in Eqs. (7.23) and (7.24), but the qualitative apphcation of these ideas is not difficult. Consider once again the styrene-vinyl acetate system ... 

The protonated azirine system has also been utilized for the synthesis of heterocyclic compounds (67JA44S6). Thus, treatment of (199) with anhydrous perchloric acid and acetone or acetonitrile gave the oxazolinium perchlorate (207) and the imidazolinium perchlorate (209), respectively. The mechanism of these reactions involves 1,3-bond cleavage of the protonated azirine and reaction with the carbonyl group (or nitrile) to produce a resonance-stabilized carbonium-oxonium ion (or carbonium-nitrilium ion), followed by attack of the nitrogen unshared pair jf electrons to complete the cyclization. 

Phenanthrene and anthracene both react preferentially in the center ring. This behavior is expected from simple resonance considerations. The c-complexes that result from substitution in the center ring have two intact benzene rings. The total resonance stabilization of these intermediates is larger than that of the naphthalene system that results if substitution occurs at one of the terminal rings. ... 

Delocalized n systems must be planar, or nearly so, for resonance stabilization to be most effective. Imposing planarity, however, may cause ring strain. 

Formation of a quinonoid carboline-type anhydro-base requires loss of resonance stabilization of the indole moiety. In the carboline anhydro-bases this is counterbalanced by the preservation of a 677 system in the hetero ring. No such balancing factor is present in the case of 3,4-dihydro-j3-carboline derivatives. Formation of the exocyclic anhydro-base in the latter case preserves benzenoid resonance. It is noteworthy that in the two cases where formation of a carboUne-type anhydro-base was reported in dihydro derivatives additional aromatic conjugation is present. 

When an azine-nitrogen and a leaving group are in the 2,3-relation to each other in monoaza- and polyaza-naphthalenes, there is a dramatic effect on the reaction rate (for 3-chloroisoquLnoline lO -lO -fold less than for its 1-chloro isomer and for 2-chloroquinoline 200-400-fold less than for 2-chloropyridine) due to restrictions imposed on the resonance stabilization of charge in the transition state by the bicyclic system ... 

It has been contended that here too, as with the benzene ring (Ref 6), the geometry is forced upon allylic systems by the a framework, and not the 7t system Shaik, S.S. Hiberty, P.C. Ohanessian, G. Lefour, J. Nouv. J. Chim., 1985, 9, 385. It has also been suggested, on the basis of ab initio calculations, that while the allyl cation has significant resonance stabilization, the allyl anion has little stabilization Wiberg, K.B. Breneman, C.M. LePage, T.J. J. Am. Chem. Soc., 1990, 112, 61. 

Protonation of the enolate ion is chiefly at the oxygen, which is more negative than the carbon, but this produces the enol, which tautomerizes. So, although the net result of the reaction is addition to a carbon-carbon double bond, the mechanism is 1,4 nucleophilic addition to the C=C—C=0 (or similar) system and is thus very similar to the mechanism of addition to carbon-oxygen double and similar bonds (see Chapter 16). When Z is CN or a C=0 group, it is also possible for Y to attack at this carbon, and this reaction sometimes competes. When it happens, it is called 1,2 addition. 1,4 Addition to these substrates is also known as conjugate addition. The Y ion almost never attacks at the 3 position, since the resulting carbanion would have no resonance stabilization " ... 

Although at first glance addition to the central carbon and formation of what seems like an allylic carbonium ion would clearly be preferred over terminal addition and a vinyl cation, a closer examination shows this not to be the case. Since the two double bonds in allenes are perpendicular to each other, addition of an electrophile to the central carbon results in an empty p orbital, which is perpendicular to the remaining rr system and hence not resonance stabilized (and probably inductively destabilized) until a 90° rotation occurs around the newly formed single bond. Hence, allylic stabilization may not be significant in the transition state. In fact, electrophilic additions to allene itself occur without exception at the terminal carbon (54). 

The key pathway branching competition in this reaction is the isomerization of the initial adduct (I) to either the resonantly stabilized four-member ring intermediate (III), or the three-member ring intermediate (II). Formation of the four-member ring has a lower energy barrier (saddle point 4), but is more entropically constrained, as all the internal rotors of the system are eliminated. 

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