Unsaturated system stability

The meaning of the word aromaticity has evolved as understanding of the special properties of benzene and other aromatic molecules has deepened. Originally, aromaticity was associated with a special chemical reactivity. The aromatic hydrocarbons were considered to be those unsaturated systems that underwent substitution reactions in preference to addition. Later, the idea of special stability became more important. Benzene can be shown to be much lower in enthalpy than predicted by summation of the normal bond energies for the C=C, C—C, and C—H bonds in the Kekule representation of benzene. Aromaticity is now generally associated with this property of special stability of certain completely conjugated cyclic molecules. A major contribution to the stability of aromatic systems results from the delocalization of electrons in these molecules. 

No such fading in methanol is observed with the ion XVIII, in which the additional phenyl group extends the unsaturated system and stabilizes the ion XVIII, as compared with the ion XIX, in which the nitrogen atom insulates the two parts of the molecule. 

HP-6 Reference to aromatic systems stability, orientation rules and their relationship with more or less unsaturated cyclohexane rings and decalin systems. 

High-level ab initio calculations have provided more precise structural details, and relative stability estimates, for members of the 7-norbornyl anion series (12-15). Far from being classical carbanions, each of the ions is stabilized by delocalization of the negative charge into accessible LUMOs of anti-parallel C—C bonds of the molecular framework and each is more stable than methyl carbanion. Consequently, it is unlikely that solution studies of the unsaturated systems will reveal any bishomo-antiaromatic character. 

During the coverage period of this chapter, reviews have appeared on the following topics reactions of electrophiles with polyfluorinated alkenes, the mechanisms of intramolecular hydroacylation and hydrosilylation, Prins reaction (reviewed and redefined), synthesis of esters of /3-amino acids by Michael addition of amines and metal amides to esters of a,/3-unsaturated carboxylic acids," the 1,4-addition of benzotriazole-stabilized carbanions to Michael acceptors, control of asymmetry in Michael additions via the use of nucleophiles bearing chiral centres, a-unsaturated systems with the chirality at the y-position, and the presence of chiral ligands or other chiral mediators, syntheses of carbo- and hetero-cyclic compounds via Michael addition of enolates and activated phenols, respectively, to o ,jS-unsaturated nitriles, and transition metal catalysis of the Michael addition of 1,3-dicarbonyl compounds. ... 

If the potential leaving group is attached to unsaturated carbon, as in vinyl chloride or phenyl chloride, attack by nucleophiles is also extremely difficult, and these compounds are very unreactive in Sn2 reactions compared with simple alkyl halides. In these cases, the reason is not so much steric but electrostatic, in that the nucleophile is repelled by the electrons of the unsaturated system. In addition, since the halide is attached to carbon through an 5p -hybridized bond, the electrons in the bond are considerably closer to carbon than in an 5/ -hybridized bond of an alkyl halide (see Section 2.6.2). Lastly, resonance stabilization in the halide gives some double bond character to the C-Hal bond. This effectively strengthens the bond and makes it harder to break. This lack of reactivity is also tme for SnI reactions (see Section 6.2). 

Michael-type addition of stabilized carbon donors to electron-withdrawing a,p-unsaturated systems is an efficient method for C-C bond construction. The nitro-group... 

Last but not least, ageing and destruction processes can be monitored in polymers under application, and structural and quantitative analysis of unknown additives (stabilizers etc.) is possible in commercial polymers using UV-vis spectroscopy. Advantage can be taken here of the fact that the position of an electronic absorption in unsaturated systems depends only weakly on the surroimd-ing medium. Even though UV-vis spectroscopy is not very specific in the absorption band, it is highly sensitive and therefore much better than NMR or IR spectroscopy to detect small amounts of chromophors. 

Nitrenes have a short lifetime (only several microseconds)86- 8 and undergo stabilization by the following reactions isomerization to imines, dimerization to azo compounds, hydrogen abstraction followed by ring closure to heterocyclic compounds, bimolecular insertion into C-H bonds to secondary amines, addition to solvent yielding ylids, and addition to unsaturated systems yielding heterocyclic compounds. Table 117-106 includes the reaction products and references for the different classes of nitrenes. 

For benzene, [6]annulene, with six it electrons (4n + 2, n = 1), the theory clearly meets both tests. As we have pointed out, there is a substantial stabilization of about 37 kcal mole-1 compared with a hypothetical localized model. The familiar chemical properties also point to a strong tendency for maintenance of the six it electron unsaturated system. The proton magnetic resonance spectrum of benzene and its derivatives shows the proton resonances in the range of 8 = +7 to +8 ppm (downfield from tetramethylsilane), 1-2 ppm lower than protons attached to nonbenzenoid double bonds. Referring to Figure 1.21a, we can see that the prediction is in agreement with this result. The induced field adds... 

In contrast to cycloaddition reactions of phosphaalkenes, cycloaddition reactions between phosphaalkynes and other unsaturated systems are comparatively rare. Indeed, there are only a limited number of reports for monophos-phacyclobutadiene) complexes, which are obtained from the corresponding phosphaalkyne. Relatively recently, the reaction of phosphaalkynes with highly electron deficient alkynes was reported <19990M4838>. Treatment of a CF3C=CGF3-coordinated dimeric rhodium complex with phosphaalkynes in hexane at — 20°C followed by warming to room temperature afforded the red air- and moisture-stable phosphete complexes 60 in ca. 50% isolated yields. When phosphaalkynes are allowed to react with a kinetically stabilized cyclobutadiene, 2-Dewar-phosphinines, for example 93 (Equation 30), are obtained <1998S1305>. 

Our interest in this chapter is in silver-catalyzed cycloisomerization reactions. Therefore, we shall present different silver-catalyzed cycloisomerization reactions as a function of the nucleophilic and electrophilic moiety. Cycloisomerization reactions including the classical ene-yne cycloisomerization (with X = CHR, Scheme 5.1), and the related heterocyclization reactions with heteroatoms embedded in unsaturated systems (X = NR, O Scheme 5.1) belong to the same reaction family. In addition, the alkynyl part can be exchanged for an allene unit. Internal or external nucleophiles (Nu) can then stabilize, through cascade reactions, the positive charge created.24... 

While the addition of stabilized carbanions to activated unsaturated systems (Michael addition) remains one of the most popular methods for the for-... 

Furthermore, the geometry of adducts is influenced by the silicon bonded X groups. It should be pointed out that the rotation about the SiY bond is obviously not much hindered therefore, the substituents at silicon are always oriented in such a way in the crystalline phase as to minimize repulsive intramolecular interactions with substituents at Y. Over and above that, it should be noticed that the formation of n-complexes of silenes stabilize the unsaturated systems. 

Another aspect of stability of the unsaturated systems concerns the n-bond strength of the double bonded fourth group elements, the experimental determination of which certainly represents problems. Nevertheless, from n-bond migration processes some insights may be derived about the relative stability of these n-bonds as a function of the double bond substituents. In this connection, I turn first to the question Are SiC-n-bonds stabilized by mesomeric effects of silicon bonded phenyl groups ... 

The formation of carbon-carbon bonds by conjugate addition of carbonucleophiles to a,/3-unsaturated systems has been studied intensively and reviewed over the past few years . Interestingly, applications with simple, unstabilized lithium enolates are relatively rare. Most reported examples are limited to the addition of stabilized enolates, such as those derived from malonates or acetoacetates. Nevertheless, some diastereo- and enantioselective versions of the conjugate addition, even with unstabilized lithium enolates, are well known. In 2004, Tomioka and coworkers studied the influence of a chiral diether (191) on the 1,4-addition of lithium ester enolates (189) to a,-unsaturated ketones (equation 51) . Their investigations showed that good enantioselectivities were obtained with cyclic enones, like 2-cyclopentenone (190) addition to a mixture of 189 and 191 gave the desired 1,4-adduct (R)-192 with 74% ee, but only 47% yield. Unfortunately, also the Peterson product 193 was formed in a yield of 22% by initial 1,2-addition of the enolate to the Michael acceptor. 

The mobile Jt-electrons of unsaturated systems, responsible for the stabilization of the carbocations, provide equally efficient stabilization for carbanions. Consequently, a retrosynthetic cleavage of a benzylic, allylic, or propargylic C-C bond has additional merits since the resulting fragments can be visualized as either an electrophile or a nucleophile. The dual synthetic value of the allylic moiety has been extensively utilized in the synthesis of a large number of natural acyclic isoprenoids. The structures of many of these compounds look like they were purposely tailored for this type of retrosynthetic analysis. In fact, the 1,5-diene system, usually present in their structure (Scheme 2.19), immediately suggests the cleavage of its central C-3-C-4 bond, which leads to two allylic precursors. 

Lycopene is a red pigment found in tomatoes, watermelon, papaya, guava, and pink grapefruit. An antioxidant like vitamin E, lycopene contains many conjugated double bonds—double bonds separated by only one single bond—that allow jt electron density to delocalize and give the molecule added stability. In Chapter 16 we learn about such conjugated unsaturated systems. 

Similar condensations can be accomplished with other types of stabilized carbanions, e.g. sulfonyl anions, as illustrated by equation (81). The resulting sulfonyl lactone (228) eliminates sulfinic acid on treatment with p-TsOH to furnish the a,3-unsaturated system (229).Spirolactonization is the result of the Reformatsky reaction of ester (231) with cyclic ketones. In equation (82), this reaction is applied to the synthesis of the lysergic acid precursor (232), which is formed stereoselectively from (230). ... 

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