Conjugated stability

Allylic conjugation stabilizes carbanions, and pAT values of 43 (in cyclohexylamine) and 47—48 (in THF-HMPA) have been determined for propene. On the basis of exchange rates with cesium cyclohexylamide, cyclohexene and cycloheptene have been found to have pAT values of about 45 in cyclohexylamine. The hydrogens on the sjp-... 

Since it is clear that the presence of an unshared pair of electrons on the sulfur of the sulfoxide group leads to no special instability in the case of the known thiirene oxides (i.e., 18a, 28a,b and the first alkyl-substituted thiirene oxide 30 recently synthesized60), the reduced antiaromatic properties of the thiirene oxides relative to that of thiirenes have been manifested experimentally. As far as the possibility of electron-attracting conjugative stabilization involving the sulfur atom in thiirene oxides is concerned, the experimental evidence accumulated so far is not decisive. Thus, the chemical shift of the vinylic carbon of... 

The higher strain energy in thiirene dioxides (19) compared to thiirane dioxides (17) is obvious. Yet, the elimination of sulfur dioxide from the latter is significantly faster than one would expect for a thermally allowed concerted process. Consequently, either aromatic-type conjugative stabilization effects are operative in thiirene dioxides2,12 or the relative ease of S02 elimination reflects the relative thermodynamic stability of the (diradical )99 intermediates involved in the nonconcerted stepwise elimination process. 

In the brosylate acetolysis sets, conjugative stabilization of the transition state is geometrically excluded in the syn form, but is anticipated in the anti form (13). That is, on structural grounds, is expected to be the parameter of choice for the syn set and for the anti set. The available data (for the OMe, Me, Cl and NO2 substituents) do indeed conform to this expectation ... 

Gygax R, Wirz J, Sprague JT, Allinger NL. Electronic structure and photophysical properties of planar conjugated hydrocarbons with a 4n-membered ring. Part III. Conjugative stabilization in an antiaromatic system The conformational mobility of l,5-bisdehydro[12]annulene. Helv Chim Acta 1977 60 2522-9. 

The proline-catalyzed reaction has been extend to the reaction of propanal, butanal, and pentanal with a number of aromatic aldehydes and proceeds with high syn selectivity.197 The reaction can also be carried out under conditions in which the imine is formed in situ. Under these conditions, the conjugative stabilization of the aryl imines leads to the preference for the aryl imine to act as the electrophile. A good yield of the expected P-aminoalcohol was obtained with propanal serving as both the nucleophilic and the electrophilic component. The product was isolated as a 7-amino alcohol after reduction with NaBH4. 

There have been many applications of the Wittig reaction in multistep syntheses. The reaction can be used to prepare extended conjugated systems, such as crocetin dimethyl ester, which has seven conjugated double bonds. In this case, two cycles of Wittig reactions using stabilized ylides provided the seven double bonds. Note the use of a conjugated stabilized ylide in the second step.250... 

Selected Substituent-Stabilized Tocopherols and Conjugatively Stabilized Ortho-Quinone Methides... 

FIGURE 6.45 Inability of 5a-substituted derivatives to form structures analogous to o-QM 3 causes increased oxidative stability as in compounds 71 and 72. 5-(p-Hydroxyphenyl)-y-tocopherol (73) is oxidized to the conjugatively stabilized o-QM 74, the phenylogous a-tocored (75). 

FIGURE 6.46 Oxidation chemistry of 5-(4-methylphenyl)- y-tocopherol (76), establishing a reaction system phenylogous to a-tocopherol (1), with quinone methide 77 and benzyl bromide 78 being the conjugatively stabilized, phenylogous counterparts of o-QM 3 and 5a-bromo-a-tocopherol (46), respectively. 

Computations performed on an N-methylated imidazole showed that the presence of the methyl group increases the proton affinity of the other nitrogen. In contrast, a peptide substituent at a carbon adjacent to the nitrogen decreases its proton affinity. This can be explained by the conjugation stabilization of the peptide-substituted neutral form, which is absent in the protonated form. This result seems to indicate that heteroatomic rings linked by... 

In Table 7 the six-membered monocyclic dienes are represented by the conjugated 1,3-cyclohexadiene and its isomer 1,4-cyclohexadiene. 1,3-Cyclohexadiene has a nonplanar equilibrium conformation that is primarily influenced by three factors -electron interaction (optimal for a planar conformation) angle strain and torsion strain (both optimal for a planar conformation). The reduced overlap between the two --orbital systems is, for the observed C=C—C=C angle of 18°, estimated at ca 10% and should therefore not influence the conjugation stabilization drastically, compared to a conformation with coplanar C=C bonds. 

We now wish to describe qualitative trends in pi-conjugated systems in terms of the localized NBO donor-acceptor stabilizations (3.109) and NRT bond orders. For this purpose we consider the representative group of molecules 1-16 shown in Fig. 3.37. Numerical bond orders and conjugative stabilization energies pertaining to these molecules are gathered in Table 3.19. 

Orientational and energetic factors in donor acceptor interactions Let us next examine the dependence of Eqs. (3.109) on the factors in the numerator and denominator that affect conjugative stabilization, taking Eq. (3.109a) as an example. 

Table 3.19. Formal n moieties, NRT bond orders of associated atoms, and conjugative stabilization energies for molecules 1-16 (shown in Fig. 3.37)...

Let us now consider other factors that are expected to affect the NBO matrix element Fa,b and conjugative stabilization according to Eq. (3.109). 

Thus, there is net weakening of conjugative stabilization in this case, as well as net delocalization of pi charge from the CN region into the CO region. 

Such one-center enhancement effects can be illustrated by formamide 5 for nb—>7Ta (3.109c) interactions. As shown in Table 3.19, the n — -7tco interaction of 5 leads to strong conjugative stabilization (59.8 kcal mol-1) and reduced C—O bond order (1.732), the famous amide resonance of peptide chemistry ... 

Exercise Justify the estimate given above for the increased conjugative stabilization (QMRE) in 15 relative to 1. 

Given these surprising experimental facts, it is obviously important to inquire why the cyclic conjugation patterns (3.122) and (3.123) should lead to such diametrically opposite changes in conjugative stability compared with acyclic analogs. A... 

We first consider benzene, 17, the prototypical aromatic molecule. From the entries in Table 3.21 and comparisons analogous to Example 3.15, one can recognize that conjugative stabilizations in benzene are significantly stronger than those of comparable species in Table 3.19. Thus, on a per-pi-bond basis, the estimated stabilizations in benzene are 40.8 kcal mol-1, more than three times those of diene 2 (12.8 kcal mol-1), twice those of the acyclic triene 11 (20.3 kcal mol-1), and about 58% greater than the most strongly stabilized polyene, 16 (25.8 kcal mol-1). 

In contrast to benzene, the pi-conjugative stabilizations of D4h-symmetric cyclobutadiene, 18, are found to be exactly zero. The absence of conjugative stabilization may also be judged from the 7ta—7rb orbital contours in Fig. 3.44. Indeed, it is... 

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