Rr-electron densities

The (thermal) decomposition of thiazol-2-yldiazonium salts in a variety of solvents at 0 C in presence of alkali generates thiazol-2-yl radicals (413). The same radicals result from the photolysis in the same solvents of 2-iodothiazole (414). Their electrophilic character is shown by their ability to attack preferentially positions of high rr-electron density of aromatic substrates in which they are generated (Fig. 1-21). The major... 

The carbonyl group withdraws rr electron density from the double bond and both the carbonyl carbon and the p carbon are positively polarized Their greater degree of charge separation makes the dipole moments of a p unsaturated carbonyl compounds signifi cantly larger than those of comparable aldehydes and ketones... 

The diminished rr electron density m the double bond makes a p unsaturated aide hydes and ketones less reactive than alkenes toward electrophilic addition Electrophilic reagents—bromine and peroxy acids for example—react more slowly with the carbon-carbon double bond of a p unsaturated carbonyl compounds than with simple alkenes... 

Figure 3 HMO calculations of rr-electron densities and bond orders...

It is apparent from simple valence bond considerations as well as from calculations of rr-electron density, " that isoindoles should be most susceptible to electrophilic attack at carbon 1. This preference is most clearly evident when the intermediate cations (85-87) from electrophilic attack (by A+) at positions 1, 4, and 5 are considered. The benzenoid resonance of 85 is the decisive factor in favoring this intermediate over its competitors. 

The nitrogen ending atoms being more electronegative possess a rr electronic density greater than that of the carbon atoms directly linked to them. 

Consider the exact definition of from Eq. (10.32). When atom fe is a sp carbon, we can safely neglect the second- and higher-order terms because the values are small, in favor of the simple approximation, Eq. (10.41). However, we must consider both (T- and rr-electron densities and their variations. The appropriate first derivatives dEf"/dNk)° are indicated in Table 10.3. 

The charge alternation indicated by quantum chemical calculations (Section IV,B and Fig. 1) is supported by NMR spectra. Different screening of C—H due to different rr-electron densities gives rise to the chemical shifts of the ring protons. This means that those positions of the five-membered ring that are directly bonded to the six-membered ring are more strongly screened. This observation is also in accordance with theoretical predictions. 

A number of workers have reported the NMR spectra of mono-substituted phenyl groups and correlated the shifts vdth molecular parameters. It has been shown for organic compounds that the chemical shift of the para-carbon in monosubstituted benzenes is linearly related to the total TT-electron density at the para position in these compounds. Also the shift separation of the meta- and para-carbons appears to be linearly related to the rr-electron density on the para-carbon due to resonance interaction with the substituent 49, 156). Spiesecke and Schneider have reported a good linear relationship between the para-carbon chemical shift of monosubstituted benzenes and the Hammett, o-para constant, but no such relationship appears to exist for the other carbon chemical shifts, except between the chemical shift for the substituted carbon atom (corrected for magnetic anisotropy effects of the substituent) and the electronegativity of the substituent 210). 

While the rr-densities predict greater reactivity to electrophilic attack at C-2 in the imidazole anion, localization energies suggest an order of reactivity of C-4 > C-2. Use of the CNDO/2 method predicts the order of electrophilic substitution in benzimidazole to be5= 7>6>4>2, whereas the observed order (for aqueous bromination) is 5>7>6,4,2 (78JCS(P2)865). Predictions of preferential nucleophilic attack at C-2 are confirmed by experiment. Table 2 lists rr-electron densities in benzimidazole. 

On CNDO/2 calculations with pK (as base) = 2.30 for triazole, the basicities of H and AH tautomers are estimated to be equal, and maximum separation of protonated nitrogens i.e. N-1 and N-4 rather than N-1 and N-2) is predicted to afford the most stable cation (68TL3727). Acidity constants of 1,2,4-triazoles correlate with total and rr-electron densities but not with the lone pair character of the pyridine-type N in MO calculations (70JCS(B)i692, 70BCJ3344). 

Engel has reportedthe synthesis of the strained oxetane (193) from the 3,-y-unsaturated ketone (192), the reaction being facilitated by the high rr-electron density of the alkene and a flexible tether, which allows the alkene it- and oxygen n-orbitals to become coplanar. A similar oxetane product (194) has been made by Cookson and Rogers. Smith and Dieter have reported the chemoselective formation of oxetane (196) from ketoalkene precursor (195), in which geometrical constraint leads to selective... 

TABLE III-4. CORRELATION BETWEEN rr ELECTRON DENSITIES AND CORRECTED CHEMICAL SHIFTS (108,110)... 

The different effect of the nitrogen atoms from the ortho, meta, and para-positions (2-N, 3-N, 4-N) relative to the ipso carbon atom on the resonance constants for azinyl groups depends on the distribution of rr-electron density over the heteroaromatic ring. The correlation of the resonance... 

Reactivity indexes and rr-electron densities of 4-aryl-l,3-dithiolylium ions have also been calculated by the Hiickel MO method (70CPB865,65ZC23). 

The formation of terpenes and other isoprenoid lipids involves the condensation of dimethylallyl pyrophosphate with isopentenyl pyrophosphate (29). In this case (Scheme 11), the leaving group is the pyrophosphate (PP) of an allylic pyrophosphate, such as dimethylallyl pyrophosphate, generating an allylic carbonium ion derivative (and inorganic pyrophosphate, which forms as an ion pair with the carbonium ion). The electron-rich agent that adds to the electron-deficient center to form the carbon-carbon bond is the rr-electron density of the isopentenyl pyrophosphate. The stereochemical course of the biosynthesis of complex natural products has been determined and is consistent with such a mechanism (30). 

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