Wheland intermediates calculations

After what we have seen to date, it surely comes as no great surprise to find that the ratio of o- to p-product obtained from substitution of C6H5Y, where Y is o-/p-directing, is seldom, if ever, the statistical ratio of 2 1. There is found to be very close agreement between calculation and n.m.r. data for the distribution of +ve charge—p-> o- m—around the ring in the cyclohexadienyl cation (57), which is the Wheland intermediate for proton exchange in benzene (cf. p. 133) ... 

The details of protonation of several alkyl-substituted phenanthrenes by superacids have been reported.73 The observed mono- and di-cations are usually in agreement with those predicted by AMI MO calculations. Molecular modelling studies have suggested a multi-step pathway for the sulfonation of toluene widi sulfur trioxide.74 Intermediate 71-complcx. Wheland intermediate and pyrosulfonate species (34) are suggested, the product (p-toluenesulfonic acid) arising from an exothermic reaction between toluene and the acid (35) fonned by a facile prototropic rearrangement of (34). The sulfur trioxide monosulfonation of isopyrene and some derivatives leads usually to sulfonated... 

Protonation of fluorobenzene in the gas phase has been studied by infrared photodissociation (IRPD) spectroscopy by Solca and Dopfer.351 F-protonated fluorobenzene was formed in significant amount when protonation was carried out with CH5+. It was found to be the most stable isomer in the gas phase by quantum mechanical calculations [B3LYP/6-311G(2df,2pd) level] separated by a large energy barrier from the four Wheland intermediates. F-protonated fluorobenzene is best described as a weakly bound ion-dipole complex between the phenyl cation and HF. 

MO studies of aromatic nitration cast doubt on the existence of jt-complexes and electron-transfer complexes in liquid-phase nitrations.14 The enthalpy of protonation of aromatic substrates provides a very good index of substrate reactivity to nitration. Coulomb interaction between electrophile and substituent can be a special factor influencing regioselectivity. A detailed DFT study of the reaction of toluene with the nitronium ion has been reported.15 Calculated IR spectra for the Wheland intermediates suggest a classical SE2 mechanism. MO calculations of cationic localization energies for the interaction of monosubstituted benzenes with the nitronium ion correlate with observed product yields.16... 

Indole (2) undergoes electrophilic substitution preferentially at the b(C3)-position whereas pyrrole (1) reacts predominantly at the a(C2)-position [15]. The positional selectivity in these five-membered ring systems is well explained by the stability of the Wheland intermediates for electrophilic substitution. The intermediate cations from 3 (for indole, 2) and a (for pyrrole, 1) are the more stabilized. Pyrrole compounds can also participate in cycloaddition (Diels-Alder) reactions under certain conditions, such as Lewis acid catalysis, heating, or high pressure [15]. However, calculations of the frontier electron population for indole and pyrrole show that the HOMO of indole exhibits high electron density at the C3 while the HOMO of pyrrole is high at the C2 position [25-28] (Scheme 3). 

Hiickel calculations of localization energies ((3CS = 0.6, as = ac) give the positional orders as 2 (thiophene) > 2 [3,2-6] > 2 [2,3-6] > 3 [2,3-6] > 3 [3,2-6] > 3 (thiophene).[82JCS(P2)295]. The reactivity of the 2-position of thiophene is clearly predicted incorrectly, and the order for the 2-positions of the thienothiophenes reflects that found for chlorination and formylation, which have transition states nearer to the Wheland intermediate. (Localization energy calculations assume, of course, that... 

Spin-coupled studies of Wheland intermediates formed by ring protonation of benzene, phenol and benzonitrile provide ab initio support [7] for the usual qualitative VB arguments used to discuss the energetics and selectivity of aromatic electrophilic substitution reactions. Analogous calculations have also been performed for the reaction between benzene and a methyl cation [9]. 

Applications of MO calculations to studies of reaction mechanisms are of limited value because of the considerable demands placed on the computational method to guarantee reliable results. It is not always easy to formulate the task properly <89CHE1321>. Nevertheless, efforts to elucidate the diazo coupling reaction of imidazole have been assisted by MNDO calculations of the heats of formation of the possible Wheland intermediates <89JCS(P2)2055> (see Section 3.02.5.3.7), and an FMO analysis of the effects of methyl- and benzo-substitution on the facility of proton transfer from C-2 of imidazole, benzimidazole, and their 1-methyl derivatives has thrown light on the mechanism <86CJC1240>. 

MO Calculations and Photoelectron Spectroscopy. Some all-valence-electron CNDO/2 SCF-MO calculations on fluorobenzene, hexafluorobenzene, pentafluoro-anisole, and some derived Wheland intermediates have been reported in a paper which is mainly concerned with derivatives of pyridine and the diazines (see p. 467). An MO-LCAO-SCF study of the electronic structure of fluorobenzene has yielded the electrostatic molecular potential and isopotential maps which are consistent with a poru-directing influence of fluorine in electrophilic substitution, ... 

High-level valence bond calculations revisited the traditional resonance model for substituted Wheland intermediates. Five structures were considered important, the three standard textbook structures in addition to (23) and (24). These additional structures explain why n donation is observed with n donors even when they are in the meta position. Several computational papers addressed the issue of regioselectivity in... 

Nitration of the toluene appears to be a three-step process, as demonstrated with DFT calculations of benzene nitration in the gas phase by Olah and coworkers." The acetyl nitrate readily transfers a nitronium -like moiety to the Jt-system of the toluene close to the para or ortho sites. This is a jt-complex with no real bonding interaction. An sp to sp hybridization of the para or ortho carbon must then occur to create a o-complex (aka Wheland intermediate or arenium cation). Finally, this 0-complex must transfer the proton from the para or ortho site to a Brpnsted acid site in the zeolite wall. Upon optimizing the H-beta structure with the acetyl nitrate site and toluene, it became clear that the most likely acid transfer site after creation of the o-complex is the All-02 site in our model, which, as we already... 

As noted earlier in this chapter, charge densities have been calculated for 6a-thiathiophthen and some of its derivatives localization energies for Wheland-type intermediates in substitution reactions have also been estimated. The results indicate, as has been inferred previously, that the 3-position should be attacked by electrophiles. The localization energies for attack at sulphur are very similar to those for attack at C(3) thus, attack at sulphur may become competitive, and indeed examples of this mode of reaction are known. The theoretical treatment further predicts that attack by nucleophiles should occur at the 2- (or 5-) position, in agreement with the experimental evidence. 

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