Molecular-orbital calculations bromination

Most electrophilic substitutions in benzimidazole (31 R = H) occur primarily in the 5-position. In multiple bromination the order followed, 5 > 7 > 6,4 > 2, parallels molecular orbital calculations. In benzimidazole itself the 4(7)- and 5(6)-positions are tautomerically equivalent. Fusion of a benzene ring deactivates C-2 to electrophilic attack to such an extent that it is around 5000 times less reactive than the 2-position of imidazole. Strong electron donors at C-5 direct halogenation to the 4-position, whereas electron-withdrawing groups favor C-4 or C-6 substitution (84MI21). 

Simple resonance theory predicts that pentalene (48), azulene (49), and heptalene (50) should be aromatic, although no nonionic canonical form can have a double bond at the ring junction. Molecular orbital calculations show that azulene should be stable but not the other two, and this is borne out by experiment. Heptalene has been prepared but reacts readily with oxygen, acids, and bromine, is easily hydrogenated, and polymerizes on standing. Analysis of its NMR spectrum shows that it is... 

Ab initio calculations on COBtj 2 symmetry) and the asymmetrically substituted carbonyl halides COCIF, COBrF and COBrCl (which have only C, symmetry) have shown that when a bromine atom is present in the molecule, the highest occupied molecular orbital has bromine lone-pair character [1857aa]. In contrast to this, the HOMO of COCIF has mixed chlorine and oxygen character. The principal orbital characteristics for the most chemically significant orbitals of these molecules are given in Tables 17.3 and 17.4 it must be remembered that the low symmetry means that there is little bar to extensive mixing of the molecular orbitals. 

Molecular orbital calculations point to the 6-position of (184 R = H, R = Et) as the most likely site of electrophilic attack. Conformably to this expectation, nitration at 0°C produces the 6-nitro-derivative (185) nearly quantitatively, and bromination and chloromercuration proceed analogously. Like monocyclic meso-ionic pyrimidines, the meso-ionic thia-zolo[3,2-a]pyrimidines react with dipolarophiles, such as dimethyl acetylenedicarboxylate, to give 7,8-dimethoxycarbonylthiazolo[3,2-a]pyri-din-5-one (186). ... 

Although cyclopropanes are far less reactive than alkenes, they can be opened by various electrophiles including protic acids, bromine, chlorine, mercury(II) salts and acetyl chloride. The ring-cleavage processes of cyclopropanes by electrophiles were studied with the aid of ab initio molecular orbital and other calculations. Early studies assumed that traditional... 

Simple HUckel molecular orbital (HMO) calculations on the pyrrolo[2,l-c][l,2,4]triazole (28) suggest that electrophilic attack would occur most readily at C-10, and this prediction was borne out by observations that acid-catalyzed deuteration and bromination by NBS in the dark both occur at this position <85JCR(S)363>. Various reactivity indices have been calculated for a number of pyrrolo[ 1,2-b][, 2,4]triazoles (29) and pyrrolo[2,1 -c][ 1,2,4]triazoles (30) using the MO LCAO method within the semiempirical SCF approximation. These indicate that the 5-position is most susceptible to electrophilic attack, followed by the 7-position <74CHE230>. 

Work on correlations of rate coefficients with ionisation potentials for the reactions of NO3 with a series of alkenes has been extended. Ionisation potentials are determined using a semi-empirical molecular orbital package (MOPAC 5.0). We are now able to estimate both room-temperature rate coefficients and Arrhenius parameters for the reaction of NO3 with a wide range of alkenes, including those containing vinylic chlorine atoms (which we earlier found difficult to handle). In an effort to improve our understanding of the physical basis of our observations, ab initio calculations have been performed on a limited number of compounds. Such calculations should overcome some of the limitations of the semi-empirical methods when applied to elements such as chlorine and bromine. 

This chapter represents an update to the previous two editions, published in 19771 and 19892, and covers the literature of the period 1989-1994 with some references to 1995 papers. It deals mainly with electrophilic additions across the C=C, C=Si and Si=Si bonds and includes both theoretical (ab initio calculations, orbital approach, molecular modelling etc.) and experimental aspects. Particular attention is paid to mechanistic studies, facial selectivity and neighbouring group participation. Synthetic utilization of electrophilic addition is discussed only if including substantial mechanistic insight purely synthetic work is not covered. Aside from the classical reactions, such as hydration, bromination etc., newly included material comprises aziridination (Section VI), attack at C=C bond by an electron-deficient carbon (Section VII) and those electrophilic reactions which utilize a transition or non-transition metal as the electrophile (Section VIII). 

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