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O zależnościach pomiędzy aromatycznością i efektem podstawnikowym w układach jednopierścieniowych

Szatyłowicz Halina, Krygowski Tadeusz Marek

Wiadomości Chemiczne

2019

[Z] 73, 3-4

243--261

Aromaticity/aromatic and substituent/substituent effects belong to the most commonly used terms in organic chemistry and related fields. They are used for more than a century, and so far are the subject of thousands publications a year. The quantitative description of the aromaticity of planar π-electron cyclic molecules is based on four criteria: (i) they are more stable than their acyclic unsaturated analogues, (ii) bonds have intermediate lengths between those for the single and double ones, (iii) external magnetic field induces π-electron ring current, and (iv) aromatic systems prefer reactions in which the π-electron structure is preserved. conserved. Quantitative characteristics based on these criteria, named as aromaticity indices, allow to relate aromaticity to the substituent effect. This latter can be described using either traditional Hammett-type substituent constants or characteristics based on quantum-chemistry. For this purpose, the energies of properly designed homodesmotic reactions and electron density distribution are used. In the first case, a descriptor named SESE (substituent effect stabilization energy) is obtained, while in the second case – cSAR (charge of the substituent active region), which is the sum of the charge of the ipso carbon atom and the charge of the substituent. The application of these substituent effect descriptors to a set of π-electron systems, such as: benzene, quinones, cyclopenta- and cyclohepta-dienes, as well as some azoles, allowed to draw the following conclusions: (i) The less aromatic the system, the stronger the substituent influences the π-electron system. Highly aromatic systems are resistant to the substituent effect, in line with the organic chemistry experience that aromatic compounds dislike reactions leading to changes in the π-electron structure of the ring. (ii) Intramolecular charge transfer (resonance effect) is privileged in cases where the number of bonds between the electron-attracting and electron-donating atoms is even. These effects are much weaker when this number is odd. Classically, it may be related to traditional para vs meta substituent effects in benzene derivatives. We should note that in electron-accepting groups, such as CN or NO2 (and others), electron-accepting atoms are second counting from Cipso. (iii) In all cases, when the substituent changes number of π-electrons in the ring in the direction of 4N+2, its aromaticity increases, for example electron-donating substituents in exocyclic substituted pentafulvene, or a halogen atom in complexes with heptafulvene.

Nowe spojrzenie na efekt podstawnikowy

Szatyłowicz H., Krygowski T. M.

Wiadomości Chemiczne

2017

[Z] 71, 7-8

497--516

Classical view on the substituent effect (SE) is associated with an empiric approach presented 80-years ago by Hammett [1]. He proposed a simple formula to represent the effect of a substituent upon the rate or equilibrium constants of a reaction in which the reacting group is in a side chain attached to the ring and introduced quantitative descriptors of the SE named substituent constants σ, defined in terms of dissociation constants of meta- and para- substituted benzoic acids. Then the Hammett’s equation relied on using them to describe SE for various physicochemical properties, P(X), by means of linear regression like P(X)=ρ·σ, where ρ is so called reaction constants describing sensitivity of a system in question on the SE. Application of the quantum chemistry modeling allowed to find descriptors (independent of empirical approaches) which are characterized by clear physical meaning and are accessible by use of standard computational packages. The oldest descriptor is based on homodesmotic reaction [X-R-Y + R = R-X + R-Y] in which energy of products is subtracted from that of substrates [32]. The model is named as SESE (substituent effect stabilization energy) and its values are usually well correlated with empirical constants σ, or their modifications. Ten years ago Sadlej-Sosnowska introduced [23, 24] an effective descriptor of SE based on atomic charges of a substituent X and the ipso carbon atom named cSAR(X) (charge of the substituent active region). Unlike atomic charges at substituent, q(X), the cSAR(X) values correlate well with the Hammett substituent constants [25]. Recently as an interesting and showing new aspects descriptor of SE appeared a model making use of population of electrons at sigma and pi orbitals of planar pi-electron systems (or their fragments), named as sEDA and pEDA [33]. Again in particular cases these descriptors correlate with the Hammett σ. This descriptor allowed to reveal how strong is SE on population of pi-electron systems in substituted derivatives of benzene, and how much is this different for para and meta substituted species. Analysis of the relation of pEDA vs sEDA for meta and para substituted derivatives of nitrobenzene revealed that sEDA values increase with a decrease of electronegativity of the linking atom [47]. The above mentioned action of the sigma structure is modulated by the remaining part of the substituent as well as its pi-electron structure. This part of substituents (including also the linking atom) is responsible for an interplay of the sigma structure with the pi-electron one. Application of cSAR(X) for series of meta- and para- substituted phenol and phenolate derivatives [36] revealed that reverse substituent effect, i.e. the effect of impact of the functional group Y on the electron accepting/donating power of the substituent in systems like X-R-Y may be as large as the overall differences in these kind of properties between NO and NMe2! In the σ constants scale this is full range of σ for uncharged substituents, 1.73 units of σ. Application of cSAR for CH2 groups in 1-X-bicyclo[2.2.2]octane derivatives and using the regression of cSAR(CH2) against cSAR(X) values allowed to document that substituent effect in these systems is inductive in nature [39]. In summary, substituent effect descriptors based on quantum chemistry modeling are usually consistent with the empirical ones, but are able to present more detailed information on physical aspects of the problem.