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Conjugation and Reactions of Conjugated Compounds
Published in Michael B. Smith, A Q&A Approach to Organic Chemistry, 2020
A pericyclic reaction is one in which electrons are transferred within a π-system to form new bonds. This type of reaction generally involves transfer of double bonds from one position to another within a molecule as well as formation of sp3-hybridized carbon–carbon bonds. What does the term [m+n] refer to in a pericyclic reaction?
Atom Economy
Published in Aidé Sáenz-Galindo, Adali Oliva Castañeda-Facio, Raúl Rodríguez-Herrera, Green Chemistry and Applications, 2020
Kunnambeth M. Thulasi, Sindhu Thalappan Manikkoth, Manjacheri Kuppadakkath Ranjusha, Padinjare Veetil Salija, Nisha Vattakkoval, Shajesh Palantavida, Baiju Kizhakkekilikoodayil Vijayan
Pericyclic reactions are concerted reactions in which reorganization of electrons occur via a single cyclic transition state. There are different types of pericyclic reactions, such as cycloaddition, electrocyclic reaction, sigmatropic rearrangement, group transfer reaction, chelotropic reaction and diatropic rearrangement. Pericyclic reactions are 100% atom economical, like simple addition and rearrangement.
Theoretical study of cyclohexadiene/hexatriene photochemical interconversion using spin-Flip time-Dependent density functional theory
Published in Molecular Physics, 2020
The CHD to cZc-HT photochemical interconversion can be depicted qualitatively using the conservation of orbital symmetry, i.e. following the Woodward-Hoffmann rules [36] for a pericyclic reaction. Similar to the description made for the ring closure of 1,3-butadiene in reference [37] using the Woodward-Hoffmann rules, we shall illustrate the photochemical interconversion between CHD and cZc-HT. Figure 2 describes an orbital correlation diagram when the absorption of a photon by the CHD leads to the excitation of an electron from the bonding orbital 2π to the antibonding orbital 3π. The conrotatory (4n+2 electron in π system) path of the excited CHD leads to the excited cZc-HT keeping the same orbital symmetry [37,38]. However, the suggestion of an intermediate excited state cZc-HT generates a problem since it is known that CHD, after photoexcitation, return to either its ground state or to the ground state of cZc-HT, i.e. without the formation of an intermediate excited state cZc-HT [4]. This discrepancy can be resolved if one considers the overall states of CHD and cZc-HT via state correlation diagram presented in Figure 3. For doing so, we used the direct product between the symmetry specie of the individual occupied orbitals satisfying the following rules: S × S = S; S × A = A and A × A = S, where S (with character 1) means symmetric with respect to a symmetry element and A (with character -1) means antisymmetric with respect to a symmetry element. All doubly occupied orbitals are always totally symmetric. Because the conrotatory path preserves the rotation around the C axis, we can classify the relevant states in terms of S and A with respect to this axis [37,38]. Since CHD (1σ,1π,2π) is correlated with cZc-HT (1π,2π,4π) (see Figure 2), it follows that the CHD (1 1 2, S) is correlated with cZc-HT (1 2 4, S), where S denotes the result of the direct product between the symmetry S × S = S and the superscript 1 means that the electrons are in a singlet configuration (see Figure 3).