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A Detailed Study on Pericyclic Reactions 1Raj Kumar Pandey, 2Dr

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A Detailed Study on Pericyclic Reactions 1Raj Kumar Pandey, 2Dr ISSN: 2348-0033 (Online) ISSN : 2249-4944 (Print) IJEAR VOL . 3, ISSU E 1, JA N - JUN E 2013 A Detailed Study on Pericyclic Reactions 1Raj Kumar Pandey, 2Dr. Neelima Jain CMJ University, Shillong, Meghalaya, India Abstract result of a compounding of the long-established sigma designation In organic chemistry, there exist Pericyclic reactions, stereochemistry from single carbon-carbon bonds and the Greek word tropos, of which depends on symmetry of MOs (Molecular Orbitals) and meaning turn. In this type of rearrangement reaction, a substituent not on the overall symmetry of the molecules. So, in this paper, moves from one part of a π-bonded system to another part in an we have done a detailed study on Pericyclic Reactions. intramolecular reaction with simultaneous rearrangement of the π system. Fig. 3 shows Sigmatropic rearrangement. Keywords Pericyclic Reactions, Molecular Orbits, Organic Chemistry, Stereo Chemistry I. Introduction Pericyclic reactions are concerted reaction that proceeds via a cyclic transition state having no distinct intermediates in the reaction, in this type of reaction, Bond forming and bond breaking steps are simultaneous but not necessarily synchronous [2]. Fig. 3: Sigmatropic rearrangement There are various methods to analyze Pericyclic reactions. Some of them are as follows: D. Group Transfer Reactions • Orbital symmetry correlation method (Woodward, Hoffmann, The concerted transfer of a group from one molecule to another Longuet-Higgins and Abrahamson) due to concomitant movement of a σ-bond (from one molecule to • The frontier orbital method (Woodward, Hoffmann and another) and formation of a new σ-bond (between two molecules) Fukui) at the expense of a π-bond is generally referred to as group transfer • Transition state aromaticity method (Dewar and Zimmerman) Pericyclic reaction. [1] II. Types of Pericyclic Reactions There are three types of Pericyclic reactions discussed below: A. Electrocyclic Reaction Formation of single bond between ends of a linear π system or the reverse process. Fig. 1 shows Electrocyclic Reaction. Fig. 4: E. Cheletropic Reactions Cheletropic reactions are a special class of cycloadditions or retro- cycloadditions in which the two σ-bonds are either made or broken to the same atom. Fig. 1: Electrocyclic Reaction B. Cycloaddition Two or more unsaturated molecules (or parts of the same molecule) Fig. 5: combine with the formation of a cyclic adduct in which there is a net reduction of the bond multiplicity. The resulting reaction F. Dyotropic Rearrangements is a cyclization reaction. Many but not all cycloadditions are The pericyclic reactions which involve concerted intramolecular concerted. Fig. 2 shows Cycloaddition. migration of two σ-bonds simultaneously are known as dyotropic rearrangements. However dyotropic reactions can also occur stepwise. Type 1: Fig. 2: Cycloaddition C. Sigmatropic Rearrangement The net result is one σ-bond is changed to another σ-bond in an uncatalyzed intramolecular process. The name sigmatropic is the Fig. 6: Two Migrating Groups Interchange Their Relative www.ijear.org INTERNATIONAL JOURNAL OF EDUCATION AND APPLIED RESEARCH 31 IJEAR VOL . 3, ISSU E 1, JA N - JUN E 2013 ISSN: 2348-0033 (Online) ISSN : 2249-4944 (Print) Positions Fig. 8: Molecular Orbitals Type 2: IV. Theories and Interpretation A. Conservation of orbital symmetry (Woodward− Hoffmann) This is the first theory that successfully explained and predicted the outcome of Pericyclic reactions. It correlates all the relevant orbitals in the starting material(s) and product(s). Fig. 7: The σ-Bonds are Migrated to New Bonding Sites Without any Positional Interchange for Groups [5]. B. Aromatic Transition State (Dewar-Zimmerman) This is an easy concept to apply to all reaction types, but it’s not Pericyclic Reactions have some rules known as symmetry rules so easy to understand why it is valid, especially in comparison which are required and are discussed in Table 1. with the FMO approach that we will generally choose [4]. Table 1: Symmetry Rules for Pericyclic Reactions [7] V. Conclusion Electron In 1950, two chemists, Otto Diels and Kurt Alder, discovered Electron State Stereochemistry Pairs general method of preparing cyclohexene derivatives that is now known as the Diels-Alder reaction. The Diels-Alder reaction is one Even Antara-con type of a broader class of reactions that are known as pericyclic Ground State number reactions. Pericyclic reactions are the concerted reactions involving Odd number Supra-dis reorganization of electrons which occur by the way of a single Even Supra-dis cyclic transition state. Excited State number Odd number Antara-con References [1] S. Sankararaman,"Pericyclic Reactions", [Online] Available: III. Frontier Molecular Orbital (FMO) http://www.nptel.iitm.ac.in/courses/IIT-MADRAS/ Frontier Molecular Orbital Theory was developed in the 1960s Engineering_Chemistry_III/pdf/5_Pericyclic%20reactions. by Kenichi Fukui who recognized that chemical reactivity can pdf often be explained in terms of interacting Highest Occupied [2] Eugen S. Andreiadis,"Stereochemical Aspects of Pericylclic MOs (HOMOs), Lowest Unoccupied MOs (LUMOs) and Singly Reactions", May 2003, [Online] Available: http://www. Occupied MOs (SOMOs). andreiadis.net/files/Andreiadis_Pericyclic-Reactions.pdf [3] Pericyclic Reactions, [Online] Available: http://www.chem. HOMO + LUMO Bonding MO purdue.edu/courses/chm651mal/Ch5_Pericyclic_Rxns.pdf HOMO + HOMO Antibonding MO [4] Tim Wallace,"Pericyclic Reactions (CHEM30412)", [Online] LUMO + LUMO Null interaction Available: http://www.personalpages.manchester.ac.uk/ SOMO + SOMO Bonding MO staff/T.Wallace/30412tw/PDFs/CHEM30412_TW_Lect1. pdf The FMO approach was developed by Woodward & Hoffmann [5] "Photochemistry and Pericyclic Reactions", National Open in the late nineteen sixties who used it to explain an apparently University of Nigeria, [Online] Available: http://www. diverse set of reactions involving π-systems, including Diels-Alder nou.edu.ng/noun/NOUN_OCL/pdf/edited_pdf3/CHM%20 cycloaddition. Hoffmann used the approach to explore transition 414%20Photochemistry%20and%20Pericyclic%20 metal complexes [6]. Fig. 8 shows Molecular Orbitals. reactions.pdf [6] Pericyclic Reaction Chemistry, [Online] Available: http:// www.fatih.edu.tr/~besat/Teaching/CHEM%20548/ Pericyclic%20Reaction%20Chemistry.pdf [7] Pericyclic Reactions, [Online] Available: http://courses. chem.psu.edu/chem39/Slides/Section11.pdf 32 INTERNATIONAL JOURNAL OF EDUCATION AND APPLIED RESEARCH www.ijear.org ISSN: 2348-0033 (Online) ISSN : 2249-4944 (Print) IJEAR VOL . 3, ISSU E 1, JA N - JUN E 2013 Table 2: Comparison of Different types of Pericyclic Reactions [5] Type Change in no. of σ bonds Change in no. of π bonds Description A cyclic product is formed; Cycloaddition +2 -2 may be intermolecular or intramolecular. Electrocyclic +1 -1 Intramolecular Intramolecular; Sigmatropic 0 0 migration of a σ-bond; rearrangement of π-electrons Intermolecular transfer of a group; migration of a σ-bond from one Group transfer +1 -1 molecule to another; formation of new σ-bond at the expense of one π-bond. A cyclic product is formed; two σ-bonds are formed Cheletropic +2 -1 (π-bond) -1 (lone pair) to same atom; a lone pair is disappeared dyotropic 0 0 Simultaneous migration of two σ-bonds www.ijear.org INTERNATIONAL JOURNAL OF EDUCATION AND APPLIED RESEARCH 33.
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