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Classification of Pericyclic Reactions ____________________________________________________________________________________________________ Subject Chemistry Paper No and Title 9; Organic Chemistry-III Module No and Title 27; Classification of Pericyclic Reactions Module Tag CHE_P9_M27 CHEMISTRY PAPER No. 9; Organic Chemistry-III MODULE No. 27; Classification of pericyclic reactions ____________________________________________________________________________________________________ TABLE OF CONTENTS 1. Learning Outcomes 2. Introduction 3. Classification of pericyclic reactions 3.1 Electrocyclic reaction 3.2 Cycloaddition reactions 3.3 Sigmatropic rearrangements 3.4 Group transfer reactions 4. Woodward-Hoffmann rules and pericyclic reactions 5. Summary CHEMISTRY PAPER No. 9; Organic Chemistry-III MODULE No. 27; Classification of pericyclic reactions ____________________________________________________________________________________________________ 1. Learning Outcomes After studying this module, you shall be able to • Know what are pericyclic reactions • Learn about classification of pericyclic reactions • Identify electrocyclic reaction, cycloaddition and sigmatropic shifts • Evaluate application of Woodward-Hoffmann rules to pericyclic reactions • Analyze which type of pericyclic mechanism is operative in a reaction 2. Introduction Based on mechanism, chemical reactions are broadly classified as ionic, free radical and pericyclic reactions. Pericyclic reactions are a unique set of reactions that takes place through a cyclic transition state in a concerted fashion and exhibit high levels of stereospecificity. Additionally, for pericyclic reactions no intermediates have been isolated and the reactions are free from changes in solvent polarity, free radical generators and even catalysts (although lately Lewis acid catalysis has been reported for some reactions). All these reactions are potentially reversible in nature. Unlike ionic reactions, for pericyclic reactions there is no definite sense of direction to the movement of electrons as the electrons move in a cyclic manner. There is also a difference between a synchronous reaction and a multi-stage concerted process as in synchronous reaction all bond-making and bond-breaking events take place simultaneously, but in a multi-stage concerted process some events precede others without producing an intermediate state. Most of the pericyclic reactions are concerted and may or may not be synchronous. Though some of the pericyclic reactions occur spontaneously, but in major of them introduction of energy either in the form of heat or light is required. Moreover product depends on the source of energy used. 3. Classification of pericyclic reactions All pericyclic reactions share common features discussed above. They are classified into following four categories 3.1 Electrocyclic reactions 3.2 Cycloaddition 3.3 Sigmatropic rearrangements 3.4 Group transfer reactions Let us understand each type of these reactions in detail CHEMISTRY PAPER No. 9; Organic Chemistry-III MODULE No. 27; Classification of pericyclic reactions ____________________________________________________________________________________________________ 3.1 Electrocyclic reactions Electrocyclic reactions are characterized by the creation of a ring from an open chain conjugated system, with a σ bond forming across the ends of the conjugated system or opening of a cyclic system under ring strain to give rise to a diene. Electrocyclic reactions are unimolecular in nature. Most of the electrocyclic reactions are ring closing and a few are ring opening in conformationally strained systems such as cyclobutene. Electrocyclic ring closing reactions have been observed for cationic and anionic polyene species such as allyl cations as well. If we look at the stereochemistry of electrocyclic reactions, in the following reaction one stereoisomer gave rise to one specific product only. H CH3 CH3 CH3 CH3 H trans, cis, trans triene cis H CH3 CH3 CH3 H CH3 trans, cis, cis triene trans CHEMISTRY PAPER No. 9; Organic Chemistry-III MODULE No. 27; Classification of pericyclic reactions ____________________________________________________________________________________________________ The stereospecificity of electrocyclic reaction can be explained based on Frontier molecular orbital (FMO) theory, especially considering the HOMO of the triene, the mode of bond rotation decides stereochemistry of product formed. There are two known modes of bond rotation namely conrotation and disrotation as shown below. If we consider termini of the HOMO of the triene system, it can be seen that the end groups must rotate in a disrotatory manner (twist in opposite directions, when observed front-on) to form the bond. On the other hand, when photoactivated, an electron moves from the HOMO of triene to the next orbital, the LUMO (now this orbital contains an electron and it is no longer unoccupied). In this photo excited system ring will close in the opposite direction as compared to the thermal system and moreover the groups will conrotate i.e., they will twist in the same way to form the sigma bond. As electrocyclic reactions are reversible, whether ring opening/closing will take place is determined by thermodynamics. However, the stereochemistry of product formed is not influenced by the thermodynamic stability rules. In another example, thermal ring opening reaction of 3,4-dimethylcyclobutene (which may exist as cis and trans isomers) gives hexadiene (which may be cis, cis/ cis, trans/ trans, trans isomers). In the reaction, cis 3,4-dimethylcyclobutene on ring opening gave only cis, trans product, while trans 3,4-dimethylcyclobutene isomer, leads to formation of trans-trans diene. These reactions take place via conrotatory motion about the C3-C4 bond in a four-member cyclic transition state. CHEMISTRY PAPER No. 9; Organic Chemistry-III MODULE No. 27; Classification of pericyclic reactions ____________________________________________________________________________________________________ 3.2 Cycloaddition reaction Cycloaddition reactions involve concerted combination of two π-electron systems to form a ring system. Cycloaddition reactions are described as [i+j] additions, when a system of “i” conjugated atoms combines with a structure consisting of “j” conjugated atoms. In cycloaddition reactions carbon–carbon bond formation takes place without any use of a nucleophile or electrophile. Among the pericyclic reactions, cycloadditions are most abundant and useful set of reactions. [4+2] cyclization is most common type of cycloaddition reaction .Diels-Alder reaction falls in this category of cycloaddtion reactions. The utility of this reaction lies in two simultaneous C-C bond formations, which, with suitable substrates may lead to creation of four new stereogenic centers in the product. In Diels-Alder terminology the two reactants are referred to as the diene and the dienophile. OAc OAc H3COOC COOCH3 [4+2] + Benzene, 80 °C COOCH3 COOCH3 OAc OAc Diene Dienophile Adduct Cycloaddition reactions involve bond formation at the ends of both the reacting molecules. This requires proper orientation of orbitals so that effective overlap may give rise to product formation. Reactions where the new bonds being formed are on the same surface are called suprafacial on that component. For the [4+2] cycloaddition reaction, since the new bonds are being formed at the same surface so the reaction is suprafacial on both the diene and dienophile as well. Following are various arrangements of orbitals in [4+2] π system that may give rise to suprafacial and antarafacial orbital overlap to give rise to bond formation. 2 1 2 1 1' 1' 3 3 Suprafacial components 4 2' 2' 4 2 1 2 1 1' 1' 3 3 Antarafacial components 4 2' 4 2' Orbital orientation for [4+2] cycloaddition CHEMISTRY PAPER No. 9; Organic Chemistry-III MODULE No. 27; Classification of pericyclic reactions ____________________________________________________________________________________________________ On the other hand, antarafacial reaction requires bond formation between orbitals at two different surfaces. Almost all pericyclic cycloaddition reactions go by suprafacial mode on both the components as antarafacial overlap would require a most unusually long and flexible conjugated system. This suprafacial approach therefore makes stereochemistry of products predictable. That is, if the diene with substituents at each end forms new bonds to one surface, the product will have a preserved stereochemistry. 1,3 dipolar cycloaddition are another important class of cycloaddition reactions. In this reaction, molecules with 1,3 dipoles (azides, ozone, nitro compounds) react with alkenes or alkynes or their hetero atom substituted analogues in a [3+2] cycloaddition fashion, giving rise to a cyclic products. As an example of [3+2] cycloaddition, substituted ethyne undergoes 1,3-dipolar addition with azides to give triazoles. N N N N 0 °C, 12-16h 98 % COOMe COOMe For photochemical cycloaddition if the total number of electrons is 4n then the cycloaddition is feasible. As a result, alkenes give four member ring in photo cycloaddition reactions by self coupling or by cross coupling. Chelotropic reactions are a special group of cycloaddition in which the two bonds are made or broken to the same atom. Thus sulfur dioxide adds to butadiene to give an adduct, for which the sulfur has provided a lone pair to one of the σ bonds and has received electrons in the formation of the other. + :SO2 SO2 It is an oxidative addition to the sulfur dioxide, changing oxidation state of
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