Concerted Pericyclic Sigmatropic Rearrangements

Chapter 12 Qualitative Molecular Orbital Theory Pericyclic Reactions 

Is the following concerted sigmatropic rearrangement thermally allowed  

Answer The process uses three arrows and so is a six-electron (4n-i-2) process. The side view shows that the rearrangement occurs on the same face and with retention. We 

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The Claisen rearrangement is the concerted pericyclic sigmatropic rearrangement which occurs when allyl aryl ethers are heated at high temperatiure without catalysts [1,2]. If an ortho position is unfilled, the reaction leads to o-allylphenols (Figure 1). 

The mechanism is a concerted pericyclic [3,3] sigmatropic rearrangement and accounts for all these facts. For the ortho rearrangement ... 

Chapter 6 looks at concerted pericyclic reactions, including the Diels-Alder reaction, 1,3-dipolar cycloaddition, [3,3]- and [2,3]-sigmatropic rearrangements, and thermal elimination reactions. The carbon-carbon bond-forming reactions are emphasized and the stereoselectivity of the reactions is discussed in detail. 

Pericyclic reactions are concerted reactions that take place in a single step without any intermediates, and involve a cyclic redistribution of bonding electrons. The concerted nature of these reactions gives fine stereochemical control over the generation of the product. The best-known examples of this reaction are the Diels-Alder reaction (cyclo-addition) and sigmatropic rearrangement. 

Pericyclic processes comprise a broad and important class of concerted reactions of both theoretical and practical interest. These transformations, which are especially useful in the construction of carbon-carbon bonds,93 include electrocyclic reactions, sigmatropic rearrangements, and cycloadditions. Because they are not typically subject to general acid-general base chemistry but can be highly sensitive to strain and proximity effects, they are attractive targets for antibody catalysis. 

The concerted reactions presented in this chapter are called pericyclic reactions. They are divided into electrocyclic reactions, cycloaddition reactions, and sigmatropic rearrangements. Some occur when energy is supplied in the form of heat others require light energy to occur. Most have strict stereochemical requirements. 

Many reactions involve a cyclic transition state. Of these, some involve radical or ionic intermediates and proceed by stepwise mechanisms. Pericyclic reactions are concerted, and in the transition state the redistribution of electrons occurs in a single continuous process. In this chapter, we will consider several different types of pericyclic reactions, including electrocyclic transformations, cycloadditions, sigmatropic rearrangements, and the ene reaction. 

Common error alert The tautomerization of an enol to a carbonyl compound appears to be a [1,3] sigmatropic rearrangement at first glance, but the reaction is catalyzed by base or acid and never proceeds by a concerted pericyclic mechanism. These observations will be rationalized in the next section. 

This reaction does not proceed via a Whitmore 1,2-shift, but instead is a concerted pericyclic [3,3] sigmatropic rearrangement that results in a cyclic ketone, which after tautomerisation, gives the phenol. Furthermore, in this reaction a C-0 bond is broken, rather than made, and a C-C bond is formed thus, it could have been considered under carbon/carbon rearrangements. 

Pericyclic reactions are concerted processes that occur by way of a cyclic transition state in which more than one bond is formed or broken within the cycle. The classic example of such a process is the Diels-Alder cycloaddition reaction, one of the most common and useful synthetic reactions in organic chemistry. Cycloaddition reactions, sigmatropic rearrangements and electrocyclic reactions all fall into the category of pericyclic processes, representative examples of which are given in Schemes 3.1-3.3. This chapter will discuss these reactions and their use in synthesis. 

Complex, Multi-step, or Non-concerted Pericyclic Processes.—Pyrolysis of labelled tricyclo[8,2,2,0 ]tetradeca-3(8),ll,13-triene (264 -n = D) at 180°C led to the formation of unlabelled 1,2-dimethylenecyclohexane, [ H jbenzene, 2,3-dideuterio-butadiene, and tetralin labelled at the four aromatic ring positions. The labelling pattern suggests that the 1,2-dimethylenecyclohexane and benzene derive from a common precursor, as do the butadiene and tetralin. The suggested reaction pathway is shown in Scheme 3 and involves preliminary [1,3] sigmatropic or biradical rearrangement of (264) into (265), which in turn is transformed into (266) by intramolecular [4 +2] cycloaddition retro-[4 +2] addition is considered to convert... 

Pericyclic reactions, most notably the Diels-Alder reaction, other cycloadditions, and certain sigmatropic rearrangements in which two or more electron pairs move in a more or less concerted manner along a cyclic pathway are a cornerstone of organic synthesis. Much of their importance derives from the efficiency with which they create two or more bonds in one step and also in a stereospecific manner. Some examples are as follows ... 

Continuous efforts for the elucidation of reaction mechanism of the aromatic Claisen rearrangement have been made since the reaction was discovered [4]. The reaction is considered to be a concerted pericyclic pathway. The first step is the [3,3]-sigmatropic rearrangement (Claisen rearrangement) resulting in the bond... 

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