Radicals pericyclic process

Domino radical reactions consisting of radical/cationic, radical/anionic, radi-cal/radical, and radical/pericyclic processes have become increasingly important... 

The mechanism of the Diels-Alder cycloaddition is different from that of other reactions we ve studied because it is neither polar nor radical. Rather, the Diels-Alder reaction is a pericyclic process. Pericyclic reactions, which we ll discuss in more detail in Chapter 30, take place in a single step by a cyclic redistribution of bonding electrons. The two reactants simply join together through a cyclic transition state in which the two new carbon-carbon bonds form at the same time. 

For the reason of comparison and the development of new domino processes, we have created a classification of these transformations. As an obvious characteristic, we used the mechanism of the different bond-forming steps. In this classification, we differentiate between cationic, anionic, radical, pericyclic, photochemical, transition metal-catalyzed, oxidative or reductive, and enzymatic reactions. For this type... 

Abstract In this chapter different types of domino-processes are described which consist of the combination of cationic, anionic, radical, pericyclic and transition metal-catalyzed as well other reactions. The methodology is used for the highly effective synthesis of carbocycles and heterocycles as well as of natural products and other interesting materials. It is also employed as an efficient tool in combinatorial chemistry. 

Although the synthetic utility of radical anion pericyclic processes is still to be explored, the recently disclosed intermediacy of radical anions in metathesis reactions with Grubb s catalyst [360] should ignite the search for further examples of this interesting class of reactions. 

To the best of our knowledge no experimental kinetic and thermodynamic studies have been performed on radical anion pericyclic processes. Interesting structural information has, however, been obtained for some [2 - - 2] cycloadducf , e.g. 4N/5e radical anions [361],... 

It is conceivable that the difference in the Doering and Cooke results can be attributed to the isopropyl group in a-thujene which would make the non-allylic radical center tertiary in the proposed intermediate. In the Cooke and Andrews work the corresponding radical center would be secondary. Possibly the difference is enough to make the biradical longer lived and better able to achieve geometrical equilibrium in the experiments of Doering and coworkers. Whether the stereochemistry observed by Cooke and Andrews is the result of involvement of one or more pericyclic processes or whether it reflects dynamic phenomena in the biradical (see next section) is an open question. 

The interconversion of butadiene radical cations and ionized cyclobutene represents a model case for a formal pericyclic process. Much work has been invested to study not only the distinguishability of these isomers and their derivatives by mass spectrometry, but also to check the role of orbital symmetry in the ionic species. Hass has addressed the latter problem in depth in a review on pericyclic reactions in radical cations in both the gas and condensed phases and no further survey on the papers mentioned there will be given here. The topic pertains also to the ring-opening of ionized benzocyclobutene to ionized ortho-quinodimethane (cf Section V) and various otha- phenyl-, methyl- and carboxy-substituted derivatives. In this context, we restrict ourselves hwe mentioning that an upper limit of 7 kcalmol only has been detemined by CE mass spectrometry for the activation barrier of the cycloreversion of the parent cyclobutene radical cations. The energy requirement for the cycloreversion of ionized 1- and 3-substituted cyclobutenes were found, by experiment, to be markedly different. Obviously, dissociation of the (in a sense bis-allylic) strained C—C bond is much more facile when the substituent is at C-3,... 

Stereoelectronic control by steric constraints Several examples provided in the following sections desalbe the application of steric factors for shaping the reactant conformational profile towards prodnclive, stereoelec-tronically activated geometries. We will illustrate the generality of these principles by providing an example of such control in cationic, anionic (both benzylic carbanion and enolates), radical, and pericyclic processes. 

As was the case in our earlier publications, the total syntheses described herein were classified according to the first step of the domino process they feature. Hence, the distinction has been made between cationic, anionic, radical, pericyclic, transition-metal-catalyzed, and reductive or oxidative domino processes. 

The major value of free radical group increments is in the prediction of stabilities of proposed radical and biradical intermediates in various thermal and photochemical reactions. For example, we might want to determine whether an observed thermal rearrangement occurs homolytically, via biradical intermediates, or by a concerted, pericyclic process. One valuable piece of information is AH° of the proposed biradical intermediate. If it is too high for the biradical to lie on the reaction path, then the biradical route can be rejected. We will see examples of this type of analysis in Chapter 15. 

While the Bronsted acid/base terms specifically refer to proton donors and acceptors, respectively, the Lewis approach (named after G. N. Lewis, who introduced the idea in 1923) greatly broadens the definitions of what is an acid and what is a base. Recall that a Lewis acid is an electron pair acceptor and a Lewis base is an electron pair donor. All common organic reactions that do not involve radicals or concerted pericyclic processes can in some manner be discussed as Lewis acid-base reactions. Similarly, all these reactions can be considered to be occurring between electrophiles and nucleophiles. Recall that an electrophile is any species seeking electrons and a nucleophile is any species seeking a nucleus (or positive charge) toward which it can donate its electrons. In this context, a Lewis base is synonymous with a nucleophile, and a Lewis acid is synonymous with an electrophile it just de-... 

A major problem in interpreting pericyclic reactions of radical ions is the difficulty of distinguishing between concerted and nonconcerted paths. The same difficulty occurs in the case of thermal reactions, but the problem here is more acute since there is less scope for the application of subtle experimental techniques. Thus cyclohexene (6) undergoes a retro-Diels-Alder reaction to (7) in the mass spectrometer but the products formed do not enable us to distinguish between a two-step process via the intermediate radical ion (8) and a pericyclic process involving the cyclic transition state (9). 

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