No-mechanism reactions

By the end of the 1950s the main features of ionic and radical reactions were reasonably well understood, but pericyclic reactions were not even recognized as a separate class. Diels-Alder reactions, and a good many others, were known individually. Curly arrows were used to show where the bonds went to in these reactions, but the absence of a sense of direction to the arrows was unsettling. Doering provocatively called them no-mechanism reactions in the early 1960s. 

We shall discuss several types of no-mechanism reactions first and some-mechanism reactions second. From an orbital viewpoint, all heterolytic rearrangements would be subsumed under sigmatropic reactions heterolytic substitutions and additions (eliminations) turn out to be versions of cycloadditions (cycloeliminations). Nevertheless, we have retained categories on the basis of mechanism, because these are familiar, and because the trend is from the relatively simple to the complex mechanism. 

The possible dilemma here is that a stereoselective or stereospecific radical reaction may be indistinguishable from a no-mechanism reaction. 

There are no charged or electron-deficient intermediates in pericyclic reactions. They used to be called, somewhat facetiously, no-mechanism reactions. However, a mechanism may involve several polar steps as well as one or more pericyclic steps, and in such cases, charged and electron-deficient intermediates may abound. 

The Cope rearrangement, like the Claisen rearrangement, is a no mechanism reaction and thus does not involve ionic or radical intermediates. For practical purposes the result is that Cope rearrangements are independent of catalysts and of the nature of the solvent, and that substituent effects are slight. Steric influences, however, are considerable cw-1,2-divinylcyclobutane rearranges to 1,5-cyclooctadiene within a few minutes at 120° ... 

There are no charged or electron-deficient intermediates in pericyclic reactions. They used to be called, somewhat facetiously, no-mechanism reactions. 

The interpretation of the chemical reactions of Scheme 4 did not merely represent a solution to a longstanding star number in problem seminars at leading universities. As well as this, the veil over the no mechanism reactions had been blown aside. But even more importantly, there was from this time on no longer any convincing justification for special treatment for electronically excited molecules and their chemical transformations in organic chem-... 

The type of pictorial, back-of-the-envelope, application of theory exemplified by the 4n + 2 rule would soon be applied toward understanding a wide range of no-mechanism reactions. The Diels-Alder reaction (see chapter 3) is one example. The reverse reaction ( retro-Diels-Alder ) also occurs readily. In one step (one transition state), three n bonds are broken and two new o bonds and one n bond are created. The simultaneous cleavage and formation of all bonds in one step is referred to as concerted and such reactions often occur under mild conditions and form specific stereoisomers. Since the bonds made and broken form a continuous cycle in the transition state, the Diels-Alder is an example of an electrocyclic reaction. In contrast to the Diels-Alder reaction, dimerizations of two alkene molecules (R2C=CR2) to cyclobutanes typically fail (as do the reverse thermal decompositions of cyclobutanes). These reactions often succeed when ultraviolet light (photochemistry) is used in place of heat (thermal chemistry). 

The Diels-Alder reaction cannot be explained adequately by the familiar polar reaction mechanism involving electron pair interactions or by a free radical mechanism. In fact in older literature, it was sometimes described as a no-mechanism reaction. There are no intermediates, and kinetic studies show that the reaction is first order in both the diene... 

Libby s proposals were essentially non-inechanistic. The formation of ethyl bromide by the recombination of an ethyl radical and a Br-atom was, in the strictest sense of the term, a no-mechanism reaction. Perhaps the difficulty with the early models was that they attempted to explain everything. The approach most prevalent today is to consider each recoil species separately by investigating the nature of the labeled products and the effects of various physical and chemical parameters on the yields of these products. As we shall see later, rationalization of product distribution in terms of mechanism has had considerable success. Application of known mechanisms of displacement reactions, insertion reactions, free radical reactions, etc. to recoil reactions has been quite useful. 

This is another example of what has variously been called a pseudo-Diels-Alder, ene, or no-mechanism reaction (Hoffmann, 1969). It is similar to the reaction written for the attack of rubber molecules by phenolic curatives or the in situ formed nitroso derivative of the quinoid (e.g., benzoquinonedioxime) vulcanization system. It is also closely related to the sulfurization scheme written for accelerated-sulfur vulcanization. Comparisons between accelerated sulfur, phenolic, quinoid, and maleimide vulcanization can then be visualized as follows ... 

Among the numerous reactions that have been developed to date, a certain class, namely the pericyclic reactions, especially stands out for several reasons. Originally termed no-mechanism reactions, these transformations have occupied one of the most prominent positions in organic synthesis. Beside ionic and radical reactions, they constitute the third distinct class of reaction mechanisms. In all kinds of pericyclic reactions - cycloadditions, sigma tropic rearrangements, electrocyclizations, and ene reactions - one can observe cyclic transition states. No intermediates are formed, and all bond-forming and bond-breaking processes take place in concert [1]. 

Hill, R.K. and Rabinovitz, M. (1964) Stereochemistry of No-mechanism reactions Transfer Asymmetry in the reactions of olefins with dieneophiles. 

No Mechanism reactions were also extremely accommodating as regards electron-pushing , a favorite pastime of many organic chemists at that time. In reactions that could be presumed to have some polar character, the directionality of charge transmission could be deduced in a more or less straightforward manner. [9] In contrast, the flow of electrons in the course of the Diels-Alder reaction, for example, could be variously depicted as ... 

In their now classic monograph [1], Wooodward and Hoffmann concentrate on three basic types of no mechanism reaction Electrocyclic reactions -notably polyene cyclizations, cycloadditions, and sigmatropic rearrangements. These three reaction types will be taken up in this and the next two chapters from the viewpoint of Orbital Correspondence Analysis in Maximum Symmetry (OCAMS) [2, 3, 4], the formalism of which follows naturally from that developed in Chapter 4. The similarities to the original WH-LHA approach [5, 6], and the points at which OCAMS departs from it, will be illustrated. In addition, a few related concepts, such as allowedness and forbiddenness , global vs. local symmetry, and concertedness and synchronicity , will be taken up where appropriate. 

To be able to understand the course of no-mechanism reactions, you need only remember that in any concerted (one barrier, one transition state) reaction, a bonding overlap must be maintained between orbitals. The rest is all detail. 

By the mid-1960s, ionic and radical reactions were well studied and their mechanisms largely understood. There were a number of reactions that did not fit into either of these categories. One example is the Diels-Alder reaction, identified in the 1930s. This reaction is notable for its stereocontrol, implying a highly ordered transition state and a concerted mechanism. Another example, electrocyclic ring closure, was identified by R. B. Woodward in his work on the synthesis of vitamin Bj. These reactions and others like them were called no-mechanism reactions. 

It must be kept in mind that although these theories have very broad predictive powers, there are many limitations. Factors such as structural constraints can prevent a reaction that is allowed on an orbital basis. Which direction an allowed reaction will go is not predicted by this theory. Often more than one allowed reaction is possible from given starting materials, and again, these theories seldom make a distinction. Furthermore, nonconcerted mechanisms may operate to give products that are forbidden by these theories, but as been demonstrated with the examples in this chapter, molecular orbitals can be used to rationalize the pericyclic reactions that were once termed no-mechanism reactions. 

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