Pericyclic mechanisms, drawing

Common error alert Students often draw fictional one-step pericyclic mechanisms for reactions that require more than one step. A good guard against this sort of error is to be sure that you can name any pericyclic step in your mechanism. For example, for the following reaction, you may be tempted to draw a four-electron, one-step mechanism involving the C C cr bond of the first starting material and the it bond of the second. Such a mechanism, however, cannot be classified in the scheme that has just been discussed, and therefore it is likely to be incorrect. 

Pericyclic reactions can proceed under acidic or basic conditions. For example, the oxy-Cope rearrangement is greatly accelerated under basic conditions, and the Diels-Alder reaction is greatly accelerated by Lewis acids. Often a series of polar reactions is used to synthesize an unstable intermediate, which then undergoes a pericyclic reaction to reveal the product. In other words, a good command of polar mechanisms (Chapters 2 and 3) is essential to understanding how to draw pericyclic mechanisms. 

Pericyclic mechanisms are undoubtedly the hardest for students to draw. The superficial similarity of the mechanistic types, the way in which seemingly reasonable steps are disallowed by theoretical considerations, the simultaneous formation of many bonds, the lack of a clearly reactive center— all these features of pericyclic reactions combine to make them anathema to many students. You can learn some useful techniques to help you work through... 

General considerations when drawing pericyclic mechanisms ... 

Whether a reaction requires stoichiometric or catalytic quantities of a transition metal has a strong bearing on how one draws a mechanism for the reaction. The mechanism of a reaction that requires stoichiometric quantities of the metal can be written in a linear fashion like a polar or pericyclic mechanism. However, the mechanisms of metal-catalyzed reactions are usually drawn in a circular fashion, showing how the original metal species is regenerated at the end of each catalytic cycle. Whether a mechanism is catalytic or stoichiometric in metal is usually clear from the reaction conditions. 

The object of this paper is to draw attention to the possible importance of concerted molecular reactions, of the type termed pericyclic by Woodward and Hoffman (1), in the mechanism of coal liquefaction. 

Another class of reaction where you can see at once that the disconnect ion is the reverse of the reaction is Pericyclic Reactions. An example would be the Diels-Alder reaction between butadiene and maleic anhydride. Draw the mechanism and the product. 

Stereospecificity, the property that the stereochemistry of the starting materials determines the stereochemistry of the product, is one of the hallmarks of pericyclic reactions. It is possible to draw two-step nonconcerted, polar or free-radical mechanisms for many pericyclic reactions, but these two-step mechanisms fail to account for the stereospecificity of the reactions. For example, a two-step polar mechanism can be drawn for the Diels-Alder reaction between 2-methoxybutadiene (a nucleophile) and ethyl cA-crotonatc (an electrophile). This mechanism proceeds through a dipolar intermediate in which one new cr bond has formed. In this intermediate, there is free rotation about the two C atoms of the dienophile, so the cis stereochemical relationship between the Me and CC Et groups is expected to be lost in the product. In fact, though, the product is exclusively cis. This finding does not completely rule out a polar mechanism— it is possible that the intermediate exists but that ring closure occurs more quickly than rotation about the cr bond—but it does limit the lifetime of the dipolar intermediate to such an extent that one can say practically that it does not exist. 

Stereospecificity, the property that the stereochemistry of the starting materials determines the stereochemistry of the product, is one of the hallmarks of pericyclic reactions. It is possible to draw two-step polar or free-radical mechanisms for many pericyclic reactions, but these two-step mechanisms fail to account for the stereospecificity of the reactions. For example, a two-step polar mechanism can be drawn for the Diels-Alder reaetion between... 

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