Rearrangements and Electrocyclizations

Cycloadditions, the subject of the last chapter, are just one of the three main classes of peri-cyclic rearrangement. In this chapter, we consider the other two classes—sigmatropic rearrangements and electrocyclic reactions. We will analyse them in a way that is similar to our dealings with cycloadditions. 

Improved reaction and activation energies of [4 + 2] cycloadditions, [3,3] sigmatropic rearrangements and electrocyclizations with the spin-component-scaled MP2 method137... 

So far, cycloadditions have been our only examples of pericyclic reactions. There are several other classes of pericyclic reactions, of which the most notable are cheletropic reactions, sigmatropic rearrangements and electrocyclic reactions. In essence, frontier orbital theory treats each of them as a cycloaddition reaction. 

Rate Constants and Gibbs Activation Energies of Walk Rearrangements and Electrocyclic Ring Openings in Ester-Substituted and Nitrile-Substituted Dimethylbicyclopentene Derivatives (25)... 

Formation of C-C Bonds by Sigmatropic Rearrangements and Electrocyclic Reactions... 

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. 

Sol 1. Isomerization of substituted cycloheptatrienes takes place through sequences of [1,5] sigmatropic rearrangements and electrocyclic reactions. 

The direct connection of rings A and D at C l cannot be achieved by enamine or sul> fide couplings. This reaction has been carried out in almost quantitative yield by electrocyclic reactions of A/D Secocorrinoid metal complexes and constitutes a magnificent application of the Woodward-Hoffmann rules. First an antarafacial hydrogen shift from C-19 to C-1 is induced by light (sigmatropic 18-electron rearrangement), and second, a conrotatory thermally allowed cyclization of the mesoionic 16 rc-electron intermediate occurs. Only the A -trans-isomer is formed (A. Eschenmoser, 1974 A. Pfaltz, 1977). 

The best way to understand how orbital symmetry affects pericyclic reactions is to look at some examples. Let s look first at a group of polyene rearrangements called electrocyclic reactions. An electrocyclic reaction is a pericyclic process that involves the cycli/ation of a conjugated polyene. One 7r bond is broken, the other 7t bonds change position, a new cr bond is formed, and a cyclic compound results. For example, a conjugated triene can be converted into a cyclohexa-diene, and a conjugated diene can be converted into a cyclobutene. 

Thus in the N-silyl substituted series, 17 and 18, which rearrange thermally to the corresponding diazo compounds, the stability increases through the series R=Me, Ph, i-Pr. As discussed below, these compound undergo the usual cycloaddition and electrocyclization reactions of nitrile imines and are not simply overstabilized curiosities. The usefulness in synthesis of those with P—C bonds is probably limited since these bonds are not easily broken, but products derived from those with C—Si and C—B bonds (e.g., 21 and 22) should be capable of further... 

Orbital correlation diagrams are useful for cycloadditions and electrocyclic reactions but not for sigmatropic rearrangements since no element of symmetry is preserved. 

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