Cycloaddition and electrocyclization

Scheme 13.1. Some Examples of Photochemical Cycloaddition and Electrocyclic Reactions...

Scheme 13.1 lists some example of photochemical cycloaddition and electrocyclic reactions of the type that are consistent with the predictions of orbital symmetry considerations. We will discuss other examples in Section 13.4. 

Density functional theory and MC-SCF calculations have been applied to a number of pericyclic reactions including cycloadditions and electrocyclizations. It has been established that the transition states of thermally allowed electrocyclic reactions are aromatic. Apparently they not only have highly delocalized structures and large resonance stabilizations, but also strongly enhanced magnetic susceptibilities and show appreciable nucleus-independent chemical-shift values. 

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... 

A general cheletropic reaction is shown in Figure 12.2. This reaction involves the addition to, or extrusion from, a conjugated system of a group bound through a single atom. The reaction usually involves the elimination of simple stable molecules such as SO2, CO, or N2. The atom to which there were two a bonds carries away a pair of electrons, usually in a spn hybrid orbital. The addition of a carbene to a simple olefin to form a cyclopropane is also a cheletropic reaction which, as discussed in Chapter 14, is not predicted to be concerted. Cheletropic reactions incorporate features of both cycloaddition and electrocyclic reactions. 

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

Photochemical activation can be used to achieve forward or reverse cycloadditions and electrocyclic reactions that are thermodynamically unfavorable or have unfavorable concerted thermal mechanisms. Thus the... 

This chapter follows on from chapter 12 where we introduced some basic ideas on stereocontrol. Since then we have met many stereospecific reactions such as pericyclic reactions including Diels-Alder (chapter 17), 2 + 2 photochemical cycloadditions (chapter 32), thermal (chapter 33) cycloadditions, and electrocyclic reactions (chapter 35). Then we have seen rearrangements where migration occurs with retention at the migrating group such as the Baeyer-Villiger (chapters 27 and 33), the Amdt-Eistert (chapter 31) and the pinacol (chapter 31). 

The decomposition of cyclic sulfones and sulfoxides has attracted increasing attention recently in view of their relationship to cyclic fragmentation reactions and of the dramatic impact of the Woodward-Hoffman rules for intermolecular cycloaddition and electrocyclic ring opening reactions. The simplest cyclic sulfone known is 2,3-diphenyl thiirene-1,1-dioxide (A) which was isolated by Carpino and McAdams by the Ramberg-Backlund base catalyzed dehydrohalogenation of a, a -dibromodibenzylsulfone, viz. 

The isoelectronic pyridinium A-ylides, e.g. 110, behave in an analogous manner to the A-oxides. Photolysis in an inert solvent leads to the diazepine 112 as the sole product, probably with intermediate formation of a diaziridine 111, followed by electrocyclic N/C-1 cleavage. Photolysis in benzene yields, in addition to the diazepine 112, the azepine 113 because of a competitive N-N cleavage and interaction of the solvent with the resulting nitrene (by a [2+1] cycloaddition and electrocyclic opening of the diazanorcaradiene) ... 

A route to a 1,4-azaphosphinine is reported through a sequence involving cycloaddition and electrocyclic ring expansion (Scheme 4) <1996CEJ68>. 

The photochemistry of alkenes and dienes has already been examined in part, since these compounds are particularly illustrative of the principles of orbital symmetry control in electrocyclic processes. The orbital symmetry rules for cycloadditions and electrocyclic processes were covered in Section 11.2. Cycloadditions are also considered, from a synthetic viewpoint, in Part B, Chapter 6, Section 6.2. This section will emphasize unimolecular rearrangements of alkenes and dienes. 

Beyond Diels-Alder reactions of indole-2,4-qui-nodimethanes to construct indoles, there is a wealth of indole ring synthesis involving Diels-Alder cycloadditions and electrocyclizations that do not begin with an indole or indolone. Indeed, a number of ingenious indole ring syntheses starting from scratch are known. 

Some examples of photochemical cycloaddition and electrocyclic reactions... 

The Diels-Alder reaction couples the ends of two separate tt systems. Can rings be formed by the linkage of the termini of a single conjugated di-, tri-, or polyene Yes, and this section will describe the conditions under which such ring closures (and their reverse), called electrocyclic reactions, take place. Cycloadditions and electrocyclic reactions belong to a class of transformations called pericyclic (peri, Greek, around), because they exhibit transition states with a cyclic array of nuclei and electrons. 

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