And -sigmatropic rearrangement

A pericyclic reaction is one that takes place in a single step through a cyclic transition state without intermediates. There are three major classes of peri-cyclic processes electrocyclic reactions, cycloaddition reactions, and sigmatropic rearrangements. The stereochemistry of these reactions is controlled by the symmetry of the orbitals involved in bond reorganization. 

Double-bond isomerization can also take place in other ways. Nucleophilic allylic rearrangements were discussed in Chapter 10 (p. 421). Electrocyclic and sigmatropic rearrangements are treated at 18-27-18-35. Double-bond migrations have also been accomplished photochemically, and by means of metallic ion (most often complex ions containing Pt, Rh, or Ru) or metal carbonyl catalysts. In the latter case there are at least two possible mechanisms. One of these, which requires external hydrogen, is called the nwtal hydride addition-elimination mechanism ... 

The fifth chapter l-(2,4,6-Trialkylphenyl)-lH-Phospholes with a Flattened P-Pyramid Synthesis and Reactivity presents the interesting chemistry of these compounds including electrophilic substitution and Diels-Alder reactions and sigmatropic rearrangements, making a variety of organophosphorus compounds accessible. 

With phenylhydroxylamine and an aldehyde, 1,2-propadienyl methyl ketone afforded indole derivatives via the intermediacy of 437 the latter was formed by a sequential addition, in situ aldol condensation and sigmatropic rearrangement process [195]. 

There are several variations of the Claisen rearrangement that make it a powerfid tool for the synthesis of y,<5-unsaturated carboxylic acids. The ortho ester modification of the Claisen rearrangement allows carboalkoxymethyl groups to be introduced at the /-position of allylic alcohols.157 A mixed ortho ester is formed as an intermediate and undergoes sequential elimination and sigmatropic rearrangement. 

The reactions of 1,2,3-triazolium 1-imide (277) with a range of alkene and alkyne dipolarophiles give rise to a variety of new ring systems (Scheme 54). Compounds (276) and (278) are obtained from (277) by reaction with acrylonitrile and DMAD, respectively. These reactions are tandem 1,3-dipolar (endo) cycloadditions and sigmatropic rearrangements which are regio- and stereospecific <90JCS(Pl)2537>. Kinetic and mechanistic studies show that these reactions are dipole-HOMO controlled. The second-order rate constants are insensitive to solvent polarity, the reaction indicates... 

Pericyclic reactions are concerted reactions that take place in a single step without any intermediates, and involve a cyclic redistribution of bonding electrons. The concerted nature of these reactions gives fine stereochemical control over the generation of the product. The best-known examples of this reaction are the Diels-Alder reaction (cyclo-addition) and sigmatropic rearrangement. 

The three principal types of pericyclic reactions are cycloaddition, electro-cyclic rearrangement, and sigmatropic rearrangement ... 

In contrast, applying frontier orbital theory to unimolecular reactions like electrocyclic reactions and sigmatropic rearrangements is inherently contrived, since we have artificially to treat a single molecule as having separate components, in order to have any frontier orbitals at all. Furthermore, frontier orbital theory does not explain why the barrier to forbidden reactions is so high—whenever it has been measured, the transition structure for the forbidden pathway has been 40 kJ mol-1 or more above that for the allowed pathway. Frontier orbital theory is much better at dealing with small differences in reactivity. 

The aromaticities of symmetry-allowed and -forbidden transition states for electrocyclic reactions and sigmatropic rearrangements involving two, four, and six r-electrons, and Diels-Alder cycloadditions, have been investigated by ab initio CASSCF calculations and analysis based on an index of deviation from aromaticity. The order of the aromaticity levels was found to correspond to the energy barriers for some of the reactions studied, and also to the allowed or forbidden nature of the transition states.2 The uses of catalytic metal vinylidene complexes in electrocycliza-tion, [l,5]-hydrogen shift reactions, and 2 + 2-cycloadditions, and the mechanisms of these transformations, have been reviewed.3... 

Recognizing Electrocyclic Reactions, Cycloaddition Reactions, and Sigmatropic Rearrangements... 

Pericyclic reactions are commonly divided into three classes electrocyclic reactions, cycloaddition reactions, and sigmatropic rearrangements. An electrocyclic reaction forms a sigma bond between the end atoms of a series of conjugated pi bonds within a molecule. The 1,3-butadiene to cyclobutene conversion is an example, as is the similar reaction of 1,3,5-hexatriene to form 1,3-cyclohexadiene ... 

The concerted reactions presented in this chapter are called pericyclic reactions. They are divided into electrocyclic reactions, cycloaddition reactions, and sigmatropic rearrangements. Some occur when energy is supplied in the form of heat others require light energy to occur. Most have strict stereochemical requirements. 

The rules for disrotatory electrocyclic reactions, cycloadditions, and sigmatropic rearrangements are summarized in the accompanying chart ... 

Reductive alkylation of 2-methoxybenzoates [12-14], carbon alkylation of the alkali metal salts of arenols [15-19], and sigmatropic rearrangements such as the Claisen reaction of allyl... 

A hetero Diels-Alder reaction of a precursor 1-9 may be involved in the biosynthesis of the lignane carpanone 1-8 (Fig. 1-3), however, there is no proof for such an assumption [32]. On the other hand, it is well known that pericyclic reactions such as electrocyclic reactions and sigmatropic rearrangements occur in nature e.g. in the biosynthesis of vitamine D, vitamine B12 [33-35] and ecto-carpene [36]. 

This rotation is the reason why you must carefully distinguish electrocyclic reactions from all other pericyclic reactions. In cycloadditions and sigmatropic rearrangements there are small rotations as bond angles adjust from 109° to 120° and vice versa, but in electrocyclic reactions, rotations of nearly 90° are required as a planar polyene becomes a ring, or vice versa. These rules follow directly from application of the Woodward-Hoffmann rules—you can check this for yourself. 

After your experience with cycloadditions and sigmatropic rearrangements, you will not be surprised to learn that, in photochemical electrocyclic reactions, the rules regarding conrotatory and disrotatory cyclizations are reversed. 

Applying frontier orbital theory to unimolecular reactions like electrocyclic ring closures and sigmatropic rearrangements is inherently contrived, since we are looking at only one orbital. To set up an interaction between frontier orbitals, we... 

Trifluoroethanol has become a much-used building-block [150] for a wide range of synthetic procedures (see also Chapter 6, Section II) (Figure 8.43b), and sigmatropic rearrangements have been exploited to move the fluorine labels to intemaT sites [151]. 

---