And electrocyclic rearrangements

Thermally or Photochemically Induced Sigmatropic and Electrocyclic Rearrangements... 

Geometrical isomerization and sigmatropic and electrocyclic rearrangements are all observed in the family of vitamin D, as shown for the case of calciferol (vitamin D2, 7) in Scheme 4.5 (Jacobs and Havinga, 1979 Mermet-Bouvier, 1973). 

The cation (26) was obtained by protonation of the corresponding azulene in dichloromethane. A tropylium ion-mediated a-cyanation of amines was described. The key step is a hydride transfer from the amine to the cation, resulting in cycloheptatriene and an iminium ion, the latter then reacting with cyanide to give the aminonitrile. The dehydrofropylium-Co2(CO)6 ion has been prepared as a BF4 salt. Various measures suggest that the ion is weakly aromatic, with about 25% of the aromaticity of the tropylium ion. Computational analysis of a number of annulenes predicts that the Mobius dication (CH)i4+ should be stable under persistent ion conditions. In particular, this dication is stable towards reactions such as cis-trans isomerization and electrocyclic rearrangement that limit the lifetime of other Mobius annulenes. 

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 ... 

In a number of rearrangements of siienes the Si=C bnd has been observed to react with a C—H bond of a methyl group that is usually attached at the ortho position of an adjacent mesityl group. Formally, these can be regarded as 2tt + 2cr reactions, although other descriptions may be possible. For example, a 1,5-H shift followed by an electrocyclic rearrangement of a Si=C with a C=C would effect the same results. Little is known about the mechanisms involved. Several examples of these types of reaction are described below, some being effected photochemically and some thermally. 

One surprising feature emerged from the reactions of methanesul-phonyl azides with aromatic solvents at 120 °C, and that was the total absence of any IV-sulphonylazepine derivatives (45) from the reaction products. The latter would have been expected from the electrocyclic rearrangement of 42 of the same type as had been observed in the reac-... 

Reaction of 2-amino-benzyl alcohol and 2-chloroH-phcnylaminopyrimidinc forms the intermediate cation 204, which contains ene and iminium functionalities and undergoes electrocyclic rearrangement to the 2-phenylamino-6//-pynmido[2,l-2 ]quinazoline 205 (Scheme 32). The cation 204 is stabilized by the aryl groups. The 2-NHPh stmcture of the product was confirmed by 111 NMR spectroscopy <2002TL1303>. 

Note that the observed high stereospecificity of the reactions in equations (70) and (71) also points to a rapid coupling of ion pair after the electrocyclic rearrangement of the DBC cation radicals in the contact ion pairs. 

SCHEME 22 Activation and reaction volumes of electrocyclic rearrangements... 

The formation of cycloheptadienones from alkoxy(cyclopropyl)carbene complexes and alkynes (Entry 5, Table 2.24) [388,389] proceeds essentially by the same mechanism as the Dotz benzannulation reaction (see Figure 2.32). The cyclopropyl group participates in the electrocyclic rearrangement as the equivalent of a vinyl group. 

The sulfur analogue of the Hauser ortho-substitution rearrangement provides access to an arylacet-ic NSAID. Reaction of the aminobenzophenone 176 with ethyl methylthioacetate and tert-butvl hypochlorite gives the intermediate 178. The reaction probably proceeds by way of formation of the S-chlorinated sulfonium derivative 177 displacement on sulfur will lead to the salt 178. Treatment with triethylamine leads initially to the betaine 179. Electrocyclic rearrangement of that transient intermediate leads, after rearomatization, to the homoanthranilic acid 180. Internal ester-amine interchange leads then to indolone 181 [45]. The thiomethyl group is then removed with Raney nickel. Saponiflcation of intermediate 182 affords bromfenac (183) [46J. 

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