Pericyclic Rearrangements

Strategy XVI Pericyclic Rearrangements in Synthesis Special Methods for Five-Membered Rings... 

Many pericyclic rearrangements show a pressure-induced acceleration which is characterized by a negative volume of activation157. The effect, which is usually smaller than that of intermolecular cycloadditions, may be explained with different packing coefficients of cyclic and acyclic states as already discussed for the pericyclic and stepwise cycloadditions. 

Polyenes are most often synthesized by cross-coupling reactions between unsaturated systems. Typically these reactions require an activated carbon, often in the form of an organometallic reagent. Enolates and phosphonium ylides, Wittig-type reagents, are also commonly employed in carbon-carbon bond formation. Pericyclic rearrangements also result in the generation of new carbon-carbon bonds and will be treated separately. 

The mechanism of nitrosation of MeCOCH2 Na+ with MeONO to give MeCOCH=NOH has been studied by HF and MP3 ab initio methods pericyclic rearrangement of first-formed adduct gives an intermediate complex, [MeCOCH2NO(OMe)] Na+, from which the product is obtained by antiperiplanar elimination of MeOH. 

Pericyclic reactions are unimolecular, concerted, uncatalyzed transformations. They take place in a highly stereoselective manner governed by symmetry proper-ties of interacting orbitals. - Characteristic of all these rearrangements is that they are reversible and may be effected thermally or photochemically. The compounds in equilibrium are usually interconverted through a cyclic transition state,224 although biradical mechanisms may also be operative. A few characteristic examples of pericyclic rearrangements relevant to hydrocarbon isomerizations are presented here. 

Experimental and computational approaches to elucidating mechanistic details in pericyclic rearrangements have been reviewed,1 as have preparations of alkenes by rearrangement reactions.2 The formation of saturated carbon atoms with no attached heteroatoms by sigmatropic or electrocyclic rearrangement reactions has been reviewed.3... 

Electrocyclic reactions are a type of pericyclic rearrangement reaction where the net result is a conversion of one Tt-bond into a G-bond (Scheme 9.16). These reactions can be either photochemically or thermally induced. Selected examples of the potential of photoinduced 6jt-electrocyclization for organic synthesis are shown below. 

The focus of the section on silene reaction kinetics is mainly on studies of bimolecular reactions of transient silene derivatives, because little absolute kinetic data exist for the reactions of stable derivatives and there have been few quantitative studies of the kinetics of unimolecular isomerizations such as ,Z-isomerization and pericyclic rearrangements, although a number of examples of such reactions are of course well known. In contrast, most of the studies of disilene reaction kinetics that have been reported have employed kinetically stable derivatives, and E,Z-isomerization has thus been fairly well characterized. The paucity of absolute rate data for unimolecular isomerizations of transient silenes and disilenes is most likely due to the fact that it is comparatively difficult to obtain reliable data of this type for transient species whose bimolecular reactions (including dimerization) are so characteristically rapid, unless the unimolecular process is itself relatively facile. Such instances are rare, at least for transient silenes and disilenes at ambient temperatures. 

The coupling of the betaines (352) with diazonium salts, however, may take an unusual course in that arylation results. The reaction may be rationalized by the formation of a covalent intermediate (353) which subsequently suffers a pericyclic rearrangement involving expulsion of nitrogen (81H(15)1349). 

Having read Chapter 36, you should be able to classify the pericyclic rearrangement reaction it is a [2,3]-sigmatropic rearrangement (make sure you can see why before you read further) and as such is the first of the pericyclic rearrangements of sulfoxides that we shall talk about. 

Provide a reasonable mechanism for the following reaction that has a pericyclic rearrangement as its last step. 

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