Sigma-bond rearrangements

Rearrangements in which a sigma bonded group is transferred from one end of a conjugated n system to the other are called sigma bond rearrangements. Some examples are ... 

Mechanistically this reaction is described as a concerted pericyclic [3,3] sigma-tropic rearrangement. A carbon-oxygen bond is cleaved and a carbon-carbon bond is formed. In a subsequent step the initial product 4 tautomerizes to the stable aromatic allylphenol 3 ... 

The key step in this sequence, achieved by exposure of 46 lo a mixture of sulfuric acid and acetic anhydride, involves opening of the cyclopropane ring by migration of a sigma bond from the quaternary center to one terminus of the former cyclo-l>ropane. This complex rearrangement, rather reminiscent of the i enone-phenol reaction, serves to both build the proper carbon. keleton and to provide ring C in the proper oxidation state. 

Woodward and Hoffmann have given the name of sigmatropic rearrangements to such reactions because they involve the movement of a sigma bond which is adjacent to one or more it system. 

An entirely different situation is found in alkynyl halides. Here only examples in which the rearrangement is supported by carbonyl groups [106, 164] or electron-withdrawing pentafluorophenyl groups [165] are known. As one example, the selective reaction of the alkynyl bromide next to the carbonyl group, in the presence of a second alkynyl bromide with a propargylic C-O sigma bond, is shown (88 —> 89) [164] (Scheme 1.38). 

Oxidative desulfonylation Chlorodimethoxyborane, 73 Oxidative hydrolysis see Hydrolysis) Oxy-Cope rearrangement see Sigma-tropic rearrangements) Oxymercuration see Addition reactions to carbon-carbon multiple bonds)... 

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 ... 

In addition to rearrangements such as the previous two, it is possible for both of the components to contain pi bonds. In the following reaction the carbon-oxygen bond is broken and the new sigma bond is formed between the carbons at the other end of each of the components. This is an example of a [3,3] sigmatropic rearrangement. 

Marks has reviewed this area. Basset has bound organometallic species to a silica surface and seen interesting alkane reactions that go via initial CH bond activation this step probably proceeds by a sigma bond metathesis pathway since the metals involved are d . Snbseqnent reactions inclnde CC bond cleavage and carbon skeletal rearrangements. 

Since its discovery in 1912, the Claisen rearrangement of allyl vinyl ethers (1 - 2 equation 1) has become one of the most powerful tools for stereoselective carbon-carbon bond formation. Much of its current popularity is due to the subsequent development of a series of new variants of this 3,3-sigma-tropic rearrangement. In 1940, Carroll reported the base-catalyzed rearrangement of -keto esters and allylic alcohols to alkenic ketones (3 4 5 equation 2). Twenty years later. Saucy and Marbet dem-... 

Saccharin, 363 Salicylic acid, 430 Sandmeyer reaction, 416 Saturated compounds, 50 Saytzeff, 91, 130 Schiff base, 411 Schmidt rearrangement, 406 Secondary carbon, 54 Semicarbazide, 315 Sigma bond, 15 Sigma complex, 205 Silyl ethers, 286 Singlet carbene, 32 Skraup reaction, 459... 

In the non-aromatic Claisen-type rearrangements, a sigma bond breaks, a sigma bond is formed, and two t-bonds migrate concertedly. The basic reaction is illustrated below... 

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