Rearrangement special

Trialkyl esters of phosphonic acid exist ia two structurally isomeric forms. The trialkylphosphites, P(OR)2, are isomers of the more stable phosphonates, 0=PR(0R)2, and the former may be rearranged to resemble the latter with catalytic quantities of alkylating agent. The dialkyl alkylphosphonates are used as flame retardants, plasticizers, and iatermediates. The MichaeUs-Arbusov reaction may be used for a variety of compound types, including mono- and diphosphites having aryl as weU as alkyl substituents (22). Triaryl phosphites do not readily undergo the MichaeUs-Arbusov reaction, although there are a few special cases. 

Special reactions of side-chain substituents rearrangements involving three-atom side chains... 

The rearrangement (automerization) of Dewar thiophene 5-oxide (61), observed by NMR, occurs so much more rapidly than that of the corresponding episulfide that special mechanisms have been invoked. The one which involves a zwitterionic intermediate (Scheme 108) is favored over a pseudopericyclic sulfur-walk mechanism in which the electrons of the carbon-sulfur o--bond and the pair of electrons on sulfur exchange places as the sulfur atom migrates around the ring (80JA2861). 

Each of the following carbocations can rearrange to a cation with special stabilization. Indicate likely routes for the rearrangement to a more stable species for each ion. 

These reactions do not appear to involve free carbocations, because they proceed effectively in nucleophilic solvents that would successfully compete with halide or similar anions for free carbocations. Also, rearrangements are unusual under these conditions, although they have been observed in special cases. 

Kubota and co-workers describe a novel oxidative rearrangement of the diosphenol (58) of 17iS-hydroxyandrost-4-ene-2,3-dione to the A-nor-A -1,2-diketone (59) in 33 % yield by the action of specially prep d manganese dioxide in boiling acetone. The rate of ring contraction is very sensitive to the source of the oxidant, and a trace of dilute sulfuric acid in the reaction mixture causes oxidative fission of ring A. 

Espenson and Livesey have solved the second-order kinetics problem (in the special case) when A is unknown. Combine Eqs. (2-16) and (2-51) and rearrange to give... 

Comforth has reviewed literature reports and independently studied the special cases of reaction of 1 with salicylaldehyde and with 2-acetoxybenzaldehyde. Coumarins (10) are afforded in the condensation of 1 with salicylaldehyde or its imine, whereas when 2-acetoxybenzaldehyde is used, acetoxy oxazolone 12 is the major product. The initial aldol condensation product between the oxazolone and 2-acetoxybenzaldehyde is the 4-(a-hydroxybenzyl)oxazolone 11, in which base-catalyzed intramolecular transacetylation is envisioned. The product 9 (R = Ac) can either be acetylated on the phenolic hydroxy group, before or after loss of acetic acid, to yield the oxazolone 12, or it can rearrange, by a second intramolecular process catalyzed by base and acid, to the hydrocoumarin, which loses acetic acid to yield 10. When salicylaldehyde is the starting material, aldol intermediate 9 (R = H) can rearrange directly to a hydrocoumarin. Comforth also accessed pure 4-(2 -hydroxyphenylmethylene)-2-phenyloxazol-5(4//)-one (13) through hydrolysis of 12 with 88% sulfuric acid. 

Oxaziranes which carry an aryl group on nitrogen occupy a special position. Sometimes, they rearrange even at room temperature, so they cannot easily be isolated. For preparative purposes, rearrangement in hot xylene is suitable. For the rearrangements of iV-alkyl-oxa-... 

The pinacol rearrangement reaction is of limited synthetic importance although it can be a useful alternative to the standard methods for synthesis of aldehydes and ketones." Especially in the synthesis of ketones with special substitution pattern—e.g. a spiro ketone like 5—the pinacol rearrangement demonstrates its synthetic potential ... 

The vinylcyclopropane rearrangement is an important method for the construction of cyclopentenes. The direct 1,4-addition of a carbene to a 1,3-diene to give a cyclopentene works only in a few special cases and with poor yield. The desired product may instead be obtained by a sequence involving the 1,2-addition of a carbene to one carbon-carbon double bond of a 1,3-diene to give a vinylcyclopropane, and a subsequent rearrangement to yield a cyclopentene ... 

The epoxidation of divinyl carbinol constitutes a special case of a dienol epoxida-tion, as the starting diene is not conjugated (Scheme 9.10). Desymmetrization by SAE, followed by a Payne rearrangement, furnishes the vinylepoxide in high yield and with excellent enantioselectivity (compare Table 9.2, Entry 1) [43]. 

This possibility can be dismissed22 because it is asymmetric. Read backward, the first step would be not the second-order axial addition of Cl" but a unimolecular rearrangement, and an improbable one at that. Aside from this, the fact that the pathway is not symmetric rules it out. If it is invalid in the special case of an exchange, then it can hardly provide the general pathway for substitution at square-planar metal centers. 

Nucleophilic substitutions of halogen by the addition-elimination pathway in electron-deficient six-membered hetarenes by sulfinate anions under formation of sulfones have been described earlier120. The corresponding electron-poor arenes behave similarly121 (equation 30). A special type of this reaction represents the inverse Smiles rearrangement in equation 31122. 

Rearrangements have been included in which sulfones participate not only as reactants but also as products. Reactions have been classified according to mechanism, but although the main emphasis has been on mechanism and stereochemistry, special attention to synthetic applications has also been given, wherever appropriate. Obviously, due to space limitations as well as the vast amount of work available, only selected and representative results of general importance, as judged by the concern of the reviewer, are presented below. Thus, the exclusion of a particular piece of work in no way passes judgement on its scientific value. 

The rearrangement of furfuryl benzenesulfinate (1) appears of special interest. In contrast with the corresponding benzyl ester, this sulfinate was found to undergo a facile rearrangement to sulfone. Furthermore, in nonhydroxylic solvents a mixture of furfuryl phenyl sulfone (2) and 2-methyl-3-furyl phenyl sulfone (3) is obtained (equation 6)40. 

Finally, it should be of interest that the rearrangements of several cyclic benzylic sulfinates have also been described in the literature by Durst46 and Hogeveen47, and seem to proceed by a special two-step mechanism retro-Diels-Alder extrusion of S02, followed by its chelotropic addition to the unstable quinodimethane intermediate (e.g. equation 10). 

Rearrangements have been included in which sulfoxides participate not only as reactants but also as products. Reactions have been classified according to mechanism, but although the main emphasis has been on mechanism and stereochemistry, special attention to synthetic applications has also been given wherever appropriate. 

We will focus on the development of ruthenium-based metathesis precatalysts with enhanced activity and applications to the metathesis of alkenes with nonstandard electronic properties. In the class of molybdenum complexes [7a,g,h] recent research was mainly directed to the development of homochi-ral precatalysts for enantioselective olefin metathesis. This aspect has recently been covered by Schrock and Hoveyda in a short review and will not be discussed here [8h]. In addition, several important special topics have recently been addressed by excellent reviews, e.g., the synthesis of medium-sized rings by RCM [8a], applications of olefin metathesis to carbohydrate chemistry [8b], cross metathesis [8c,d],enyne metathesis [8e,f], ring-rearrangement metathesis [8g], enantioselective metathesis [8h], and applications of metathesis in polymer chemistry (ADMET,ROMP) [8i,j]. Application of olefin metathesis to the total synthesis of complex natural products is covered in the contribution by Mulzer et al. in this volume. 

Trifluoropropene is a compound of special interest in this series. Some transformations of the intermediate telomer radicals had been observed in this case, which prompted to study this reaction in more details. In addition to telomers C6H5CH2(CH2CHCp3)nBr (T Br, n = 1,2), the authors have found compound PhCH=CHCH(CF3)CH2CH2Cp3 and explained its formation by rearrangement of the intermediate radical with two monomer units followed by easy... 

An intramolecular mechanism of the rearrangement has been shown in the special ESR study (refs. 21, 22), conducted on the model radicals, generated by abstraction of a bromine atom from T2 " the rate constant K2, equal to (5.0 + 0.3) X 104 sec- at 22°C, has been also determined. In addition, fragmentation of radical... 

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