Carbanions 2,3 -sigmatropic

The first asymmetric synthesis of an a-mercapto y-unsaturated phosphonate (203) using the readily available chiral dimenthylphosphonyl ester group and a carbanionic [2,3] sigmatropic rearrangement was achieved. Absolute configuration of the newly formed chiral centre of nonracemic thiol (203) was determined. 

Well known carbanionic sigmatropic rearrangements, applied to mixed P and S compounds, regio- and/or stereoselectively lead to new (a-sulfanylalkyl) or P-sulfanylaryl) phosphonates, phosphine oxides, or phosphorodiamidates. In these difunctional compounds, chirality can be either introduced on the phosphorus, on the a-carbon, or on the sulfur atom. 

Thia-[2,3]-Wittig sigmatropic rearrangement of lithiated carbanions 47, obtained by deprotonation of the S-allylic sulfides 46, affords the thiols 48 or their alkylated derivatives 49. The corresponding sulfonium ylides 51, prepared by deprotonation of the sulfonium salts 50 also undergoes a [2,3]-sigmatropic shift leading to the same sulfides 49 [36,38] (Scheme 13). As far as stereochemistry is concerned, with crotyl (R R =H,R =Me) and cinnamyl (R, R =H,R =Ph) derivatives, it has been shown that the diastereoselectivity depends on the nature of the R substituent and on the use of a carbanion or an ylide as intermediate. 

Subsequently, Kametani and coworkers observed a similar allylic sulfoxide-sulfenate-sulfoxide rearrangement. These authors reported the exceptionally facile ringopening reaction of condensed cyclobutenes facilitated by arylsulfinyl carbanion substituents. For example, treatment of sulfoxide 68 with butyllithium in tetrahydrofuran at — 30°C for 10 min, followed by normal workup, results in the formation of product 71, which can be explained by the intervention of a double [2,3]-sigmatropic rearrangement of the initial product 69 via 70 (equation 32). A similar double [2,3]-sigmatropic rearrangement of 1,4-pentadienylic sulfoxides has also been reported by Sammes and coworkers. ... 

Deprotonation of allylic aryl sulfoxides leads to allylic carbanions which react with aldehyde electrophiles at the carbon atom a and also y to sulfur . With benzaldehyde at — 10 °C y-alkylation predominates , whereas with aliphatic aldehydes at — 78 °C in the presence of HMPA a-alkylation predominates . When the a-alkylated products, which themselves are allylic sulfoxides, undergo 2,3-sigmatropic rearrangement, the rearranged compounds (i.e., allylic sulfenate esters) can be trapped with thiophiles to produce overall ( )-l,4-dihydroxyalkenes (equation 24). When a-substituted aldehydes are used as electrophiles, formation of syn-diols 27 occurs in 40-67% yields with diastereoselectivities ranging from 2-28 1 (equation 24) . ... 

The rearrangements of allylic sulfoxides, selenoxides, and amine oxides are an example of the first type. Allylic sulfonium ylides and ammonium ylides also undergo [2,3]-sigmatropic rearrangements. Rearrangements of carbanions of allylic ethers are the major example of the anionic type. These reactions are considered in the following sections. 

Synthetically valuable [2,3]-sigmatropic rearrangements include those of allyl sulfonium and ammonium ylides and a -carbanions of allyl vinyl ethers. 

In sharp contrast to carbonium ions, the carbanions undergo (1, 2) sigmatropic shifts very rarely if ever. An example is as given below ... 

Ethyl cyanoacetate in the presence of piperidine may also be used as the carbanionic component in reactions with salicylaldehyde. The initial Knoevenagel condensation is followed by a [l,7]-sigmatropic shift and cyclization to the 2-iminochromene derivative which adds another cyanoacetate molecule (67AP1). 

These rearrangement reactions are interpretable in terms of [2.3] sigmatropic shifts of the intermediate ylides. A number of such rearrangements of open-chain systems have been described, involving sulfonium ylides [43] [44] [45], ammonium ylides [46] [57], anions in a-position to oxygen (Wittig rearrangement) [48] [49], and fluorenyl carbanions [50]. 

When we come to use the Woodward-Hoffmann rules on these [2,3]-sigmatropic rearrangements, we find something new. We have a K bond and a o bond and a carbanion. How are we to represent a carbanion (or a carbocation) that is just a p orbital on an atom The new symbol we use for a simple p orbital is to. A carbanion is an component and a carbocation is an m0 component as it has zero electrons. If the two new bonds are formed to the same lobe of the p orbital of the carbanion, we have an m2s component but, if they are formed to different lobes, we have an m2a component. 

Treatment of the silole 19 with at least one Si-H function with KH leads first to penta-coordinated anion 20, followed by an allowed [l,5]-sigmatropic hydrogen migration (formally [l,2]-shift) to form the carbanion 21 <1994JA8428, 19990M3813>. The latter reacts with an electrophile to give the 2,5-dihydrosilole derivative 22 as a mixture of two diastereomers. 

The book is divided into three parts. Part I deals with typical complex organic reactions such as (i) reactions involving carbocations and carbanions, (i/) Pericyclic and electrocyclic reactions and (ii/) Sigmatropic and Chelotropic reactions. This part also includes material useful for characterization of products from structural point of view such as Geometrical isomerism, Stereochemistry and Conformation. Part II is concerned with spectroscopic methods of structure determination such as U.V.,... 

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