Carbanions tosylates

One of the more important approaches to 1-azirines involves a similar base-induced cycloelimination reaction of a suitably functionalized ketone derivative (route c. Scheme 1). This reaction is analogous to route (b) (Scheme 1) used for the synthesis of aziridines wherein displacement of the leaving group at nitrogen is initiated by a -carbanionic center. An example of this cycloelimination involves the Neber rearrangement of oxime tosylate esters (357 X = OTs) to 1-azirines and subsequently to a-aminoketones (358) (71AHC-(13)45). The reaction has been demonstrated to be configurationally indiscriminate both syn and anti ketoxime tosylate esters afforded the same product mixture of a-aminoketones... 

The domino reaction is initiated by the chemoselective attack of the carbanion 2-458 on the terminal ring carbon atom of epoxyhomoallyl tosylate 2-459 to give the alkoxides 2-460 after a 1,4-carbon-oxygen shift of the silyl group. The final step to give the cyclopentane derivates 2-461 is a nucleophilic substitution. In some cases, using the TBS group and primary tosylates, oxetanes are formed as byproducts. 

Selenide anions are powerful nucleophiles that can displace halides or tosylates and open epoxides.233 Selenide substituents stabilize an adjacent carbanion so that a-selenenyl carbanions can be prepared. One versatile procedure involves conversion of a ketone to a bis-selenoketal which can then be cleaved by w-butyl lithium.234 The carbanions in turn add to ketones to give /f-hydroxyselenides.235 Elimination gives an allylic alcohol. 

The Michael reaction is the nucleophilic addition of a carbanion to ot,p-unsaturated carbonyl compounds. It is a useful way to make C-C and C-hetero atom bonds. Karodia s group studied the use of the ionic liquid ethyltri- -butylphosphonium tosylate ( -Bu3PEtOTs) as a solvent for... 

If 3-chloro-N-nitrosopiperidine is used instead of the tosylate, 3,4-dehydro-N-nitrosopiperidine is formed in high yield. Another, perhaps more general, route to the vinylnitrosamines makes use of the acidity of the hydrogens on the a-carbon of nitrosamines (18). The resulting carbanion reacts smoothly with phenyl-selenyl chloride. The adduct is then oxidized with m-chloroper-benzoic acid (19). 

The most important reactions of alkyl substituents a and y to the ring heteroatom are those which proceed via base-catalyzed deprotonation. Treatment of 2- and 4-alkyl heterocycles with strong bases such as sodamide and liquid ammonia, alkyllithiums, LDA, etc., results in an essentially quantitative deprotonation and formation of the corresponding carbanions. These then react normally with a wide range of electrophiles such as alkyl halides and tosylates, acyl halides, carbon dioxide, aldehydes, ketones, formal-dehyde/dimethylamine, etc., to give the expected condensation products. Typical examples of these transformations are shown in Scheme 17. Deprotonation of alkyl groups by the use of either aqueous or alcoholic bases can also be readily demonstrated by NMR spectroscopy, and while the amount of deprotonation under these conditions is normally very small, under the appropriate conditions condensations with electrophiles proceed normally (Scheme 18). 

A very promising new method for converting oxiranes, as well as ketones, into oxetanes has recently been reported. This method uses the carbanion of dimethyl(N-tosyl)sulfoximine and gave good yields in the several cases reported. When this reagent is employed with ketones, oxirane formation is presumably an intermediate stage, but the oxirane is not isolated. The method thus provides an excellent synthesis of spiro-oxetanes from ketones, as the example with camphor in equation (85) shows (79JA6135). 

The formation of carbanions, according to Scheme 6, has been much studied but has proved to be of little preparative use. The benzyl anion, generated from benzyl-dimethylsulphonium tosylate, reacted with acrylonitrile but the addition product was formed in only low yield Similarly the reactive ylid formed by deprotonation of trimethylsulphonium salts has been cathodically generated and trapped by several aldehydes and ketones as well as ethyl maleate and fumarate examples are given in Scheme 7. For the best case (benzophenone), the epoxide was formed in 40%... 

Thus polystyryl carbanions and polyacrylonitrile carbanions prepared by anionic polymerization were reacted with cellulose acetate or tosylated cellulose acetate in tetrahydrofuran under homogenous reaction conditions. The carbanions displaced the acetate groups or the tosylate groups in a S v2-type nucleophilic displacement reaction to give CA-g-PS and CA-g-PAN. Mild hydrolysis to remove the acetate/tosylate groups furnishes the pure cellulose-g-polystyrene (Figure 3). 

Carbanions. These add to the C-3 position of (5)-(l), affording epoxides (3) after intramolecular displacement of the tosylate group and in situ ring closure of the ring-opened intermediate (eq 4). Deprotonation of oxirane (3) leads to rearrangement to cyclopropane derivatives. ... 

There are many examples of BF3-OEt2 promoted openings of (1) by carbanions, including sulfone-stabilized anions, vinylic anions, allylic anions, and phosphonate-stabilized anions. For example, the lithium anion of trans-1,2-Bis(tributylstannyl)ethylene opens (5)-(l) in the presence of BF3-OEt2 in THF at —78°C, affording fra/w-l-(tributylstannyl)-5-tosyl-4-hydroxypent-l-ene in 50% yield the latter is converted into oxirane (3) in 76% yield on treatment with powdered Sodium Hydroxide in monoglyme. ... 

A Sulfone Assisted Carbanion and a Tosyl Group Jointly Take Away a Carbonyl Group... 

In strong bases such as the one provided by sodium hydride and dimethyl sulfoxide (DMSO)—namely, dimsyl sodium—one should expect the formation of carbanions at sites of acidic protons. Ketones are attractive as potential sources of carbanions. However, ketone I features two blocked a carbons, without protons. Conversely, the tosyl group is ill suited for carbanion stabilization. The last functionality one may appeal to is the phenyl sulfone substituent at the end of the jec-pentyl chain. Recent investigations have revealed their potential as carbanion precursors, adding an important feature to their considerable usefulness in organic synthesis That is, sulfones can be removed under such mild conditions that carbonyl groups are not affected, and unconstrained a-sulfonyl carbanions have the unusual quality of retaining the asymmetry of their precursors in a wide variety of experimental conditions. ... 

It seems likely as a consequence that a carbanion may be formed under these experimental conditions at the carbon vicinal to the sulfone group. This anion then would be conveniently placed at a six-carbon-atom distance from the electrophilic carbonyl, thus providing an expedient base to form the C-C bond clearly required for the construction of the six-membered ring of II. However, ketones become tertiary alcohols upon attack by carbon nucleophiles. Consequently, one of the alpha C-C bonds next to this ketone must be broken in order to preserve the ketone of the final product. The electron reorganization that would follow is consistent with a concomitant extrusion of tosylate anion. This is illustrated by three-dimensional structures IV and V (see Scheme 11.1)... 

One may overcome this inpasse by changing the direction of approach of the arylsulfone carbanion to the ketone in IV from the endo to the exo side of the molecule, as depicted in IV (see Scheme 11.3). This is not only sterically feasible, but in addition the alkoxy and tosylate units would be parallel to one another at close to bonding distance. Their collapse to the oxetane postulated previously would occur easily as portrayed in XII - XIII. 

Diazo transfer occurs when azides attached to good leaving groups react with nucleophiles. Use of carbanions is nucleophiles results in the formation of diazo derivatives. The process was first successfully utilized by Doering and DePuy , who reacted cyclopentadienyl lithium with tosyl azide and obtained diazocyclopentadiene (90) in... 

The reaction of alkyllithium reagents with acyclic and cyclic tosylhydrazones can lead to mixtures of elimination (route A) and addition (route B) products (Scheme 22). The predominant formation of the less-substituted alkene product in the former reaction (Shapiro Reaction) is a result of the strong preference for deprotonation syn to the N-tosyl group. Nucleophilic addition to the carbon-nitrogen tosyl-hydrazone double bond competes effectively wiA a-deprotonation (and alkene formation) if abstraction of the a-hydrogens is slow and excess organolithium reagent is employed. Nucleophilic substitution is consistent with an Su2 addition of alkyllithium followed by electrophilic capture of the resultant carbanion. 

Alkylations of boron-stabilized carbanions have been carried out with primary alkyl halides containing acetal, alkene, alkyne, chloride, cyano, ester and tosylate groups, though a ketone group was not tolerated. ... 

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