Rearrangement carbanionic

It is to be noted that after the formation of the carbanion the migrating group goes to the carbanion carbon without a electron pair. This is in sharp contrast to re-arrangements in carbocations where the migrating group moves with the electron pairs. The above rearranged carbanion may combine with a proton to complete the reaction. 

The methods developed for the synthesis of 1-alkynylphosphonates use synthetic concepts such as charge affinity inversion and positive halogen abstraction. Four main reaction categories allow the preparation of 1-alkynylphosphonates the apparent Michaelis-Arbuzov and Michaelis-Becker reactions, the propargyl phosphite-allenylphosphonate rearrangement, carbanionic displacements at quinquevalent phosphorus centers SNPfV), and conversions of vinyl- to alkynylphosphonates by addition-elimination reactions. 1-Alkynylphosphonates possess remarkable potential, and cycloaddition reactions appear to offer very attractive synthetic procedures. ... 

Addition of lithiated trimethylsilyldiazomethane to aldehydes provided adduct 103 which was shown through in situ IR measurements and quenching experiments to be in equilibrium with the 1,3-Brook-rearranged carbanion 104. Addition of methanol or methyl iodide provided 105 and 106, respectively. Stereocontrolled hydride shift upon formation of the rhodium-stabilized carbenoid provided (2)-selective formation of silyl enol ethers 107 and 108. ... 

OC-All lation of Carbonyl Compounds and Derivatives. The organoborate iatermediates generated by the reaction of alkylboranes with carbanions derived from a-halocarbonyl compounds and a-halonitriles rearrange to give a-alkylated products. 

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... 

Perfluoropolyenes also can rearrange to four-membered ring products upon fluoride ron or Lewis acid catalysis [112, II3, 114] (equations 46 and 47) These intramolecular cycloadditions are multistep processes involving carbanion or carbocation intermediates... 

The carbanionic species thus formed is protonated to give the final product 3. The use of an alkoxide as base leads to formation of a carboxylic ester as rearrangement product use of a hydroxide will lead to formation of a carboxylic acid salt ... 

The rearrangement of an ether 1 when treated with a strong base, e.g. an organo-lithium compound RLi, to give an alcohol 3 via the intermediate a-metallated ether 2, is called the Wittig rearrangement. The product obtained is a secondary or tertiary alcohol. R R can be alkyl, aryl and vinyl. Especially suitable substrates are ethers where the intermediate carbanion can be stabilized by one of the substituents R R e.g. benzyl or allyl ethers. 

Acrylamides represent still another interesting class of monomers.6 Their anionic polymerization may be initiated by strong bases, like, e.g., amides. The growing chain contains the unit —CH2—CH —CO—NH2 and intramolecular proton transfer competes efficiently with its carbanionic growth. Since the rearrangement... 

More than two dozen radical clocks are now known. They span a range of lifetimes from 10 to 10"7 s.12 The investigator must be aware of the possibility that the clock rearrangement is due to a side reaction or that the radical induced an efficient chain mechanism (Chapter 8). Also, radicals are not the only entities that can rearrange in this fashion. Carbanions, for example, have been shown to rearrange under certain conditions. 

The reaction of the phenylsulphinyl allylic lithium a-carbanion 342 with oxiranes was found by Guittet and Julia to give, after rearrangement and desulphurization, dihydroxy-dienes 343427 (equation 197). Demoute and coworkers have described the alkylation reaction of a very sophisticated 2-alkenyl sulphoxide 344 as a part of the total synthesis of a juvenile hormone 345428 (equation 198). Since the allylic sulphoxide carbanion has an ambident character, the alkylation may occur sometimes also at the y-position. This direction of alkylation is observed in the case of acyclic allylic sulphoxide anions 346, and results in the formation of the corresponding allylic sulphoxide 347 and vinylic sulphoxide 348423 (equation 199). 

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