Anionotropic migration

The reaction of trialkylboranes with iV-chloroalkylamines can be used to synthesize a wide variety of functionally substituted dialkylamines in good yields [66,67], and it complements the synthesis of secondary amines via the reaction of trialkylboranes with organic azides. The reaction is analogous to the reaction of chloramine with organoboranes, and presumably occurs via an anionotropic migration of an alkyl group from boron to nitrogen (Scheme 24). 

Certain vinylphosphonium salts substituted by an electron-withdrawing group (Z = CH= =CH2, Ph, PhCO, RCO) react with aqueous bases to give phosphine oxides resulting from an anionotropic migration of a phenyl group from phosphorus to the a-carbon (SNpmig) (Scheme 42)P< ... 

Substituted biguanides (19) also react with benzil on heating in alcohol in the absence of any catalyst to give 2-substituted guanilydene-5,5-diphenylhydantoins (20), whose formation involves an anionotropic migration of a phenyl group.41 N-Amidino-O-alkylisoureas and 1-aryl- or 1-alkyl-3-amidino-2-thioureas behave similarly.42... 

The structural closeness of 1-boraadamantane (5) and 1-azaadamantane (113) induced the attempts to convert (5) into (113). The first reported synthesis has been carried out according to the following synthetic scheme. Treatment of (5) with A,iV-dichloroamines leads to the formation of an intramolecular complex compound (114). A second anionotropic migration in (114) gives rise to compound (115) standard oxidation of the latter provides 3-alkyl-3-azabicyclo[3.3.1]nonanes (116) (Scheme 42) <8UOM(220)l>. 

The semibenzilic mechanism requires (Scheme 12) a nucleophilic addition at the carbonyl carbon. Then, in a concerted process, the halogen is ejected by the 1,2-anionotropic migration of the CJ-C bond. 

Schlosser670 proposed an equivalent mechanism at the phosphorane 64 stage, formed from the corresponding ylide and not from the phosphonium salt the driving force for the migration comes from the carbenoid nature of the carbon in the a-position to the phosphorus. This is consistent with the anionotropic nature of the migration. The mechanism is corroborated by a similar migration from the ylide 65670 (reaction 198). 

The final product of the deprotonation depends strongly on the deprotonation reagent and/or the reaction conditions. Thus, in THF and using MeLi (or NaH) as base, an anionotropic l,3-Si,0-trimethylsilyl migration occurs in the alkoxymethylsilane 191 with formation of the silyl anion 192 instead of elimination of silanolate. Therefore, after hydrolytic work-up only trimethylsiloxy(bis(trimethylsilyl)silyl)alkanes 193 were obtained (equation 48).108,117... 

A review on anionotropic 1,2-rearrangements of borate complexes evidences the most important factors that are responsible for which group migrates, by showing a broad spectrum of reactions involving borate complexes.17... 

Anionotropic rearrangements and displacement reactions yield allenes Allenes are also formed in basic media (Scheme 3.32). The reaction is particularly easy if the migrating hydrogen is adjacent to a second multiply bonded carbon. 

A number of interesting and useful organic reactions involve isomerizations of substances having one or more carbon-carbon double bonds. This chapter deals with the kinetics of reactions of alkenes, cycloalkenes and substituted alkenes which involve migration of carbon-carbon double bonds, with or without structural alteration of the carbon skeleton of the starting materials. These reactions include prototropic and anionotropic rearrangements, several concerted unimolecular isomerizations such as the Cope and Claisen rearrangements, and a number of non-concerted thermal isomerization reactions. 

Anionotropic rearrangements involve migrations of electronegative groups. They comprise two mechanistically related groups of reactions substitutions and isomerizations. Allylic substitution reactions in which skeletal rearrangement accompanies nucleophilic substitution, viz. 

Allylic alcohols, esters, and ethers undergo acid-catalyzed isomerizations, and allylic alcohols and ethers isomerize at appreciable rates only in the presence of acids. The migrating groups in these reactions are uncharged molecules derived from the oxonium ion conjugate acids of the starting materials. For this reason, and because they are mechanistically related to anionotropic isomerizations, Braude named these reactions oxotropic rearrangements. 

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