Dianion intermediate

In aqueous alkaline conditions with chloroacetic acid the pyrido[4,3- f]pyrimidinethione (80) undergoes facile ring opening, attributed to the resonance stabilization of a delocalized covalent hydrate dianion intermediate (81) (82). Pyrido[2,3- f]pyrimidine-4-thiones (and... 

Reduction Conversion of Nitriles into Amines Reduction of a nitrile with LiAIH4 gives a primary amine, RNH . The reaction occurs by nucleophilic addition of hydride ion to the polar C=N bond, yielding an imine anion, which still contains a C=N bond and therefore undergoes a second nucleophilic addition of hydride to give a dianion. Both monoanion and dianion intermediates are undoubtedly stabilized by Lewis acid-base complexafion to an aluminum species, facilitating the second addition that would otherwise be difficult Protonation of the dianion by addition of water in a subsequent step gives the amine. 

Because hydride ion is a base as well as a nucleophile, the actual nucleophilic acyl substitution step takes place on the carboxylate ion rather than on the free carboxylic acid and gives a high-energy dianion intermediate. In this intermediate, the two oxygens are undoubtedly complexed to a Lewis acidic aluminum species. Thus, the reaction is relatively difficult, and acid reductions require higher temperatures and extended reaction times. 

Under the conditions of the Birch reduction, IV-Boc amides such as 60 can be reductively alkylated in high yields, presumably via a dianion intermediate which is protonated by ammonia at C-5 leaving an enolate anion at C-2 <96JOC7664>. Quenching the reaction with alkyl halides or ammonium chloride then affords the 3-pyrrolines 61. 

The stereochemistry is controlled by a stereoelectronic preference for protonation perpendicular to the enolate system and, given that this requirement is met, the stereochemistry normally corresponds to protonation of the most stable conformation of the dianion intermediate from its least hindered side. 

The mechanism of the chemical reduction of enones with metal (Li, Na, etc.) in liquid ammonia can be described by the following equation in which the substrate 212 receives two electrons from the metal to give the dianion intermediate 213. This intermediate is then successively transformed into the enolate salt 214 and the ketone 2T5 with an appropriate proton donor source. It can readily be seen that the stereochemical outcome of this reaction depends on the stereochemistry of the protonation step 213 - 214. An excellent review on this topic has been recently written by Caine (60). This subject will be only briefly discussed here. 

Terminal alkynyl groups, for example, are deprotonated the use of a second equivalent of base allows the generation of 1,2-rearrangement products via dianion intermediates. 

The classical Bamford-Stevens reaction6 is a two-step elimination process involving a hydrazone dianion intermediate. The latter sequentially... 

ESR study of the electrochemical dehalogenation of 2-halo-5-nitrofurans showed that the first step involves the formation of radical anions the dehalogenation of which proceeds via a dianion intermediate for chloro compounds, for the iodo compounds. Both routes were involved for the bromo compounds. The stability of the intermediate anion radicals of 2-halo-5-nitrofurans increased in the order I < Br < Cl208. 

In path A, it is possible that a second electron transfer from a second equivalent of Sml2 reduces the ketyl, generating a metallaoxirane species. This reactive dianionic intermediate then adds to a second carbonyl substrate. 

The lithiation of phthalans with an excess of lithium in the presence of a catalytic amount of DTBB at -78 °C led to the formation of dianionic intermediates, whose reaction with different electrophiles gave rise to ring-opened products as shown in the scheme below <07TL3379>. 

Stable dioxadiazasilepines and dioxadiazastannapines 216 were prepared in 60-77% yields from vicinal oximes 215 via dianion intermediates, which are intramolecularly trapped with dielectrophilic diorganodichlorosilanes and diorganodichlorostannanes, respectively (Equation 43) <2005TL315>. 

The reductive coupling of dithiobenzoate esters results in the predominant formation of Z-l,2-dialkylthio-l,2-diphenylethylenes in good yields when alkyl halides are added after completion of the electrolysis [Eq. (27)] [202]. A reaction mechanism via a dianion intermediate is postulated, but the stereochemical course has remained ambiguous. 

The earliest studies on this reaction began with an attempt to generate simple symmetrical pinacols. Reaction of 1 equiv. of ytterbium metal with 2 equiv. of a diaryl ketone in IllF/HMPA provided excellent yields of the corresponding symmetrical pinacols (equation 98). Interestingly, when equimolar quantities of ytterbium metal and benzophenone were employed, the sole product isolated after aqueous work-up was benzhydrol. When D2O was utilized to quench this reaction mixture, C-deuterated benzhy-drol was formed (equation 99). These latter results in cated that a discrete ketone dianionic intermediate was generated in the reaction between ytterbium metal and diaryl ketones. 

In the laboratory of J.D. Winkler, the synthesis of the carbon framework of the eleutherobin aglycon was developed using a tandem Diels-Alder reaction and a Grob fragmentation as key steps.The tricyclic fragmentation precursor was subjected to potassium carbonate in DMF at 75 °C to afford the fragmentation product in 68% yield via a dianion intermediate that underwent a spontaneous hemiketalization. 

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