Intermediate dianionic

Cycttzation of dianions of bis(diphenyl thiocetals). Treatment of I with 2-4 equiv. of methyllithium and of TMEDA results in formation of 2 (major product) and 3. The paper presents evidence that these cyclizations involve an intermediate dianion, which loses a thiophenoxide ion to give an anionic sulfur-stabilized carbene. 

Another interesting aspect in the reduction of 37 is the fact that the intermediate dianion (372-) was not detected by NMR spectroscopy. This is somewhat peculiar because the cyclic voltammogram indicated the presence of four redox steps, which lead to the tetraanion. Moreover, the radical species (37 ) and 372- were observed in the UV-VIS spectrum. A simple explanation is given to this in all stages before reaching 374 there exists a fast electron exchange between two states, radical and dianion, which prevents the observation of the NMR spectrum (Figure 10)56. 

Treatment of allyl isothiocyanate with the system lithium diisopropylamide (LDA)//-BuOK will result in annulation generating the intermediate dianion 150, which may subsequently be reacted with water, followed by iodo-methane, to provide the 2-(alkylthio)pyrrole 151 in a respectable yield (Scheme 18) <1997TL7247>. 

The S—S bond of 1,2-dithioles is easily cleaved by reduction and, for example, dithioles (249) can be converted into acyclic methyl dithiolates (251) by electrochemical reduction, followed by alkylation of the intermediate dianions (250). Benzodithioles (252) are more stable and the heterocycle survives reduction with zinc and trifluoroacetic acid, although the exocyclic double bond is saturated to give 2-al-kyldithiolanes (253). ... 

Naphthalene-catalyzed lithiation of l,3-dimethyl-2-phenylimidazolidine leads to cleavage of the benzylic carbon-nitrogen bond, with formation of an intermediate dianion. The dianion could be trapped with several electrophiles, including primary and secondary alkyl halides, as well as enolizable and nonenolizable carbonyl derivatives, affording diamines 485 in satisfactory yields (Scheme 112) <2005T3177>. 

Reductive cleavage of imidazolidines 641 was implicated in the one-pot synthesis of N,N,N -trisubstituted ethylenediamines 643 from V,V -disubstituted ethylene diamines and an aldehyde R CHO. Presumably the intermediate iminium ion 642 is reduced by NaBH4 (Scheme 154) <2003SC3193>. Naphthalene-catalyzed lithiation of l,3-dimethyl-2-phenylimidazoline 644 leads to benzylic C-N bond cleavage. The intermediate dianion can be trapped with electrophiles (HjO, alkyl halides, ketones, and aldehydes) to afford diamines 645 <2005T3177>. 

The strong activation effect by the carboxyl group allows reduction to occur when only one equivalent of alcohol is present or even without an alcohol. In these cases, the intermediate dianion persists in solution and can be trapped with electrophilic reagents to generate a quaternary carbon center. 

The common nucleophile in ribonuclease enzymes, and thus in relevant models, is the 2 -OH group of the central nucleotide. The work of the Williams group [31] confirmed the mechanism of hydrolysis of uridyl esters (Scheme 2.14, base = U) with good, substituted-phenol leaving groups as a relatively simple process, described by the simple general base catalysis mechanism (Bronsted p = 0.67), with 2.10 2.11 as the rate determining step (Scheme 2.14), followed by rapid breakdown of the presumed phosphorane (pentacovalent addition) intermediate dianion... 

Metallation of the acyclic imidate salt (47) followed by reaction of the intermediate dianion with aldehydes leads to the production of the oxazoles (48 Scheme 19). This procedure constitutes a useful modification of the Comforth oxazole synthesis. 

The p-halogenotetradecachlorotriphenylmethyl radicals. In the carbanion synthesis of the radical HOCO—PTM- from HOCO—PTM—H in the conventional manner, the radical I—PTM- is a minor by-product. It appears that the intermediate dianion OCO—PTM reacts rapidly with iodine, giving the radical OjC—PTM- (Ballester ct a/., 1982b Ibanez, 1972). High concentrations of iodine and/or long reaction times cause the formation of the radical I—PTM- as a major product by oxidative decarboxylation. It has been found that the dianion "O2C—PTM is stable under the reaction condition (Ballester et al., 1982b Ibanez, 1972). These results may be accounted for as shown in (142) through the intermediacy of acyclic,... 

MMA) to form intermediate dianions that are rapidly protonated by the solvent THE to form the monoanion initiating species as shown in Scheme 7.4 [51, 52]. Eor the nuclide-initiated polymerization of MMA, although there was good agreement between calculated and observed molecular weights, the molecular weight distributions were broad = 1.2-1.6) [52]. 

Ramage and coworkers (537) have developed a biomimetic synthesis of pulvinic acids which relies for its success on the facility with which dioxolanones of type (110) undergo nucleophilic attack at the lactone carbonyl group with subsequent extrusion of cyclohexanone. In the synthesis of xerocomic acid (Scheme 15) the dioxolanone (110), obtained as the predominant isomer from reaction between the phos-phorane (109) and methyl (3,4-dibenzyloxyphenyl)glyoxalate, was cleaved with the lithium enolate of /-butyl (4-benzyloxyphenyl)acetate. The intermediate dianion (111) probably exists at first as the chelate (112) which is then broken down on aqueous work up and subsequently cyclised specifically at the less hindered carbonyl group to produce the ester (113). The dianion (111) is analogous to the hypothetical... 

Chloropropenal has been converted into the dianion (58) and used to synthesize variously substituted allylic alcohols (Scheme 16).These intermediate dianions (58) show no tendency to undergo -elimination to allenes, although under appropriate conditions... 

In 1984 Fujisawa et al. observed a directing effect due to a CT oxygen stereocenter (Scheme 4.29) [32]. Tandem deprotonation and silylation of the j8-hydroxy allylic ester presumably gave the intermediate sUyloxy silyl ketene acetal. The Z-config-uration of the silyl ketene acetal was a consequence of chelation of the intermediate dianion by the Id cation. The authors postulated that the allylic alkene prefer-... 

Nuclides have been shown to react with monomers such as styrene and methyl methacrylate to form intermediate dianions that are rapidly protonated hy the solvent THF to form the monoanion initiating species, as shown below (30,31) ... 

When using low water concentrations, the reduction of Rh(iii) back to Rh(i) is slower, but so is the formation of [Rhl4(CO)2] due to the lower HI content available for reactions (3) and (4). The promotional effect of Li salts is attributed to the coordination of either acetate or iodide to 58 forming a highly nucleophilic intermediate dianion, [Rh(GO)2l2X] (X = I or OAc) and also to the kinetic and thermodynamic parameters of this new organic cycle, in which water has been replaced by LiOAc. ... 

---