1,1-vinyl dianions

In the last section, the access to a-selanyl vinylmetals is considered. Besides the classical methods involving vinylic selenides and ketene selenoacetals, the hydrometallation (M = Sn, Zr, B) of acetylenic selenides constitutes an efficient method. These species can be considered as vinyl dianion equivalents. Some examples of cross-coupling reactions are presented. 

In this concluding section, we will recall that a-selanylvinylmetal derivatives can be produced by a-deprotonation of vinylic aryl selenides 11, cleavage of ketene selenoacetals 12 or bromine/lithium exchange of a-bromovinyl selenides 13 (Scheme 49). These strategies involve vinylic selenides whose syntheses are not simple. More recent works have used organocuprate additions, hydrometala-tions (M = Sn, Zr) or hydroborations of readily available acetylenic selenides 14 (Scheme 49). The process can be regio- and stereocontrolled and produce the a-selanylvinylmetal derivatives under mild conditions. The latter can be considered as vinyl dianion equivalents and are very useful intermediates for cross-coupling reactions. 

In the more general enone synthesis 89 - 102 - 105, the original anion of the bis-(phenyl-thio) acetal 89 behaves as a vinyl dianion (R CH=C2- ), an interesting complement to syn-thons 37 and 58... 

Dianion formation from 2-methyl-2-propen-l-ol seems to be highly dependent on reaction conditions. Silylation of the dianion generated using a previously reported method was unsuccessful in our hands. The procedure described here for the metalation of the allylic alcohol is a modification of the one reported for formation of the dianion of 3-methyl-3-buten-l-ol The critical variant appears to be the polarity of the reaction medium. In solvents such as ether and hexane, substantial amounts (15-50%) of the vinyl-silane 3 are observed. Very poor yields of the desired product were obtained in dirnethoxyethane and hexamethylphosphoric triamide, presumably because of the decomposition of these solvents under these conditions. Empirically, the optimal solvent seems to be a mixture of ether and tetrahydrofuran in a ratio (v/v) varying from 1.4 to 2.2 in this case 3 becomes a very minor component. 

Metalated epoxides can react with organometallics to give olefins after elimination of dimetal oxide, a process often referred to as reductive alkylation (Path B, Scheme 5.2). Crandall and Lin first described this reaction in their seminal paper in 1967 treatment of tert-butyloxirane 106 with 3 equiv. of tert-butyllithium, for example, gave trans-di-tert-butylethylene 110 in 64% yield (Scheme 5.23), Stating that this reaction should have some synthetic potential , [36] they proposed a reaction pathway in which tert-butyllithium reacted with a-lithiooxycarbene 108 to generate dianion 109 and thence olefin 110 upon elimination of dilithium oxide. The epoxide has, in effect, acted as a vinyl cation equivalent. 

It seems likely that aryl, vinyl, and cyclopropyl halides react by an alternative mechanism, since the corresponding radicals are less stable than alkyl radicals. It has been suggested that these halides may react through a dianion.13... 

A domino process based on the twofold addition of alkenyl anions to a squarate ester was used by Paquette and coworkers [96] for the total synthesis of the triqui-nane sesquiterpene hypnophilin (4-284). The three-component reaction of 4-281,4-282 and vinyl lithium gave primarily the trans- and cis-adducts A and B, which furnished D either by an electrocyclic ring opening/ring closure via C or a dianionic oxy-Cope rearrangement (Scheme 4.60). Further transformations led to E and F, which resulted in the formation of 4-283 on treatment with acid. 

Okamoto and Mita studied the anionic polymerization of 1,4-DIPB in THF [261]. They found the reactivity of the pendant vinyl groups by about three to four orders of magnitude lower than that of the vinyl groups of the monomers. Popov et al. compared the reactivities of 1,4-DVB and 1,4-DIPB in the reaction with polystyryl dianions in THF/benzene mixtures [262]. While addition of 1,4-DVB to the dianion solution caused an immediate macrogelation, no gel formation was observed on the addition of 1,4-DIPB. Anionic polymerization of 1,3-DIPB was also studied by several research groups [263-265]. They reported formation of low molar mass species. 

Subsequently, Tselinskii et al. found the dianion of Compound 2a to be stable and reacted it with a variety of electrophiles including picryl chloride, acetic anhydride, methyl iodide and vinyl ketone. They synthesized its dinitro derivative, dinitrodifurazano piperazine (2b) (DNDFP) [Structure (2.63)] by reacting the dianion (2a) with nitrogen oxide in CH3CN. The dinitro derivative which is quite reactive, was isolated by column chromatography and confirmed by mass spectroscopy [272]. 

Three structures may be proposed for diazapyracylene an A/,N-bipyrryl core with two vinyl bridges (307a), a cyclododecahexaene dianion with an internal cross-linked N=N unit (307b) and a peripheral [12]annulene loosely linked to an internal hydrazine unit (307c). Structure (307c) does not correlate well with the isoelectronic pyracylene dianion (308). 

The formation of pyrroles 54 and 57 is likely to be a result of decomposition of the oximes 55 and 59 in the form of dianion by analogy with retroaldol condensation (Scheme 33). As far as the formation of pyrrole 57 (Schemes 28 and 29) is concerned, this seems to result from -elimination of water and vinyl alcohol molecules from the oximes 55 and 58 respectively or from the corresponding intermediate 2-hydroxy(vinyloxy)propyl-1-vinylpyrroles. 

However, it has turned out that there are a few synthetic methods having broad scope, that is, (a) for the synthesis of dioxepins and dithiepins (i) transacetalization of the corresponding diol or dithiol, respectively, with an open chain acetal (ii) treatment of the corresponding diol or dithiol with sodium borohydride and reaction of the resulting dianion with a gem-dihalogen derivative. The latter method is particularly useful for the synthesis of dioxepins and dithiepins unsubstituted in the 2-position (b) for the synthesis of 4,5-dihydro-l,3-dioxepins (i) metal- or base-catalyzed double-bond isomerization and (ii) Heck vinylation or arylation, respectively, for the synthesis of 6-substituted 4,5-dihydro-l,3-dioxepins. 

The dimer cation was supposed to have a sandwich structure in which the orbitals of one molecule overlapped with those of the other molecule. The band at 450 nm (B) is due to the bonded dimer cation (St—St T) the formation of this species corresponds to the initiation step of the polymerization. The bonded dimer cation may be formed by the opening of the vinyl double-bonds. Egusa et al. proposed that the structure was a linked head-to-head type I or II, by the analogy of the dimeric dianions of styrene and a-methylstyrene. Table 1 summarizes the assignment of absorption bands observed in pulse radiolysis of 1,1-diphenylethylene in dichloromethane, which is a compound suitable for studying monomeric and dimeric cations [28],... 

The stannylcupration of alkynes has been widely studied. Reaction of alkynes with lithium bis(tributylstannyl) cuprate leads to r -2-(tri butyl stannyl) vinyl cuprates, which are synthetically equivalent to cis- 1,2-ethylene dianions. Addition of the tin-copper reagent across the triple bond occurs i>7/-stereospecifically, thus providing Z-vinylstannanes. Phenylacetylene reacts with the tin cuprate with a regiochemistry opposite to that of 1-decyne.294 The intermediate cuprates react well with the various electrophiles.295 For example, the reaction with ethylene oxide gives primary alcohols, and further treatment of their />-toluenesulfonates with butyllithium gives 1-substituted cyclobutenes (Equation (120)) 294... 

The 7-hydroxy telluride 161 is prepared by hydrotelluration of methyl vinyl ketone, followed by in situ reduction with sodium borohydride.154 Treatment of 161 with 2 equiv. of nBuLi, followed by capture of the dianion 162 with electrophiles, gives the corresponding alcohols 163.252 Dianions like 162 can be transmetallated with CeCf , leading to organometallics of the type 164, which on reaction with lactones give spiroketals (Scheme 94).253... 

The reaction of tellurium with acetylene and iodobenzene in the presence of potassium hydroxide, hexamethylphosphoric triamide, tin(II) chloride, and water under 12 atm acetylene in an autoclave at 100-120° produced diphenyl tellurium, phenyl vinyl tellurium, and divinyl tellurium4. Diphenyl tellurium could be the product of the arylation of the telluride dianion by phenyl iodide. 

Axially chiral vinyl sulfoximines have been prepared with high diastereoselec-tivity (> 99 1) by asymmetric elimination of LiOSiMe3 from 3-siloxy sulfoximines.72 For example, addition of lithiated (+)-(S)-2a to ketone 94 gave the (3-hydroxy sulfoximines 95 with a 99 1 diastereoselectivity. When the dianion of 95 was quenched at -78 °C with chlorotrimethylsilane, the vinyl sulfoximine 97 was... 

Carbolithiation reactions of ketone a,-dianions, generated by the above amine-free method with several alkenes, such as styrenyl derivatives, vinyl sulfides and vinylsilanes, can lead to the generation of ketone a,5-dianions (Scheme 15)14. For example, when one equivalents of triphenylvinylsilane was treated with a ketone a,-dianion, in THF, at 0 C for 1 h and the resulting reaction mixture was quenched by 2.2 mol equivalents of trimethylchlorosilane, the corresponding bis-silylated enol silyl ether was obtained. Substituted styrenyl derivatives, such as 1,1-diphenylethylene and cinnamyl alcohol, also underwent a smooth carbolithiation to give the corresponding ketone a,5-dianions. Similar addition reactions of ketone a,f)-dianions to vinyl phenyl sulfide took place smoothly to give a,5-dianions with a sulfur attached in the 5-position. 

It is a modification of the Shapiro olefin synthesis3 which allows the vinyl anion intermediate to be trapped with primary halides and other electrophiles. Use of triisopropylbenzenesulfonylhydrazones as the vinyllithium precursor4 is an improvement over previously4 used toluenesulfonylhy-drazones,5 6 which can be employed in the sequence provided excess sec-butyllithium (typically 4.5 equiv) and alkyl halide (3.0 equiv) are used. Methyl ketones (e.g., acetone, acetophenone, 2-octanone) can also be used and can be converted into their dianions using 2.2 equiv of the weaker base, n-butyllithium. The conditions described above, with the slight modifications noted, have been used for a variety of ketones as shown in Table I. 

Two minor processes sometimes operate competitively with that illustrated in the scheme. One of these involves 1,4-addition of the second vinyl anion to give a reactive intermediate that differs structurally from 1, but is capable of setting into motion a closely related sequence of chemical events leading to an isomeric diquinane.4 This is the route followed to produce the minor product characterized here. The other option consists of cis-t, 2-addition, an event that is followed by a dianionic oxy-Cope rearrangement via a boat-like transition state.4 When sufficient substitution is present to allow the installation of multiple stereogenic centers, the adoption of this pathway is easily distinguished from the electrocyclic alternative since a cis relationship between relevant substituents is in place, instead of the trans arrangement required by the electrocyclization cascade. 

In principle, dimerization is reversible and the equilibrium is rapidly established. The equilibrium concentration of radical ions derived from vinyl monomers is so small that it cannot be detected by ESR (for monomer concentrations 10-1 mol dm-3 it is < 10-7 mol dm-3). The rate constant of dissociation of the dimeric dianion of a-methylstyrene (aMeS)... 

Carbanions can react with Cl in PVC macromolecules [295] and with the ester group of PMMA [284]. The rates of the two reactions are probably not very different by the addition of a-methylstyrene tetramer dianion to a PVC + PMMA solution, the copolymer poly(vinyl chloride)-gro/ir-poly-(methyl methacrylate) was obtained [296]. Macrocations formed by the reaction of strong acids with polyalkenes (see Chap. 3, Sect. 3.2) react with polyethers (polysiloxanes) yielding graft and block copolymers, e. g. poly(propylene)-0ra/ir-poly (oxyethylene) [297], poly(propylene)- /ocA -... 

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