Vinylic anions inversion

A reasonable idea of the stability of the stereoisomeric trigonal vinyl cations can be gained from the behavior of vinyl anions and radicals. It is known that the interconversion between stereoisomeric vinyl anions is fairly slow, with an activation energy of the order of 18-24 kcal/mole (171). On the other hand, inversion of stereoisomeric vinyl radicals is reasonably rapid, even at fairly low temperatures, with an activation energy of the order of 2-8 kcal/mole (172). Hence, extrapolating from the electron-rich vinyl anion through the neutral vinyl radical to the electron-deficient vinyl cation, one would expect rapid interconversion between stereoisomeric vinyl cations and only a small amount (if any) of stereospecificity. To put it differently, the vinyl cation should be mostly linear with an empty p orbital and very little trigonal character. 

The energy barrier calculated for inversion of the vinyl anion (2---> 21) by changing... 

The question of configurational stability has been investigated first for vinylidene carbenoids and, more recently, for alkylcarbenoids. Vinyl anions are usually considered to be configurationally stable" ° the calculated inversion barrier of the ethenyl anion 10 (R = H) is about 35 kcal mol (equation 4)" . Concerning lithioalkenes, this configurational stability has been confirmed experimentally for a-hydrogen, a-alkyl and a-aryl substituted derivatives . The inversion of vinylidene lithium carbenoids was already... 

Carbanions have been less studied, apart from CH3, 167,168 but included in a more recent set of calculations169 on several carbanions (CH3-, C2H5-, and ethynyl anion) are calculations on C2H3-. For reliable calculations on this type of molecule, diffuse orbitals must be added to the basis set. Several different basis sets were used, but the geometry of the neutral parent molecule was used in some of the calculations. The main aim of this paper was to investigate the electron density and difference densities, electron affinities, and proton affinities. The inversion barrier in the vinyl anion was ca. 142 kJ mol-1 which was in good agreement with that found by Lehn et a/.148 in an earlier calculation. 

Stereoelectronic features of vinyl anions are associated with their bent geometries and usual configurational stabilities in solution. Theory predicts a sizable inversion barrier for the parent vinyl anion (28.7kcal/ mol at the MP2/6-31+G(d)//HF/6-31 -nG(d) level). The two conformers of 1-propenyl anion can be formed stereospecifically in the gas phase (via desilylation of E- and Z-l-trimethylsilylpropene) and show different reactivity patterns. ... 

The character of deformations of the transition state structurally may readily be explained by the preferability of tram addition. Indeed, for a cis addition the stage of topomerization of the vinyl anion XIX is needed, which requires overcoming the energy berrier of the planar inversion amounting for the vinyl anion to 35 kcal/mol. The tram distortion of the C—H bonds exhibited by the triple bond when it is approached by a nucleophile is one more reason for the remarkable ease with which the nucleophilic addition reactions of this type proceed. 

In sharp contrast to the above mentioned conversions of 73a, its reaction with 0-ethyl thiophosphate is nonstereoselective (90% inversion and 10% retention). Moreover, with a fluoride anion, complete loss of stereochemistry was observed [60], An efficient synthesis of diastereomerically pure vinyl phosphonates lOOa-e was... 

In a recent series of studies focused on the synthetic utility of alkenyliodo-nium salts, ( )-/J-phenylethenyl(phenyl)- and ( )-l-hexenyl(phenyl)iodonium tetrafluoroborates, 22 and 23, were utilized for alkenylations of a range of soft, anionic nucleophiles (Scheme 45) [128-135]. In all cases but one, alkenylations with 22 occurred with retention of configuration, while alkenylations with 23 occurred with inversion of configuration. Only the dialkyl phosphoroselenolate salts gave mixtures of (Z)- and (fj)-products with 22 [132]. Furthermore, although cuprous iodide was used to catalyze the reactions of 22 and 23 with the phosphorothioate and -dithioate salts, the stereochemical results were the same [131,133]. It was generally assumed that retention was an outcome of the ligandcoupling or addition-elimination pathways, while stereochemical inversion was attributed to the vinylic... 

Lithiation of compound 560 with s-BuLi-TMEDA in THF at —78 °C following an inverse addition protocol provided the anion 561. It reacts with primary alkyl iodides and triflates, silyl chlorides, diphenyl disulfide, epoxides, aldehydes, ketones, imines, acyl chlorides, isocyanates and sulfonyl fluorides to yield the expected compounds 562 (Scheme 152). The transmetallation of compound 561 with ZnBr2 allowed the palladium-catalyzed cross-coupling reaction with aryl and vinyl bromides837. When the reaction was quenched with 1,2-dibromotetrafluoroethane, the corresponding bromide 562 (X = Br) is obtained838. 

The polymerization of a water-soluble monomer such as AM, acrylic acid (AA), sodium acrylate (NaA), or 2-hydroxyethylmethacrylate (HEMA), can be carried out easily in inverse microemulsion or/and bicontinuous microemulsion. These water-soluble monomers also act as cosurfactants, increasing the flexibility and the fluidity of the interfaces, which enhances the solubilization of the monomer. A cosurfactant effect during the polymerization of vinyl acetate in anionic microemulsions has also been reported [12]. 

This method has also been quite useful for the synthesis of triorganoarsenic compounds. Arsenide anions, which can be prepared by several methods (vide infra), react with alkyl and aryl halides. Several examples are shown in equations 18 , 19 , 20 °, 21 , 22 23 , 24 and 25 . In the reaction with alkyl halides inversion of configuration at the sp carbon atom takes place and in the reaction of vinyl halides retention of configuration has been observed (equations 18 and 21). Photostimulated reaction with aryl halides has been reported to proceed via the Sjj l mechanism thus, the ketone group is compatible with the reaction conditions as shown in equation 23 . 

In the C-0 bond cleavage reaction of vinyl epoxides by a palladium complex, formic acid acts as a good proton and hydride donor evolving CO2 (Eqs. 3.24 and 3.25). Zero-valent palladium complex favors the attack at an allylic carbon in an Sn2 manner to give i/ -allylpalladium(II) complex with inversion of configiuation, and formate anion coordinates to the palladium center (Scheme 3.50). Then, decarboxylation of the formate affords palladium hydride, which attacks the ry -allyl moiety from the endo side. Thus, 1,2-addition of hydrogen atoms takes place regioselectively with inversion at the allylic carbon [94]. 

Stereoselectivity of radical addition is not limited to sulfonyl radicals. The (rani-addition has also been observed for tin, bromine, chlorine, and silyl radicals. Varying degrees of selectivity has been observed for addition of carbon-centered radicals, depending on the substituents size and effect on the inversion barrier. Because the importance of negative hyperconjugation decreases for radicals in comparison to the anions, the barriers for inversion decrease in parallel. As a result, the selectivity can erode under conditions when trapping of the radical is slower than the equilibration, e.g. in the thiol-yne click reaction, which often provide a mixture of E and Z-vinyl sulfides. ... 

The base-catalysed cis-trans isomerization of l-aryl-2-phenylcyclopropanes has been subjected to kinetic analysis and a complexed anion, e.g. (190), is thought to be produced initially. Configurational inversion of the cyclopropyl sulphoxide (173) can also be effected by base. A kinetic analysis of the hydrolysis of 2- and 2,2-di-substituted bromocyclopropanes suggests that for 2-vinyl-substituted compounds considerable progress towards an allyl cation has been been made at the transition state. Metallation studies show that, for nortricyclanes at least, the cyclopropyl hydrogen atoms are most readily attacked by pentylsodium in the presence of potas-... 

While one would expect that polymerizations of a monomer in a heterogeneous system with ultraviolet radiation would be difficult because of the blocking effect of the continuous phase, emulsion polymerizations of vinyl chloride have been carried out. The design of a small-scale pilot plant has even been reported upon [180]. With anionic emulsifers, the process is actually mechanistically similar to chemically initiated latex polymerization of VCM [181]. At high conversions, inversion of the latex and the formation of a free-flowing powder containing 35-70< /b PVC may take place [182]. Stable latexes were also formed in the presence of cationic surfactants [183]. 

---