Intermediate, allyl anion

Notice that the final result is a 1,3 charge affinity inversion Umpolung) of an allylic derivative via a FGI of a functional group of type E by a group of type A, followed by a [2,3]-sigmatropic rearrangement. If the intermediate allyl anion reacts with a carbonyl compound as the electrophile the result is then a 1,4-D system, such as ... 

For many reactions the type of intermediate that is involved may be deduced from a study of a family of reactants. For example, by noting that in allylic oxidation the order of reactivity is isobutene > trans-2-butene > cis-2-butene > 1-butene one may deduce that an allyl radical or cation is an intermediate. For other oxidations, if the reaction rate order is primary > secondary > tertiary, then an anionic intermediate is implicated. However, care must be taken that the formation of the intermediate is involved in the ratedetermining step and that there are no adsorption equilibrium effects. To rule out the latter, the reaction should be carried out at conditions of low coverage. 

Base-catalyzed (NaOMe) isomerizations of 1H- and AH-azepines to 3H-azepines are well known and most likely proceed via allylic anion intermediates (80TL595,72CB982). 

During the preparation of allylic zinc reagents, the formation of Wurtz-coupling products may be observed, especially if the intermediate allylic radical is well stabilized. However, the direct insertion of zinc foil to allyl bromide in THF at 5 C is one of the best methods for preparing an allylic anion equivalent. Allylic zinc reagents are more convenient to prepare and to handle than their magnesium and lithium counterparts... 

Selectivities to various isomers are more difficult to predict when metal oxides are used as catalysts. ZnO preferentially produced 79% 1-butene and several percent of a -2-butene [624-64-6] (75). CdO catalyst produced 55% 1-butene and 45% ar-2-butene. It was also reported that while interconversion between 1-butene and or-2-butene was quite facile on CdO, cis—trans isomerization was slow. This was attributed to the presence of a 71-allyl anion intermediate (76). High or-2-butene selectivities were obtained with molybdenum carbonyl encapsulated in zeolites (77). On the other hand, deuteration using Th02 catalyst produced predominandy the 1,4-addition product, trans-l-bu. en.e-d2 with no isotope scrambling (78). 

The transformation was believed to involve participation of a nuw-merie anion as shown in Eq. (336) Formation of an olefin from the intermediate allylic alcohol is reasonable in view of the recognis d lability of allylic C—0 bonds. 

The protonation leads specifically to the trans-decalin system, though reduction could apparently give rise to two stereoisomeric products. The guiding principle appears to be that protonation of the intermediate allylic anion 12 takes place axially, orthogonal to the plane of the double bond, and to the most stable conformation of the carbanion which allows the best sp3-orbital overlap on the /Tcarbon with the -orbital system of the double bond. 

Unusual -branched Baylis-Hillman adducts have been prepared by Li and coworkers by a novel Et2AlCl promoted domino Michael-aldol reaction of propynoates 50 with organo-cuprates and chiral p-toluenesulfinimines 52 (Scheme 10) [42], These condensations proceeded with very good diastereoselectivity to give allylic amines 53. The selectivity can be explained through the chairlike transition state 54. The anion intermediate approaches the sulfinimine from the sterically less hindered side of the lone pair of electrons. The nucleo-... 

Telluriranes were postulated as intermediates in the conversion of 2-R-2-chloromethyl-oxirancs by tclluridc anions to allyl alcohols3. 

OL-Silyl ketones,l Silyl enol ethers with sterically hindered silyl groups rearrange to a-silyl ketones in the presence of /i-BuLi (2 equiv.) and KO-f-Bu (2.5 equiv.). Tri-methylsilyl ethers do not undergo this rearrangement, but triisopropylsilyl (TIPS) and diisopropylmethylsilyl (DIMS) ethers do if they contain an allylic a-proton. The silyl group rearranges preferentially to the less hindered terminus of the intermediate allyl anion. The rearrangement is less useful with acyclic substrates because of side reactions. 

Palladium-catalyzed reductive silylation has also been reported. The cathodic reduction of allylic acetates in the presence of silylating agents and a catalytic amount of (Ph3P)4Pd [53] gives the corresponding allylsilanes. The initially formed rr-allyl-palladium(II) complex seems to be reduced at the cathode to generate the allyl anion intermediate, which reacts with chlorosilane to give the final product. 

Functionally substituted benzylic, allylic, and vinylic compounds containing alkoxides, esters, ethers, nitriles, or amides can be reacted with halosilanes under Barbier conditions using HMPT to yield C- and O-silylated products, 1,2- or 1,4-addition products, as well as reductive dimers. Radical and anionic intermediates are postulated, based on SET reactions from the metal, and multiple silated species can be obtained. The use of the TMSCl-Mg-HMPT system has been extensively investigated by Galas group [85] at the University of Bordeaux, and their work has greatly advanced the science of the Barbier reaction with silanes. 

Unlike the cyanocyclopropyl anion <77JA263l>, the cyclic diphosphiranyl anion intermediate is not detectable, but the 1,3-diphosphaallyl anion is observed when the progress of the reaction is followed by 31P NMR at low temperature. It appears at 5 31P = 64.4 with a yP Li = 40 Hz, characteristic of covalent phosphorus-lithium bonds <89NJC315>. The exo-exo configuration of this intermediate was confirmed by x-ray diffraction <93PS(77)230>. This anion, recrystallized with two molecules of diethylether, has a rr-allyl structure (d c = 1.71 A) and the lithium atom is located in... 

With em-dihalogenodiphosphiranes, the reaction gives quantitative yields of 1,3-diphosphaallene probably involving the same mechanism as Scheme 8 and an unstable allylic anion intermediate <92JOM(436)169>. 

By using a Pd/Sml2 system as catalyst, allylic acetates can be converted regioselectively to the corresponding allylic selenides, most probably by selenation of allylic anion intermediates (Scheme 15.80) [160]. 

A stereo- and regio-selective synthesis of rrans-2,5-dialkylpyrroiine structures is accomplished via the A-substituted allylic anion intermediates (Scheme 12). The a-regioselective alkylation is presumably due to the presence of the electron-withdrawing methoxycarbonyl group. The piperidine ring system... 

Figure 3. Allyl anion intermediate in toluene-butadiene telomerization...

Another possible explanation of the results invokes steric effects in the intermediate allyl anion (Figure 3). The steric interaction between the bulky chelated cation and the benzyl group probably pushes the... 

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