Alkenes allyl anion

A secondary orbital interaction has been used to explain other puzzling features of selectivity, but, like frontier orbital theory itself, it has not stood the test of higher levels of theoretical investigation. Although still much cited, it does not appear to be the whole story, yet it remains the only simple explanation. It works for several other cycloadditions too, with the cyclopentadiene+tropone reaction favouring the extended transition structure 2.106 because the frontier orbitals have a repulsive interaction (wavy lines) between C-3, C-4, C-5 and C-6 on the tropone and C-2 and C-3 on the diene in the compressed transition structure 3.55. Similarly, the allyl anion+alkene interaction 3.56 is a model for a 1,3-dipolar cycloaddition, which has no secondary orbital interaction between the HOMO of the anion, with a node on C-2, and the LUMO of the dipolarophile, and only has a favourable interaction between the LUMO of the anion and the HOMO of the dipolarophile 3.57, which might explain the low level or absence of endo selectivity that dipolar cycloadditions show. 

Base removes bepzylic H, retro- allyl anion+alkene, all-s gives trans-cyclooctene retro [2+2], s on both a-bonds makes trani-double bond. This is an exercise in drawing believable transition structures like 6.42. 

Dipolar cycloadditions 6.12 + 6.13 —> 6.14, however, are a large group of [4 + 2] cycloadditions isoelectronic with the allyl anion+ alkene reaction. There is much evidence that these reactions are usually concerted cycloadditions. They have a conjugated system of three p orbitals with four electrons in the conjugated system, but the three atoms, X, Y, and Z in the dipole 6.12 and the two atoms A and B in the dipolarophile 6.13, are not restricted to carbon atoms. The range of possible structures is large, with X, Y, Z, A and B able to be almost any combination of C, N, O and S, and with a double 6.12 or, in those combinations that can support it, a triple bond 6.15 between two of them. 

The 1,3-dipolar molecules are isoelectronic with the allyl anion and have four electrons in a n system encompassing the 1,3-dipole. Some typical 1,3-dipolar species are shown in Scheme 11.4. It should be noted that all have one or more resonance structures showing the characteristic 1,3-dipole. The dipolarophiles are typically alkenes or alkynes, but all that is essential is a tc bond. The reactivity of dipolarophiles depends both on the substituents present on the n bond and on the nature of the 1,3-dipole involved in the reaction. Because of the wide range of structures that can serve either as a 1,3-dipole or as a dipolarophile, the 1,3-dipolar cycloaddition is a very useful reaction for the construction of five-membered heterocyclic rings. 

In the 1,3-dipolar cycloaddition reactions of especially allyl anion type 1,3-dipoles with alkenes the formation of diastereomers has to be considered. In reactions of nitrones with a terminal alkene the nitrone can approach the alkene in an endo or an exo fashion giving rise to two different diastereomers. The nomenclature endo and exo is well known from the Diels-Alder reaction [3]. The endo isomer arises from the reaction in which the nitrogen atom of the dipole points in the same direction as the substituent of the alkene as outlined in Scheme 6.7. However, compared with the Diels-Alder reaction in which the endo transition state is stabilized by secondary 7t-orbital interactions, the actual interaction of the N-nitrone p -orbital with a vicinal p -orbital on the alkene, and thus the stabilization, is small [25]. The endojexo selectivity in the 1,3-dipolar cycloaddition reaction is therefore primarily controlled by the structure of the substrates or by a catalyst. 

Hydrocarbons lacking directing substituents are not very reactive toward metal-lation, but it has been found that a mixture of n-butyllithium and potassium r-butoxide66 is sufficiently reactive to give allyl anions from alkenes such as isobutene.67... 

Lee and Squires determined the gas-phase acidities of a number of cyclic alkenes and dienes including the bicyclic compounds 4, 5, 6 and 715. Their values are summarized in Table 5 and have estimated uncertainties of 1-2 kcal mol 1. The relatively high acidity of 4 was attributed to bishomoconjugation of the double bond with the allyl anion, as shown in 815. 

Recent advances include alkyl iodides as substrates that can be activated by metal complexation. Also Jt-allyl "anions", when co-ordinated to palladium, are activated toward attack by nucleophiles. This is very similar to the activation of co-ordinated alkenes and it shows the very high electrophilicity of palladium. The valence state of palladium, and/or the charge on palladium, and therefore also the ligands attached to it are very important ... 

The high selectivities found in the protonation experiments of the nitronate ions 44 suggested that also allyl anions 54 can be regioselectively protonated by a general acid protonation. Therefore, some lithium allyl compounds (Structures 6) were generated by deprotonation of alkenes with n-butyl lithium. 

Draw the frontier orbital interactions for the all-suprafacial cycloaddition of an allyl anion to an alkene and for an allyl cation to a diene showing that they match, and show that the alternatives, allyl cation with alkene and allyl anion with diene are symmetry-forbidden. 

Intramolecular hydroamination of cyclohexa-2,5-dienes has afforded the corresponding bicyclic allylic amines with high selectivity (Scheme 13).80 The reaction does not proceed through a direct hydroamination of one of the diastereotopic alkenes but more likely involves a diastereoselective protonation of a pentadienyl anion, followed by addition of a lithium amide across the double bond of the resulting 1,3-diene and a highly regioselective protonation of the final allylic anion. 

The mechanism does not proceed through a direct hydroamination of one of the diastereotopic alkenes, but involves a series of very selective processes including a deprotonation of (22), diastereoselective protonation of (26), intramolecular addition of lithium amide (27) to the 1,3-diene moiety, and final regioselective protonation of the allyl anion (28), all mediated by a substoichiometric amount of n-BuLi. 

In isomerization reactions, an alkene is deprotonated to form an allyl anion, which is reprotonated to give the more stable alkene (double-bond migration). The most simple example is the isomerization of 1-butene producing a mixture of cis- and trans-2-butene (Scheme 3). Because the stability of the cis-allyl anion formed as an intermediate is greater than for the trans form, a high cis/trans ratio is observed for base-catalyzed reactions whereas for acid-catalyzed reactions the ratio is close to unity. Thus, the cis/trans ratio of the products has frequently been used as an indication of base-catalyzed reaction mechanisms. The carbanions formed in the course of such superbase reactions are not freely mobile in solution,... 

Temperature variations of 1H- and 13C-NMR spectra of allyl and pentadienyl compounds of the alkali metals have given information about barriers to rotation about the C—C bonds. The endo and exo isomers of the allyl anions [Eq. (1)] are formed stereospecifically at low temperatures from Z-and E-alkenes, respectively (75,76). 

Fig. 2.6 Cmde estimates of the coefficients of the n orbitals of an X-substituted alkene as an unweighted sum of the coefficients of an allyl anion 2.5 and an alkene...

Diels-Alder reactions are classified as [4 + 2] cycloadditions, and the reaction giving the cyclobutane would be a [2 + 2] cycloaddition. This classification is based on the number of electrons involved. Diels-Alder reactions are not the only [4 + 2] cycloadditions. Conjugated ions like allyl cations, allyl anions and pentadienyl cations are all capable of cycloadditions. Thus, an allyl cation can be a 2-electron component in a [4 + 2] cycloaddition, as in the reaction of the methallyl cation 6.2 derived from its iodide 6.1, with cyclo-pentadiene giving a seven-membered ring cation 6.3. The diene is the 4-electron component. The product eventually isolated is the alkene 6.4, as the result of the loss of the neighbouring proton, the usual fate of a tertiary cation. This cycloaddition is also called a [4 + 3] cycloaddition if you were to count the atoms, but this is a structural feature not an electronic feature. In this chapter it is the number of electrons that counts. 

Phen and the radical anion of the alkene. Secondary electron transfer from allylsilane to Phen produces the radical cation of allylsilane and neutral Phen. The radical cation of allylsilane is cleaved by assistance of acetonitrile to generate an allyl radical. The allyl radical adds to the radical anion of the alkene to give the allylated anion which is converted into the product upon protonation. Alkyl and arylmethyl radicals can be generated in a similar manner from tetraalkyl tin compounds and arylmethylsilanes, respectively [124]. These radicals add regioselectively to the -position to the cyano groups in the radical anions of alkenes. 

The addition of methoxide or cyanide ions to cyclopropene 280 gives ° mixtures of isomeric alkenes upon methylation or protonation via allyl anions. However, triphenyl-phosphine, -arsine, or -stibine, and dimethyl sulphide afford the corresponding ylides 281. Photochromic l,8a-dihydroindolizines result from reaction of spiroannelated cyclopropenes of the type 280 with pyridines. The synthesis proceeds to ylides of the type 281 which cyclize to give the observed products. 

Based on these results and results for reduction of other combinations of allyl halides and activated alkenes, it has been suggested that when the allyl halide is more easily reduced than the alkene, the allylic anion (2-F reduction) adds to the activated double bond of the alkene, giving predominantly the terminal alkene [Eq. (32)]. In contrast, initial formation of the radical anion of the (di)activated alkene may lead to an S>j2 reaction between the radical anion and the allyl halide followed by further reduction of the intermediate radical and final protonation [Eq. (33)] [190,191]. However, electron transfer between the alkene radical anion and especially allyl iodide followed by coupling of the allyl radical and a radical anion cannot be ruled out. 

These diastereoselectivities can rationalized by transition state (64 Scheme 9). A pyramidal configuration of the anionic center is assumed (c/. nonplanarity of a benzylic or an allylic anion ). This leaves more space for an equatorial substituent EWG (64) than for an axial EWG (65). The resulting product structure (63) agrees well with the general trends of Table 4. The finer issue — why (64) is preferred over (65) in the case of Z) vs. ( )-alkenes — remains unexplained. 

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