1.3- Dienes allylic anions

Hexaaza-1,5-dienes RN=NNRNRN=NR, derivatives of 15 [96], are unusual high-energy molecules. Very recently, Cowley, Holland, and co-workers [101] fairly well stabilized the dianion RN R "16 as a ligand in a transition metal complex. These species are stabilized by such conjugations as those in allyl anions, which are special conjugations of the n-tr conjugations. 

In the last decade, a new aspect of nickel-catalyzed reactions has been disclosed, where nickel serves to selectively activate dienes as either an al-lyl anion species or a homoallyl anion species (Scheme 1). These anionic species are very important reactive intermediates for the construction of desired molecules. Traditionally they have been prepared in a stoichiometric manner from the corresponding halides and typical metals, e.g., Li, Mg. In this context, the catalytic generation method of allyl anions and homoallyl anions disclosed here might greatly contribute to synthetic organic chemistry and organotransition metal chemistry. 

This review focuses on the recent developments in nickel-catalyzed functionalizations of conjugated dienes as allyl anions and homoallyl anions. 

Nickel(O) complexes are extremely effective for the dimerization and oligomerization of conjugated dienes [8,9]. Two molecules of 1,3-butadiene readily undergo oxidative cyclization with a Ni(0) metal to form bis-allylnickel species. Palladium(O) complexes also form bis-allylpalladium species of structural similarity (Scheme 2). The bis-allylpalladium complexes show amphiphilic reactivity and serve as an allyl cation equivalent in the presence of appropriate nucleophiles, and also serve as an allyl anion equivalent in the presence of appropriate electrophiles. Characteristically, the bis-allylnickel species is known to date only as a nucleophile toward carbonyl compounds (Eq. 1) [10,11],... 

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. 

The reduction of the tetrasilacyclohepta-1,2-diene with a sodium mirror in Et20 gave dark red crystals that were shown by an X-ray structure determination to be the sodium salt of the allyl anion 61 produced by a series of intramolecular rearrangements of the initially formed radical anion (Fig. 37). The Na-C distances are 265.6(5)-288.2(5) pm.98... 

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. 

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. 

The essential features of the Diels-Alder reaction are a four-electron n system and a two-electron it system which interact by a HOMO-LUMO interaction. The Diels-Alder reaction uses a conjugated diene as the four-electron n system and a it bond between two elements as the two-electron component. However, other four-electron it systems could potentially interact widi olefins in a similar fashion to give cycloaddition products. For example, an allyl anion is a four-electron it system whose orbital diagram is shown below. The symmetry of the allyl anion nonbonding HOMO matches that of the olefin LUMO (as does the olefin HOMO and the allyl anion LUMO) thus effective overlap is possible and cycloaddition is allowed. The HOMO-LUMO energy gap determines the rate of reaction, which happens to be relatively slow in this case. 

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. 

If the donor is a sulfur atom, its lone pair is practically nonbonding (the electronegativities of sulfur and carbon are similar), above 2 and scheme c is no longer valid. If the sulfur is modeled by a carbanion, the sulfur-substituted diene and the dienophile are represented by the pentadienyl and the allyl anions, respectively. Their HOMOs will both be nonbonding. Hence FO theory predicts that placing the sulfur on the dienophile and the attractor on the diene may be slightly more favorable than the opposite substitution pattern. 

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. 

For dienes with substituents at C-2, similar arguments can be used and similar results obtained, as seen in Fig. 6.21. For the Z-substituent, some allyl cation character is mixed into the orbitals of 2-vinylbutadiene, and, for an X-substituent, some allyl anion character is mixed into the orbitals of butadiene. 

Treatment of the anion generated from the diene cyclobutadiene-colbalt complex 239 and MeLi with racemic oxaziridine 33 gave hydroxyl products 240 and 241 in 44% yield <2004JOC2516>. These products were obtained as a 1 1 mixture of diastereomers arising from exo-facc addition of the oxygen electrophile to the Jt-allyl anion. 

It was suggestedthat bicyclobutane formation from conjugated dienes occurs in a concerted fashion from vibrationally relaxed singlet having an allyl anion-methyl cation electronic configuration. 

We are not restricted to the reactions of neutral molecules. Allyl anions can react with olefins, allyl cations with dienes, and pentadienyl cations with olefins, as we can see from the examples in Fig. 4-6, in which we can also see why the reactions of allyl cations with olefins, of allyl anions with dienes, and of pentadienyl cations with dienes are not observed. 

Substituted Dienes. The C-, Z-, and X-substituents on the 2-position can be thought of as affecting the 7r-bond to which they are attached more than they affect the other 7r-bond. Thus, to take just one case, the HOMO of the 2-X-substituted diene is made by mixing a butadiene and an allyl anion, as... 

We have considered a variety of related intermediates in this chapter as extended versions of enolate ions 191 and as dienes. Another legitimate way to consider them is as acylated allyl anions 191c, and this leads us to the next chapter where we consider how to use allyl anions in synthesis. 

Reactions alkylations, reactions with epoxides and aldehydes, conjugate additions Heterocyclic synthesis with allyl silanes Reactions with Co-stabilised cations An Allyl Dianion The Role of Tin in Anion Formation Halide Exchange with Chelation Indium Allyls Allyl Anions by Deprotonation The synthesis ofall-trans dienes The synthesis ofall-trans retinol... 

Perhaps the most useful of the r 3 allyl complexes (cf. 24) are the Jt-allyls of nickel.15 The simplest type 61 are rather unstable and form the bromide-bridged complex 62 on treatment with HBr. These are stable compounds officially complexes of Ni(I) but better regarded for our purpose as dimers of r 3 complexes of allyl anions and Ni(II), much as allyl Grignard reagents 2 can be regarded as o-complexes of allyl anions and Mg(II). Direct exchange of Mg(II) for Ni(II) gives the unstable complexes 61, but the stable dimer 62 can be made by oxidative insertion of Ni(0), as its cyclo-octa-1,5-diene (COD) complex, into allyl bromide 1. 

We shall finish this chapter by considering two ways to achieve condensation between an allyl anion and an aldehyde (or ketone) in the synthesis of dienes 163. The reagent 165 is an allyl anion stabilised by a heteroatom (Z) that can be lost with the OH group after addition to R CHO. 

Allyl anions (38a),(38b) and (38c) exhibit a similar regioselectivity toward electrophiles, and thus serve as homoenolate synthons. Addition of titanium tetraisopropoxide to (39) followed by condensation with aldehydes gives the anti adducts exclusively, which can be converted to the (Z)-1,3-dienes upon treatment with methyl iodide (Scheme 18). The ( -1,3-dienes can be prepared from the lithiated allyldiphenylphosphine oxide. The stabilized allylic phosphonate anion (40) condenses with carbonyl... 

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