Allyl anions structure

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. 

The 1,3-dipoles consist of elements from main groups IV, V, and VI. The parent 1,3-dipoles consist of elements from the second row and the central atom of the dipole is limited to N or O [10]. Thus, a limited number of structures can be formed by permutations of N, C, and O. If higher row elements are excluded twelve allyl anion type and six propargyl/allenyl anion type 1,3-dipoles can be obtained. However, metal-catalyzed asymmetric 1,3-dipolar cycloaddition reactions have only been explored for the five types of dipole shown in Scheme 6.2. 

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. 

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],... 

In 1989 we reported on the synthesis and structure of the first l,3-diphospha-2-sila-allylic anion 3a [4], mentioning its value as a precursor for phosphino-silaphosphenes. In analogy to 3a we obtained the anions 3b-f [5] by treatment of 4 equivalents of the lithium phosphide 1 with the adequately substituted RSiC, of which 3b and 3c were investigated by X-ray analyses. The very short P-Si bond lengths (2.11-2.13 A) of 3a-c and the almost planar arrangement of Pl-Sil-P2-Lil indicate the cr-character of the Lithium P-Si-P allyl complex. 

FIGURE 2. Structures of calculated allyl cations, 10, and allyl anions, 11... 

With less polar solvents and more basic allyl anions the compounds are present as ion pairs. The carbon-metal bond with the alkali and alkaline earth metals are known to have high ionic character. The allyl compounds behave accordingly as salts. The structures of allyl compounds of the alkali and alkaline earth metals are of two fundamental types, a 41 (or metal cation is associated closely with a single terminal allylic carbon, and the rf 1 (or ji) type, 15, in which the cation bridges the two terminal allylic positions. 

For the allyl anion, a localized Lewis structure for the n system is... 

An example of a more strongly delocalized species is the allyl anion, which is conventionally described in terms of two resonance structures ... 

For the allyl anion the NRT weightings are found to be 47.5% each for the two leading structures (cf. Example 1.10), plus a smattering of small values (<0.5%) for 16 other structures. These weightings lead to C—C bond orders of 1.511, nearly equal to the classical value 1.5 predicted by the two leading structures alone. The C—H bond orders are 0.972 on the central C and 0.986 and 0.990 on each terminal C. The total valency of the central C is therefore... 

Even the allyl anion can be seen as an example of resonance-enhanced coordination. As shown in Section 4.9.2, r -CsHs- complexation is accompanied by a shift toward the localized H2C —CH=CH2 resonance structure that places maximum anionic character at the metal-coordinated carbon atom. In effect, the carbanionic lone pair nc is shared between intramolecular nc 7icc (allylic resonance) and intermolecular nc—>-n M (metal coordination) delocalizations, and the former can be diminished to promote the latter, if greater overall stabilization of the metal-ligand complex is achieved thereby. 

Germylene 185 can be reduced with substoichiometric amounts of KCg to give the cyclotrigermenyl radical 186, which has been structurally characterized (Equation (329)), while reaction of 185 with an excess (2 equiv.) of KC8 produces 187 which is the germanium analog of the allyl anion (Equation (330)).400 The structure of 187 has been obtained as well. 

In Chapter 11 of Part A, the mechanistic classification of 1,3-dipolar cycloadditions as a type of concerted cycloadditions was developed. Dipolar cycloaddition reactions are useful both for the synthesis of heterocyclic compounds and for carbon-carbon bond formation. Table 6.2 lists some of the types of molecules that are capable of dipolar cycloaddition. These molecules, which are called 1,3-dipoles, have n-electron systems that are isoelectronic with allyl anion, consisting of two filled and one empty orbital. Each molecule has at least one charge-separated resonance structure with opposite charges in a... 

No other synthetic method Is known that achieves the equivalent transformation. Rather elaborate procedures using an allylic anion type of the homoenolate "equivalents" or homoenolate radicals have been reported, but their tolerance to the structure of the enone acceptor is much narrower. 

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. 

Due to the low solubility of the concave pyridines 3 in diethyl ether, the corresponding pyridine buffers could not be compared with the experiments of Table 4. But when the protonations were carried out in other solvents, no influences of the acids (including the acids of Table 4) on the regioselectivity could be found. The exchange of diethyl ether by other solvents caused a color change of the allyl anion solution which indicated different structures for the anions in diethyl ether and in other, more polar solvents [44],... 

In the case of the protonation of allyl anions 54 (Structures 6), no systematic governing of the selectivity is possible at present. Due to the existence of complex mixtures of lithium allyl aggregates, these systems are extremely sensitive to small changes in reaction conditions. 

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