Allyl anions configuration

If R1 differs from R2. the preparation may lead to both regioisomers. In these cases, a synthetic route which does not rely on allyl anion substitution is often the most advantageous one. Thus, the best results are recorded for allylboronates and -silanes which also possess the required constitutional and configurational stability. 

Hard nucleophiles (alkyl anions) give predominant retention of configuration. At the opposite extreme, softer nucleophiles, such as benzyl or allyl anions, lead mainly to inversion. The following examples are quite significant ... 

Figure 15-12 also shows the electronic configuration of the allyl anion, which differs from the allyl radical in having an additional electron in tt2, the nonbonding orbital with its electron density divided between Cl and C3. 

The molecular orbitals of the allyl system are formed by the overlap of three atomic p orbitals. Because there is an odd number of atomic orbitals, one of the molecular orbitals is a nonbonding orbital, whose energy is comparable to that of the isolated p orbitals from which it was derived. Note that if there were degenerate molecular orbitals in the allyl system, the electronic configurations of various allyl species would be different. For example, if P2 3 for the allyl system had identical energy levels, the allyl anion would have two unpaired electrons. 

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. 

Figure 29.7. Allyl system. Configuration of tt electrons in cation, free radical, and anion.

In accordance with this model one finds diastereoselectively anti products on reaction of aldehydes with ( )-allyl compounds, whereas allyl systems with the (Z)-configuration give mainly syn products and it is even possible to effect asymmetric induction. As the double bond of the product can be oxidatively cleaved to a CW3 group, the reaction can be regarded as a stereoselective aldol reaction, an aspect which explains the widespread interest in this type of reaction. With heterosubstituted allylic anions it is sometimes possible to effect predominantly y-attack with different electrophiles by the choice of the heteroatom.2 For instance it is well known that with sulfur substituents like —SR, —SOR or —SOjR the a-attack dominates, but doubly lithiated allenethiol possesses high y-reactivity and can be used as a homoenolate anion equivalent in reaction with electrophiles such as alkyl halides (Scheme 7). ... 

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. 

In previous chapters the influence of the gegenion and the solvent on rearrangements of carbanions mostly concerned the rate of a certain reaction (and thus kinetic aspects), as, e.g., in the case of the cyclopropyl-allyl anion rearrangement (Sect. 2.1), the ally anion isomerization (Sect. 2.2), the cyclization of 5-hexenyl alkali metal compounds (Sect. 4) or the configurational isomerization of a-substituted vinyl-lithium compounds (Sect. 5). 

Direct generation of a cyclopropoxy anion from a siloxycyclopropane and fluoride ion has been applied to seven-membered ring formation. Michael addition of the resulting allyl anion can account for the cyclization. An uncyclized i ,y-unsaturated ketone with the E configuration is also formed as a byproduct. 

No thermal ring openings are observed for conventional cyclopropyl anions, even if they carry two phenyl groups to provide stabilization of the potential terminus of the allyl anion. CH acidic optically active precursors of cyclopropyl anions exhibit a very high degree of retention of configuration when treated with bases in polar protic solvents and in most cases in aprotic solvents, too. - ... 

Allyllithium is a softer nucleophile than alkyllithiums, so it can lead to inversion of configuration (Table 12). When the Li+ cation is complexed, the extent of inversion is always increased (Table 25) (137). The above data may be explained as follows. In the case of allyllithium, Clark, Jemmis, and Schleyer (163) have shown that the valence orbitals are centered toward Li+ and have some sp3 character. In contrast, the valence orbitals of the naked allyl anion have a pure p character, and thus are more diffuse (Scheme 33). In the latter case, the un-... 

There are several important structural differences for polydienyl anions in polar media versus hydrocarbon solvents (1) chain ends are generally not associated into higher aggregates in polar media compared to hydrocarbon (2) the charge distribution of unsymmetrical allylic anions is a function of solvent, counterion, and temperature (3) the kinetic and equilibrium distribution of chain-end configurations can vary with solvent and counterion (4) the distribution of contact ion pairs, solvent-separated ion pairs, and free ions can vary with solvent, counterion, and temperature. 

As we shall see. Frontier Configuration (FC) theory tells us that stereoselectivity in 4N + 3 electron systems should be qualitatively the same as stereoselectivity in 4N electon systems. Thus, for example, we expect that 1,3-butadiene, allyl anion, and allyl radical will all undergo electrocyclization in a conrotatory fashion. While there is every indication that this is the case in the first two systems, the ring closure of allyl radical-type systems is predicted by computations to be near disrotatory with indicative (but not definitive) experimental results pointing in the same direction. What is Wrong ... 

Addition of the anion of l-(diphenylphosphinyl)-2-octene to 4-/er/-butoxy-2-cyclopentenone occurs exclusively opposite to the rwr-butoxy moiety to give a mixture of. syn- and twr/ -adducls. As discussed above, the geometry of the alkcnc in the allylic moiety determines the relative configuration of the newly created stereogenic centers1. 

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