Allylic anions reaction with electrophiles

However, it can be expected that anions of allyl sulfoxides maintain, at low temperature, the chiral integrity of the sulfoxide group and that reactions with electrophiles could have regio-and stereochemical implications. 

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). ... 

Heteroatom-substituted allylic anions can serve as homoenolate anion equivalents in reaction with electrophiles, when y-attack can be realized and the formed vinyl heterocompound can be hydrolyzed to an aldehyde (Scheme 76). ... 

The discussion on pp. 29-31 established the many reasons why an allyl anion and an allyl cation react with electrophiles and nucleophiles respectively at C-l (and C-3) of the allyl system. The force of these arguments is less when they are applied to the reaction of an allyl radical with a radical. Although the frontier orbital interaction (Fig. 5-10a) will still favour attack at C-l in the usual way, the interaction of the lowest filled 7E-orbital will not be negligible, especially with a radical having a low-energy SOMO (Fig. 5-10b). Since the lowest filled 7t-orbital has the larger coefficient on C-2, reaction at this site... 

The most useful of all allyl anion equivalents are the allyl silanes.20 This is because it is easy to make them regioselectively, because they do not undergo allylic rearrangement (silicon does not do a [1,3] shift) and because their reactions with electrophiles are very well controlled addition always occurring at the opposite end to the silicon atom. Symmetrical allyl silanes can be made from allyl-lithiums or Grignards by displacement of chloride from silicon. A useful variant is to mix the halide with a metal, e.g. sodium, and Me3SiCl in the same reaction, rather after the style of the silicon acyloin reaction,21 as in the synthesis of the acetal 80. 

From this point on, the regioselectivity of substituted allyl anions is much less regular, and somewhat less explicable. For a start, X-substituted allyl anions react with carbonyl electrophiles with a selectivity. This is explicable, but it is determined by the site of coordination by the metal, not by the frontier orbitals. We can contrast the reaction of the oxygen-substituted lithium anion 4.57 with an alkyl halide, which is y selective, as usual, and the reaction of the zinc anion 4.58 with a ketone, which is a selective.304 The oxygen substituent coordinates to the zinc cr-bound at the y position, and the aldehyde is then delivered to the a position in a six-membered cyclic transition structure 4.59. The same reaction with the lithium reagent 4.57 gives a 50 50 mixture of a and y products, and so lithium is not so obviously coordinated in the way that the zinc is. This type of reaction is often brought under control in the sense 4.59 for synthetic purposes by... 

Reaction of (50) with alkyl halides gives exclusive a-alkylation. With aldehydes and ketones, a-addi-tion again takes place to give (52) via intramolecular silyl transfer with concomitant loss of lithium cyanide (c/. 25 Scheme 30). Treatment of (52) with p-Ts0H H20 gives the cyclopentenone annelation product. The allylic cyanohydrin anion (53) also gives a-adducts upon treatment with aldehydes and ketones at -78 °C, whereas reaction with electrophiles at 0 C affords -y-adducts (c/. 25). 

Nonanoylzirconocene chloride (1), readily prepared by the reaction of 1-octene and zirconocene hydrochloride (Schwartz reagent) followed by exposure to 1 atm of CO, serves as an umnasked acyl anion and provides ketones or a-diketones in moderate to good yields by the reaction with electrophiles (aryl iodide, benzyl bromide, acyl chloride, allyl chloride, aUyl acetate) in the presence of 5% of PdCl2(PPh3)2 (Scheme 47). ... 

Regioselectivity in reactions of heterocyclic anions of allyl-type with electrophiles 99CRV665. 

Regioselectivity of syntheses of heterocycles in reactions of allyl-type anions with electrophiles 99CRV665. 

The Pd(0)-catalyzed displacement of allylic acetates (297) with various nucleophiles via the allylic Pd(II) complex (298) is a well-established procedure (Scheme 114). Through attack of electrons (+2e ) in place of nucleophiles, (298) is expected to undergo a reductive cleavage providing allylic carbanions (299) and the acetate anion along with Pd(0) complexes. The latter can then be captured by various electrophiles (polarity inversion. Scheme 114) leading to (300) [434]. This procedure is useful for the deprotection of allyl esters under neutral conditions. Recently, a mechanistic study of the Pd-catalyzed reaction of allylic acetate (297), using carbonyl compounds as an electrophile, has been reported [435]. 

Contrary to Uthiated alkyl carbamates, which hold a pyramidalized sp -hybridized carb-anionic centre and thus generally react with electrophiles under retention of configuration (see Section n.B.l), lithiated ( )-allyl carbamates 310 have a high tendency for antarafa-cial reactions, which seem to be enforced by the bulky sparteine as the lithium ligand (equation 79). Fortunately, in these reactions, the Af,Af-diisopropylcarbamoyloxy group prefers the ewdo-position to lead to the a-product 311 (inversion) and to the y-product (Z)-312 [sometimes besides small amounts of (E)-ent-312)]. 

Telechelic polymers, containing one or more end groups with the capacity to react with other molecules, are useful for synthesizing block and other copolymers [Fontanille, 1989 Hsieh and Quirk, 1996 Nuyken and Pask, 1989 Pantazis et al., 2003 Patil et al., 1998 Quirk et al., 1989, 1996 Rempp et al., 1988]. Living anionic polymers can be terminated with a variety of electrophilic reagents to yield telechelic polymers. For example, reaction with carbon dioxide, ethylene oxide, and allyl bromide yield polymers terminated with carboxyl, hydroxyl, and allyl groups, respectively. Functionalization with hydroxyl or carboxyl groups can also be achieved by reaction with a lactone or anhydride, respectively. Polymers with amine end... 

In order to illustrate this point, we report the opposite behavior of the ally and n -butyl anions in the substitution of an optically active fluorosilane, 1-NpPhMeSi—F. The first leads to inversion (91% IN) and the latter to retention (98% RN) (49), when the reactions with allyl- and n -butyllithium are carried out in the presence of a cryptand specific for lithium cation. The use of a cryptand makes it possible to study the difference in behavior of the anion species alone, since it avoids the possibility of electrophilic assistance by Li+ which could affect the apicophilicity of the fluorine atom. 

Whereas an allylsilane can serve as an allyl anion synthon, the reaction of 1,3-bis(silyl)propene with electrophiles can afford the 1,3-disubstituted propene. Thus, treatment of a mixture of (If)- and (Z)-2-aryl-l,3-bis(trimethylsilyl)propenes 5 with 2 equivalents of NBS at —78 °C stereoselectively yields the corresponding (Z)-2-aryl-l,3-dibromopropene 6. When 1 equivalent of NBS is employed, the monobromo product 7 is obtained (equation 5). The reactions apparently proceed via the pattern of sequential displacement of allylsilane moieties40,41. 

These thioacetals can be deprotonated by K2C03 in the presence of Aliquat 336 and the anions obtained react with various electrophiles including allylic halides. In contrast to reactions with NaH as the base, reaction with an allylic bromide results in allylic transposition. 

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