Allylic carbanions, formation

Most dienones that have been reduced have structures such that they cannot give epimeric products. However, reduction of 17 -hydroxy-7,17a-dimethyl-androsta-4,6-dien-3-one (63) affords 17 -hydroxy-7j9,17a-dimethylandrost-4-en-3-one (64), the thermodynamically most stable product, albeit in only 16% yield. The remainder of the reduction product was not identified. Presumably the same stereoelectronic factors that control protonation of the / -carbon of the allyl carbanion formed from an enone control the stereochemistry of the protonation of the (5-carbon of the dienyl carbanion formed from a linear dienone. The formation of the 7 -methyl compound from compound (63) would be expected on this basis. 

Deprotonation of allylic aryl sulfoxides leads to allylic carbanions which react with aldehyde electrophiles at the carbon atom a and also y to sulfur . With benzaldehyde at — 10 °C y-alkylation predominates , whereas with aliphatic aldehydes at — 78 °C in the presence of HMPA a-alkylation predominates . When the a-alkylated products, which themselves are allylic sulfoxides, undergo 2,3-sigmatropic rearrangement, the rearranged compounds (i.e., allylic sulfenate esters) can be trapped with thiophiles to produce overall ( )-l,4-dihydroxyalkenes (equation 24). When a-substituted aldehydes are used as electrophiles, formation of syn-diols 27 occurs in 40-67% yields with diastereoselectivities ranging from 2-28 1 (equation 24) . ... 

It was recognized in early examples of nucleophilic addition to acceptor-substituted allenes that formation of the non-conjugated product 158 is a kinetically controlled reaction. On the other hand, the conjugated product 159 is the result of a thermodynamically controlled reaction [205, 215]. Apparently, after the attack of the nucleophile on the central carbon atom of the allene 155, the intermediate 156 is formed first. This has to execute a torsion of 90° to merge into the allylic carbanion 157. Whereas 156 can only yield the product 158 by proton transfer, the protonation of 157 leads to both 158 and 159. 

The relative amounts of these produced at various temperatures are shown in Table VII. The formation of these products may be explained using carbanionic mechanisms. The cyclic material may form by addition of an allylic carbanion to a molecule of the styrene, followed by a cyclization to yield a benzylic carbanion [Reaction (33a, b, c)]. 

The isomerization of olefins via er-alkyl species is a structure-requirement reaction, while the isomerization via alkyl cation is a structure-nonrequirement reaction. By the same reasoning, the isomerization via allyl carbanion or allyl carbonium ion is expected to be a structure-nonrequirement reaction. In fact, the formation of alkylallyl carbanions on basic surfaces seems to... 

The 7r-electrons in 3-methylallyl carbanion are distributed with more electron density on the a carbon atom than on the y carbon, but the formation of 1-butene from ris-2-butene indicates the kinetic feasibility of the attack of a proton on the y carbon of 3-methylallyl carbanion. Accordingly, the hypothesis that the hydrogenation of butadiene through a 7t-allylic carbanion would prefer a priori the 1,4-addition has no scientific basis (57). 

A predominant isomerization of metalloles to transoidal dienes occurs when t-BuLi is used34. This isomerization involves the formation of an allylic carbanion. which is protonated by water at the position a to Si (or Ge) and is silylated at the exocyclic carbon... 

During this reaction an aldehyde functionality is introduced selectively at C-6 of the piperidine. The advantages of the BOC group as activator and director of lithiation are its applicability to carbanion formation at otherwise inactivated or-positions and its convenience for addition and removal.6 Equatorially substituted 2-piperidines are well recognized to be less stable than their axially substituted isomers because of allylic 1,3-strain.7 Therefore 4 would be expected to undergo conformational equilibration to 4 . Directed lithiation via 16 and subsequent reaction with dimethylformamide finally yields 5 as an 8 1 mixture of trans and cis isomers. They can be separated by careful silica gel chromatography. 

To determine the diastereoselectivity of the above bora-ene reaction, boronate 193 derived from a-pinene was synthesized. Reaction of a-pinene 192 with Schlosser s base (BunLi + KOBu ) furnishes the allyl carbanion, which upon treatment with triisopropyl borate and subsequent transesterification with pinacol yields a-pinanyl pinacol boronate 193. Bora-ene reaction with this allyl boronate and S02 at — 78 °C in CH2CI2 yields the mixed anhydride 194 as a 2.3 1 mixture of diastereomers upon removal of excess S02. Treatment of this mixture of anhydrides with aryl Grignard led to the formation of two diastereomers of aryl sulfoxides 195 in 3.2 1 ratio (Scheme 33) <2006TL2783>. 

Simple sulfonyl carbanions which do not contain additional carbanion-stabilising groups, e.g. carbonyl groups or heteroatoms, can be readily alkylated in high yield by modern techniques with the use of alkyllithiums and lithium amide bases. A number of allylic halides have been successfully used. In allylic halides, the halogen directly attached to the double-bonded carbon is relatively inert towards nucleophilic attack (Scheme 41). In this way, sulfones (96) can be transformed via desulfonation into vinyl halides (97) or into ketones (98) by hydrolysis (Scheme 41). In contrast to ordinary alkyl sulfones, triflones (99) can be alkylated under mildly basic conditions (potassium carbonate in boiling acetonitrile) (Scheme 42). The ease of carbanion formation from triflones (99) arises from the additional electron-withdrawing (-1) effect of the trifluoromethyl moiety. 

Allylic protons of alkenes undergo base-catalyzed exchange much more rapidly than either vinylic or saturated aliphatic protons . The enhanced kinetic acidity of such protons is due to the fact that their removal results in formation of resonance-stabilized allylic carbanions. These carbanions react with proton donors to form the original alkenes and sometimes one or more isomeric alkenes, viz. 

Cythpropanes. The reaction of y,8-unsaturated epoxides with lithium tliflhylamidc and lIMPT results in formation of disubstituted cyclopropanes.A Ivpieal example is shown in equation (IX), which also indicates the importance III a pol.u solvent The reaction involves foiinalion of the allylic carbanion... 

Dimerisation of unsaturated nitriles or carbonyls is catalysed by isonitriles in the presence of copper(i) compounds. The mechanisms involve intermediate formation of vinyl or allyl carbanions. ... 

Thorium oxide is one of the catalysts active for hydrogenation by anionic intermediates. 1,3-Butadiene and 2-methyl-l,3-butadiene undergo hydrogenation by 1,4 addition of H atoms to form trarw-2-butene and 2-methyl-2-butene, respectively. The formation of alkanes is negligibly small even after complete consumption of the reactants. The intermediates are allylic carbanions, and the skeletal structures of carbon are retained during the reaction. Detailed mechanisms are described in section 4.15. 

The reaction of the phenylsulphinyl allylic lithium a-carbanion 342 with oxiranes was found by Guittet and Julia to give, after rearrangement and desulphurization, dihydroxy-dienes 343427 (equation 197). Demoute and coworkers have described the alkylation reaction of a very sophisticated 2-alkenyl sulphoxide 344 as a part of the total synthesis of a juvenile hormone 345428 (equation 198). Since the allylic sulphoxide carbanion has an ambident character, the alkylation may occur sometimes also at the y-position. This direction of alkylation is observed in the case of acyclic allylic sulphoxide anions 346, and results in the formation of the corresponding allylic sulphoxide 347 and vinylic sulphoxide 348423 (equation 199). 

Sulfoxides (R1—SO—R2), which are tricoordinate sulfur compounds, are chiral when R1 and R2 are different, and a-sulfmyl carbanions derived from optically active sulfoxides are known to retain the chirality. Therefore, these chiral carbanions usually give products which are rich in one diastereomer upon treatment with some prochiral reagents. Thus, optically active sulfoxides have been used as versatile reagents for asymmetric syntheses of many naturally occurring products116, since optically active a-sulfinyl carbanions can cause asymmetric induction in the C—C bond formation due to their close vicinity. In the following four subsections various reactions of a-sulfinyl carbanions are described (A) alkylation and acylation, (B) addition to unsaturated bonds such as C=0, C=N or C= N, (C) nucleophilic addition to a, /5-unsaturated sulfoxides, and (D) reactions of allylic sulfoxides. 

---