A-Sulfonyl lithium carbanions

Similarly, the reaction of 1-chloropentyhnagnesium chloride (56), derived from 1-chloro-l-iodopentane, with a-sulfonyl lithium carbanions (52) affords 1,2-di- or 1,1,2-trisubstituted olefins (57) in moderate to good yields (equation 14). This reaction represents an elegant preparation of olefins from sulfones. 

TABLE 1. Reaction of a-sulfonyl lithium carbanions (52) with chloromethylmagnesium chloride (53)... 

Thus, magnesium carbenoid 38, generated from 1-chloroalkyl phenyl sulfoxide (37) in THF at —65°C with 2.8 eq of i-PrMgCl, reacts with a-sulfonyl lithium carbanion to lead to 1,2-di- and 1,1,2-trisubstituted olefins (60). Yields are better in such conditions as compared to the reaction described in equation 14. 

The proposed mechanism of this reaction is composed by an initial S v2-type nucleophilic substitution reaction of 113 with the nucleophilic a-sulfonyl lithium carbanion to give the alkylmagnesium species (114) having a sulfonyl group at the / -position. Then, a -elimination reaction of magnesium sulfinate from the intermediate (114) occurs... 

TABLE 4. Synthesis of alkylidenecyclopropanes (118) from 1-chlorocyclopropyl phenyl sulfoxide (116) and a-sulfonyl lithium carbanions through magnesium cyclopropy-Udene (117)... 

The electrophilic reaction of magnesium alkylidene carbenoids with other nucleophiles than the original Grignard reagent can also be carried out. For example, treatment of magnesium alkylidene carbenoid 157, derived from 147, with a-sulfonyl lithium carbanion afforded allenes 159 in moderated yields (equation 39/. ... 

The proposed mechanism is as follows First, the a-sulfonyl lithium carbanion attacks the electrophilic carbenoid carbon atom to give the vinyhnagnesium intermediate (158). As the sulfonyl moiety is a good leaving group, a /-elimination takes place to afford the allenes (159). 

X-ray crystallography is by now one of the most important methods for the determination of carbanion structures. From the very large number of recently characterized carbanions only a few can be mentioned here. They include preparatively important species such as the first lithiated carbamide [29], the lithium and tetra- -butyl ammonium salts of a a-sulfonyl substituted carbanion [30], and a protoype of the synthetically very useful Lochmann bases. [31] On the other hand structural work - both by X-ray crystallography and by various modem NMR spectros-... 

Intramolecular cyclization of 2-phenysulfonylmethyl lactam 3 took place upon reaction with lithium hexamethyldisilazan via generating its a-sulfonyl carbanion to give a cyclized postulated intermediate that can be quenched with trimethylchlorosilane to afford the stable silyl ketal 4. The later ketal was desulfonylated by Raney-Ni and desilylated through treatment with tetrabutyl ammonium fluoride (BU4NF) to afford the carbacephem 5 (94M71) (Scheme 1). 

Potassium or lithium derivatives of ethyl acetate, dimethyl acetamide, acetonitrile, acetophenone, pinacolone and (trimethylsilyl)acetylene are known to undergo conjugate addition to 3-(t-butyldimethylsiloxy)-1 -cyclohexenyl t-butyl sulfone 328. The resulting a-sulfonyl carbanions 329 can be trapped stereospecifically by electrophiles such as water and methyl iodide417. When the nucleophile was an sp3-hybridized primary anion (Nu = CH2Y), the resulting product was mainly 330, while in the reaction with (trimethylsilyl)acetylide anion the main product was 331. 

The a-sulfonyl carbanions can be trapped with a variety of electrophiles19. The method provides a synthetically useful synthon for a propylene 1,3-dipole. Reductive cleavage of the sulfone 28 thus prepared, with lithium phenanthrenide in THF, furnishes bicyclooctane 29 (equation 19)16. 

In the course of oxidation of the sulfonyl group in 32, lithium di isopropyl ami do is used to form the a-sulfonyl carbanion 33 This anion reacts with MOPH via complex 34 to give natural product 1. The MOPH complex utilized here is crystalline and stable to air over brief periods of time, so it is relatively convenient to employ. It has become a standard reagent tor a-hydroxylation of CH-acidic compounds.1 ... 

Certain substituted sulfones may be obtained by special methods for instance, vinyl sulfones (82) may be formed by addition of a sulfonyl carbanion (83) to a carbonyl compound followed by elimination (Scheme 34). An example when X = H is the synthesis of methyl styryl sulfones (84) by the Knoevenagel condensation of an aromatic aldehyde with a methanesulfonylacetate (85) followed by dealkylation-decarboxylation of the intermediate product by treatment with lithium iodide in DMF (Scheme 35). 

This sulfoxide-magnesium exchange reaction could be successfully applied to a new synthesis of allenes (Scheme 3.12) [6]. The procedure is a novel method for synthesis of allenes from three components, ketones, chloromethyl p-tolyl sulfoxide, and sulfones, in relatively short steps. A key step is an attack of the lithium a-sulfonyl carbanion on the electron-deficient carbene carbon. /1-Elimination of the sulfoxyl group then occurs to give the allene. 

Removal of the protons adjacent to the sulfone functionalities was effected with n-butyl lithium at temperatures ranging from 0 to 30°. The formation of the a-sulfonyl carbanions could be followed visually upon addition of one equivalent of n-butyl lithium, the polymer solution turned red, further addition of base caused a darkening of the solution to a greenish brown coloration. Quenching experiments with methyl iodide, followed by NMR analysis of the products, confirmed that these colorations were due to the appearance of mono- and dianions, respectively. In most cases, the polyanions remained in solution throu out the reaction sequence. 

Gais HJ, HeUmann G. Stereochemistry of chiral, nonracemic lithium-salts of acyclic a-sulfonyl carbanions-the asymmetric induction exerted by the lithium-coordinated sulfonyl group. 7. Am. Chem. Soc. 1992 114(ll) 4439-4440. 

In Entry 5, the carbanion-stabilizing ability of the sulfonyl group enables lithiation and is then reductively removed after alkylation. The reagent in Entry 6 is prepared by dilithiation of allyl hydrosulfide using n-bulyl lithium. After nucleophilic addition and S-alkylation, a masked aldehyde is present in the form of a vinyl thioether. Entry 7 uses the epoxidation of a vinyl silane to form a 7-hydroxy aldehyde masked as a cyclic acetal. Entries 8 and 9 use nucleophilic cuprate reagents to introduce alkyl groups containing aldehydes masked as acetals. 

Nitroalkenes react with lithium dianions of carboxylic acids or with hthium enolates at -100 °C, and subsequent treatment of the Michael adducts with aqueous acid gives y-keto acids or esters in a one-pot operation, respectively (Eq. 4.52).66 The sequence of Michael addition to nitroalkenes and Nef reaction (Section 6.1) provides a useful tool for organic synthesis. For example, the addition of carbanions derived from sulfones to nitroalkenes followed by the Nef reaction and elimination of the sulfonyl group gives a,P-unsaturated ketones (Eq. 4.53).67... 

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