Carbanions lithiated

The most frequently used carbanionic lithiating agents, RLi, are MeLi, n-BuLi, s-BuLi and t-BuLi. The RLi compounds are aggregates, the degree of aggregation depending on the bulk of R, concentration and solvent. They are electron-deficient compounds, i.e., electron-pair acceptor acids, which on coordination with electron-pair donor bases, in particular ethers and amines, depolymerize to smaller units e.g., addition of TMED to hexameric n-BuLi in hydrocarbons forms the coordinatively saturated, monomeric n-BuLi-TMED. An exception is MeLi, for which the tetramer persists even in THF or on addition of TMED. 

With titanated 2-alkenyl carbamates, the opposite regioselectivity can also be observed. Lithiated l-(4-methylphenylsulfonyl)-2-alkenyl diisopropylcarbamates, after metal exchange with chlorotriisopropoxytitanium, add to aldehydes y-selectivelylls. The less reactive titanat-ing reagent tetraisopropoxytitanium does not apparently react with these stabilized lithium carbanions, because in its presence a-selectivity is retained (Section 1.3.3.3.1.3.2.). 

Treatment of a-lithionitriles with vinylic sulfones resulted in the formation of cyclized products, i.e., 3-oxothian-l, 1-dioxides 346 or cyclopropane derivatives 348. When a-lithiated aliphatic nitriles were used, carbanions 343, formed by the nucleophilic addition,... 

The oxidative dimerization of the anion of methyl phenyl sulfone (from a Grignard reagent) in ethereal solution in the presence of cupric chloride in 5% yield has been reported47. Despite the reported48 poor stability of the a-sulfonyl C-centered radicals, Julia and coworkers49 provoked the dimerization (in 13 to 56% yields) of the lithiated carbanion of alkyl phenyl sulfones using cupric salts as oxidants. The best results are obtained with cupric triflates in THF-isobutyronitrile medium (56% yield for R = H). For allyl phenyl sulfones the coupling in the 3-3 mode is predominant. 

Thia-[2,3]-Wittig sigmatropic rearrangement of lithiated carbanions 47, obtained by deprotonation of the S-allylic sulfides 46, affords the thiols 48 or their alkylated derivatives 49. The corresponding sulfonium ylides 51, prepared by deprotonation of the sulfonium salts 50 also undergoes a [2,3]-sigmatropic shift leading to the same sulfides 49 [36,38] (Scheme 13). As far as stereochemistry is concerned, with crotyl (R R =H,R =Me) and cinnamyl (R, R =H,R =Ph) derivatives, it has been shown that the diastereoselectivity depends on the nature of the R substituent and on the use of a carbanion or an ylide as intermediate. 

Tertiary amides with carbanion stabilization at the (3-carbon give (3-lithiation.61... 

Sulfur compounds are useful as nucleophilic acyl equivalents. The most common reagents of this type are 1,3-dithianes, which on lithiation provide a nucleophilic acyl equivalent. In dithianes an umpolung is achieved on the basis of the carbanion-stabilizing ability of the sulfur substituents. The lithio derivative is a reactive nucleophile toward alkyl halides and carbonyl compounds. 11... 

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. 

Additions of stabilized carbanions to imines and hydrazones, respectively, have been used to initiate domino 1,2-addition/cyclization reactions. Thus, as described by Benetti and coworkers, 2-subshtuted 3-nitropyrrolidines are accessible via a nitro-Mannich (aza-Henry)/SN-type process [165]. Enders research group established a 1,2-addition/lactamization sequence using their well-known SAMP/ RAMP-hydrazones 2-308 and lithiated o-toluamides 2-307 as substrates to afford the lactams 2-309 in excellent diastereoselectivity (Scheme 2.72) [166]. These compounds can be further transformed into valuable, almost enantiopure, dihydro-2H-isoquinolin-l-ones, as well as dihydro- and tetrahydroisoquinolines. 

Protons are easily detached from the furan nucleus, especially when some activating group is present. Even the carbonate ion will catalyze the deuter-ation of position 5 in 2-furoic acid salt.177 And 40% NaOD in D20, will catalyze the replacement of all the nuclear protons giving the fully deuterated product. The acid can be converted into d4-furan by treatment with mercury salts, and then DC1.178 For general synthetic purposes it is now usual to obtain nuclear carbanions by lithiation as described in Section IV. 

Lithiation of to form 184 was reported by the Harmata group to be the first example of a sulfoximine-stabilized vinyl carbanion. The resulting organolithium species 184 reacted with various electrophiles to supply structurally diverse benzothiazines <88TL5229>. However, the diastereoselectivity of the reactions with aldehydes was low (Scheme 48). 

Reviews covering the chemistry of group 2 metal complexes with phosphorus-stabilized carbanions,279 and of molecular clusters of magnesium dimetallated primary phosphanes, are available.2 u Magnesium phosphanes remain rare compounds.281 Lithiation of bromide 98 with BuLi in the presence of tmeda in pentane produces a lithium phosphine dimer subsequent treatment with MgCl2 in EtzO gives the phosphane 99 in 69% overall yield (Equation (19)). The centrosymmetric 99 has Mg-C = 2.217 A Mg-P = 2.77 A (av.).282... 

In a direct comparison of the reactivity of 1-alkyl- and 2-alkylbenzotriazoles, compound 393 was lithiated in the presence of benzophenone with 1 equiv of LDA to give a mixture of alcohol 394 and dimer 395 (Equation 12) <1996LA745>. No reaction was detected at the carbon adjacent to the benzotriazol-l-yl moiety. When benzaldehyde was used instead of benzophenone, only dimer 395 was obtained. This suggests that a-benzotriazol-2-yl carbon radical reactions are much faster than those of a-benzotriazol-l-yl) carbanions. 

Related to these diphosphine dichalcogenides are the triphosphine trisulfides [R2P(S)]3CH (12) which can be prepared from lithiated methylene diphosphine disulfides upon treatment with thiophosphinic chlorides (Equation 9). Deprotonation of 12 with tBuLi gives a resonance-stabilised anion 13 containing a planar central carbanion whose charge is delocalised onto the three neighbouring phosphorus and sulfur atoms.32... 

A variety of localized lithiated carbanions, such as aryllithiums and sulfur- and silicon-substituted alkyllithiums, have been found, by application of C, Li, and Li NMR techniques, to form triple ions in THF-HMPA solution. Thus, change to triple ion structures (18a-g) could be discerned as HMPA (2-5 equiv.) was added to solutions of monomeric structures (17a-g) in 4 1 THF-diethylether. The amount of triple ion is sensitive to ortho substitution monomeric (17a) and (17b) form 65-80% triple ion in presence of 1-3 equiv. HMPA whereas (17c) and (17e) form less than 20% at 5 equiv. HMPA. Pyridylthio-substituted carbanion (19) forms bis-chelated triple ion (20). 

A review entitled a-heteroatom-substituted 1-alkenyllithium regents carbanions and carbenoids for C-C bond formation has addressed the methods of generation of such species, illustrated the carbenoid reactivity of a-lithiated vinyl halides and vinyl ethers, and emphasized the synthetic potential of the carbanion species in asymmetric synthesis of a-hydroxy- and a-amino-carbonyl compounds. ... 

An example where the presence of a counterion makes a difference between the gas phase and solution phase pathways involves the intriguing carbanion produced on deprotonation of 1,3-dithiane at C-2. In solution, this species, almost invariably produced by reaction of the dithiane with butyllithium, is widely used as an acyl anion equivalent in synthetic chemistry. Its importance for the present work is that this is a configurationally stable lithiated species in solution the carbanion stays sp -hybridized, and the lithium prefers the equatorial position, even to the extent of driving a terr-butyl group on the same acidic C-2 carbanion to the axial position in the lithiocarbon species. The carbanion is thought to be stabilized primarily by orbital overlap with the C-S antibonding orbitals, as opposed to more conventional polar and 7t-resonance stabilization. ... 

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