Lithium dimsylate

Anionic Claisen rearrangement. Acceleration of the Claisen rearrangement of allyl vinyl ethers was originally observed with potassium hydride in HMPT, but even milder conditions are possible using sodium or lithium dimsylate. The catalyzed rearrangement is as stereoselective as the thermal counterpart. Thus vicinal asymmetric centers are formed selectively on rearrangement of (E)- and (Z)-crotyl ethers (equations I and II). 

Claisen rearrangement The Claisen rearrangement of highly substituted allyl vinyl ethers can proceed readily even when two vicinal quaternary centers are formed. Thus addition of an allylic alcohol (1) to an allenic sulfone (2) provides the allyl vinyl ether 3, whose anion rearranges at 50° to 4. Lithium dimsylate is the preferred base other counterions promote an undesirable side reaction.1... 

A high degree of asymmetric induction has been realized in the carbanion-accelerated Claisen rearrangement of phosphorus-stabilized anions. Treatment of 1,3,2-oxazaphosphorinane (166) with freshly prepared lithium dimsylate led to a 95 5 ratio of a-methyl ketones (167) and (168) (Scheme 33). Li coordination combined with steric interactions provide the necessary control elements for stabilization of the highly organized allyl anion conformation (169). 

The formation of the above anions ("enolate type) depend on equilibria between the carbon compounds, the base, and the solvent. To ensure a substantial concentration of the anionic synthons in solution the pA" of both the conjugated acid of the base and of the solvent must be higher than the pAT -value of the carbon compound. Alkali hydroxides in water (p/T, 16), alkoxides in the corresponding alcohols (pAT, 20), sodium amide in liquid ammonia (pATj 35), dimsyl sodium in dimethyl sulfoxide (pAT, = 35), sodium hydride, lithium amides, or lithium alkyls in ether or hydrocarbon solvents (pAT, > 40) are common combinations used in synthesis. Sometimes the bases (e.g. methoxides, amides, lithium alkyls) react as nucleophiles, in other words they do not abstract a proton, but their anion undergoes addition and substitution reactions with the carbon compound. If such is the case, sterically hindered bases are employed. A few examples are given below (H.O. House, 1972 I. Kuwajima, 1976). 

Treatment of dimethylsulfoxide (DMSO) with sodium hydride generates methylsulfinyl carbanion (dimsyl ion), which acts as an efficient base in the production of ylides. The Wittig reaction appears to proceed more readily in the DMSO solvent, and yields are generally improved over the reaction with -butyl lithium (i). Examples of this modification are given. 

To obtain complete conversion of ketones to enolates, it is necessary to use aprotic solvents so that solvent deprotonation does not compete with enolate formation. Stronger bases, such as amide anion ( NH2), the conjugate base of DMSO (sometimes referred to as the dimsyl anion),2 and triphenylmethyl anion, are capable of effecting essentially complete conversion of a ketone to its enolate. Lithium diisopropylamide (LDA), which is generated by addition of w-butyllithium to diisopropylamine, is widely used as a strong... 

Table 4.4 lists some common bases used in organic chemistry. Although butyl-lithium behaves as a very strong base in many reactions, it also exhibits other chemistry, so it is usually used to prepare other strong bases listed in the table. Lithium diisopropylamide, sodium amide, dimsyl anion, and sodium hydride are often used to prepare the conjugate bases of aldehydes, ketones, and esters for use in reactions. Potassium fert-butoxide is employed when a base somewhat stronger than the conjugate bases of most alcohols is needed. 

The use of DMSO in recent studies has been largely upon the premise that the problem of base association would be avoided. Some doubt as to the validity of this assumption arises when the results of a conductometric study are considered (Exner and Steiner, 1974). Ion-pairing constants for lithium, sodium, potassium and cesium t-butoxide in DMSO have been evaluated as 108, 106, 270 and 200 M-1 respectively. Not only do these results suggest that there is base association in DMSO but they also imply that base-catalysed reactions involving alkali metal alkoxides in DMSO should be affected by the nature of the cation. If these conclusions are valid and if the possible involvement of the dimsyl anion in these reactions is also taken into consideration, then the choice of DMSO to remove the problem of base association can be a poor one, especially if the base is a lithium or sodium salt of a hindered alkoxide. It is far better to avoid association effects by the use of crown ethers (Bartsch et al., 1973, 1974, 1975). On the other hand, the use of lithium and potassium t-butoxide in DMSO solvent might aid in distinguishing reactivities of free ions and of ion pairs in certain processes. 

Streitwieser et al. [160] and BordweU et al. [161] used the lyate ions of organic solvents such as cyclohexylamine and dimethyl sulfoxide in the determination of the C—H acidity of weak organic carbon acids. Using super base systems such as alkali metal salts of cyclohexylamine [i.e. lithium and cesium cyclohexylamides) [160] and dimethyl sulfoxide (sodium dimsyl) [161] in an excess of these non-HBD solvents, relative acidity scales for weak carbon acids have been established. In this way, values for the ionization of over a thousand Bronsted acids in dimethyl sulfoxide have... 

Other examples of carbon nucleophiles which have been added to a glycidol include the lithium salt of 1-trimethylsilyl-3-phenylthioprop-l-yne (eq 13), the lithium salt of 1-phenylsulfonyl-2-trimethylsilylethane, the lithium salt of pentacarbonyl(methoxymethylcarbene)chromium, the dimsyl anion, and the lithium salt of acetone dimethylhydrazone. ... 

A lot of progress in this area is due to the work of Trost, who introduced diphenylsulphonium cyclopropylide and phenylthiocyclopropyl lithium as extremely versatile C3-building blocks. The first reagent is easily available from the corresponding sulphonium salt by deprotonation with suitable bases (either under irreversible conditions with dimsyl sodium, or, preferably, in a reversible manner by employing potassium hydroxide in DMSO). The ylide adds to a,) -unsaturated carbonyl compounds forming... 

The chemistry of the methylsulphinyl carbanion, MeS(0)=CHa, has been reviewed by Durst and by Field. Szmant has briefly reviewed the stereochemistry of sulphinyl carbanions. Martin has reported the isolation of etherate complexes of dimsyl-lithium but reported the uncomplexed material to be shock-sensitive. 

Rearrangement reactions of 1,4-benzodiazepines are numerous and have been the subject of a review. A novel rearrangement of the chlor-diazepoxide analogue (217) is now reported whereby reaction is induced by dimsyl-lithium. The products vary depending upon whether the reaction is quenched by water or dimethyl sulphate, see Scheme 4, but all may be accounted for in terms of intermediate (218). 

The 32 possible derivatives of methyl a- and -D-fmctofuranosides bearing four chromophores, chosen from 4-bromobenzoate or 4-methoxycinnamate, have been prepared for c.d. studies. All nine methylated/benzoylated derivatives 2 of 1,5-anhydro-D-fucitol have been synthesized by partial methylation (dimsyl lithium, DMSO, methyl iodide) followed by in situ benzoylation. The corresponding acetates were available by debenzoylation/acetylation of compounds 2. ... 

Two groups have developed means of extending the condensation of sulphinyl carbanions with carbonyl compounds, such that it becomes an alternative to the Wittig reaction, through conversion of the initial carbinol adducts into olefins. Dimsyl-lithium with benzophenone gave the initial adduct as a lithium salt, which, when treated with a chlorophosphite at -80 °C, afforded a 91% yield of diphenylethylene." The isolated initial adduct (97) could be converted into undecene by reaction first with MeS—CH CH—(CH2)sCH3... 

Petrus, L., Cray, D. C., BeMiller, J. N. (1995). Homogeneous alkylation of cellulose in lithium chloride/dimethyl sulfoxide solvent with dimsyl sodium activation. A proposal for the mechanism of cellulose dissolution in LiCl/MezSO. Carbohydrate Research, 268, 319-323. 

---