[36049-90-8]  · C4H7LiS2  · 2-Lithio-1,3-dithiane  · (MW 126.19)

(umpolung; C-C bond formation)

Physical Data: pKa values for several 1,3-dithianes;1e,2 measurements/calculations on relative acidity of axial vs. equatorial C-2 hydrogens;3 crystal structure for 2-methyl- and 2-phenyl-2-lithio-1,3-dithiane;4 low-temperature 13C NMR spectra of 6Li- and 13C-labeled reagent.5

Solubility: sol ether, THF; slightly sol pentane.

Analysis of Reagent Purity: solutions of the reagent are colorless, with the exception of solutions of the 2-vinyl- and 2-phenyl-substituted analogs, which are yellow; solutions of the reagent should be titrated before use by one of the standard methods, e.g. MeOD quenching studies (NMR).

Preparative Methods: the parent compound is most conveniently prepared from commercially available 1,3-Dithiane by metalation with one equivalent of n-Butyllithium at -20 °C in THF.6 2-Lithio-1,3-dithianes with a substituent at C-2 can be prepared similarly; the reaction time for metalation varies.6 Alternatively, Sn/Li transmetalation (LDA or MeLi) of 2-trimethylstannyl- or 2,2-bis(trimethylstannyl)-1,3-dithiane7 is a much faster process than the direct metalation and occurs within a minute at -78 °C. This allows preparation of substituted 2-lithio-1,3-dithianes, in which the substituent has electrophilic sites, which otherwise would not survive the metalation process.7 Some sodium8 and potassium9 analogs have been reported.1e A comprehensive review on the preparation of dithianes is covered under 1,3-Dithiane.

Handling, Storage, and Precautions: must be prepared and transferred under inert gas (Ar or N2) to exclude oxygen or moisture; solutions in THF are stable for weeks at -20 °C; at room temperature, 2-lithio-1,3-dithiane does not decompose via a carbenoid pathway, but abstracts a proton from the solvent.1f Handle in a fume hood.


On treatment with primary, secondary, or allylic alkyl halides, or primary sulfonates, the corresponding substituted dithianes are obtained, generally in good yield (eq 1).1b -f,10,11

Formally, these transformations give access to ketones from aldehydes (or aldehydes from Formaldehyde, R = H),12 i.e. the dithiane moiety here functions in a reactivity umpolung mode,13 as acyl anion A or acyl dianion equivalent B.

Allylic alcohols, after appropriate in situ activation, can be used as alternatives to the alkyl halides (eq 2).14

Considerable chemoselectivity is observed in reactions with multifunctionalized electrophiles,1b-f e.g. epoxide vs. primary halide (eq 3),15 or allylic vs. nonallylic halide,16 or iodide vs. allylic mesylate (eq 6).17

Cycloalkylation products can be obtained from reactions with bifunctional electrophiles, an approach which has found use in the preparation of a variety of macrocycles (acyl dianion equivalent B, eq 4).1b-f Alternatively, dialkylation with the appropriate electrophiles can also be used for the straightforward preparation of large-ring compounds (eq 5)1e,f,18 or to set the stage for the synthesis of polycyclic structures (eq 6).17,19 More recent examples of alkylation of 2-lithio-1,3-dithianes include the preparation of backbone-rigid diamides,20 the synthesis of (-)-ε-cadinene,21 methyl ketones which undergo asymmetric reduction in the presence of the dithiane moiety,22 aminoalkynyldithianes,23 and dioxaspiro[5.5]undecane ring systems.24

2-Lithio-1,3-dithiane reacts with arene-metal complexes to give the corresponding arene substituted derivatives (eq 7).25 Some regiochemical control can be achieved; side products often limit the utility of these reactions.26

Nitroalkenes (eq 8), as do nitroarenes, react with 2-lithio-1,3-dithianes by conjugate addition.27 Vinyl sulfones, as described for an oxygenated cyclopentene (eq 9)28 and an a,b-(phenylseleno)vinyl selenoxide29 example, display a similar reactivity pattern.

Vinylphosphonium salts react with 2-lithio-1,3-dithiane to form the b-phosphonio substituted derivative. The inherent properties of the dithiane moiety then allow for the preparation of b-phosphinyl carboxylic acids (eq 10)30 or other functionalized phosphorus compounds,31 valuable building blocks in organic synthesis.


The reagent reacts with activated imines to afford the corresponding a-aminoalkylated derivatives (eq 11). Typical reaction partners are phenylazirines (eq 12)32 or iminium salts.33 In the case of the former, only the C-2 substituted reagent gives the a-aminoalkylated compound; the parent reagent will afford b-aminoketene dithioacetals instead (eq 13).32a Recently, the reaction with N-tosylated aziridines has been reported, which leads to the b-amino substituted ketone derivatives.34 1-Methyl-4-quinolone as reaction partner gives the a-aminoalkylated product, which in this case derives from a 1,4-addition reaction (see also a-Hydroxyalkylation/g-Ketoalkylation below).35


Epoxide ring opening with 2-lithio-1,3-dithianes proceeds in general with good regio- and stereochemical control, following the rules for SN2-type processes, and affording the b-hydroxyalkylated products in good to excellent yield, routinely exceeding 80% (eq 14).1b-f

More recent examples include stereoselective epoxide opening to access enantiomerically pure hydroxylated compounds,36 e.g. propanal-type aldol products.37 This reaction sequence also proved to be useful for the synthesis of C-3 formyl sugar derivatives,38 chiral 4-hydroxycyclopentenones from glucose,39 and was employed as a key feature in the construction of the Kishi intermediate40 for aplysiatoxin (eq 15).41 Noteworthy is the high chemoselectivity displayed by the reagent in these reactions.

An unusual example, in which the reagent reacts preferentially with a fulvene functionality over epoxide opening, has been reported (eq 16).42


Both types of product, obtained from the addition of 2-lithio-1,3-dithianes to aldehydes and ketones, or to the conjugate unsaturated analogs, represent one of the unique features available from the reactivity pattern of the reagent. By virtue of its role as acyl anion equivalent A, this reaction allows establishment of a 1,2-arrangement of oxygen functional groups, not so readily achieved by using classical reagents (eq 17). It is this feature of umpolung1d of the normal carbonyl reactivity pattern, which earned 2-lithio-1,3-dithianes so much attention in synthetic organic chemistry, and accounts for a significant proportion of all reactions involving the use of the reagent.1b-f The yields observed for these addition reactions are in general very good; product formation in the 70-90% range is not uncommon.

The scope of the reagent was broadened after a thorough investigation established the factors which influence the regioselectivity of the addition of the reagent to a,b-unsaturated aldehydes and ketones.1d As a complement to the standard 1,2-addition mode described above, 2-lithio-1,3-dithianes can also be used deliberately to establish a 1,4-arrangement of oxygen functional groups (eq 18), again by functioning as an acyl anion equivalent A.1b-f,35,43 The outcome of the addition to enones can be influenced not only by the choice of solvent,1d but also with the help of alkene-complexing agents, as has been demonstrated for the tricarbonyl(tropone)iron complex.44

Recent literature reports the addition reaction of 2-lithio-1,3-dithiane to aldehydes for assembling the C(10)-C(19) moiety of FK506,45 and the possibility of achieving asymmetric induction in this reaction sequence has been investigated (eq 19).46

Cyclic hemiacetals can be used in place of the free carbonyl compound with equal success.47 The stereoselectivity of the reaction can be influenced by neighboring polar groups, as illustrated in the example with (R)-pantolactone (eq 20).47


The characteristic features of the reagent, as indicated in the foregoing section on addition reactions to carbonyl derivatives, also dominate the reactivity pattern observed for the reaction with carboxyl derivatives. The role of 2-lithio-1,3-dithianes as acyl anion equivalent A, establishing a 1,2-relationship between the functional groups, is the dominating feature (eq 21).

2-Lithio-1,3-dithiane reacts with acyl derivatives, such as esters or lactones, amides or aziridinones,48 to afford the corresponding acyl dithiane in variable yield (eq 21).1b-f Little investigation has yet been carried out into the regioselectivity for the reaction with the corresponding a,b-unsaturated derivatives.49,50 With an electron releasing substituent on C-2 the reagent reacts with butenolides in a 1,4-conjugated fashion (eq 22), a scheme specifically exploited in several syntheses of steganin lignans.49 Also, unsaturated amides, thioamides, and nitriles are reported to react with the reagent in the 1,4-mode indicated in eq 22.50c-f

Acid chlorides will give the double addition carbinol product (eq 25),51 nitriles afford either amino ketene dithioacetals (eq 24)52,53 or ketones (eq 23),54 nitrile oxides lead to oxime derivatives (eq 26), activated imides can be opened regioselectively (eq 29),55 activated ketene acetals give the corresponding enol-type compound (eq 30),56 and isocyanides afford the cyano substituted dithiane (eq 27).57

Carboxylation by reaction with Carbon Dioxide proceeds in excellent yield (eq 28).1d,e,58 The reaction products find use as intermediates in a notable lactone synthesis.59

The reaction of 2-lithio-1,3-dithiane with carboxyl derivatives can be reversed. This result allows a synthetically useful C-C bond cleavage (eq 31).1e

2-P, Sn, Si, Ge, and Ti Derivatives.

The versatility of 2-lithio-1,3-dithiane as an acyl anion A or acyl dianion equivalent B is further amplified by the possibility of preparing the phosphorus,60 tin,1f,7 silicon,60,61 germanium,1f,62 or titanium63 analogs, e.g. on simple treatment with the corresponding chlorides. The phosphorus and silicon analogs deserve special attention, as they allow a convenient entry to ketene dithioacetals (see below). Also, the silicon analog was used in the preparation of formyl-61 and acylsilanes.11 The tin and germanium, as well as silicon, derivatives react with acid chlorides to give 2-acyl-1,3-dithianes.62 The special role of the tin derivative has been outlined already (see Preparation above);7 the titanium analog was found to be more stable than the reagent, and as a consequence more selective.63

Ketene Dithioacetals.

The lithio derivative obtained from deprotonation of 2-Trimethylsilyl-1,3-dithiane deserves special mention. On reaction with carbonyl compounds, ketene dithioacetals are obtained, compounds which are responsive to both electrophilic as well as nucleophilic attack. Other methods for the synthesis of ketene dithioacetals have been reviewed.60 In addition to their normal reactivity pattern, these derivatives offer an exceptionally broad umpolung-type reactivity pattern, i.e. acyl anion A, enolate carbanion C, homoenolate anion D, and chemistry of the homolog E becomes available (eq 32). The

richness offered by this class of compounds in terms of their potential as strategical and useful intermediates in organic synthesis is documented amply, and reference to review articles have to suffice in this context.1a,60

Related Reagents.

Bis(methylthio)(trimethylsilyl)methane; Bis(phenylthio)methane; N,N-Diethylaminoacetonitrile; 1-Ethoxyvinyllithium; RL031-; Methoxy(phenylthio)methane; 1-Methoxyvinyllithium.

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Michael Kolb

Marion Merrell Dow, Cincinnati, OH, USA

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