[598-30-1]  · C4H9Li  · s-Butyllithium  · (MW 64.06)

(strong base capable of lithiating weak carbon acids;1 useful for heteroatom-facilitated lithiations;2-4 reagent of choice for ortho lithiations5)

Physical Data: colorless to pale yellow liquid; slowly eliminates LiH; d25 0.783; bp 90 °C/0.05 mmHg;6,7 13C NMR, 1H NMR, 6Li NMR, and MS studies have been reported.8-11

Solubility: sol hydrocarbon and ethereal solvents, but should be used at low temperature in the latter solvent type: half-lives in ethereal solvents have been reported;12 reacts violently with H2O and other protic solvents.

Form Supplied in: commercially available as approximately 1.3 M solution in cyclohexane. Mostly tetrameric in hydrocarbons;9,13 mostly monomeric in THF;14 when used in combination with tertiary polyamines such as N,N,N,N-Tetramethylethylenediamine (TMEDA) and 1,4-Diazabicyclo[2.2.2]octane (DABCO), reactivity is usually increased.1,15

Analysis of Reagent Purity: since the concentration of commercial solutions may vary appreciably it is necessary to standardize solutions of the reagent prior to use. A recommended method for routine analyses involves titration of the reagent with s-butyl alcohol using 1,10-phenanthroline or 2,2-biquinoline as indicator.16 Several other methods have been described.17

Preparative Method: may be prepared in high yield from s-butyl chloride and Lithium metal in hydrocarbon solvents.13

Handling, Storage, and Precautions: solutions of the reagent are pyrophoric and the reagent may catch fire if exposed to air or moisture. Handling of the reagent should be done behind a shield in a chemical fume hood. Safety goggles, chemical resistant gloves, and other protective clothing should be worn. In case of fire, a dry-powder extinguisher should be used: in no case should an extinguisher containing water or halogenated hydrocarbons be used to fight an alkyllithium fire. Bottles and reaction flasks containing the reagent should be flushed with N2 or preferably Ar and kept tightly sealed to preclude contact with oxygen or moisture. Standard syringe/cannula techniques for air- and moisture-sensitive chemicals should be applied when transferring the reagent.18 For detailed handling techniques see Wakefield.1b


s-Butyllithium is a powerful metalating agent which is used frequently to convert a variety of organic compounds into their lithio derivatives. The resulting organolithium products may be subsequently functionalized with a large number of electrophiles, hence allowing the preparation of, for example, alcohols, carboxylic acids, ketones, esters, organosilicon compounds, and other organometallic compounds.1 The metalations of relatively strong carbon acids, such as terminal alkynes, triarylmethanes, and methyl heteroaromatics, may be regarded as simple acid-base reactions and these are done most conveniently using the less basic n-Butyllithium,19 which is also less reactive and easier to handle than s-BuLi. However, the use of s-BuLi is warranted for deprotonation of weaker carbon acids (e.g. preparation of aryl-,15 vinyl-,20,21 and certain allyllithiums,15,21,22) and those that require a less nucleophilic lithiating agent.5,23

Lithiations employing s-BuLi are most often conducted in electron-donating solvents such as Et2O, THF, and DME which coordinate to lithium much more strongly than do the alkyl groups of hydrocarbon solvents and thus enhance the reactivity of the organolithium species. It is generally agreed that this effect is the consequence of the depolymerization of higher order organolithium aggregates (tetramers) into smaller units (dimers and monomers).1 The increased reactivity of organolithiums in ether solvents can, in fact, lead to a-deprotonation of the solvent molecules at elevated temperatures; in diethyl ether, for example, s-BuLi has a maximum lifetime of about an hour at +20 °C and THF is attacked even more readily.12 Hence, to ensure the integrity of the organolithium reagents in these solvents, the use of low-temperature conditions (typically -78 °C) is imperative.

s-Butyllithium is often used in the presence of various lithium complexing ligands such as Hexamethylphosphoric Triamide, TMEDA, and DABCO, which serve to further enhance the reactivity of this reagent.15 The s-BuLi/TMEDA complex is an extremely powerful lithiating agent, effecting rapid deprotonation of many compounds, e.g. benzene, tetramethylsilane, and propene (eq 1),15 which are unreactive toward s-BuLi alone. Compared to the analogous n-BuLi/TMEDA complex, s-BuLi/TMEDA is considerably more potent as a metalating agent; e.g. the lithiation of Me4Si proceeds ~1000 times faster with s-BuLi/TMEDA than with n-BuLi/TMEDA.15a

Metal alkoxides, such as Potassium t-Butoxide, are also used in combination with s-BuLi to facilitate the metalation of weak carbon acids including aromatic compounds,24 vinylic systems,25 and others.26,27 The products of these metalations are organopotassium derivatives, but they can be readily converted into the corresponding organolithium compounds by the addition of Lithium Bromide.26,27

s-Butyllithium is often used to effect regiospecific and rapid deprotonation of heteroatom-containing compounds.2-4,28,29 Heteroatoms such as oxygen, sulfur, nitrogen, phosphorus, and halogens enhance the acidity of a- or b-protons through either inductive or coordination effects and hence facilitate deprotonation. For example, the a-lithiation of vinyl thioethers,30 vinyl chlorides,31 and vinyl thioesters32 is effected at low temperatures using s-BuLi in the presence of HMPA or TMEDA. The vinyllithium compounds derived from thioethers are important acyl anion equivalents which, after alkylation, are readily converted into substituted ketones (eq 2).30

Lithiation of a-disubstituted alkenes is directed to the b-site in cases where the a-substituents are capable of activating the b-position through either inductive effects or chelation (eqs 3 and 4).20,33 Internal coordination to the lithium by the electron-donating b-substituent in eq 4 permits deprotonation and subsequent electrophilic substitution to take place stereoselectively.20

The preparation of a variety of a-heteroatom-substituted alkyllithiums is conveniently achieved using s-BuLi and in many cases this reagent exhibits highly original reactivity. (Methoxymethyl)trimethylsilane, for example, undergoes facile lithiation on treatment with s-BuLi in THF at -78 °C to give Me3SiCHLiOMe, which is readily hydroxyalkylated by the addition of aldehydes or ketones (eq 5).34 With other butyllithium reagents, this reaction takes a completely different course: n-BuLi undergoes a nucleophilic attack on the Si atom with concomitant loss of the CH2OMe group, and the use of t-Butyllithium gives LiCH2SiMe2CH2OMe through preferential deprotonation of one of the trimethylsilyl protons.34 In a similar reaction, (Chloromethyl)trimethylsilane is lithiated a to the chlorine atom using s-BuLi to furnish synthetically useful a,b-epoxytrimethy lsilanes after treatment with aldehydes or ketones (eq 6).23 Again, s-BuLi appears to be a superior reagent for this lithiation: the use of n-BuLi results in loss of the chloride anion, presumably through initial nucleophilic attack of this reagent upon the silicon atom and subsequent alkyl migration, and t-BuLi leads to the predominant formation of Trimethylsilylmethyllithium, formally derived from Li-Cl exchange.23 Selenide Bis(trimethylsilylmethyl) selenide,35 Methylenetriphenylphosphorane,36 phosphonic acid triamides,37 and 1,3-oxathianes38 are also readily lithiated on treatment with s-BuLi. The final ketene O,S-acetal product in eq 7 is formed via the Peterson alkenation process after addition of 2-methylpropanal to the lithio intermediate.38a

Metalation of N-methylpiperidine, N-methylpyrrolidine, and Trimethylamine takes place exclusively at the methyl group when s-BuLi is used in combination with t-BuOK (eq 8),26 and the same mixture can be used to metalate t-butyl methyl ether (eq 9).27 The intermediate organopotassium product in eq 9 is converted into the corresponding organolithium derivative by addition of LiBr to give t-BuOCH2Li, the synthetic equivalent of hydroxymethyllithium (LiCH2OH).27

Many allylic or benzylic heteroatom-containing compounds are often lithiated more conveniently using the less reactive n-BuLi rather than s-BuLi. However, the more basic s-BuLi is a better reagent for the deprotonation of alkyl allyl ethers39 and certain allyl thioethers21b that react slowly (if at all) with n-BuLi in THF at low temperatures (<-65 °C). The allyllithium derivatives formed in these reactions may be alkylated or hydroxyalkylated either at the a- or the g-position, depending on the electrophile and the structure of the allylic anion. With metalated allyl ethers, primary alkyl halides tend to attack the g-position to afford alkylated enol ethers, but carbonyl compounds react to give products derived mainly from a-attack (eq 10).39,40 The opposite tendency is generally observed with analogous lithiated allyl thioethers21,41,42 which, upon addition of alkyl halides, give primarily a-alkylated products (eqs 11 and 12).21b,42 Generally, soft electrophiles (e.g. RI) exhibit a higher propensity for g-attack than do hard electrophiles (e.g. RCl).42

Proton removal adjacent to a heteroatom is further facilitated if the lithium can be internally coordinated to proximate electron donors, such as the carbonyl oxygen, permitting the formation of dipole-stabilized carbanions.28 Thus lithiations of various amides,43 thioamides,44 imides,45 esters,46 Boc derivatives of cyclic amines (pyrrolidines, piperidines, and hexahydroazepines),47 thioesters,32 N,N-dialkylthiocarbamates,32 and various formamidine derivatives48 are achieved conveniently using s-BuLi (eqs 13-17).32,43b,44,47a,48a Subsequent addition of electrophiles followed by hydrolytic removal of the activating carbonyl, carbamoyl, or formamidine moiety provides a valuable synthetic route to a variety of a-substituted amines, alcohols, and thiols.28,46 Successful alkylation of the dipole-stabilized carbanions may require the conversion of the initial lithio carbanions into their organocuprate derivatives, e.g. by the addition of n-PrC&tbond;CCu.48a

Lithiation takes place more readily if the dipole-stabilized anions are further activated by an adjacent aromatic ring or a double bond and, in fact, many such compounds are most conveniently deprotonated using the less reactive n-BuLi.28

Stereoselective s-BuLi-promoted a-lithiations have been accomplished with various piperidine and isoquinoline derivatives bearing chiral oxazoline or formamidine substituents on the nitrogen (eqs 18 and 19).49,50 Asymmetric deprotonations a to oxygen in carbamates and a to nitrogen in Boc-protected pyrrolidines have been effected using s-BuLi in the presence of (-)-Sparteine, which is a homochiral lithium-complexing ligand.51,52 These deprotonations lead to the formation of chiral dipole-stabilized carbanions which react with electrophiles under strict stereocontrol to give substituted products in high enantiomeric excess, typically >95% ee. In the case of the N-oxazoline derivatives of piperidine and isoquinoline, enantiomerically enriched secondary amines are obtained upon hydrolytic removal of the oxazoline moiety.49a These reactions are of considerable synthetic value and this methodology has been successfully applied to the asymmetric synthesis of 2-piperidines49 and 2-pyrrolidines (eq 20),52 2-alkanols,51a 2-hydroxyalkanoic acids,51a alkanediols,51b,c and lactones (e.g. (R)-pantolactone) (eq 21).51b

ortho Lithiations.

s-Butyllithium is a commonly used reagent for ortho lithiation of aromatic rings bearing heteroatom-containing substitutents. This methodology has been exploited extensively as a route to a wide variety of polysubstituted aromatic compounds, including natural products53 (eq 22),53e and several excellent reviews have been published on the topic.2,5,54 Included in the list of groups commonly used for s-BuLi-promoted ortho lithiations are OMe,55 CH2NEt2,55 NMe2,56 CONEt2,57 OCONEt2,57 SO2NR2,58 2-oxazolinyl,57 and groups that contain acidic hydrogens and themselves undergo deprotonation prior to ring lithiation, e.g. CONHR57,58 and SO2NHR.57,58 Depending on the substituent, both inductive and coordination effects can be invoked to account for the observed regiochemistry.2,59 Although a large number of ortho lithiations may be conducted with n-BuLi,2 the use of s-BuLi is generally preferred, especially when the reactions involve aromatic tertiary amides or O-aryl carbamates which are susceptible toward nucleophilic additions with the latter. For example, N,N-dimethyl- and -diethylbenzamides have been shown to afford primarily aryl butyl ketones upon treatment with n-BuLi.55,60 The recommended procedure for ortho lithiation involves slow addition of the aromatic substrate in anhydrous THF to a slight excess of 1:1 s-BuLi/TMEDA in THF at -78 °C.55 Under these conditions, the lithiation is usually complete within 5 min.

When two ortho-directing groups are in a meta relationship, lithiation is generally directed to their common site. Notable exceptions to this behavior are found in cases where CONEt2, CONR, or oxazoline substitutents are meta to a dialkylamino group.5 These systems exhibit nearly total reversal of the general trend and, as a result, almost exclusive metalation of the 6-position is observed (eq 23).56 Considerable amounts of 6-substituted products are also obtained in cases where OCONEt2 and OMe groups are in a meta relationship.57 When two ortho-directing substituents are ortho or para to each other, the regiochemistry of the reaction depends on the relative directing abilities of these groups (eq 24).57

A large number of substituted polyaromatic and heteroaromatic compounds are also accessible through s-BuLi-induced ortho lithiation reactions. These include, for example, derivatives of naphthalene,57 furan,61 thiophene,62 pyridine,63 quinoline,64 and N-protected imidazoles.65 The smooth ortho lithiation of pyridyl (eq 25)63 and quinolinyl systems is notable given the well-known tendency of organolithium compounds to undergo nucleophilic addition reactions with the pyridine nucleus.1

Lithium-Halogen Interchange and Transmetalation Reactions.

The lithium-halogen interchange involves the exchange of halogen by lithium in a reversible reaction, with the most stable organolithium species being favored at equilibrium. Hence, the most synthetically useful lithium-halogen exchanges take place between alkyllithiums and aryl halides, cyclopropyl halides, and vinyl halides. These reactions are accomplished most conveniently using n-BuLi rather than s-BuLi and, as a result, the latter has been employed only in a relatively few cases. However, the formation of select a-halo- and a-dihalolithiums,66 aryllithium compounds,67 and vinyllithium derivatives68 using s-BuLi has been reported. The preparation of simple alkyllithium compounds by lithium-halogen exchange is usually limited to the generation of primary alkyllithiums from primary alkyl iodides by treatment with the more reactive tertiary alkyllithiums (such as t-BuLi).69

The lithium-selenium exchange has been exploited extensively for the generation of a variety of organolithiums from selenides, selenoacetals, and mixed S,Se-acetals, with n-BuLi being the most commonly employed reagent for these reactions.3,70 Although s-BuLi can be used for all of these conversions, the primary role of this reagent is to provide a more reactive lithiating agent for the generation of synthetically useful3,71 a-selenoalkyllithiums from selenoacetals of sterically hindered ketones.72 For example, 2,2-bis(methylseleno)adamantane is completely unreactive toward n-BuLi in THF at -78 °C, but it undergoes a facile (30 min) Li-Se exchange on treatment with s-BuLi under identical conditions to afford the expected a-selenoorganolithium product in high yield (eq 26).72

The following order of reactivity of organolithium reagents toward selenoacetals has been established: t-BuLi/THF-hexane &AApprox; s-BuLi/THF-hexane > n-BuLi/THF-hexane &AApprox; s-BuLi/ether-hexane >> MeLi/THF-ether > t-BuLi/hexane &AApprox; n-BuLi/hexane.72 As seen from this order, the reactivity of s-BuLi in ether-hexane is comparable to that of n-BuLi in THF-hexane. The a-selenoalkyllithiums generated in ether-hexane are more stable than those prepared in THF. In this medium they also exhibit higher reactivity toward certain carbonyl compounds, especially hindered73 or highly enolizable ketones.74 The 1-methyl-1-phenylselenoethyllithium product in eq 27 (generated in ether-hexane) undergoes nucleophilic 1,2-addition with a cyclic a,b-unsaturated ketone to afford ring-expanded ketones after treatment with TlOEt.75

Organolithium compounds are also accessible through the replacement of tin and tellurium by lithium, with the various organotin compounds being particularly important precursors for otherwise inaccessible organolithium derivatives (e.g. a-amino- and a-alkoxy-substituted organolithiums). Although s-BuLi is well suited for these transformations,52,76 most of the synthetic applications exploiting this methodology involve the use of n-BuLi.


Arylsulfonylhydrazones undergo formal elimination reactions on treatment with 2 equiv of s-BuLi to give vinyllithium compounds.77,78 Tosylhydrazones should be avoided with this reagent because they are susceptible to ring lithiation and benzylic deprotonation; instead, the use of hindered 2,4,6-triisopropylbenzenesulfonylhydrazones is recommended (eq 28).77 In cases where two regioisomers can be produced, s-BuLi appears to promote the formation of the more substituted vinyllithium compounds. In contrast, the use of n-BuLi leads to the less substituted vinyllithium product.78 Other notable s-BuLi-induced elimination reactions are the a-eliminations involving bis(phenylthio)methyllithium-containing cycloalkanol derivatives, which have been exploited as a route to ring-expanded ketones (eq 29),79 and b-eliminations of ortho-lithiated 2-carboxamide-substituted furan derivatives, which have been used to prepare ring-opened enyne products.80

Formation of Enolate Anions and Enolate Equivalents.

s-Butyllithium may be used for the generation of enolate anions or enolate equivalents from active hydrogen compounds provided that nucleophilic addition of this reagent to the electrophilic carbon center is avoided. Thus the less reactive amides and imines are often suitable substrates for these reactions. For example, the bicyclic lactam in eq 30 undergoes two consecutive enolization/alkylation reactions in a highly diastereoselective fashion to afford a dialkylated product,81 and the N-t-butylimine of 2-heptanone is deprotonated using s-BuLi to afford a mixture of two isomeric ketones after alkylation and hydrolysis (eq 31).82 The less substituted ketone was obtained as the only isomer when s-BuLi was used with HMPA. Similar reactions have been reported for a-nitro imines,83 and thiolactams.84


Salicylamides are available from various aryl carbamates, including those derived from Pyridine and naphthalene,63 through the s-BuLi-mediated O-C 1,3-carbamoyl migration reactions (Snieckus rearrangement).57 This regiospecific rearrangement is the anionic equivalent of the Fries rearrangement and it involves low-temperature (-78 °C) ortho lithiation of aryl carbamates with s-BuLi/TMEDA/THF followed by warming of the reaction mixture to rt (eq 32).85 Benzylic carbamates rearrange to give products derived from either 1,4- or 1,2-carbamoyl migration (eq 33) following treatment with s-BuLi in THF,86 and lithiated phenolic esters rearrange to furnish acyl phenols even at low temperatures (eq 34).67 s-Butyllithium is also capable of initiating various Wittig rearrangements involving allylic ethers, but these reactions are done more conveniently using the less reactive n-BuLi.

Related Reagents.

n-Butyllithium; t-Butyllithium; Potassium Hydride-s-Butyllithium-N,N,N,N-Tetramethylethylenediamine.

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Timo V. Ovaska

Connecticut College, New London, CT, USA

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