n-Butyllithium1

[109-72-8]  · C4H9Li  · n-Butyllithium  · (MW 64.05)

(strong base capable of lithiating carbon acids;1 useful for heteroatom-facilitated lithiations;2,3 useful for lithium-halogen exchange;1,4 reagent of choice for lithium-metal transmetalation reactions1a,c,5)

Physical Data: colorless liquid; stable at rt; eliminates LiH on heating; d25 0.765; mp -76 °C; bp 80-90 °C/0.0001 mmHg; dipole moment 0.97 D.6 13C NMR, 1H NMR, 6Li NMR8 and MS studies have been reported.7-9

Solubility: sol hydrocarbon and ethereal solvents, but should be used at low temperature in the latter solvent type: half-lives in diethyl ether and THF have been reported;10 reacts violently with H2O and other protic solvents.

Form Supplied in: commercially available as approximately 1.6 M, 2.5 M, and 10.0 M solution in hexanes and in cyclohexane, approximately 2.0 M solution in pentane, and approximately 1.7 M, and 2.7 M solution in n-heptane. Hexameric in hydrocarbons;1c tetrameric in diethyl ether;1c dimer-tetramer equilibrium mixture in THF;11 when used in combination with tertiary polyamines such as TMEDA and DABCO, reactivity is usually increased.1,12

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.15 Several other methods have been described.16

Preparative Methods: may be prepared in high yield from n-butyl chloride13 or n-butyl bromide14 and Lithium metal in ether or hydrocarbon solvents.

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. For detailed handling techniques see Wakefield.1b

Lithiations.

n-Butyllithium is a commonly used reagent for deprotonation of a variety of nitrogen (see Lithium Diisopropylamide), oxygen, phosphorus (see Diphenylphosphine), and carbon acids to form lithium salts. Compared to its s- and t-butyl analogs, n-BuLi is less basic17 and less reactive but it is usually the reagent of choice for deprotonation of relatively strong carbon acids. These lithiations are most favorable when the conjugated bases are stabilized by resonance or when the carbanion forms at the sp hybridized carbon of a triple bond. Thus indene,18 triphenylmethane,19 allylbenzene,20 and methyl heteroaromatics (e.g. Pyridine, quinoline, and isoquinoline derivatives)21 are readily lithiated with n-BuLi at the benzylic position. Allenes are lithiated at C-1 or C-3 depending on the number and size of the alkyl groups at these positions,22 and terminal alkynes react with n-BuLi to give lithium acetylides.4 Propargylic hydrogens can also be removed,23 and treatment of terminal alkynes with 2 equiv of n-BuLi results in the lithiation of both propargylic and acetylenic positions (eq 1).23a

The metalating ability of n-BuLi (and other organolithiums) is greater in electron-donating solvents than in hydrocarbons. Electron-donating solvents such as diethyl ether or THF provide coordination sites for the electron deficient lithium and promote the formation of lower-order organolithium aggregates.1a,c These exhibit significantly higher levels of reactivity than do the higher-order oligomers present in hydrocarbon solvents. n-Butyllithium is often used in the presence of added lithium complexing ligands, such as N,N,N,N-Tetramethylethylenediamine (TMEDA) and 1,4-Diazabicyclo[2.2.2]octane (DABCO), which further enhance the reactivity of this reagent.12 Many compounds, normally unreactive toward n-BuLi alone (e.g. benzene), are readily lithiated24 and even polylithiated25 by a combination of n-BuLi and one of these additives. Allylic26,27 and benzylic sites bearing no additional activating groups at the a-position (e.g. toluene)12c are also lithiated in the presence of lithium complexing donors (eq 2).26

Metal alkoxides are often employed as additives to enhance the metalating ability of n-BuLi. Particular use has been made of the n-Butyllithium-Potassium t-Butoxide mixture,28 which is capable of effecting rapid metalation of benzylic29 and allylic30 systems as well as aromatic rings.28 Although the products of these metalations are not organolithium compounds they do, nevertheless, readily react with electrophiles. Alternatively, they may be converted into the corresponding organolithium derivatives by the addition of Lithium Bromide.30a,31 Using t-BuOK/n-BuLi for deprotonation, the one-pot synthesis of 2-(4-isobutylphenyl)propanoic acid (ibuprofen) from p-xylene, through a sequence of metalations and alkylations, has been achieved in 52% overall yield.29 Dimetalation of arylalkynes with n-BuLi/t-BuOK followed by addition of electrophiles has been used as a route to ortho-substituted arylalkynes (eq 3).31b

Facile and regioselective a-deprotonation is often effected by treatment of heteroatom-containing (e.g. oxygen, sulfur, nitrogen, and the like) compounds with n-BuLi, and these reactions have been extensively reviewed.2,3,32,33 Thus sulfones,34 certain sulfides,35 and sulfoxides36 can be lithiated adjacent to sulfur under various conditions, and a-heterosubstituted vinyllithium compounds are often available from the corresponding ethers,37 thioethers,38 chlorides,39 and fluorides40 via lithiation using n-BuLi. Isocyanides41 and nitro compounds42 have been lithiated adjacent to nitrogen with n-BuLi in THF at low temperatures (less than -60 °C) and the formation of various a-phosphorus alkyllithiums has been reported.43 Simple ethers are also susceptible to lithiation at the a site by n-BuLi, especially at elevated temperatures.44 The initial proton abstraction in these systems is generally followed by various cleavage reactions resulting from a,b- and a,b-eliminations as well as the Wittig rearrangement.45 Tetrahydrofuran, for example, is rapidly lithiated by n-BuLi at 35 °C (t1/2 = 10 min) to give ethylene and the lithium enolate of acetaldehyde.46

Due to the stabilizing effect of two sulfur atoms, 1,3-dithianes are easily lithiated at the a-position on treatment with n-BuLi (see 2-Lithio-1,3-dithiane).47 The 2-lithio-1,3-dithianes constitute an important class of acyl anion equivalents, permitting electrophilic substitution to occur at the masked carbonyl carbon.47 Hydrolysis of the 1,3-dithiane functionality into a carbonyl group is effected in the presence of mercury(II) ion (eq 4).48

When conducted at sufficiently low temperatures (less than -78 °C), a-deprotonation can occur faster than nucleophilic addition to an electrophilic center present in the same molecule. This strategy, which involves initial lithiation followed by intramolecular nucleophilic addition of the newly generated C-Li bond to an electrophilic moiety, is useful for the construction of carbocycles including medium and large ring systems (eqs 5 and 6).49,50

A large number of heteroaromatic compounds,2 such as furans (see Furan),51 thiophenes (eq 7),52 oxazoles,53 and N-alkyl- and N-aryl substituted pyrroles (see 2-Lithio-N-phenylsulfonylindole),54 pyrazoles,55 imidazoles,56 triazoles,57 and tetrazoles,58 are lithiated under various conditions a to the ring heteroatom using n-BuLi. However, pyridine and other nitrogen heteroaromatics bearing the pyridine, pyrimidine, or pyrazine nucleus are generally not lithiated. Indeed, they have a tendency to undergo nucleophilic addition reactions with this reagent1b (see 2-Lithiopyridine).

When the a-position is benzylic, propargylic, or allylic, deprotonation takes place more readily and n-BuLi is generally the reagent of choice for these reactions.59,60 However, the more basic s-Butyllithium is a better reagent for deprotonation of alkyl allyl ethers61 and certain alkyl allyl thioethers62 which react slowly (if at all) with n-BuLi in THF at low temperatures (less than -65 °C).

Proton removal adjacent to a heteroatom is further facilitated if the lithium can be coordinated to proximate electron donors, such as a carbonyl oxygen, permitting the formation of dipole-stabilized carbanions.33 Thus various 2-alkenyl N,N-dialkylcarbamates undergo rapid a-deprotonation adjacent to oxygen on treatment with n-BuLi/TMEDA at -78 °C.63,64 The resulting dipole-stabilized lithium carbanions react with ketones and aldehydes in a highly regioselective fashion providing g-hydroxyalkylated enol esters (eq 8) which, following cleavage of the carbamoyl moiety (Titanium(IV) Chloride/H2O or MeOH), afford d-hydroxy carbonyl compounds (homoaldols) as lactols or lactol ethers.65 Similarly, aliphatic or aromatic amides66,67 (eq 9),67a phosphoramides,68 and some formamidine derivatives (e.g. 1,2,3,6-tetrahydropyridine,69 thiazolidine,69 1,3-thiazine,69 and tetrahydroisoquinolines70) are selectively lithiated a to the nitrogen at the activated position. Electrophilic substitution of the intermediate organolithiums followed by hydrolytic cleavage of the amide or formamidine group provides a synthetically valuable route to a-substituted (or g-substituted69) secondary amines.33 Use of the more basic s-BuLi (or t-Butyllithium) is generally required for deprotonation of the analogous nonbenzylic or nonallylic systems.

n-Butyllithium is also used for the stereoselective a-lithiation of chiral sulfonyl compounds,71 chiral 2-alkenyl carbamates,64,72 various heterocyclic amine derivatives with chiral auxiliaries appended on the nitrogen (e.g. oxazoline73 or formamidine74,75 groups) (eq 10),75 and chiral oxazolidinones derived from benzylamines (eq 11).76 These elegant reactions have been applied to the asymmetric syntheses of a number of natural products.77,78

Ortho Lithiations.

Heteroatom-containing substituents on aromatic rings facilitate metalation by organolithium reagents and direct the metal almost exclusively to the ortho position. This effect, usually referred to as ortho lithiation, is of considerable synthetic importance and the topic has been extensively reviewed.2,4,79 Although n-BuLi (typically in THF or Et2O with added TMEDA)80 is capable of effecting a large number of ortho lithiations, the use of this reagent is somewhat limited by its tendency to undergo nucleophilic carbonyl additions with some of the most potent and useful ortho directors, particularly tertiary amide and carbamate functionalities (e.g. CONEt2 and OCONEt2).79,81 For example, N,N-dimethyl- and -diethylbenzamides afford primarily aryl butyl ketones upon treatment with n-BuLi (see 2-Lithio-N,N-diethylbenzamide).82 The reagent of choice for these lithiations is s-Butyllithium, which is less nucleophilic than n-BuLi and hence more tolerant of electrophilic functional groups.79 There are, however, a variety of ortho-directing groups that are well suited for n-BuLi-promoted lithiations. These include NR2,80 CH2NR2,80 CH2CH2NR2,80 OMe (see o-Lithioanisole),83 OCH2OMe,84 SO2NR2,85 C=NR,86 2-oxazolinyl,85,87 F,88 CF3,80 and groups that contain acidic hydrogens and themselves undergo deprotonation prior to lithiation of the aromatic ring (thus requiring the use of 2 equiv of n-BuLi), e.g. CONHR,89 CH2OH,80 NHCO-t-Bu,90 and SO2NHR80 (eq 12).91 n-Butyllithium is also used frequently for the ortho lithiation of heterocyclic aromatic rings,2 including those that contain the pyridine nucleus (eq 13).92

Formation of Enolate Anions and Enolate Equivalents.

Owing to its tendency to undergo nucleophilic addition with carbonyl groups and other electrophilic carbon-heteroatom multiple bonds (C=NR, C&tbond;N, C=S),1 n-BuLi is usually not the reagent of choice for the generation of enolate anions or enolate equivalents from active hydrogen compounds. This is done most conveniently using the less nucleophilic lithium dialkylamides (e.g. Lithium Diisopropylamide (LDA), Lithium 2,2,6,6-Tetramethylpiperidide (LiTMP), and Lithium Hexamethyldisilazide (LTSA)) prepared (often in situ) from sterically hindered secondary amines, typically by treatment with n-BuLi.93 However, less reactive carbonyl compounds such as amides (eq 14)94 and carboxylic acids95 as well as those containing carbon-nitrogen or carbon-sulfur multiple bonds, e.g. imines,96 oxazines,97 nitriles,98 some hydrazones,99 and thioamides,100 can be lithiated a to the electrophilic carbon with n-BuLi under various conditions. b-Keto esters can be alkylated at the a-carbon using Sodium Hydride for the first deprotonation and n-BuLi for abstraction of the less acidic a-proton followed by addition of alkyl halides.101 Lithiation of unsymmetrical imines using n-BuLi takes place regioselectively at the most substituted a-carbon (eq 15).96 In contrast, LDA directs metalation and subsequent alkylation predominantly to the less substituted a-position in similar systems.102 Lithium enolates of camphor imine esters, prepared by addition of n-BuLi, undergo highly diastereoselective Michael additions with a,b-unsaturated esters (eq 16).103 The tightly chelated structures of the intermediate enolates permit selective re face approach of the Michael acceptors, giving rise to the high degree of distereoselectivity observed.

Metal-Halogen Interchange and Transmetalation Reactions.

The metal-halogen interchange reaction which involves the exchange of halogen and lithium atoms is an important method for the preparation of organolithium compounds not readily accessible through metalation. In particular, the generation of aryl-, vinyl-, and cyclopropyllithium derivatives from the corresponding bromides or iodides on treatment with n-BuLi (usually in ethereal solvents at or below -78 °C) is of considerable synthetic utility (eqs 17-19).104-106 The relative rates of exchange depend on the halide and decrease in the order I > Br > Cl > F. Fluorides and chlorides, with the exception of some polychlorinated aliphatic107 and aromatic compounds,108 are quite resistant to lithium-halogen interchange; instead, they tend to promote ortho and a lithiations.79

gem-Dihalocyclopropanes react with n-BuLi in a highly stereoselective fashion (eq 19), although subsequent isomerization can take place.109 Alkyl-substituted vinyllithiums can be prepared with retention of configuration105,110 and even aryl-substituted vinyllithium compounds retain their configuration under controlled conditions (<-78 °C in THF or at rt in hydrocarbon solvents).111 Simple alkyllithiums are generally not accessible by this route because of the unfavorable interchange equilibrium that ensues when primary or secondary halides are treated with n-BuLi, except in cases where the initially formed organolithium is rapidly consumed in a subsequent, irreversible reaction (eq 20).112 Primary alkyllithiums may be prepared, however, from the corresponding alkyl iodides (but not from the bromides)113 on treatment with the more reactive t-Butyllithium which renders the exchange operationally irreversible.114

n-Butyllithium is the reagent of choice for effecting a number of transmetalation reactions involving the replacement of tin, selenium, tellurium, or mercury by lithium.1a,1c,5 These reactions are typically conducted at low temperatures (less than -60 °C) in ethereal solvents, with THF being the most commonly employed reaction medium. The tin-lithium exchange is a particularly important operation, providing a convenient route to aryl- and vinyllithiums,115 as well as functionalized lithio derivatives which are not readily accessible by other means, such as a-alkoxylithium compounds,116 oxiranyllithiums,117 and amino-substituted organolithiums.118 The Sn-Li exchange in these systems and subsequent trapping of the intermediate lithio derivatives proceed stereoselectively with retention of configuration at the tin-bearing carbon (eqs 21 and 22).116c,118c Consequently, enantiomerically enriched functionalized products are available through this methodology from homochiral a-hetero-substituted stannanes.119

Appropriately substituted alkenic a-alkoxylithiums, derived from the corresponding tri-n-butyltin compounds on treatment with n-BuLi, have also been used to initiate 5-exo-trig cyclization reactions to give tetrahydrofuran derivatives (eq 23).120 Other related ring-forming reactions have been reported.121

Organoselenium compounds, particularly selenoacetals or selenothioacetals, undergo facile lithium-selenium exchange reactions on treatment with n-BuLi, typically in THF at -78 °C.122-124 The intermediate a-seleno organolithium derivatives formed in these reactions are readily trapped with electrophiles to afford a variety of synthetically useful a-functionalized selenide products3,125 (eq 24).126 Studies on cyclohexyl selenoacetals have shown that Li-Se exchange takes place almost exclusively at the axial position123 and that equatorial a-lithio sulfides, derived from mixed Se,S-acetals by Li-Se exchange, epimerize within minutes at -78 °C to give the more stable axial lithio isomers.124 n-Butyllithium may be used for the majority of Li-Se exchanges although selenoacetals derived from sterically hindered ketones react quite slowly with this reagent, and in these cases the use of the more reactive s-Butyllithium is warranted.123

A variety of organolithiums, including benzylic, vinylic, alkynic, and 1-alkoxylithium compounds, are also accessible through the lithium-tellurium exchange, which involves the treatment of diorganotellurides with n-BuLi at -78 °C in THF (eq 25);127 vinyllithium derivatives have been prepared from the corresponding organomercurials by essentially the same methodology.128

Rearrangements.

n-Butyllithium is used frequently to promote various anionic rearrangement reactions. 2,3-Wittig rearrangements are commonly effected by treatment of allylic and propargylic ethers with n-BuLi,129 and 1,2-, 1,4-, as well as 3,4-rearrangements with similar systems are also known.130 These reactions are most often initiated at low temperatures either by a-deprotonation adjacent to oxygen or via Li-Sn exchange, and the rearrangement occurs upon warming of the reaction mixture to 0 °C. Additional benzylic, propargylic, or allylic stabilization is necessary for deprotonation by n-BuLi; however, the Li-Sn method does not suffer from this limitation.131 2,3-Wittig rearrangements involving (Z)-allylic ethers proceed generally with high syn stereoselectivity (eq 26),131 whereas the opposite tendency is observed with (E)-allylic ethers.132 The regiochemistry of unsymmetrical bis-allylic ethers depends on the degree of substitution at the a- and g-carbons, with the less substituted allylic position being the preferred site of deprotonation (eq 27).133 The cyclic variant of the 2,3-Wittig rearrangement can be used for ring contraction reactions and it provides a useful method for the construction of macrocyclic ring systems (eq 28).134

Secondary 1,2-epoxy alcohols may be prepared through the Payne rearrangement in aprotic media by treatment of primary 2,3-epoxy alcohols with n-BuLi in THF with catalytic amounts of Lithium Chloride,135 and a,b-unsaturated ketones are obtained from trimethylsilyl substituted propargylic alcohols through the Brook rearrangement followed by alkylation of the intermediate allenyllithium products and acidic workup (eq 30).136 Quaternary ammonium salts, such as benzyltrimethylammonium ion, react with n-BuLi to deliver nitrogen ylides which can undergo either the Stevens rearrangement to give tertiary amines or the Sommelet-Hauser rearrangement to afford ortho alkyl-substituted nitrogen-containing aromatics, depending on the reaction conditions (eq 31).137 Dibenzyl thioether has been reported to undergo a Sommelet-Hauser type rearrangement on treatment with n-BuLi/TMEDA in HMPA,138 and appropriately substituted sulfonic ylides, prepared from sulfonium salts on treatment with n-BuLi, undergo 2,3-sigmatropic rearrangements (eq 32).139

Elimination Reactions.

A number of eliminations can be effected using n-BuLi. The formation of phosphorus, nitrogen, and sulfur ylides1a and the generation of benzyne intermediates from aromatic halides140 are well established processes, and a-eliminations resulting in the formation of carbenes have been used to prepare cyclopropanes,141 oxiranes,142 and other products via subsequent carbenoid rearrangements (eq 33).143 Vinylidene dihalides undergo dehalogenation with concomitant formation of a triple bond on treatment with 2 equiv of n-BuLi.144 The initial product of this reaction is a lithium acetylide which can be quenched with methanol to give a terminal alkyne or alkylated in situ by addition of alkyl halides to afford internal alkynes (eq 34).145 The decomposition of (arylsulfonyl)hydrazones of aldehydes or ketones upon treatment with at least 2 equiv of n-BuLi (Shapiro reaction) is a useful method for the generation of vinyllithium compounds (eq 35).146 In cases where two regioisomers can be produced, n-BuLi appears to promote the formation of the less substituted vinyllithium via deprotonation of the kinetically more acidic proton, whereas the use of s-BuLi reportedly leads to the more substituted vinyllithium product.147

Related Reagents.

s-Butyllithium; t-Butyllithium; n-Butyllithium-Boron Trifluoride Etherate; n-Butyllithium-Potassium t-Butoxide; Methyllithium; Tungsten(VI) Chloride-n-Butyllithium.


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Timo V. Ovaska

Connecticut College, New London, CT, USA



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