Borane-Tetrahydrofuran1

[14044-65-6]  · C4H11BO  · Borane-Tetrahydrofuran  · (MW 85.94)

(hydroborating and reducing agent)

Physical Data: d420 = 0.898 g cm-3 for 1 M solution in THF.

Solubility: sol THF; reacts violently with water.

Form Supplied in: solution in THF 1 M in BH3, stabilized with 0.005 M sodium borohydride.

Analysis of Reagent Purity: 11B NMR: d -0.7 ppm (JB-H = 103 Hz).5 Active hydride is determined by hydrolysis of an aliquot in a mixture of glycerol and water (1:1) and measuring the hydrogen evolved according to a standard procedure.6a Boric acid formed by hydrolysis is determined by titration with standard sodium hydroxide in the presence of mannitol.

Preparative Methods: Diborane is conveniently generated by the reaction of Sodium Borohydride with Boron Trifluoride Etherate in diglyme and absorbed in THF.2 The solution, ca. 2 M in BH3, may contain traces of boron trifluoride, which poses problems only on rare occasions.3 Diborane synthesis has been reviewed.4

Handling, Storage, and Precautions: use in a fume hood; air- and moisture-sensitive, flammable liquid; handle and store under nitrogen or argon. Commercial solutions are stable over prolonged periods when stored at 0 °C under nitrogen. Unstabilized solutions can be kept for several weeks under those conditions. However, in such solutions the BH3 concentration may slowly decrease due to the cleavage of THF by borane.6b

Introduction.

The stoichiometry, directive effects, stereochemical and mechanistic aspects of the hydroboration of alkenes, dienes, alkynes, and functional derivatives are discussed in detail in monographs and reviews1,7,8 (see also Borane-Dimethyl Sulfide). Here, transformations of major synthetic importance involving organoboranes produced by hydroboration with borane-THF are outlined.

Hydroboration-Oxidation: Synthesis of Alcohols.

Oxidation of the intermediate organoborane with alkaline Hydrogen Peroxide, the standard procedure, proceeds with complete retention of configuration to give alcohols in excellent yields (eq 1).9

Many functional groups are tolerated.10 Other oxidation procedures are known.11 Generally, functional groups in isolated positions do not affect the hydroboration of double or triple bonds. However, proximate substituents may dramatically influence the regioselectivity of addition and stability of the organoborane product.12-15

Hydroboration-Elimination: Synthesis of Alkenes.

Enamines,16 certain homoallylic alcohols,17 and heterocyclic compounds18 have been transformed into alkenes or dienes by elimination or fragmentation of the organoborane intermediate (eq 2).17

Synthesis of Hydroperoxides.

The low-temperature autoxidation of organoboranes in THF leads to the formation of diperoxyboranes, which provide the corresponding alkylhydroperoxides in excellent yields upon treatment with hydrogen peroxide. Two of the three alkyl groups on boron are utilized (eq 3).19 The reaction involves a radical chain and the configurational integrity of the alkyl group is not retained.

Hydroboration-Protonolysis: Noncatalytic Stereospecific Hydrogenation.

Trialkylboranes undergo protonolysis by treatment with carboxylic acids.20 The first alkyl group is readily cleaved. Complete removal of alkyl groups requires heating with acid. The protonolysis proceeds with retention of configuration (eq 4). Internal alkynes can be transformed into (Z)-alkenes (see also Borane-Dimethyl Sulfide).

Contrathermodynamic Isomerization of Alkenes.

Thermal isomerization of alkylboranes at 150-170 °C results in migration of the boron atom to the least hindered position.21 In aliphatic systems, boron migrates along the carbon chain to the terminal position; however, it cannot pass quarternary centers. Treatment of the isomerized organoborane with a high boiling alkene brings about displacement,22 and oxidation yields the primary alcohol (eq 5).23

Functional groups unreactive to borane, e.g. ethers, do not interfere; however, the direction of migration may be influenced by reactive groups, e.g. hydroxy.24

Amination: Synthesis of Primary and Secondary Amines.

Trialkylboranes are converted by hydroxylamine-O-sulfonic acid to primary amines. The reaction proceeds with complete retention of configuration (eq 6).25a

Other aminating reagents are also known.26 Secondary amines can be prepared by the reaction of trialkylboranes with organic azides.27 For better utilization of R groups in these reactions, RBMe2 and RBCl2 can be used, respectively.25b,26

Halogenolysis.

Alkylboranes are readily converted into the corresponding alkyl chlorides by radical reaction with Trichloramine.28a The reaction is not stereospecific; however, with other reagents, e.g. dichloramine T, the stereochemical integrity of the alkyl group is retained.28b Brominolysis and iodinolysis of trialkylboranes proceed under mild conditions in the presence of a base.29 The yields of primary alkyl bromides and iodides are excellent. Three alkyl groups are utilized in the brominolysis and two in the iodinolysis. Secondary alkyl groups react with predominant inversion of configuration (eq 7). In contrast, the dark bromination proceeds with complete retention of configuration (eq 7).30

Mercuration.

Mercury(II) salts react with primary trialkylboranes to yield organomercurials (eq 8).31 Mercuration of secondary alkylboranes is sluggish.

Sulfuridation: Synthesis of Thioethers, Alkyl Thio- and Selenocyanates.

Trialkylboranes are cleaved by dialkyl and diaryl sulfides, producing mixed thioethers (eq 9).32 The reaction is catalyzed by air. Alkyl thio- and selenocyanates are obtained by treatment of trialkylboranes with iron(III) thio- and selenocyanate, respectively.33

Synthesis of Mono- and Dialkylboranes.34

Several valuable hydroborating and reducing agents,34 e.g. Thexylborane, Disiamylborane, Dicyclohexylborane, 9-Borabicyclo[3.3.1]nonane (9-BBN, eq 10),35 and borinane,36 are synthesized by hydroboration of the corresponding alkenes or dienes with the reagent.

Single Carbon Homologation: Carbonylation, Cyanidation, Dichloromethyl Methyl Ether (DCME) Reaction; Synthesis of Alcohols, Aldehydes, and Ketones.

Trialkylboranes react with carbon monoxide at 100-125 °C in diglyme. The reaction proceeds stepwise and the migration of alkyl groups can be controlled to give products of one, two, or three group transfers (eq 11).37 Cyanidation38 and the DCME reaction39a lead to the same products under milder conditions. Highly hindered tertiary alcohols can be prepared. Annulation and spiroannulation of 1-allylcyclohexenes have been achieved.39b All reactions proceed with retention of configuration of alkyl groups. Functional groups, such as ether, ester, nitro, and chloro are tolerated. Unsymmetrically substituted ketones can be obtained by the reaction of trialkylboranes with acyl carbanion equivalents.40

Coupling; Synthesis of Alkanes, Cycloalkanes and (E)-Alkenes.

Treatment of trialkylboranes with alkaline Silver(I) Nitrate solution results in coupling of the alkyl groups.41 Yields for coupling of primary and secondary groups are in the range of 60-80% and 35-50%, respectively.

a-Alkylation of Esters, Ketones and Nitriles.

Trialkylboranes react with carbanions generated from a-halo esters, ketones, and nitriles, transferring the alkyl group from boron to carbon.42 Two different groups can be introduced consecutively, starting with a,a-dihalonitriles.42d The anions are generated by hindered bases, e.g. t-butoxide, or better, 2,6-di-t-butylphenoxide for sensitive compounds (eq 12).

a-Diazocarbonyl compounds react with trialkylboranes directly in the absence of bases at -25 °C.43 Mild conditions are advantageous when functionalities labile to bases are present (eq 13).43a

a-Bromination-Alkyl Group Transfer: Synthesis of Hindered Tertiary Alcohols.

The photochemical a-bromination of trialkylboranes with bromine proceeds readily. Weak bases such as water or THF are sufficient to induce the migration of alkyl groups from boron to the a-brominated carbon. All three alkyl groups may be utilized. It is possible to halt the reaction after the first group migration. Highly hindered tertiary alcohols can be prepared (eq 14).44

Addition to Carbonyl Compounds: Synthesis of Unsymmetrical Ketones.

Unlike Grignard and alkyllithium compounds, trialkylboranes are inert to carbonyl compounds. The air-catalyzed addition to formaldehyde is exceptional.45 Alkyl borates are more reactive and can transfer primary alkyl groups to acyl halides. The reaction provides highly chemoselective boron-mediated synthesis of unsymmetrical ketones (eq 15).46

b-Alkylation of Conjugated Carbonyl Compounds and Oxiranes.

Trialkylboranes undergo radical conjugate addition to various a,b-unsaturated carbonyl compounds, such as enals, enones, quinones, unsaturated nitriles, crotonaldimines, and ynones, and also to alkenyl- and alkynyloxiranes47 (eq 16).47b

Functional Group Reductions.48

The reactivity order of representative functional groups toward borane-THF is49a carboxylic acids > aldehydes > ketones > alkenes >> nitriles > epoxides > esters > acid chlorides. Acid anhydrides, amides, lactones, acetals, oximes, oxime ethers, imines, and hydrazones are also reduced. Nitro compounds, organic halides, sulfones, sulfonic acids, disulfides, thiols, alcohols, phenols (hydrogen evolution), and amines are stable to borane-THF.49

Carboxylic Acids and Derivatives.

Borane-THF is the reagent of choice for the reduction of carboxylic acids to alcohols.50-56 Selective reduction in the presence of esters, halogen derivatives, nitrileLane, C.s, amides, lactones, nitro compounds, amino, phenolic, and other groups is possible (eq 17).51 Carboxylic acid salts are reduced with 2 equiv of BH3.THF.52

Aromatic acids containing electron-donating groups may undergo overreduction to hydrocarbons.57 Other carboxylic groups can also be converted into the methyl group via the reduction of N-acyl-N´-tosylhydrazines.58 Isotopically labeled chiral methyl groups have been obtained by a sequence of reactions involving BD3.THF.59

Acid anhydrides are reduced to alcohols at a slower rate than carboxylic acids.60 Aliphatic esters and lactones react slowly (6-12 h at 0 °C) to give alcohols or ethers.49,61-65

The reduction of amides to amines is another major synthetic application of the reagent. The reactivity order is tertiary > secondary >> primary. All types of amides and lactams are reduced rapidly and quantitatively by excess of borane in refluxing THF.66-69

A large-scale preparation of an intermediate in the synthesis of nitrobenzyl-DOTA, an efficient chelating agent for metal ions, used in biochemical studies, is shown in eq 18.70

Nitriles are readily reduced with an excess of borane in refluxing THF to give the corresponding amines upon acid hydrolysis of the intermediate borazines.66,71 Less reactive functional groups are tolerated. The initial intermediate N-borylimine can be alkylated to give carbinamines (eq 19).72 Cyanohydrins are reduced to amino alcohols; however, a-aminonitriles undergo decyanation.73

Ketones and Derivatives.

Although aldehydes and ketones are readily reduced by borane,49 other reagents of higher selectivity are widely used for that purpose. In some cases, however, the use of borane may be advantageous, as demonstrated in the synthesis of ciramadol involving a stereoselective reduction of 2-(a-dimethylamino-m-hydroxybenzyl)cyclohexanone74a and in highly diastereoselective keto boronate reduction (eq 20).74b

The asymmetric reductions of prochiral ketones are now dominated by catalytic reactions75 and highly selective stoichiometric reagents.76 Borane-THF reacts with chiral amino alcohols to give asymmetric reducing agents and serves as a source of BH3 in enantioselective reductions of prochiral ketones catalyzed by oxazaboralidines and oxazaphospholidines (eq 21).77 Catalysts of type 2 are best studied and new modifications are prepared.75,78

Oximes react with borane in refluxing THF to give the corresponding hydroxylamine derivatives (see also Borane-Dimethyl Sulfide).79 Oximes of aryltrifluoromethyl ketones are reduced at room temperature (not with BMS).80 Oxime ethers and acetates react under mild conditions.81 Aluminum Hydride (alane) is also a convenient reagent for these reductions.82 Reagents for asymmetric reduction of prochiral ketoxime O-alkyl ethers and imines are prepared from borane-THF and chiral amino alcohols.75,77a,83

Cleavage of Single Bonds.

Epoxides react slowly with borane-THF to yield mixtures of products.84 In the presence of small amounts of sodium borohydride or boron trifluoride, the reaction is greatly accelerated and the epoxide ring is cleanly opened. Although anti-Markovnikov products predominate, the regioselectivity is high only in certain cases, e.g. styrene and indene epoxides give 2-phenylethanol and indan-2-ol, respectively.84,85 In contrast, the reduction of s-cisoid a,b-unsaturated epoxides with borane-THF proceeds stereoselectively with the double bond shift to give allylic alcohols of (Z) configuration (eq 22).86 Similarly, a,b-unsaturated aziridines are reduced to (Z) allylic amines.87 Acetals and tetrahydropyranyl ethers undergo reductive cleavage88 (eq 23).88c

Imidazolidines and oxazolidines are also cleaved,89 whereas thiazolidines appear to be stable to the reagent at rt.90

Nitro Compounds.

The nitro group itself is stable to the reagent; however, aci nitro salts and a,b-unsaturated nitro compounds are reduced to the corresponding hydroxylamines.91,92

Other Applications.

Borane-THF reacts with Grignard reagents, arylmercury, arylthalium, and allyl- and propargyllithium compounds to give organoboranes which can be oxidized to the corresponding alcohols, phenols, and 1,3-diols.93 The reagent is used for blocking tertiary amine groups in Friedel-Crafts cyclizations and oxidative phenol coupling reactions in alkaloid syntheses.94 It activates a,b-unsaturated acids in the reaction with 1,3-dienes95 and is used in the synthesis of catalysts for asymmetric Diels-Alder and aldol reactions.75 Trialkylboranes are useful intermediates in the synthesis of compounds labeled with various isotopes for medical applications.96

Related Reagents.

Borane-Ammonia; Borane-Dimethyl Sulfide; Borane-Pyridine; Diborane.


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Marek Zaidlewicz

Nicolaus Copernicus University, Torun, Poland

Herbert C. Brown

Purdue University, West Lafayette, IN, USA



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