[7429-90-5]  · Al  · Aluminum  · (MW 26.98)

(reducing agent for many functional groups;1 can aluminate double bonds2 or insert into carbon-halogen bonds; promotes propargylation of carbonyl compounds; in combination with metallic Sn or PbBr2 mediates Barbier-type allylation of carbonyl compounds or imines)

Physical Data: mp 660.37 °C; bp 2467 °C; d 2.702 g cm-3; E°(aq) Al3+/Al0 = -1.66 V.

Solubility: reacts with dil HCl, H2SO4, KOH, and NaOH, hot AcOH; insol conc HNO3.

Form Supplied in: silver-white, malleable, ductile metal; widely available in foil, granules, ingot, pellets, powder, rod, shot, wire.

Handling, Storage, and Precautions: Al foil is moisture sensitive, powder is moisture sensitive and flammable. Aluminum is reputed to be practically nontoxic.

Functional Group Reductions.

Aluminum will reduce ketones or aldehydes to alcohols or saturated hydrocarbons, nitro compounds and Shiff bases to amines, and alkyl halides to alkanes. Aluminum also effects reductive dimerization of ketones and dehalogenation of polyhalogenated compounds. In combination with Nickel(II) Chloride hexahydrate in THF, metallic Al will reduce acyl chlorides, anhydrides and nitriles, epoxides, and disulfides. Moreover, aluminum exhibits pronounced chemoselectivity in reduction of polyfunctionalized compounds.3

Aromatic ketones are especially readily reduced by aluminum. Reduction of 2-methyl-2-phenyl-1-indanone with Al in i-PrOH proceeds with diastereoselectivity (trans/cis ratio in alcohol formed is 90:10) better than that observed with Li, Na, or K.4 Quinone (1) is reduced with Al in 85% H2SO4 to the 6,13-dihydro product (2) (eq 1) while the 6-hydroxy derivative is formed in reaction of (1) with Cu in 96% H2SO4.5

Reductive dimerization of tetralone to 3,3,4,4-tetrahydro-1,1-binaphthyl is mediated by Al foil activated with Mercury(II) Chloride.6 A facile reduction of aromatic ketones and aldehydes into corresponding alcohols is readily achieved employing Al powder in combination with NiCl2.6H2O in THF.3

Reduction of (thiazolyl)(acylaminomethyl)ketones with the reagent prepared from Al dissolved in i-PrOH in the presence of Aluminum Chloride,7a and oxidation of aluminum tris(3-pentanoxide-1,2-d5) (from the alcohol, Al shot, and a trace of HgCl2) with benzophenone,7b can be considered special cases of modified Meerwein-Ponndorf-Verley and Oppenauer reactions.

Aromatic nitro compounds are reduced to anilines with Al powder in AcOH-HCl8 or with Al-NiCl2.6H2O.3 Reactions of nitrobenzene with Al turnings (heating in aq H2SO4)9a or Al-Fe alloy (in 50% NaOH)9b afford p-aminophenol and N,N-diphenylhydrazine, respectively.

Al foil activated with HgCl2 and Hg is capable of reducing both aliphatic10a and aromatic Shiff bases10b to secondary amines. For aromatic substrates, the process is substantially complicated by reductive dimerization of the azomethines. It is this particular reagent (not other metals such as Na, Mg, or Zn) that causes unusual ring contractions of lumazines11a and pterines11b to theophyllines (eq 2) and guanines respectively, via a radical anion formed upon C=N double bond reduction.

1,2-Dehalogenation occurs when polyhalogenated substrates react with Al turnings in the presence of AlCl3.12 For alicyclic compounds it is a stereospecific process (eq 3).12b

When 2-5% NaOH is added, Al (powder or turnings) reduces 1,1-dibromocyclopropanes or 7,7-dibromonorcaranes into monobromides with modest cis or exo stereoselectivity.13 A small amount of Al powder promotes halide exchange and 3-chlorophthalide is converted almost quantitatively into 3-bromophthalide in a reaction with dry HBr.14

On refluxing with an excess of an alkyl or aryl halide, Al inserts into the carbon-halogen bond.15 Primarily a mixture of sesquihalides (3) is formed (eq 4).15 With alkyl halides,16 the sesquihalide mixture formed in situ can be easily reduced to produce trialkylaluminum compounds (eq 5).16c In case of aryl halides, (3) is an excellent reagent for transfer of an aryl moiety from Al to other elements (P, Sn) (eq 6).17 Mixture (3) also reacts with organic halides to form organoaluminum compounds. However, a more versatile method of synthesis involves the reductive replacement of Hg from organomercury compounds with Al (eq 7).18

Reduction of element-halogen bonds by Al or Al-promoted cleavage of other bonds is widely used in synthesis of organoboron,19 organosilicon,20 organogermanium,21 and organophosphorus compounds,22 as well as p-complexes of titanium,23 vanadium,23b and zirconium.24 Al powder in combination with AlCl3 and Iodine (or Iodomethane) induces borylation of C6H6 with Boron Trichloride to form PhBCl2.19a Mixtures of amines and triarylborates are reduced to borazanes by aluminum-iodine.19b On electrochemical reduction with a sacrificial Al anode, chlorosilanes undergo dimerization20a or cross-coupling if two different chlorosilanes react.20b Tetraalkylgermanes are cleaved by the system Al-I2 to yield iodogermanes.21

A mild and neutral reducing system of Al-NiCl2.6H2O-THF reduces enones to saturated aldehydes (eq 8),3 nitriles to amines, acid chlorides and anhydrides to aldehydes, disulfides to thiols, and epoxides to the alcohols. Isolated double bonds, carboxylic acids, esters, lactones, primary, benzyl and allyl halides, aliphatic aldehydes and ketones as well as aliphatic nitro compounds are inert to this agent.3

In combination with Antimony(III) Chloride, metallic Al smoothly reduces both aliphatic and aromatic aldehydes to alcohols (yield 50-98%). Reduction proceeds chemoselectively and PhCHO is preferably reduced in competition with PhCOMe. Unlike Al-NiCl2, reduction of a,b-unsaturated aldehydes with this system occurs only at the C=O group, leaving C=C double bonds intact.25

Reactions of Aluminum Alkoxides and Phenoxides.

A facile method of alcohol dehydration is based on thermal decomposition of the derived Al alkoxide which, depending on the alcohol structure, commences in the range of 200-270 °C.26 Al foil (HgCl2-activated) in i-PrOH promotes deoxygenation of epoxides, giving the corresponding alkenes.27

Friedel-Crafts Alkylation Catalyst.

Aluminum catalyzes ortho-alkylation of anilines and phenols with alkenes.28,29 Preformed aluminum anilides or phenoxides exhibit high susceptibility toward alkene attack and usually o,o-dialkylated products predominate.29a,b Selective ortho-alkylation of phenol can be accomplished using aluminum soft drink cans as the reagent (eq 9).28

Alumination Agent.

A facile hydroalumination of carbon-carbon double bonds yielding trialkylaluminum compounds takes place in a three-component mixture consisting of an alkene, hydrogen, and Al (eq 10).2 The process involves intermediate formation of a dialkylaluminum hydride which adds to the third alkene molecule, affording the trialkylaluminum. To elucidate the mechanism of the reaction, direct synthesis of organoaluminum compounds in the condensed phase from Al vapor and alkene is of noticeable importance.30

Selective Propargylation.

The problem of propargyl group introduction in various substrates without propargyl-allenyl rearrangement has been addressed using alumination (Al in the presence of HgCl2) of propargylic bromides followed by addition of a carbonyl substrate to the organoaluminum reagent formed in situ.31 This approach has been used to prepare propargylic alcohols from ketones32a and aldehydes,32b propargylic ethers from a-chloro ethers33a and acetals,33b acetals of propargylic carbaldehydes from orthoformates (eq 11),34 homopropargylic ethers and sulfides from chloromethyl ethers and sulfides.35 Propargylic compounds prepared in this manner have been used in the synthesis of spirilloxanthin, 3,4-dehydro-rhodopin,32a ecdysteroid analogs,35 and lycorine precursors.36

Aluminum is presumably a unique partner for condensations of this type because, for example, Zn always gives a mixture of alkynic and allenic alcohols.31,32a

Aluminum-Tin(0) and Aluminum-Tin(II) Systems.

A strong impetus for current applications of Al in organic synthesis has resulted from a study on allylation of carbonyl compounds mediated by the combination of stoichiometric amounts of metallic Al and Tin, and the observation of extensive acceleration in the presence of water.37 Typical procedures suggest that a carbonyl compound, allylic bromide, Al, and Sn should be used in a ratio 1:1.2-2:1:1. Allylation with unsymmetrically substituted allylic halides proceeds with complete rearrangement in the allylic unit and shows considerable diastereoselectivity (eq 12).37a Intramolecular versions of the reaction have been used in syntheses of five- and six-membered cyclic alcohols.37c

A combination of Al powder (2 equiv) and Tin(II) Chloride (0.1 equiv) in an organic solvent is even more active than the Al-Sn couple and addition of water to the solvent enhances the allylation rate remarkably.38 Carbonyl compounds are almost inert to allylation if 1 equiv of Al and only 0.1 equiv of Sn is used,38 whereas metallic Sn or SnCl2 in aprotic solvents convert allylic halides to allylstannanes in situ, which readily allylate carbonyl substrates39 with complete rearrangement in the entering allylic unit.39a It has been established that Al effects both oxidative addition of metallic Sn to allyl halide and reductive regeneration of Sn0 from SnII and SnIV for a recycle use.38 Thus all allylation reactions in Al-Sn or Al-SnII systems can be considered as aluminum-promoted and tin-recycled processes.

Al-Sn mediated condensation of aldehydes with a-(bromomethyl)acrylates40 represents a facile synthesis of a-methylene-g-butyrolactones (eq 13).40a This procedure is more efficient than allylations promoted by CrII (see Chromium(II) Chloride and Lithium Aluminum Hydride).40b

Almost exclusive threo diastereoselectivity is characteristic of the reactions of cinnamyl chloride with aldehydes promoted by Al-Sn41a or Al-SnCl2.41b Aldehydes containing a chiral center adjacent to the C=O group afford threo Cram and threo anti-Cram products with the former predominating (eq 14) in contrast to magnesium-mediated reactions of the same partners, which give all four possible diastereomers.41a

Unlike other allylation processes mediated by Al-Sn, no isomerization of the allylic unit is observed in the reaction of aromatic aldehydes with CH2=CHCF2Cl in the presence of Al-10% SnCl2 in EtOH-AcOH-H2O. This reaction regiospecifically leads to 1-aryl-2,2-difluoro-3-buten-1-ols.42

Aluminum-Lead Bromide.

Three main processes, Barbier-type allylations, reductive coupling of carbonyls, and addition of polyhaloalkanes to carbonyls are mediated by this system which has been extensively studied in the past decade. PbBr2 is used in a catalytic amount (molar ratio Al foil:PbBr2 = 100:1-5), of great importance because of the hazardous properties of lead.

Al-PbBr2 promoted allylation of various carbonyl compounds can be performed at ambient temperature in DMF, aqueous THF, or aqueous MeOH while nonaqueous THF, MeOH, and MeCN are not suitable. A moderate excess of allylic bromide (10-100% relative to carbonyl substrate) is recommended. Under these conditions the addition to a,b-enones proceeds regiospecifically in a 1,2-fashion (eq 15).43

Metallic Al itself does not promote allylation which is, however, mediated by Pb-Bu4NBr.44 The observed stoichiometry (carbonyl substrate:CH2=CHCH2Br:Pb = 1:1:1) suggests that the reaction involves an active organolead(II) rather than organolead(IV) reagent, in contrast to the tin promoted reactions.38,39 The in situ generated Pb0 is much more effective than commercially available Pb and turnover of the Pb0 catalyst in the range 14-77 is attained.43

This type of Barbier-type allylation has been applied to C-3 chain elongation of cephalosporins.45

Acetals RCH(OMe)2 (4) as masked carbonyl compounds also undergo Barbier-type allylation mediated by Al-PbBr2-AlBr3 (molar ratio 1:0.03:0.1) to form homoallylic ethers (5).46 The presence of Aluminum Bromide is critical because no allylation product is formed in the absence of this co-catalyst. Reaction pathways depend strongly on quantities of reactants and reagents, especially in the case of acetals of aromatic aldehydes. An increase of AlBr3 from 0.1 to 0.5 equiv is accompanied by reductive homocoupling to 1,2-diaryl-1,2-dimethoxyethanes (6). If the ratio CH2=CHCH2Br to (4) is higher than 2:1, commonly used in such processes, diallylation and reductive homocoupling of monoallylated products effectively compete with monoallylation. The results reveal formation of cations (7) in the presence of AlBr3 as an acid catalyst. The cations are either further allylated or reduced by Pb0 to furnish dimers (6) (eq 16).46 Replacement of AlBr3 with Trifluoroacetic Acid (TFA) (1 equiv) causes a complete shift of the reaction toward formation of homocoupling products (6) from arene and heteroaromatic carbaldehyde acetals.47 To promote homocoupling, although less efficiently, VCl3 or Cobalt(II) Chloride or Tin(II) Bromide can be also employed instead of PbBr2 in the Al-PbBr2-TFA redox system.

An extremely facile procedure for C-allylation of imines involves direct mixing of imines and CH2=CHCH2Br with a catalytic amount of PbBr2 (0.03 equiv) and Al foil (1 equiv) in ether containing Boron Trifluoride Etherate (1.1 equiv) (eq 17).48 With almost the same efficiency, aromatic and aliphatic aldimines undergo electroreductive Barbier-type allylation using an Al anode/PbBr2/Bu4NBr/THF/Pt cathode system where a combination of sacrificial Al anode and Pb0/PbII operates as a mediator for anodic and cathodic electron transfer processes, respectively.49

Regarding the chemical allylation of imines, PbBr2 and the BF3.OEt2 couple can be replaced with a catalytic amount (0.05 equiv) of Titanium(IV) Chloride. This system is highly advantageous in chirality transfer from L-valine into a homoallylic amine (diastereomeric ratio greater than 20:1) (eq 18).50

Similar to the transformations observed for acetals,46,47 introduction of TFA in the Al-PbBr2 system allows reductive dimerization of N-alkylimines to vicinal diamines.51

Reductive dimerization of allylic as well as benzylic bromides is mediated by the Al-PbBr2 system in DMF. In the case of allylic bromides dimerization is complicated by allylic rearrangement and gives a mixture of isomeric 1,5-hexadienes. Benzylic bromides only give 1,2-diarylethanes.52 The same reactions are also induced by metallic Pb-Bu4NBr in DMF.52

An Al/PbBr2/NiCl2(bipy) (molar ratio 0.7:0.1:0.1) system promotes reductive dimerization of 2-arylvinyl halides to form 1,4-diaryl-1,3-butadienes, precursors of terphenyl derivatives (eq 19). The process is almost stereospecific and both double bonds in the dimer retain the C=C bond configuration of the starting vinyl halide. The reaction is carried out in DMF or MeOH at ambient temperature with KI (1.5 equiv) added. No dimerization is observed if THF, MeCN, CH2Cl2, or C6H6 are used as solvent or in the absence of either NiCl2 or PbBr2. The latter can be replaced by BiCl3, SnBr2, or GeCl4, but dimer yields are remarkably decreased.53

Both aromatic and aliphatic aldehydes undergo Al-PbBr2 mediated reductive addition of polyhalomethanes CX3Y (X = Cl, Br; Y = Cl, Br, CN, CONH2). The reaction (aldehyde:CX3Y:Al:PbBr2 = 1:2-4:1.2:0.1) is performed in DMF, yielding a-(trihalomethyl)carbinols (8) in almost quantitative yield. These can be subjected to further reductive elimination with Al-PbBr2 in MeOH containing aq HCl to produce 1,1-dihaloalkenes (9). When carbinols (8) react in DMF, reduction of the trihalomethyl group to a dihalomethyl occurs instead of elimination (eq 20).54 This approach has been used in the stereospecific synthesis of pyrethroid insecticide precursors54 and arylacetic acids.55

In the same way, a CF3CCl2 unit can be introduced into aldehydes via a reaction with CF3CCl3. This process is chemospecific and no adducts resulting from C-F bond cleavage are formed.56

The Al-PbBr2 system is also capable of mediating a facile reductive removal of bromine atoms from 6-bromo- and 6,6-dibromopenicillanate derivatives (in MeOH/aq 1% HBr (9/1)),57 and has been used in prenylation reactions of p-benzoquinones and naphthoquinones.58

Aluminum-Bismuth(III) Chloride.

Al powder in combination with catalytic amounts of BiCl3 in aqueous THF at ambient temperature promotes efficient Barbier-type allylation of aldehydes.59 The reaction occurs with allylic rearrangement in the entering unit and stereoselectively gives the erythro product. The reaction involves allylbismuth reagent formation through the oxidative addition of an allylic halide to Bi0 generated by the reduction of BiCl3 with Al. Metallic Bi is also capable of allylating aldehydes to homoallylic alcohols in aprotic DMF, but not in an aqueous solvent.60a Metallic Zn or Fe also can be used with BiCl3 instead of Al.60b

The system also induces alkylation of immonium cations. The procedure is experimentally simple due to its being performed in aqueous media.61 Among other alkylation agents, methyl and benzyl halides have been used for the first time in this Barbier reaction system (eq 21).61b

Aluminum-Antimony(III) Chloride and Aluminum-Indium(III) Chloride.

Both systems promote Barbier-type allylation of aldehydes. However, in contrast to InCl3, which is required only in a catalytic amount for a reaction in aqueous THF,62 more than stoichiometric amounts of SbCl3 are used in DMF-H2O (3:1) and NaI is added to activate allylic bromides (aldehyde:allylic halide:Al:SbCl3:NaI = 1:1.2:2:1.5:1.2).63 Similar to the Al-PbBr2 system,58 Al-InCl3 also mediates prenylation of naphthoquinones.62 Al in combination with a catalytic amount of SbCl3 induces acetalization of carbonyl compounds.63

Aluminum-Nickel Alloy.

In alkali medium this reagent is employed for chemical reductions.64 Al affects liberation of hydrogen which actually plays the role of reducing agent (eq 22).

Besides reduction of carbonyl compounds, organic halides, and nitriles,64b in recent years numerous compounds containing N-N and N-O bonds (hydroxylamines, hydrazines, N-nitrosoamines, N-oxides) have been reduced to amines.65 A few heterocycles are also reduced by this reagent, and high temperatures, high pressures, or a hydrogen atmosphere are not required. Corresponding tetrahydro derivatives are produced from pyridines, quinolines, and pyrazines, whereas pyrimidines, pyridazines, oxazoles, and isoxazoles undergo reductive cleavage of C-N, N-N, or N-O bonds to yield saturated diamines or diamino alcohols (see Raney Nickel).66

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Emmanuil I. Troyansky

Institute of Organic Chemistry, Russian Academy of Sciences, Moscow, Russia

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