1,8-Diazabicyclo[5.4.0]undec-7-ene1

[6674-22-2]  · C9H16N2  · 1,8-Diazabicyclo[5.4.0]undec-7-ene  · (MW 152.24)

(organic soluble base for elimination reactions, isomerizations, esterifications, amidations, etherifications, condensations, carboxylations/carbonylations, and halogenations)

Alternate Name: DBU.

Physical Data: bp 259-260 °C; d 1.0192 g cm-3.

Solubility: readily sol water, ethanol, benzene, acetone, ethyl acetate, carbon tetrachloride, diethyl ether, dioxane, 1,4-butanediol, dimethyl sulfoxide; hardly sol petroleum ether.

Form Supplied in: clear, light yellow liquid.

Handling, Storage, and Precautions: stable at ambient temperatures, but is hygroscopic; overexposure to atmosphere results in water absorption which can lead to hydrolysis; rate of hydrolysis to N-(3-aminopropyl)-ε-caprolactam (10 wt% soln; molar ratio 1:76) is 3 × 10-4 mol%-1 min-1; half-life for a 0.657 molar soln is 33 min at 35 °C; contact with undiluted product may cause skin irritation or burns.

Introduction.

1,8-Diazabicyclo[5.4.0]undec-7-ene is an organic soluble amidine base which has been used effectively, and under relatively mild conditions, for a variety of base-mediated organic transformations including eliminations, isomerizations, esterifications, amidations, etherifications, condensations, carboxylations/carbonylations, and halogenations. A related reagent, 1,5-Diazabicyclo[4.3.0]non-5-ene (DBN), is used for similar reactions.

Elimination Reactions.

DBU has been used widely for dehydrohalogenations as well as for the introduction of unsaturation by elimination of sulfonic acids. Reactions generally proceed under mild conditions and without side reactions. Typical procedures use equimolar base and elevated reaction temperatures (generally 80-100 °C). Reaction solvents such as DMF, benzene, and DMSO have been used with reaction times varying from several hours to several days. Products may be distilled directly from the reaction mixture or separated from the DBU salt byproduct by extraction with a nonpolar solvent. Terminal as well as internal double bonds can be introduced with a high degree of regioselectivity. Additionally, this method has been used to prepare functionalized alkenes such as vinyl halides and vinyl ethers. Alkynes are not typically prepared using DBU-mediated eliminations; propargyl ethers, however, are an exception.

Oediger and Möller introduced DBU in 1967 for the dehydrohalogenation of bromoalkanes. DBN was found to be less effective.2 For example, treatment of 4-bromoheptane with equimolar DBU at 80-90 °C gave 3-heptene in 91% yield; with DBN the yield was 60%. Similar treatment of 2-bromooctane gave a 4:1 mixture of 2-octene and 1-octene in 84% yield; with DBN the yield was 40%.

DBU was used in the following one-pot procedure for converting b-disubstituted primary alkyl iodides to terminal alkenes.3 DBN was also used effectively for this procedure. The THP-protected tosylate (1) was converted to the corresponding iodide with NaI in DMF; subsequent addition of 1.5 equiv of DBU and heating at 80 °C for 3-4 h afforded terminal alkene (2) in 82% overall yield from (1) (eq 1). For comparison, when (1) was transformed into the bromide and treated with Potassium t-Butoxide in DMSO at 50 °C, a 3:2 mixture of the terminal alkene and its rearranged trisubstituted isomer resulted.4

DBU was found to be an effective base for converting piperidine (3) into 3,4-dehydropiperidine (4) without formation of the undesired 2,3-dehydropiperidine in a synthesis of the alkaloid sedinine (eq 2).5

DBU has also been used to prepare (E)-1-iodo-1-alkenes from 1,1-diiodoalkanes.6 In a representative procedure (eq 3), diiodobutane (5) was combined with equimolar DBU and heated to 100 °C until appearance of a brown solid (15-20 min). The product, (E)-1-iodo-1-butene (6), was isolated in 80% yield by distilling directly from the reaction mixture. Higher-boiling vinyl iodides required DMSO as reaction solvent and product extraction with pentane.

DBU is not typically used to convert vinyl halides into alkynes; in general, this conversion requires alkoxides, solid alkali, or alkali metal amides.7 However, the (Z)-vinyl bromide (7: R = H) was nearly quantitatively converted into terminal alkynes (8) in 2 h with DBU in refluxing benzene; the corresponding (E) isomer did not react (eq 4).8 When a (Z/E) mixture of (7) was treated with Potassium Carbonate in refluxing methyl ethyl ketone (MEK), only starting material was recovered. With b-oxygen substitution (R = CH2OH), vinyl bromides (7) were quantitatively converted into alkynes in 1 h with either DBU in refluxing benzene or potassium carbonate in refluxing MEK.

DBU has also been used to prepare alkenes from sulfonates. Heating tosylate (9) in solvent DBU for 30 min at 100 °C afforded the cis vinyl ether (10) in 72% yield (eq 5).9 When (9) was treated with potassium t-butoxide in t-butanol for 30 min at 80 °C, the vinyl ether product was a 3:7 cis:trans alkene isomer mixture (72% yield).

Otter used DBU to effect the elimination of methanesulfonic acid in the final step of his preparation of 1,3-dimethyl-6-propyluracil, a synthetic pyrimidine nucleoside (eq 6).10

Isomerizations.

1,8-Diazabicyclo[5.4.0]undec-7-ene is used for base-mediated double bond migrations and epimerizations. Isomerizations generally require proton abstraction at a carbon a to a carbonyl group (or related functionality) and are thermodynamically controlled.

DBU was used to equilibrate a mixture of substituted pyrrolidin-2-ones in the final step of a herbicide synthesis.11 The mixture of isomers (11) was allowed to stand for 1 h at rt with DBU in toluene to give the pure 3,4-trans isomer (12) in 96% yield (eq 7).

Amidine bases have been used extensively for the equilibration of b-lactams. Although DBN appears to be the base of choice for such epimerizations,12-17 the 7a-(dimethylamidino)-3-cephem ester 1a-oxide (14) was prepared via DBU epimerization of the corresponding 7b-isomer (13) (eq 8).18

DBU has been employed to convert esters with b,g-unsaturation into the corresponding a,b-unsaturated isomers.19 Thus 3-pentenoate (15) underwent up to 60% isomerization to 2-pentenoate (16) in the presence of DBU at 100 °C (eq 9). Corresponding exposure of pure cis-2-pentenoate (16) to DBU at 130 °C for 4 h afforded a similar product mixture (53% trans-2-pentenoate, 40% of 3-pentenoate (15), and 7% recovered starting material), suggesting thermodynamic equilibrium.20 For comparison, in a continuous process using 4-Dimethylaminopyridine at reflux, up to 78% of the thermodynamically favored (16) was converted to (15) over the course of 30 h.

b,g-Unsaturated nitriles (e.g. 17) have been isomerized to the thermodynamically favored a,b-unsaturated nitriles (e.g. 18) in the presence of catalytic DBU or DBN (eq 10).21

As part of a study directed toward the synthesis of dodecahedrane, DBU was used to effect the isomerization of bis-enone (19) into (20) in 90% yield (eq 11).22 The mechanism was presumed to involve formation of the b,g-unsaturated isomer of bis-enone (19).

Several biomimetic methods for the conversion of amines to carbonyl compounds have used amidine bases to effect equilibration of the intermediate imine.23-25 Rapoport used DBU in the oxidation of amines with 4-formyl-1-methylpyridinium benzenesulfonate (FMPBS).25 As an example, phenylacetaldehyde (22) was obtained in 83% yield from b-phenylethylamine (21) after treatment with FMPBS and DBU in CH2Cl2/DMF (eq 12). Triethylamine was not effective as a DBU replacement except when the amine b-carbon had electron withdrawing substituents (e.g. acetophenone from a-phenylalanine).

Esterifications, Amidations, and Etherifications.

DBU has been used to prepare esters26 and amides27 from carboxylic acids as well as ethers,28 esters,29 and carbamates30 from alcohols. These procedures involve proton abstraction followed by reaction of the carboxylate or alkoxide with an alkyl halide, acylating agent, or other suitable electrophile. Esterifications and amidations are generally conducted at or near rt, whereas etherifications require elevated temperatures (60-80 °C).

In 1978, Ono reported a convenient procedure for the esterification of carboxylic acids using DBU.26 Esters were produced in high yield from acids, alkyl halides, and DBU. The advantage of this procedure is that it provides mild conditions for esterification, it is not necessary to prepare the carboxylate anion in a separate step, and side reactions, especially dehydrohalogenation, are avoided. Amino acids have been esterified without racemization using this procedure. As a representative example, benzoic acid reacted with ethyl iodide in the presence of DBU for 1 h to give ethyl benzoate in 95% yield. The same reaction using triethylamine instead of DBU afforded essentially no ethyl benzoate. With benzyl bromide, benzoic acid, and DBU in DMSO, benzyl benzoate was formed quantitatively at 30 °C in 10 min.31 Triethylamine, when substituted for DBU, afforded benzyl benzoate in 81% after 1 h at 30 °C; Pyridine afforded a 15% yield of benzyl benzoate after 6 h.

The high yields of ester afforded by this method make it attractive for polyester synthesis.31 Thus isophthalic acid reacted with m-xylylene dibromide in the presence of 2 molar equiv of DBU in DMSO at 30 °C for 3 h to afford polyester (23) in high yield and viscosity (eq 13). Other organic bases such as triethylamine, pyridine, N,N-dimethylaminopyridine, or a DBU-pyridine mixture did not afford any polymer.

This method has also been used for the esterification of polymers.28,32 As an example, poly(methacrylic acid) (24) reacted with p-bromomethylnitrobenzene in the presence of equimolar DBU in DMSO at 30 °C for 3 h to afford poly(methacrylate) (25) at 97 mol% esterification (eq 14).

The etherification of poly(4-hydroxystyrene) (26) is related.32 On treatment with equimolar p-bromomethylnitrobenzene and DBU in HMPA at 60 °C for 24 h, (26) was converted into poly[4-(4-nitrobenzyloxy)styrene] (27) at 98 mol% etherification (eq 15). Solvents such as DMF, DMSO, and NMP were less effective for this reaction, requiring longer reaction times, higher reaction temperatures, and excess reagent. For example, a high degree of etherification (>80%) using DMF as solvent required 2 molar equivalents each of p-bromomethylnitrobenzene and DBU at 80 °C for more than 24 h. No reaction was observed using triethylamine or pyridine instead of DBU.

Polyimides containing pendant carboxylic acids have also been esterified using 1-phenethyl bromide and DBU.33,34

In an alternative approach to esterification, DBU was used to accelerate the reaction of N-acylimidazoles with t-butanol in a one-pot conversion of carboxylic acids into their t-butyl esters. The general reaction is outlined in eq 16. Acids were treated with 1 molar equiv of N,N-Carbonyldiimidazole in DMF under a nitrogen atmosphere at temperatures from 40 to 80 °C and reaction times of 5 to 24 h. Products were extracted from the reaction mixtures with diethyl ether. In this manner, t-butyl benzoate, t-butyl cinnamate, and t-butyl heptanoate were prepared in 91%, 64%, and 68% yield, respectively. With sodium t-butoxide rather than DBU, there was competitive formation of 3-oxoalkanoic esters with acids having one or two protons at C-2.35 A similar limitation was noted for the conversion of N-acylimidazoles to t-butyl esters using t-butanol and NBS.36

Enolizable acyl cyanides have been converted into 1-cyano-1-alkenyl esters upon treatment with tertiary amines (e.g. DBU, pyridine, dimethylamine, and 1,4-Diazabicyclo[2.2.2]octane) and carboxylic acid chlorides or anhydrides.37 With acid chlorides, equimolar base was required, whereas only a catalytic amount of base was necessary for reactions involving acid anhydrides. As an example, propionyl cyanide was treated with a stoichiometric amount of DBU and acetyl chloride in methylene chloride at rt to afford 1-cyanovinyl acetate in 61% yield.

DBU has also been used for certain deesterifications. In the case of acetates, deacetylation with DBU occurs under relatively mild conditions (rt to 80 °C; 5-45 h).38 The method only works for esters derived from acetic acid. Methanol is the solvent of choice, although dichloromethane or benzene may be added to improve reactant solubility. It was speculated, though not confirmed, that the mechanism involves formation of the desired alcohol by elimination of ketene. As an example, acetate (28) was deacetylated in 93% yield to alcohol (29) with DBU in methanol at rt for 24 h (eq 17). The same reaction using DBU in xylene afforded only starting material.39

Methyl esters are cleaved with DBU. High reaction temperatures and extended reaction times are required; however, the corresponding acid is generally obtained in high yield (>90%) without the use of ionic nucleophiles such as Lithium Iodide, lithium thiolate, or potassium t-butoxide.39 Thus a solution of methyl mesitoate, 10 equiv of DBU, and 10 equiv of o-xylene was heated to 165 °C for 48 h, affording mesitoic acid which was isolated in 95% yield after ether extraction (eq 18).

DBU was one of several effective bases used in an amide synthesis from N,N-bis(2-oxo-3-oxazolidinyl)phosphorodiamidic chloride, primary or secondary amines, and carboxylic acids.27 Other bases included 1-Ethylpiperidine, triethylamine, and N-ethylmorpholine. Reactions were conducted at rt for 1-2 h, avoiding racemization of optically pure substrates. Thus 3,3-dimethylacrylic acid and the acid salt of (S)-(-)-a-methylbenzylamine in DMA were treated with 2 equiv of DBU at rt over 30 min, then allowed to react at rt for 75 min to afford amide (30) in 75% yield (eq 19). Other methods require higher reaction temperatures (alkyl carbamates and alkyl amines afford amides at 200 °C in the presence of tertiary amines40), longer reaction times (1,3-Dicyclohexylcarbodiimide41 and Diphenylphosphinic Chloride42 procedures require up to 12 h and result in only modest yields for similar conversions), or excess reagents or reactants (the cyanuric chloride method43 requires excess acid and the o-nitrophenyl thiocyanate/Tri-n-butylphosphine method44 uses excess reagent and amine).

Condensations.

DBU has been used to effect condensations of active methylene compounds and other substrates containing active hydrogen. Reactions generally use equimolar DBU and aprotic solvents such as THF or benzene. Reaction times and temperatures vary.

DBU was shown to be an effective base for the Michael reaction of diethyl acetamidomalonate (31) with methyl acrylate in a synthesis of glutamic acid (eq 20).45 1,1,3,3-Tetramethylguanidine and DBN were found to be equally effective. All resulted in the formation of glutamic acid derivative (32) quantitatively.

In the Knoevenagel condensation of Malonic Acid with hexanal, the b,g-unsaturated isomer (33) was obtained with 94% selectivity and 56% yield after 10 h at 90 °C in the presence of equimolar DBU (eq 21).46 Other bases selective for (33) included triethanolamine, triethylamine, ethylpiperidine, N,N-dimethylaniline, and 2,6-Lutidine. For comparison, pyridine, 3-methylpyridine, and 4-methylpyridine showed ~90% selectivity for the a,b-unsaturated isomer. A b,g-unsaturated carbonyl compound was also obtained from the condensation of Formaldehyde with pentenone in the presence of catalytic DBU or DBN.47 These results are interesting in view of the literature reports that b,g-unsaturated carboxylates19,20 and nitriles21 are isomerized to the thermodynamically favored a,b-unsaturated isomers in the presence of DBU.

Diesters of homophthalic acid condensed with aromatic aldehydes in the presence of equimolar DBU in refluxing benzene for 6-10 h to give excellent yields of cinnamic esters. The reaction of dimethyl homophthalate with 3-benzyloxy-4-methoxybenzaldehyde to afford cinnamic ester (34) is representative (eq 22).48 Sodium hydride, sodium alkoxides, and potassium acetate were all found to be ineffective for this reaction.

DBU catalyzed the asymmetric synthesis of d-oxocarboxylic acids from (2R,3S)-3,4-dimethyl-5,7-dioxo-2-phenylperhydro-1,4-oxazepine (35).49 The Michael reaction of (35) with 2-cyclopenten-1-one afforded (+)-3-cyclopentanoneacetic acid (36) in moderate yield (43%) and high optical purity (96%) after hydrolysis and decarboxylation (eq 23). Trityllithium (Triphenylmethyllithium) or potassium t-butoxide afforded (36) in similar yields (30-50%) but poorer optical purity (7-76%).

Carbonylations and Carboxylations.

DBU has been used to prepare amides or imides50-52 and esters53 via the sequential carbonylation/alkylation of amines and alcohols, respectively. Similarly, the carboxylation of amines and alcohols affords urethanes and carbonates.54 Reactions use stoichiometric base and are catalyzed by palladium or nickel complexes. Product yields are generally high (80-100%). Carbonylations have been conducted in DMA at elevated temperatures (115-150 °C), whereas carboxylations have been performed at rt in a variety of solvents including methylene chloride, DMSO, THF, and glyme. Eqs (24) and (25) show the preparation of N-phenylbenzamide and allyl benzylethylcarbamate by these procedures. The efficacy of various bases was compared in the carboxylation/alkylation of benzylethylamine using a catalytic amount of tris(dibenzylideneacetone)dipalladium (Pd2(dba)3). DBU, N-cyclohexyl-N,N,N,N-tetramethylguanidine, and 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene were preferred. DBN afforded a modest 49% yield of urethane and essentially no urethane was formed using Diisopropylethylamine.

Methods for the preparation of thiocarbamates from alcohols, carbon disulfide, and alkylating agents55 or via the sulfur-assisted carbonylation of alcohols56 are related; these do not require a metal catalyst. Thus S-benzyl O-n-butylcarbonothionate was isolated in 86% yield after the carbonylation of n-butanol in THF at 80 °C for 4 h in the presence of 3 equiv of powdered sulfur and 5 equiv of DBU followed by esterification with 1.2 equiv of benzyl bromide (eq 26).

DBU has also been used in the nickel- or palladium-catalyzed coupling of alkenes with Carbon Dioxide.57,58 As an example (eq 27), isoprene was treated with Pd(acac)2, a phosphine ligand, DBU, and tributyltin ethoxide for 84 h at 80 °C under CO2 to afford an isomeric mixture of C-10 carboxylic acids (68% after esterification and purification). It was found that DBU and tributyltin ethoxide independently promote the reaction, but not as effectively as when used as a combination.

Halogenations.

DBU-based brominating agents such as DBU/bromine,59 DBU/hydrobromide perbromide,60 and DBU/bromotrichloromethane,61,62 have been used to brominate enolizable substrates and aromatic compounds. As an example, 3-halomethylcephems (e.g. 38), convenient intermediates for the synthesis of 3-substituted cephalosporins, were prepared by treatment of exo-methylene cephems (e.g. 37) with DBU/bromine in THF over the temperature range -80 to 0 °C (eq 28).

Miscellaneous.

A synthesis of phthalocyanines and metallophthalocyanines, interesting optical and electronic materials, involves the DBU-mediated reaction of alcohols with phthalonitrile at elevated temperature.63-67 The alkoxy-3-iminoisoindolenine (39) is presumed to be an intermediate (eq 29). Thus phthalonitrile was treated with equimolar DBU in refluxing ethanol to afford the corresponding phthalocyanine as a blue crystalline compound. DBN was also shown to be effective for this transformation; neither pyridine nor 1,4-diazabicyclo[2.2.2]octane promoted the formation of phthalocyanine.

Carboxylic acids have been phosgenated in the presence of a variety of amine bases, including DBU, to afford acid chlorides.68 Reactions are conducted with 2 mol% base at 80-100 °C. Yields of acid chlorides are generally greater than 90%.

DBU was used to promote the in situ formation of hydrogen selenide from Selenium, Carbon Monoxide, and water for selective reduction of a,b-unsaturated carbonyl compounds.69 N-Methylpyrrolidine was also found to be effective. Thus benzylideneacetone afforded 4-phenylbutanone in 82% yield in the presence of DBU, selenium, and carbon monoxide at 50 °C or N-methylpyrrolidine at 80 °C after 24 h. Isophorone underwent selective reduction of the conjugated double bond and b-ionone was reduced to 4-(2,6,6-trimethyl-1-cyclohexenyl)-2-butanone.


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Ann C. Savoca

Air Products and Chemicals, Allentown, PA, USA



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