· Aluminum Bromide
· (MW 266.72)
(Lewis acid catalyst similar to AlCl3; useful in oxacycle,1 cyclopentanone,3 and conjugated cyclopentenone4 synthesis; halogenations;6 reductive deoxygenation of ketones and secondary alcohols;7 selective ether cleaving reagent;8 isomerization catalyst;12 Friedel-Crafts catalyst16)
Physical Data: mp 97 °C; bp 268 °C; d 3.01 g cm-3.
Solubility: sol CS2, alcohols, ether, acetone, hydrocarbons, nitrobenzene, Br(CH2)2Br.
Form Supplied in: commercial grade white to yellowish-brown deliquescent solid; colorless is pure. Also available as a 1.0 M solution in CH2Br2. Exists as a dimer (Al2Br6) in solid and liquid phases.
Handling, Storage, and Precautions: use in a fume hood; fumes strongly in air; violent reaction with H2O; corrosive to skin. Keep tightly closed and protected from moisture. Decomposes upon heating in air to Br2 and alumina.
Unsaturated alcohols react with aldehydes in the presence of AlBr3 to give 4-bromooxacycles in a stereoselective manner (eq 1).1 Tetrahydropyrans are formed in the all-cis configuration. Seven-membered rings are produced as a mixture of isomers at the bromide position. This method has a synthetic advantage over similar allylsilane procedures2 in that larger rings can be prepared.
Ring expansion of 1-acyl-1-(alkyl or arylthio)cyclobutanes to cyclopentantones occurs readily in the presence of 1 equiv AlBr3 (eq 2).3 This method has been used to provide good yields (68-86%) of 2-, 2,4-, and 2,5-substituted cyclopentanones. Aluminum Chloride and Iron(III) Chloride were also effective in this conversion; however, BF3.Et2O and protic acids do not work.
Conjugated Cyclopentenone Synthesis.
Decomplexation of 1,3-butadieneiron tricarbonyl complexes by AlBr3 leads to conjugated cyclopentenones under mild conditions.4 The diene must be unsubstituted at the 4-position but substitution at all other positions is tolerated. This cyclocarbonylation is stereospecific, depending only on the configuration of the diene complex (eq 3). Spirocyclic compounds can be formed using an appropriate precursor (eq 4). The method serves as a valuable alternative to the intramolecular Pauson-Khand reaction. One apparent limitation is that bicyclic cyclopentenones with an angular alkyl group cannot be prepared.
A novel photochemical ring contraction of 1-naphthols to 3-halomethylindanones is promoted by AlBr3 or AlCl3 (eq 5).5 The halogen substituent is derived from the halogenated solvent. In CH2Cl2, chlorides are the major product; in CH2Br2, bromides are produced regardless of whether AlBr3 or AlCl3 is used.
Thiophenes can be regioselectively brominated by AlBr3 in the presence of Benzeneseleninyl Chloride (eq 6).6a Furans are halogenated in low yield and pyrroles are unreactive.
Bridgehead halogen exchange in polycyclic substrates is catalyzed by AlBr3 in halogenated solvents and proceeds readily in fair to good yield (50-90%).6b For bromination, CH2Br2 or CHBr3 solvent is used. Iodinations are carried out in MeI or CH2I2 and chlorinations in CHCl3 or CCl4. Bromoalkylation of 1-bromoadamantane occurs when subjected to ethylene or vinyl bromide in the presence of AlBr3 to give 1-bromo- or 1,1-dibromo-2-(1-adamantyl)ethane, respectively.6c Yields are dependent on the quality of the AlBr3. Bromination of the bridgehead position of adamantanone by AlBr3/t-BuBr (so-called
sludge catalyst) provides an improved, high yielding synthesis of 1-bromo-4-adamantanone.6d
C-O Bond Cleavages.
Reductive deoxygenation of ketones and secondary alcohols to the corresponding methylene hydrocarbons in excellent yield can be accomplished by the Diisobutylaluminum Hydride/AIBr3 reagent system.7 In some cases, the addition of a catalytic amount of Cp2MCl2 (M = Ti, Zr) or Nickel(II) Acetylacetonate is required. Diaryl, alkyl aryl, or dialkyl ketones and secondary alkyl or benzylic alcohols undergo this reaction but primary alcohols or phenols do not.
Anhydrous AlBr3 in MeCN or CS2 is useful for the removal of methyl ethers and in some instances selective cleavage is possible.8 The 5-MeO group in 3,5,6,7-tetramethoxyflavones and the 3-MeO group in 3,6,7-trimethoxy-5-tosyloxyflavones are selectively cleaved in nearly quantitative yield without debenzylation (eq 7).8a It is superior to AlCl3 in some ether cleavages8b since it is a stronger Lewis acid and more soluble in organic solvents. An excellent reagent for phenol ether cleavage (except diphenyl ether) is the pyridinium salt of AlBr3.8c
AlBr3 is an efficient reagent for epoxide cleavage giving high yields of the bromohydrin. It does not show much regioselectivity in the case of substituted substrates.9 The hydroxylation of cyclic acetals with Hydrogen Peroxide can be catalyzed by AlBr3 as well as with other reagents (especially SeBr4) to give esters of the type RCO2(CH2)
nOH (R = H, aryl, alkyl; n = 2-3).10 Disulfide cleavage with AlBr3 has been reported to give cyclized products in certain systems (eq 8).11
The AlBr3 sludge catalyst has been extensively used as an effective isomerization reagent in numerous adamantane, diamantane, and triamantane syntheses (eq 9).12 A wide variety of polycyclic hydrocarbons can be rearranged to these ring systems. Perhydrogenated phenanthrene rearranges to give mainly trans,syn,trans-perhydroanthracene by the action of AlBr3/HBr in dimethylcyclohexane.13
A facile rearrangement of a-bromoethyldiethylborane (eq 10) occurs when treated with Lewis acids.14 AlBr3 is very effective for this conversion. AlCl3 or Silver(I) Tetrafluoroborate also give high yields. The half-life for rearrangement is 0-5 min with these catalysts.
Diphenylacetylene undergoes dimerization with AlBr3 to give 1,2,3-triphenylazulene.15 The yield is very dependent on the purity of AlBr3 used. Yields are enhanced by the addition of a small amount of V2+ or Ni2+, whereas Ti3+, V4+, Cr2+, Cr3+, Fe3+, or Zn2+ almost completely suppress azulene formation.
AlBr3 is a superior reagent for intramolecular Friedel-Crafts cyclization of o-phenylalkanoic acid chlorides (Ph(CH2)
nCOCl; n = 8-15) to the paracyclophanes (eq 11).16a Under high dilution conditions, the yields are generally twice as high as that with AlCl3. Negligible yields of medium-sized cyclophanes (n = 8, 5.3%) are obtained. Yields tend to increase with increasing ring size (n = 15, 70%). Other examples of its use as a Friedel-Crafts catalyst and in the Fries rearrangment of phenol esters appear in the literature.16b-f
- 1. Coppi, L.; Ricci, A.; Taddei, M. JOC 1988, 53, 911.
- 2. (a) Coppi, L.; Ricci, A.; Taddei, M. TL 1987, 28, 973. (b) Wei, Z. Y.; Li, J. S.; Wang, D.; Chan, T. H. TL 1987, 28, 3441.
- 3. Yamashita, M.; Onozuka, J.; Tsuchihashi, G.; Ogura, K. TL 1983, 24, 79.
- 4. Franck-Neumann, M.; Michelotti, E. L.; Simler, R.; Vernier, J.-M. TL 1992, 33, 7361.
- 5. Kakiuchi, K.; Yamaguchi, B.; Tobe, Y. JOC 1991, 56, 5745.
- 6. (a) Kamigata, N.; Suzuki, T.; Yoshida, M. PS 1990, 53, 29. (b) McKinley, J. W.; Pincock, R. E.; Scott, W. B. JACS 1973, 95, 2030. (c) Stetter, H.; Goebel, P. CB 1962, 95, 1039. (d) Klein, H.; Wiartalla, R. SC 1979, 9, 825.
- 7. Eisch, J. J.; Liu, Z.-R.; Boleslawski, M. P. JOC 1992, 57, 2143.
- 8. (a) Horie, T.; Kawamura, Y.; Tsukayama, M.; Yoshizuki, S. CPB 1989, 37, 1216. (b) Adams, R.; Mathieu, J. JACS 1948, 70, 2120. (c) Prey, V. CB 1942, 75, 537.
- 9. Eisch, J. J.; Liu, Z.-R.; Ma, X.; Zheng, G.-X. JOC 1992, 57, 5140.
- 10. Zlotsky, S. S.; Nazarov, M. N.; Kulak, L. G.; Rakhmankulov, D. L. JPR 1992, 334, 441.
- 11. Campaigne, E.; Heaton, B. G. CI(L) 1962, 96.
- 12. (a) Hollywood, F.; Karim, A.; McKervey, M. A.; McSweeney, P. CC 1978, 306. (b) Kafka, Z.; Vodicka, L. CCC 1990, 55, 2043. (c) Williams, V. Z., Jr.; Schleyer, P. v. R.; Gleicher, G. J.; Rodewald, L. B. JACS 1966, 88, 3862. (d) Nomura, M.; Schleyer, P. v. R.; Arz, A. A. JACS 1967, 89, 3657. (e) Graham, W. D.; Schleyer, P. v. R.; Hagaman, E. W.; Wenkert, E. JACS 1973, 95, 5785. (f) Gund, T. M.; Osawa, E.; Williams, V. Z., Jr.; Schleyer, P. v. R. JOC 1974, 39, 2979. (g) Robinson, M. J. T.; Tarratt, H. J. F. TL 1968, 5. (h) Gund, T. M.; Williams, V. Z., Jr.; Osawa, E.; Schleyer, P. v. R. TL 1970, 3877. (i) Fort, R. C., Jr.; Schleyer, P. v. R. CRV 1964, 64, 277. (j) Schleyer, P. v. R.; Donaldson, M. M. JACS 1960, 82, 4645.
- 13. Schneider, A.; Warren, R. W.; Janoski, E. J. JOC 1966, 31, 1617.
- 14. Brown, H. C.; Yamamoto, Y. CC 1972, 71.
- 15. Meijer, H. J. D.; Pauzenga, U.; Jellinek, F. RTC 1966, 85, 634.
- 16. (a) Huisgen, R.; Ugi, I. CB 1960, 93, 2693. (b) Olah, G. A.; Kobayashi, S.; Tashiro, M. JACS 1972, 94, 7448. (c) Hausigk, D. CB 1970, 103, 659. (d) Dawson, I. M.; Gibson, J. L.; Hart, L. S.; Waddington, C. R. JCS(P2) 1989, 2133. (e) Dewar, M. J. S.; Hart, L. S. T 1970, 26, 973. (f) Erre, C. H.; Roussel, C. BSF(2) 1984, 449.
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