Weygand/Hilgetag Preparative Organic Chemistry

394 Cleavage of carbon-oxygen bonds
aryl ethers then mostly undergo the Friedel-Crafts reaction. Detailed studies
of the course of the reaction, together with yields, have been reported by
Meerwein and his co-workers for a number of Lewis acids. 18 ZnCl2 and SnCl4
appear to be the best catalysts for this process. For instance, when 0.1 mole
of SnCl4 is used per mole of benzoyl chloride, diethyl ether gives, in 4 hours
at 110°, an 80% yield of alkyl chloride and a 90% yield of ethyl benzoate.^ 9
The action of acid chlorides on tetrahydrofuran in the presence of ZnCl2
gives the 4-chlorobutyl esters. 20 Acid bromides or, more readily, acid iodides
cleave ethers without the need for a catalyst. 21
Aliphatic ethers are cleaved by acetic anhydride containing boron trifluoride
etherate plus lithium iodide at 20° : 22 the alcoholic parents of the ether
are obtained as main products, together with small amounts of olefin; for
instance, cyclohexyl acetate and 3/?-cholestanyl acetate are formed from
alkoxycyclohexanes and 3/?-alkoxycholestanes, respectively.
Aluminum halides have preparative importance, particularly for removal
of methyl and ethyl groups; it does not attack diaryl ethers. The ether-cleaving
action of aluminum chloride was discovered with fission of anisole. 23 A
crystalline double compound is formed in the reaction and decomposes to
alkyl halide and aluminum phenoxide when warmed.
Anisole (10 g) is heated with aluminum chloride (15 g) for 3 h at 120°, methyl chloride
being evolved. The residual phenoxide is decomposed by water, the mixture is acidified,
and the phenol is taken up in ether and worked up in the usual way.
Aluminum bromide is superior to the chloride for cleavage of alkyl aryl
ethers; it is soluble in indifferent solvents such as benzene, carbon disulfide,
and light petroleum. According to Pfeiffer and Haack, 24 the reaction takes
place according to the scheme:
ROCH3 + AlBr3 • ROCH3-AlBr3 • ROAlBr2 + CH3Br
ROAlBr2 + H2O > ROH + AlBr2OH
If the molecule contains carbonyl groups, each adds an additional m6le
of aluminum chloride. 24 In simple cases it suffices to heat the substance with
the requisite amount of aluminum bromide under reflux in thiophene-free
benzene. 25
For the preparation of /j-hydroxybenzoic acid, /?-anisic acid (0.79 g; dried over P2O5) is
dissolved in warm benzene (35 ml), treated with a solution of AlBr3 (4.9 g) in benzene (30ml),
and boiled for 4.5 h under reflux. After cooling, the solution is decomposed with concentrated
hydrochloric acid and extracted with ether, the hydroxy acid is removed from the ether by
sodium hydroxide solution, liberated from the alkaline solution by acid, taken up again in
ether, and recovered on evaporation (0.37 g).
Compounds such as papaverine and piperonaldehyde have been similarly dealkylated.
18 H. Meerwein and co-workers, /. Prakt. Chem., [ii], 134, 51 (1932); 147, 257 (1937).
19 L. M. Smorgonskii, Zh. Obshch. Khim., 17, 416 (1947); Chem. Abstr., 42, 858 (1948).
20 M. E. Synerholm, Org. Syn., 29, 30 (1949).
21 E. L. Gustus and P. G. Stevens, /. Amer. Chem. Soc, 55, 378 (1933); N. O. V. Sonntag,
Chem. Rev., 52, 353 (1953).
22 R. D. Youssefyeh and Y. Mazur, Tetrahedron Lett., 26, 1287 (1962).
23 C. Hartmann and L. Gattermann, Ber. Deut. Chem. Ges., 25, 3531 (1892).
24 P. Pfeiffer and E. Haack, Ann. Chem., 460, 156 (1928).
25 P. Pfeiffer and W. Loewe, /. Prakt. Chem., [ii], 147, 293 (1936).