Weygand/Hilgetag Preparative Organic Chemistry

Cleavage of ether bonds 395
Ether fission can also be brought about by Mgl2, 26 BC13, 27 SiCl4, 28 or
LiAlH4, 29 but these reagents have so far been little used for this purpose.
Boron tribromide is an effective dealkylating agent for aryl ethers. 30
The ether and BBr3 are mixed in CH2C12 or pentane at —80° and allowed to warm slowly
to 20°, then poured into water. The phenol formed is, if necessary, extracted with ether, and
is purified by recrystallization.
Certain arylalkyl ethers, with which cleavage by acid fails or leads to undesired
by-products, can be decomposed by alkalis or alkoxides at temperatures
above 150°, preferably with addition of ethanol or a higher-boiling alcohol.
Veratrole, for instance, can be partially converted into guaiacol by 3 hours*
heating with equal parts of potassium hydroxide and ethanol; 31 and o-methoxyphenyl
phenyl ether is cleaved to 0-hydroxyphenyl phenyl ether in 91%
yield by 10 hours' boiling with sodium hydroxide in diethylene glycol. 32
As expected, the ease of fission of aryl ethers parallels their ease of formation;
ethers of nitrophenols, for example, can be hydrolysed by even gentle warming
with alcoholic potassium hydroxide. Polynitrophenyl ethers are often cleaved
even by water, ammonia, or primary or secondary amines. 33 Ethers of hydroxy
quinones are also decomposed by cold dilute sodium hydroxide solution. 34
A further method of cleaving C-O-C bonds consists in treating them with
organometallic compounds. Grignard compounds can react with ethers according
to the equation:
PhOR + R'MgX > PhOMgX + R—R'
This reaction is occasionally used for fission of alkyl aryl ethers, particularly
benzyl ethers. In general, temperatures between 150° and 200° are required, 35
but fission of /?,y-unsaturated phenyl ethers occurs already at 55°. 5
Other organometallic compounds may be more effective than Grignard
compounds. For instance, cholesteryl ethyl, isopropyl, and tert-butyl ether
are converted into cholesterol, pentane, and olefin by pentylsodium at 10°,
thus: 36
Choi—O—C2H5 + CsHnNa > Choi—ONa + C5H12 + C2H4
Cholestadiene is formed as by-product by the following reaction:
Choi—OR + C5HnNa > Cholestadiene + C5H12 + RONa
If there is no /J-hydrogen atom in the alkyl group R, as is the case with cholesteryl
methyl or benzyl ether, then cholestadiene is the sole product. According
26 V. Grignard and J. Ritz, Bull. Soc. Chim. France, [v], 3, 1181 (1936).
27 W.' Gerrard and M. F. Lappert, Chem. Rev., 58, 1091 (1958).
28 R. Schwarz and W. Kuchen, Chem. Ber., 89, 169 (1956).
29 P. Karrer and O. Riittner, Helv. Chim. Ada, 33, 812 (1950); cf. V. Tweedie and M.
Cuscurida, Angew. Chem., 66, 646 (1954).
30 J. F. W. McOmie and M. L. Watts, Chem. & Ind. {London), 1963, 1658.
31 L. Bouveault, Bull. Soc. Chim. France, [iii], 19, 75 (1898).
32 H. E. Ungnade and K. T. Zilch, /. Org. Chem., 15, 1109 (1950).
33 R. J. W. Le Fevre, S. L. Saunders, and E. E. Turner, /. Chem. Soc, 1927, 1168.
34 U. Nef, /. Prakt. Chem., 42, 168 (1890).
35 E. Spath, Monatsh. Chem., 35, 319 (1914).
36 D. H. Gould, K. H. Schaaf, and W. L. Ruigh, /. Amer. Chem. Soc, 73, 1263 (1951).