Page 267 - Advanced Organic Chemistry Part B - Reactions & Synthesis

from the ether and the electrophilic boron reagent. The cleavage step can occur by 239
either an S 2oranS 1 process, depending on the structure of the alkyl group.
N
N
SECTION 3.3
+ + Cleavage of
R O R + BBr 3 R O R R O R + Br – Carbon-Oxygen Bonds
in Ethers and Esters
– BBr BBr
+ 3 2
–
R O R Br R O BBr + RBr
2
BBr 2
R O BBr + 3 H O ROH + B(OH) + 2 HBr
2
3
2
Good yields are generally observed, especially for methyl ethers. The combination of
boron tribromide with dimethyl sulfide has been found to be particularly effective for
cleaving aryl methyl ethers. 91
The boron trifluoride–alkyl thiol reagent combination also operates on the basis
of nucleophilic attack on an oxonium ion generated by reaction of the ether with boron
trifluoride. 90
+
R O R + BF 3 R O R
– BF
+ – 3
R O R + R′SH ROBF + RSR′ + H +
3
– BF 3

Trimethylsilyl iodide (TMSI) cleaves methyl ethers in a period of a few hours
at room temperature. 89 Benzyl and t-butyl systems are cleaved very rapidly, whereas
secondary systems require longer times. The reaction presumably proceeds via an
initially formed silyl oxonium ion.

+
–
R O R′ + (CH ) SiI R O R′ + I R O Si(CH ) + R′I
3 3
3 3
Si(CH )
3 3
The direction of cleavage in unsymmetrical ethers is determined by the relative ease
of O−R bond breaking by either S 2 (methyl, benzyl) or S 1(t-butyl) processes.
N
N
As trimethylsilyl iodide is rather expensive, alternative procedures that generate the
reagent in situ have been devised.
CH 3 CN
) SiCl + NaI (CH ) SiI + NaCl
(CH 3 3 3 3
Ref. 92
PhSi(CH ) + I 2 (CH ) SiI + PhI
3 3
3 3
Ref. 93
91
P. G. Williard and C. R. Fryhle, Tetrahedron Lett., 21, 3731 (1980).
92 T. Morita, Y. Okamoto, and H. Sakurai, J. Chem. Soc., Chem. Commun., 874 (1978); G. A. Olah,
S. C. Narang, B. G. B. Gupta, and R. Malhotra, Synthesis, 61 (1979).
93
T. L. Ho and G. A. Olah, Synthesis, 417 (1977); A. Benkeser, E. C. Mozdzen, and C. L. Muth, J. Org.
Chem., 44, 2185 (1979).