Organic Reactions, Volume 2

SYNTHETIC APPLICATION 17
The para rearrangement is also a first-order reaction, and the rate is
not greatly affected by acetic acid or dimethylaniline. 70 The non-occurrence
of inversion, and the atomic distances involved, make a cyclic
mechanism improbable. The rearrangement may go through a firstorder
dissociation of the allyl ether into either radicals or ions, which
must then be assumed to recombine, with ,the allyl group entering the
para position, before any secondary reactions can take place. If -allyl
radicals (or ions) actually were free during the reaction, they should combine
with a reactive solvent such as dimethylaniline, and the yield of rearrangement
product would be low, which is contrary to the observed
facts. A study 70 ° of the decomposition of quaternary ammonium com-
+
pounds of the type [Me2N(C6H5)C3H5] [OAr]~ indicates that ions are
not intermediates in the Claisen rearrangement. From the rearrangement
of benzyl phenyl ether in quinoline at 250°, Hickinbottom n
isolated benzylquinolines, hydroxyphenylquinolines, and toluene, indicating
the intermediate formation of benzyl radicals. There^is no evidence
for the formation of similar products in the Claisen rearrangement.
Synthetic Application
The usefulness of the Claisen rearrangement in synthetic work depends
on the following facts. The allyl aryl ethers, such as phenyl allyl ether
(LX), can be prepared easily in high yields and can be transformed
readily in good yields to the 2-allylphenols (LXI). The reaction thus
OCH*CH=CH2
LX LXI LXII
furnishes a convenient method of introducing allyl groups into a wide
variety of phenolic compounds. Among the naturally occurring allyl-
72> 73
phenols which have been synthesized by this method are elemicin,
eugenol,* croweacin, 33 and dill apiole. 74 The allylphenols serve as easily
accessible starting materials for dther synthetic operations. % Reduction
converts them to propyl (or substituted propyl) phenols (LXII), and this
* See p. 118 of the article cited in reference 11.
70 Tarbell and Kdneaid, /. Am. Chem. Soc., 62, 728 (1940).
7 °° Tarbell and Vaughan, J. Am. Chem. Soc., 66, 231 (1943).
71 Hickinbottom, Nature, 143, 520 (1939).
72 Mauthner, Ann., 414, 250 (1917).
78 Hahn and Wassmuth, Ber., 67, 696 (1934).
74 Baker, Jukes, and Subrahmanyam, J. Chem. Soc, 1934, 1681.