1,3-BRIDGED
J. Org. Chern., VoL 58, No. 6, 19W
AROMATIC SYSTEMS
1,3-Bridged Aromatic Systems. VIII.
1207
Rearrangements of Strained Systems'
WILLIAME. PAR HAM,*^ DAVID C. EGBERG, AND W. CHARLES MONTGOMERY School of Chemistry of the University of Minnesota, Minneapolis, Minnesota 66466, and The Paul M . Gross Chemical Laboratory, Duke University, Durham, North Carolina 2'7706 Received October 24, I972 The strained metacyclophane 3b is converted quantitatively into the rearranged naphthalene 6b by action of a mixture of p-toluenesulfonic acid and trifluoroacetic acid in hot benzene. The mechanism of this rearrangement has been established by W-tagging experiments and has been shown not to involve phenyl migration as previously suggested for the analogous naphthalene 6a. The cyclophane 3c has been prepared and its properties are compared with its analogs 3b, 3d, and 3e.
Metacyclophanes of type 3, where n = 2, 4,6 , have been prepared3 in high yield by addition of the elements of dihalocarbene t o the corresponding indenes, and the effect of constraint and strain caused by the highly compacted methylene bridge on chemical reactivity has been discussed. The practical limiting value of n in 3 is 2; attempts3bt o prepare 3a (n = 1) gave the cleavage and rearranged products 5a and 6a, SCHEME I
Q
respectively. It was suggestedab thak the rearranged product 6a was formed by phenyl migration in the ion 2 (path B, Scheme I) and that the ring-opened product 5a was formed from 3 through the intermediate 4. It was also shown that the bromo analog of 3b (C1 replaced by Br) reacts readily with hydrogen bromide in hot benzene to give the ring-opened product corresponding to 5b (C1 replaced by Br). While the mechanism suggested in path B of Scheme I for the formation of the rearranged product 6a is reasonable, an alternate mechanism is shown in Scheme 11. This mechanism assumes that the ring-opening SCHEME I1
1
Q
4 2 phenyl migration path B
IlH+
c1
c1 I
-"+/
3a,n=l
b,n=2 c,n=3
d,n = 4 e,n=6 6a,m=5 b. vz= 6
5 a , m = 5 ; X=C1 b, m =6; X = Br
P 4
(1) Supported by the National Science Foundation Grant No. GB11918. (2) Correspondence should be addressed to Duke Univeruity, Durham, N . C. 27706. (3) (a)W. E. Parham and J . I
