5040
may be strongly catalyzed by sodium hydroxide, and we are exploring this possibility.
Table I
(6) National Science Foundation Postdoctorate Fellow at Purdue University, 1967-1968.
Herbert C. Brown, Michael W. Rathke,6 Milorad M. Rogid R. B. Wetlierill Laboratory Purdire Utiiuersity, Lafayette, Indiana 47907 Receiued Jidy 16, 1968
The Electronic Structure and Reactivity of Small-Ring Compounds. 111. Mechanistic Studies of the Bicyclobutane-Benzyne Reaction’ Sir: We recently reported that the reaction of bicyclobutane with benzyne gives 3-phenylcyclobutene and benzobicyclo[2.1. Ilhex-2-ene. We wish now to report some studies on the mechanism of these reactions. 2-Deuteriobicyclobutane (I) was prepared by the method of Friedman and Wiberg2 by the thermal decomposition of cyclopropanecarboxaldehyde p-tosylhydrazone in ethylene glycol-0,0-d2 containing 0.9 equiv of the conjugate base of the solvent. The bicyclobutane I was shown by nmr spectroscopy to contain 0.83 deuterium per molecule, with 81% endo and 19 % exo. That is, I had 0.67 deuterium endo and 0.16 deuterium exo at the 2 position. When I was allowed to react with benzyne, from the thermal (45 ”) decomposition of o-benzenediazoniumcarboxylate in ethylene dichloride, and the products were separated by vlpc,* it was determined by nmr analysis that the cycloadduct I1 had all of the deuterium in the endo p ~ s i t i o n . ~In , ~ addition, the “ene” synthesis product, deuterated 3-phenylcyclobutene (III), had half of the original deuterium at the 4 position cis to the phenyl ring.6 The other half of the deuterium was attached to the phenyl ring, as a result of “ene” synthesis with abstraction of the deuteron. Thus, if there is an isotope effect in this reaction it must be very small (see Table I for a summary of the data). The conclusion, therefore, is that both the “ene” synthesis and cycloaddition reaction occur by bottomside attack of benzyne on bicyclobutane (eq 1). We use the term bottomside to refer to the endo direction. The cycloaddition reaction thus results in double inversion at the bridgeheads, whereas the “ene” synthesis gives a single inversion. One mechanistic possibility for the formation of I11 is a concerted reaction as indicated in eq 2. The p
Obsd nmr hydrogen ratio
Compound
Hydrogen
Bicyclobutane (I)
Bridgehead exo-2 endo-2 Aromatic Bridgehead
2.00 1.84 1.33 a
exo-5
1.84 1.34 4.66 1.90 1 .oo 0.94
Benzobicyclo[2.1.1]hex-2-ene (11)
2.00
endo-5 Aromatic
3-Phenylcyclobutene (111)
Vinyl Benzylic-3 trans-4 cis-4
a
Not obtained.
0.67
Calcd ratio
for bottomside attack
4.00 2.00 1.84 1.33 4.66” 1 . 92b 1 . OOh 0 .92b 0. 66b
* Calculated assuming no isotope effect.
orbitals which are utilized8 are in the proper orientation for such a reaction. The “ene” synthesis employing olefins has usually been assumed to be c ~ n c e r t e d but ,~
I
I1
(1)
I11
X = 0.67 D;Y = OJ6 D
111
has recently, in the case of bicyclo[2.1 .O]pentane, been questioned. lo The alternative possibility in the present case would involve a diradical (IV) as pictured in eq 3.
(1) Part 11: M. Pomerantz, J . Am. Chem. SOC.,88, 5349 (1966).
( 2 ) F. Cook, H . Shechter, J. Bayless, L. Friedman, R . L. Foltz, and R. Randall, ibid., 88, 3870 (1966): J. H . Bayless, Jr., Ph.D. Thesis, Case Institute of Technology, Cleveland, Ohio, 1967; I
