RESEARCH
Metals catalyze cubane isomerizations Catalysis of symmetry-restricted reactions by transition metal is a nonconcerted process The concept of orbital symmetry conservation encompassed by the Hoffmann-Woodward rules has played a prominent role in recent years in interpretation of reactivity patterns in organic chemistry. Formulated in 1965 by Dr. Roald Hoffmann of Cornell University, Ithaca, N.Y., and Dr. Robert B. Woodward of Harvard University, Cambridge, Mass., these rules are a powerful and remarkably successful framework for predicting reactivities of concerted pathways of many reactions, such as cycloaddition, and for explaining differences between thermal and photochemical reactivity patterns of such reactions. The latest work of Dr. Luigi Cassar, Dr. Philip E. Eaton, and Dr. Jack Halpern, of
fins. Such observations have led to speculation that metal d-orbitals, by functioning as "symmetry exchangers," may relax the constraints of the Hoffmann-Woodward rules and open up otherwise symmetry-restricted, concerted pathways for these chemical reactions. An alternative interpretation, however, considered by the Chicago group, is that the catalytic pathways in question are not concerted, but involve instead the initial opening of only one cyclobutane bond through an oxidative addition step. Rhodium and other D 8 transition metals are well known to undergo this type of reaction. Cubane. To determine which of these mechanisms is correct, the Chi-
Metal catalysis mediates thermally forbidden reaction
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A Rh(l)
1
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University of Chicago, contributes further to understanding of this subject by revealing the origin of modifications of some organic reactivity patterns by transition metals [J. Am. Chem. Soc, 92, 3515 (1970)]. Among the simplest reactions whose concerted pathways are "thermally forbidden" according to the HoffmannWoodward orbital symmetry conservation rules is the suprafacial—accomplished without twisting—1,2-cycloaddition of two olefins to form a cyclobutane ring. Consistent with this view, such reactions and the corresponding cyclobutane-diolefm cycloreversions are generally found to be very slow. Several chemists have found, however, that certain transition metal compounds, notably of rhodium ( I ) , catalyze rearrangements of highly strained cyclobutane derivatives such as quadricyclene to the corresponding diole42 C&EN JUNE 15, 1970
cago chemists examined the rhodium(I)-catalyzed isomerization of cubane to sf/n-tricyclo[4.2.0.0 2 ' 5 ]octa-3,7diene. Their conclusion is that the catalytic mechanism is a nonconcerted process involving an intermediate organ orhodium adduct, formed in an initial oxidative addition step. The argument has four parts. First, the oxidative addition of rhodium (I) compounds to cubanes was actually demonstrated by reacting cubane with equimolar [Rh(CO) 2 Cl] 2 to give a product with an acylrhodium —Rh— Co— bridge inserted into one edge of the cube. Treatment of this organorhodium(III) compound with triphenylphosphine expels rhodium and gives homocubanone—a molecule with a carbonyl group inserted into a cube edge. The second point of the argument is that rhodium (I) catalysis of cubane isomerization and formation of the or-
ganorhodium compound above are similar reactions. They both have second order kinetics; each reaction depends on both rhodium and hydrocarbon getting together at or before the slow step. Substituents. Moreover, the rate constants of the two reactions are similarly affected by substituents. The University of Chicago workers use cubane, methyl cubanecarboxylate, and dimethyl cubane-1,4-dicarboxylate to show that the ratio of isomerization rate constant to organorhodium formation rate constant is itself nearly a constant for each of the three cubanes studied. Fourth, examination of ring opening versus organorhodium formation reactions with an unsymmetrically substituted cubane shows similar product distributions. In cubane itself there are six faces in the cube, and opening any two opposite edges on any face will give the same tricyclooctadiene or homocubanone. With methyl cubanecarboxylate, however, two diolefins and three homocubanones are possible on isomerization or on ketone formation. Ratios of the two analogous product pairs from each reaction are almost the same. These ratios are what are expected statistically for random attack on any cube edge (one homocubanone is not formed at all because of steric unfavorability). This analysis of product ratios indicates that each reaction begins by insertion of rhodium into an edge of the cube. Other rhodium (I)-catalyzed cyclobutane-diolefm transformations, such as valence isomerization of quadricyclene to bicyclo[2.2.1]heptadiene, observed by Dr. H. Hogeveen and Dr. H. C. Volger of Shell Research, Amsterdam, the Netherlands, apparently also proceed through similar nonconcerted mechanisms. This view is indicated by the Chicago group's finding that quadricyclene also reacts stoichiometrically with [Rh(CO)Xl] L > to give an acylrhodium oxidative adduct, analogous to that obtained with cubane. A further important feature of the cubane work is the demonstration that the cyclopropane rings present in quadricyclene are not required for reaction with rhodium. The cyclobutane face of cubane is sufficiently reactive.
