When a properly constructed molecular orbital correlation diagram for a thermally activated chemical transformation links all bonding levels in starting material with bonding levels in product, the reaction is a symmetry-allowed process. Orbital symmetry relationships endow the reaction with a strong stereochemical bias: other things being equal, the symmetry-allowed stereochemical alternative will be favored over a symmetry-forbidden one. Reactions, then, tend to occur with conservation of orbital symmetry.2-4The fundamental simplicity and physical significance of these hypotheses have been partially obscured through differing understandings of what a concerted reaction is: too often concerted and allowed have been used as interchangeable synonyms. This Account attempts to make clear a distinction between the terms, and thus refute the mistaken supposition that all allowed reactions are concerted, and all forbidden reactions nonconcerted. Orbital Symmetry and State ConservationReactions such as the thermally activated valence isomerizations of even-electron closed-shell hydrocarbons usually occur with high stereoselectivity in the sense predicted by orbital symmetry theory.4 Cyclobutenes, for instance, rearrange thermally to butadienes in a conrotatory fashion, it being the stereochemical alternative giving correlation between bonding molecular orbitals in reactant and product .More fundamentally, this stereochemical mode gives direct correlation between the most stable electronic states of starting material and product. The ground state of cyclobutene, with a molecular orbital configuration or level occupation given by .... 2 2, correlates with the ground configuration of butadiene. . . . 2 22 in the conrotatory mode only.6'6The total activation energy for such processes may be small or large, depending on all contributing factors, but there is no orbital symmetry imposed barrier. The state correlation diagram for a reaction predicted to John E. Baldwin, born in Berwyn, III., graduated with an A.B. from Dartmouth College in 1969 and took his Ph.D. at the California Institute of Technology with John D. Roberts. He then joined the faculty of the University of Illinois, and in 1968 was appointed Professor of Chemistry at the University of Oregon. He was a Fellow of the Alfred P. Sloan Foundation (1966) and John Simon Guggenheim Memorial Foundation Fellow (1967. His research interests center on the course and mechanisms of molecular rearrangements and cycloaddition reactions.A. Harry Andrist received his Ph.D. from the University of Illinois in 1970. He is currently at the University of Colorado working on the stereochemistry of electrophilic additions to small-ring compounds. Robert K. Pinschmidt received his Ph.D. from the University of Oregon in 1971. He now holds a 2-year staff appointment at the Wroclaw Polytechnical University, Poland, where he is investigating the chemistry of strained hydrocarbons. be orbital-symmetry allowed is schematically a straight line (Figure 1).

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The thermal rearrangement of 2-methylbicyclo-[2,1,0]pent-2-ene gives 1-methylcyclopentadiene, rather than the 2-methylcyclopentadiene required by the usually assumed diradical mechanism for this valence isomerization. THERMAL conversion of bicyclo [Z, 1 ,O]pent-Qene (I) intocis, cis-cyclopentadiene (II)1 must, it would seem, be a disrotatory, symmetry-forbidden, and diradical two-step process.2 This view, based on the tacit assumption that the rearrangement is a valence isomerization involving the C(l)-C(4) single bond and the C(Z)-C(3) double bond, has not been disparaged by gas-phase kinetic3$* and isotopic labelling result^.^ Both the activation parameters3** and deuterium labelling experiments establishing the durability of the C(5) methylene unit throughout the rearrange-ment5 are interpretable through the diradical hypothesis.

We have observed an unusually low-energy ultraviolet transition for bicyclo[2.1.0]pent-2-ene (1), +» 263 nm (e -~440).2•3 This highly strained bicyclic Table I. Ultraviolet Absorption Maxima and Activation Energies for Rearrangement of Cyclic Olefins a, cm'1 £a, Olefin Product Xmax, nm X 10'3 kcalmol'1 300" 33.3 ~22"•6