Four-membered rings can be cleaved thermally, photochemically, or catalytically into two n: bonded fragments. Theoretical calculations, kinetic studies, and investigations of stereoand regioselectivity have been undertaken to clarify the question of whether the reaction involves one or two steps and to permit predictions on its course. [2 + 2]-Cycloreversions have been used to clarify the structure of four-membered rings, to prepare highly reactive n: electron-systems and-in combination with a [2 + 2]-cycloaddition-t0 protect double bonds.The combination of a cycloaddition and -reversion can be used to convert a carbonyl group into an olefin. Starting with compounds containing annelated four-membered rings, compounds with two functional groups or large ring systems can be prepared. (2 + 2]-Cycloreversions have also been discussed in connection with storage of solar energy. 3 5 We will classify a reaction as a [2+2]-cycloreversion only when two opposing o bonds of a carbo-or heterocyclic four-membered ring 3 are cleaved to give 112 or 4/5.Other terms used in the literature, "retrograde [2 + 21-cycloaddition", and "[4+2 + 2]-cycloelimination" are synonymous with [2 + 2]-cycloreversion. On the other hand, the concept "metathesis" has come to be used for the disproportionation of alkenes catalyzed by transition-metal compounds (cf. Section 4.1).If in 1-5, a/d and b/c are identical then the educts and products of the overall reaction of 112 to give 4/5 belong to the same chemical type, and only an exchange of substituents occurs. This specific type of cycloaddition and cycloreversion has been called an "exchange reaction" by Ul-The complete reaction often takes place in situ Angew. Chem. Inr. Ed. Engf. 21 (1982) 225-247 0 Verlag Chemie GrnbH, 6940 Weinheirn. 1982 0570-0833/82/0404-0225 S 02.50/0
Mechanism of 12 + 21-CycloreversionsAccording to the principle of microscopic reversibility the same considerations for the reaction mechanism must apply to the [2 + 2]-cycloreversion as to the [2 + 21-cycloaddition. If the two o-bonds are broken simultaneously, then according to the Woodward-Hoffmann rules['] for a thermally induced cleavage this corresponds to a [,2, + 02J process having transition state 11 in which the fragments separate from each other in a "corkscrew"[51 motion. A geometrically more favorable [,2, + ,2,]-reaction path via transition state l2[I4] is thermally "forbidden" i. e. would be associated with a much higher activation energy. In the case of photochemical [2 + 2]-cycloreversions the reverse relationships are expected, i. e. 12 would be favored over ll"].
