Carbonyl bimolecular cycloaddition

Orbital Symmetry Conservation in Bimolecular Cycloadditions. The cycloaddition reactions of carbonyl compounds to form oxetanes with ethylenes, as well as those of enones and their derivatives to form cyclobutanes, are examples of reactions which originate from triplet excited states and lead in the first step to biradical intermediates. Such reactions are of course not concerted, and they show little or no stereo-specificity. 

An intramolecular variant of the five-membered ring carbonyl ylide cycloaddition has also been explored [89]. When the specially crafted a-diazo ketoester 68 was decomposed in the presence of Rh2(OAc)4, an intramolecular cycloaddition product 69 was realized in good yield (Scheme 20). On the other hand, if DMAD was present in the reaction mixture, the bimolecular adduct 70 was isolated. 

The [2 + 2] photodimerization is a well-known photochemical reaction, for which many examples have been reported, in particular for arylalkenes and a, 3-unsatu-rated carbonyl or carboxyl derivatives. In solution, EjZ isomerization competes with the bimolecular reaction in the solid state, on the other hand, cycloaddition may be quite effective, as has long been known.Relations between the arrangement of the molecules in the crystal lattice and the reaction occurring have... 

The scope of photochemical [2 + 2] cycloadditions of enones to alkenes in solution is limited by the failure of most acyclic oc,/ -unsaturated carbonyl compounds to undergo bimolecular reactions in competition with rapid unimolecular cisjtrans photoisomerization. 

The Diels-Alder cycloaddition is an important reaction class involving the reaction of unsaturated carbonyl compounds with conjugated dienes. The carbon atoms at the 1- and 4-positions of the conjugated diene system become attached to the doubly bonded carbons of the unsaturated carbonyl compound to form a ring stiucture. Diels-Alder chemistry is used in the synthesis of a variety of complex organic compounds of commercial interest, including insecticides, fragrances, plasticizers, and dyes (92). These bimolecular reactions have been used... 

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