[reaction: see text] A novel aza-(vinylcyclopropane-cyclopentene) photochemical rearrangement is reported. 1-Pyrrolines are easily synthesized in good yields from N-cyclopropylimines. Hydrogen, alkyl, and aryl groups can be placed anywhere within the system, and the reaction proceeds regiospecifically.
A new class of photoremovable protecting groups, based on a photoinduced decarboxylation reaction coupled with the elimination of the caged molecule, is described for 2-phthalimido-3-hydroxy-propionate derivatives. When derived from enantiopure N-phthaloyl- serine or threonine, the chirality of the starting amino acid is transmitted to the protected (caged) molecule. These photocages possess good properties for their use in biological systems, and the introduction of chirality opens new possibilities for the study of diastereoselective photochemistry and stereodifferentiation processes involving the release of the caged molecule.
We present here a combined experimental and computational investigation into the photorearrangement of N-cyclopropylimines to yield pyrrolines. We show that the photochemistry, regiochemistry, and stereochemistry of the reaction can be understood in terms of a mechanism involving barrierless evolution in three different (S(2), S(1), S(0)) singlet states and sequential decay through two different (S(2)/S(1), and S(1)/S(0)) conical intersection funnels. We provide evidence that the reaction mechanism involves the generation of a nonequilibrated (i.e., transient) excited state diradical, whose decay can lead not only to pyrrolines but also to cyclopropylimine isomers. It is concluded that the reaction outcome depends on the details of the structure of such transient diradical and on the nature of the dynamics of its decay through the S(1)/S(0) conical intersection.
The scope and limitations of the photorearrangement of N-cyclopropylimines to 1-pyrrolines are presented. The influence on the reactivity of different substituents throughout the cyclopropane ring and at the iminic position of the N-cyclopropylimine structure is discussed. The observed effects are interpreted from computational studies. The principal findings relate to (1) the enhanced reactivity of 1-substituted compounds toward rearrangement, (2) the lack of reactivity of crowded cyclopropanes, and (3) the high chemoselectivity of the process.
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