On the Mechanism of the Selenolactonization Reaction with Selenenyl Halides

Observation of direct sulfenium and selenenium group transfer from thiiranium and seleniranium ions to alkenes

Sulfenium and selenenium ions undergo a stereospecific transfer from the corresponding three-membered ring species ("-iranium ions") to unactivated alkenes with varying facility. The thiiranium and seleniranium hexafluoroantimonates could be generated by treatment of the corresponding chloro sulfides or selenides with silver hexafluoroantimonate, followed by removal of the silver chloride by filtration. Clean (1)H, (13)C, and (77)Se NMR spectra could be recorded for these species. Treatment of the S-phenylthiiranium ion with an alkene leads to a slow transfer of the sulfenium group at 0 degrees C. However, the S-methylthiiranium ion did not transfer the sulfenium group, even at room temperature. On the other hand, both the Se-phenyl- and Se-butylseleniranium ions transferred the selenenium moiety instantaneously at -70 degrees C. By measuring the equilibrium position for these transfers from both directions, the relative stability of the 1-phenylseleniranium ions could be established: cis-tetramethylene < trans-2,3-dipropyl approximately trans-2,3-diisopropyl < cis-hexamethylene.

Metal-catalyzed 1,2-shift of diverse migrating groups in allenyl systems as a new paradigm toward densely functionalized heterocycles

A general, mild, and efficient 1,2-migration/cycloisomerization methodology toward multisubstituted 3-thio-, seleno-, halo-, aryl-, and alkyl-furans and pyrroles, as well as fused heterocycles, valuable building blocks for synthetic chemistry, has been developed. Moreover, regiodivergent conditions have been identified for C-4 bromo- and thio-substituted allenones and alkynones for the assembly of regioisomeric 2-hetero substituted furans selectively. It was demonstrated that, depending on reaction conditions, ambident substrates can be selectively transformed into furan products, as well as undergo selective 6-exo-dig or Nazarov cyclizations. Our mechanistic investigations have revealed that the transformation proceeds via allenylcarbonyl or allenylimine intermediates followed by 1,2-group migration to the allenyl sp carbon during cycloisomerization. It was found that 1,2-migration of chalcogens and halogens predominantly proceeds via formation of irenium intermediates. Analogous intermediate can also be proposed for 1,2-aryl shift. Furthermore, it was shown that the cycloisomerization cascade can be catalyzed by Brønsted acids, albeit less efficiently, and commonly observed reactivity of Lewis acid catalysts cannot be attributed to the eventual formation of proton. Undoubtedly, thermally induced or Lewis acid-catalyzed transformations proceed via intramolecular Michael addition or activation of the enone moiety pathways, whereas certain carbophilic metals trigger carbenoid/oxonium type pathway. However, a facile cycloisomerization in the presence of cationic complexes, as well as observed migratory aptitude in the cycloisomerization of unsymmetrically disubstituted aryl- and alkylallenes, strongly supports electrophilic nature for this transformation. Full mechanistic details, as well as the scope of this transformation, are discussed.