Strained Heterocycles Opening Reactions

S. A. Blum, V. A. Rivera, R. T. Ruck, F. E. Michael, R. G. Bergman, Synthetic and mechanistic studies of strained heterocycle opening reactions mediated by zirconium(IV) imido complexes, Organometallics 24 (2005) 1647. 

Despite the fact that the history of hetarynes is older than that of the benzynes (cf. 2), physical data on these compounds are scarce. Numerous trapping experiments furnished evidence for the formation of brradicaloid intermediates in the field of five-membered heterocycles (didehydrofurans, -thiophenes, and -pyrroles)." Direct spectroscopic data on these species, however, do not exist, which may be attributable to the increased ring strain in the five-membered o-arynes, associated with a strong tendency to undergo ring-opening reactions. 

Epoxides are particularly reactive because ring opening releases ring strain, driving the reaction forward. However, we can tell you about some chemistry of the most important simple oxygen heterocycle,... 

Reactions of imines. Silyl alkynyl ethers and aldimines undergo catalyzed condensation, the strained heterocycles that ensue open to afford conjugated amides. YbfOTfjj is also a useful catalyst for the reaction of organocuprates with N-sulfonylimines. ... 

In four-membered heterocycles, the ring strain is less than in the corresponding three-membered compounds and is approximately equal to that found in cyclobutane. Nevertheless, ring-opening reactions forming acyclic products predominate. At the same time, analogy with the reactivity of the corresponding aliphatic compounds (ethers, thioethers, secondary and tertiary amines, imines) becomes more evident. 

With this large group of heterocycles, ring strain is of little or no importance. Ring-opening reactions are, therefore, rarer than in three- and four-membered heterocycles. The crucial consideration is rather whether a compound can be regarded as a heteroarene or whether it has to be classified as a heterocycloalkane or heterocycloalkene (see p 2). Various aromaticity criteria apply to heteroarenes, and as a consequence, different opinions have been expressed on this matter [1]. As will be shown by means of examples of the various systems, the nature and number of heteroatoms are the critical factors. The parent compound of the five-membered heterocycles with one oxygen atom is furan. 

SET reduction of ketones Reductive amination Reductive coupling of imines Reformatsky reaction Ring-opening of strained heterocycles... 

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