New catalytic ring-expansion reactions of strained ring (hetero- and carbocyclic) substrates reported in the last six years (2006–2012) are presented. As evident from the diversity of new approaches, this is a vibrant area of research. Metals ranging from magnesium to gold have been used as catalysts. Some of these reactions allow access to enantioenriched products by employing catalysts decorated with chiral organic motifs (primarily C-2 symmetric).

Unsaturated cyclic ethers can be mildly and selectively reduced with catalytic amounts of B(C(6)F(5))(3) in the presence of an alkylsilane. The allylic position is preferentially reduced with minimal or no scrambling of olefin geometry. For electronically equivalent substrates, steric factors guide the reducing agent to the least substituted site.

Density functional theory computations of the Cu-catalyzed ring expansion of vinyloxiranes is mediated by a traceless dual Cu(I)-catalyst mechanism. Overall, the reaction involves a monomeric Cu(I)-catalyst, but a single key step, the Cu migration, requires two Cu(I)-catalysts for the transformation. This dual-Cu step is found to be a true double Cu(I) transition state rather than a single Cu(I) transition state in the presence of an adventitious, spectator Cu(I). Both Cu(I) catalysts are involved in the bond forming and breaking process. The single Cu(I) transition state is not a stationary point on the potential energy surface. Interestingly, the reductive elimination is rate-determining for the major diastereomeric product, while the Cu(I) migration step is rate-determining for the minor. Thus, while the reaction requires dual Cu(I) activation to proceed, kinetically, the presence of the dual-Cu(I) step is untraceable. The diastereospecificity of this reaction is controlled by the Cu migration step. Suprafacial migration is favored over antarafacial migration due to the distorted Cu π-allyl in the latter.

Detailed in this account are our efforts toward efficient oxacycle syntheses. Two complementary approaches are discussed, with both employing chemoselective allyl ether activation and rearrangement as the key step. Vinyl substituted oxiranes and oxetanes provide a single step access to dihydropyrans and tetrahydrooxepines. Oxiranes proved to be poor substrates, while oxetanes were slightly better. An alternative approach using substituted allyl ethers proved successful and addressed the limitations encountered in the ring expansions.

We report how mechanistically inspired metal additive choices result in acceleration of the copper catalyzed vinyl aziridine ring expansion reaction. Most importantly, we demonstrate how the use of in situ reducing agents significantly accelerates the reaction, suggesting a copper(I) active species. These acceleration results were confirmed using Cu(hfacac)(cod) as catalyst. NMR kinetic studies revealed the relative importance of olefin and sulfonamide electronics on the reaction rate and established the order of both catalyst and substrate, which together favored a new copper(I) insertion mechanism.Scheme 1 Proposed cationic copper(II) catalytic ring expansion.