Four-membered Rings with One Boron or Other Atom

Abstract

Recent advances in the synthesis, characterization, reactivity, and applications of four-membered heterocyclic ring systems comprised of one boron or transition metal atom are reviewed. The published literature since 1993 is reviewed comprehensively, with sufficient references to earlier work to construct a coherent and accurate narrative. Coverage includes both theoretical (computational) and experimental investigations, including significant structural parameters, crystallographic and spectroscopic data, and thermodynamic factors relevant to this structural class. The synthesis and reactivity of boracyclobutane, metallacyclobutane, boracyclobutene, metallacyclobutene, and metallacyclobutadiene compounds constitute the principal focus of the review. The synthetic literature is categorized by reaction type, with significant subsections devoted to the [2+2] cycloaddition of metal alkylidene complexes with alkenes or alkynes, transmetallation methodology, cyclization by γ-hydrogen elimination reaction, oxidative carbon–carbon bond activation of cyclopropane derivatives, ring contractions and ring expansions of larger and smaller metallacycles, and central carbon alkylation of transition metal η 3 -allyl and η 3 -propargyl/allenyl complexes. Developments in the latter area have been most substantial and are discussed in the greatest detail. The reactivity of each structural subclass is further divided in terms of thematic content: protonolysis of the organic fragment, transmetallation, migratory insertion of small unsaturated molecules (CO, alkenes, alkynes, etc.), [2+2] cycloreversion to produce metal alkylidene and alkylidyne complexes, cyclopropanation, β-hydrogen elimination, and other structural isomerizations and rearrangements. Significant applications to organic synthesis are also discussed, explicitly excluding the thoroughly reviewed area of alkene (and alkyne) metathesis. Both stoichiometric and catalytic applications are reviewed, as is the prognosis for further developments of utility in organic synthesis and materials chemistry.