Impact of Ligands and Metals on the Formation of Metallacyclic Intermediates and a Nontraditional Mechanism for Group VI Alkyne Metathesis Catalysts.

The intermediacy of metallacyclobutadienes as part of a [2 + 2]/retro-[2 + 2] cycloaddition-based mechanism is a well-established paradigm in alkyne metathesis with alternative species viewed as off-cycle decomposition products that interfere with efficient product formation. Recent work has shown that the exclusive intermediate isolated from a siloxide podand-supported molybdenum-based catalyst was not the expected metallacyclobutadiene but instead a dynamic metallatetrahedrane. Despite their paucity in the chemical literature, theoretical work has shown these species to be thermodynamically more stable as well as having modest barriers for cycloaddition. Consequentially, we report the synthesis of a library of group VI alkylidynes as well as the roles metal identity, ligand flexibility, secondary coordination sphere, and substrate identity all have on isolable intermediates. Furthermore, we report the disparities in catalyst competency as a function of ligand sterics and metal choice. Dispersion-corrected DFT calculations are used to shed light on the mechanism and role of ligand and metal on the intermediacy of metallacyclobutadiene and metallatetrahedrane as well as their implications to alkyne metathesis.

Anion receptors with nitrone C-H hydrogen bond donors.

We report the use of nitrone C-H groups as hydrogen bond donors for binding anions. Acyclic anion receptors with two nitrone moieties were synthesized by condensation reactions between aryl-aldehydes and m-phenylenedihydroxylamines. The solid-state structure of dinitrones revealed C-H⋯O hydrogen bonds. Anion binding properties were investigated using NMR titration with halide and phosphate salts.

Selective multi-electron aggregation at a hypervalent iodine center by sequential disproportionation.

We demonstrate that sequential disproportionation reactions can enable selective aggregation of two- or four electron-holes at a hypervalent iodine center. Disproportionation of an anodically generated iodanyl radical affords an iodosylbenzene derivative. Subsequent iodosylbenzene disproportionation can be triggered to provide access to an iodoxybenzene. These results demonstrate multielectron oxidation at the one-electron potential by selective and sequential disproportionation chemistry.

Oxygen-atom transfer photochemistry of a molecular copper bromate complex.

We report the synthesis and oxygen-atom transfer (OAT) photochemistry of [Cu(tpa)BrO3]ClO4. In situ spectroscopy and in crystallo experiments indicate OAT proceeds from a Cu-O fragment generated by sequential Cu-O bond cleavage and OAT from BrOx to [Cu(tpa)]+. These results highlight synthetic opportunities in M-O photochemistry and demonstrate the utility of in crystallo experiments to evaluating photochemical reaction mechanisms.

Scalable synthesis of [8]cycloparaphenyleneacetylene carbon nanohoop using alkyne metathesis.

Large scale synthesis of cycloparaphenyleneacetylenes has been challenging due to low macrocyclization yields and harsh aromatization methods that often decompose strained alkynes. Herein, a cis-stilbene-based building block is subjected to alkyne metathesis macrocylization. The following sequence of alkene-selective bromination and dehydrobromination afforded a [8]cycloparaphenyleneacetylene derivative in high yield with good scalability. X-Ray crystal structure and computational analysis revealed a unique same-rim conformation for the eight methyl groups on the nanohoop.

Size-Selective Synthesis of Large Cycloparaphenyleneacetylene Carbon Nanohoops Using Alkyne Metathesis.

Size selective synthesis of large cycloparaphenyleneacetylene carbon nanohoops was achieved using alkyne metathesis. The large nanohoops were stable in ambient conditions due to their reduced strain. The nanohoops exhibited blue fluorescence with high quantum yields.