The Role of Ate Complexes in the Copper‐Mediated Trifluoromethylation of Alkynes

Trifluoromethylation reactions have recently received increased attention because of the beneficial effect of the trifluoromethyl group on the pharmacological properties of numerous substances. A common method to introduce the trifluoromethyl group employs the Ruppert–Prakash reagent, that is, Si(CH₃)₃CF₃, together with a copper(I) halide. We have applied this method to the trifluoromethylation of aromatic alkynes and used electrospray‐ionization mass spectrometry to investigate the mechanism of these reactions in tetrahydrofuran, dichloromethane, and acetonitrile as well as with and without added 1,10‐phenanthroline. In the absence of the alkyne component, the homoleptic ate complexes [Cu(CF₃)₂]^? and [Cu(CF₃)₄]^? were observed. In the presence of the alkynes RH, the heteroleptic complexes [Cu(CF₃)₃R]^? were detected as well. Upon gas‐phase fragmentation, these key intermediates released the cross‐coupling products R?CF₃ with perfect selectivity. Apparently, the [Cu(CF₃)₃R]^? complexes did not originate from homoleptic cuprate anions, but from unobservable neutral precursors. The present results moreover point to the involvement of oxygen as the oxidizing agent.     

Helly-type theorems for hollow axis-aligned boxes

A hollow axis-aligned box is the boundary of the cartesian product of compact intervals in . We show that for , if any of a collection of hollow axis-aligned boxes have non-empty intersection, then the whole collection has non-empty intersection; and if any of a collection of hollow axis-aligned rectangles in have non-empty intersection, then the whole collection has non-empty intersection. The values for and for are the best possible in general. We also characterize the collections of hollow boxes which would be counterexamples if were lowered to , and to , respectively.