Pd(IV)-fluoride complexes, some of which are remarkably insensitive to water, have been synthesized and used in the title reaction, which proceeds with high selectivity to give the product of the C(sp(3))-F coupling. Preliminary mechanistic studies implicate a pathway involving dissociation of pyridine followed by direct C-F coupling at the Pd center.

This article describes the rational design of 1st generation systems for oxidatively-induced Aryl–CF3 bond-forming reductive elimination from PdII. Treatment of (dtbpy)PdII(Aryl)(CF3) (dtbpy = di-tert-butylbipyridine) with NFTPT (N-fluoro-1,3,5-trimethylpyridium triflate) afforded the isolable PdIV intermediate (dtbpy)PdIV(Aryl)(CF3)(F)(OTf). Thermolysis of this complex at 80 °C resulted in Aryl–CF3 bond-formation. Detailed experimental and computational mechanistic studies have been conducted to gain insights into the key reductive elimination step. Reductive elimination from this PdIV species proceeds via pre-equilibrium dissociation of TfO− followed by Aryl–CF3 coupling. DFT calculations reveal that the transition state for Aryl–CF3 bond formation involves the CF3 acting as an electrophile with the Aryl ligand acting as a nucleophilic coupling partner. These mechanistic considerations along with DFT calculations have facilitated the design of a 2nd generation system utilizing the tmeda (N,N,N’,N’-tetramethylethylenediamine) ligand in place of dtbpy. The tmeda complexes undergo oxidative trifluoromethylation at room temperature.

The terminal nitride complexes NW(OC(CF 3) 2Me) 3(DME) ( 1-DME), [Li(DME) 2][NW(OC(CF 3) 2Me) 4] ( 2), and [NW(OCMe 2CF 3) 3] 3 ( 3) were prepared in good yield by salt elimination from [NWCl 3] 4. X-ray structures revealed that 1-DME and 2 are monomeric in the solid state. All three complexes catalyze the cross-metathesis of 3-hexyne with assorted nitriles to form propionitrile and the corresponding alkyne. Propylidyne and substituted benzylidyne complexes RCW(OC(CF 3) 2Me) 3 were isolated in good yield upon reaction of 1-DME with 3-hexyne or 1-aryl-1-butyne. The corresponding reactions failed for 3. Instead, EtCW(OC(CF 3)Me 2) 3 ( 6) was prepared via the reaction of W 2(OC(CF 3)Me 2) 6 with 3-hexyne at 95 degrees C. Benzylidyne complexes of the form ArCW(OC(CF 3)Me 2) 3 (Ar = aryl) then were prepared by treatment of 6 with the appropriate symmetrical alkyne ArCCAr. Three coupled cycles for the interconversion of 1-DME with the corresponding propylidyne and benzylidyne complexes via [2 + 2] cycloaddition-cycloreversion were examined for reversibility. Stoichiometric reactions revealed that both nitrile-alkyne cross-metathesis (NACM) cycles as well as the alkyne cross-metathesis (ACM) cycle operated reversibly in this system. With catalyst 3, depending on the aryl group used, at least one step in one of the NACM cycles was irreversible. In general, catalyst 1-DME afforded more rapid reaction than did 3 under comparable conditions. However, 3 displayed a slightly improved tolerance of polar functional groups than did 1-DME. For both 1-DME and 3, ACM is more rapid than NACM under typical conditions. Alkyne polymerization (AP) is a competing reaction with both 1-DME and 3. It can be suppressed but not entirely eliminated via manipulation of the catalyst concentration. As AP selectively removes 3-hexyne from the system, tandem NACM-ACM-AP can be used to prepare symmetrically substituted alkynes with good selectivity, including an arylene-ethynylene macrocycle. Alternatively, unsymmetrical alkynes of the form EtCCR (R variable) can be prepared with good selectivity via the reaction of RCN with excess 3-hexyne under conditions that suppress AP. DFT calculations support a [2 + 2] cycloaddition-cycloreversion mechanism analogous to that of alkyne metathesis. The barrier to azametalacyclobutadiene ring formation/breakup is greater than that for the corresponding metalacyclobutadiene. Two distinct high-energy azametalacyclobutadiene intermediates were found. These adopted a distorted square pyramidal geometry with significant bond localization.

The direct and single-step conversion of benzene, ethylene, and a Cu(II) oxidant to styrene using the Rh(I) catalyst (DAB)Rh(TFA)(η-CH) [DAB = N,N'-bis(pentafluorophenyl)-2,3-dimethyl-1,4-diaza-1,3-butadiene; TFA = trifluoroacetate] has been reported to give quantitative yields (with Cu(II) as the limiting reagent) and selectivity combined with turnover numbers >800. This report details mechanistic studies of this catalytic process using a combined experimental and computational approach. Examining catalysis with the complex (DAB)Rh(OAc)(η-CH) shows that the reaction rate has a dependence on catalyst concentration between first- and half-order that varies with both temperature and ethylene concentration, a first-order dependence on ethylene concentration with saturation at higher concentrations of ethylene, and a zero-order dependence on the concentration of Cu(II) oxidant. The kinetic isotope effect was found to vary linearly with the order in (DAB)Rh(OAc)(η-CH), exhibiting no KIE when [Rh] was in the half-order regime, and a k/k value of 6.7(6) when [Rh] was in the first-order regime. From these combined experimental and computational studies, competing pathways, which involve all monomeric Rh intermediates and a binuclear Rh intermediate in the other case, are proposed.

A long-standing debate in the Cu-O2 field has revolved around the relevance of the Cu(III) oxidation state in biological redox processes. The proposal of Cu(III) in biology is generally challenged as no spectroscopic or structural evidence exists currently for its presence. The reaction of synthetic Cu(I) complexes with O2 at low temperature in aprotic solvents provides the opportunity to investigate and define the chemical landscape of Cu-O2 species at a small molecule level of detail; eight different types are characterized structurally, three of which contain at least one Cu(III) center. Simple imidazole or histamine ligands are competent in these oxygenation reactions to form Cu(III) complexes. The combination of synthetic structural and reactivity data suggests (i) that Cu(I) should be considered as either a one or two electron reductant reacting with O2, (ii) that Cu(III) reduction potentials of these formed complexes are modest and well within the limits of a protein matrix and (iii) that primary amine and imidazole ligands are surprisingly good at stabilizing Cu(III) centers. These Cu(III) complexes are efficient oxidants for hydroxylating phenolate substrates with reaction hallmarks similar to that performed in biological systems. The remarkable ligation similarity of the synthetic and biological systems makes it difficult to continue to exclude Cu(III) from biological discussions.