Search

Insights into the factors controlling the site selectivity of transition metal-catalyzed C–H bond functionalization reactions are vital to their successful implementation in the synthesis of complex target molecules. The introduction of fluorine atoms into substrates has the potential to deliver this selectivity. In this study, we employ spectroscopic and computational methods to demonstrate how the “ortho-fluorine effect” influences the kinetic and thermodynamic control of C–H bond activation in manganese(I)-mediated reactions. The C–H bond activation of fluorinated N,N-dimethylbenzylamines and fluorinated 2-phenylpyridines by benzyl manganese(I) pentacarbonyl BnMn(CO)5 leads to the formation of cyclomanganated tetracarbonyl complexes (2a–b and 4a–e), which all exhibit C–H bond activation ortho-to-fluorine. Corroboration of the experimental findings with density functional theory methods confirms that a kinetically controlled irreversible σ-complex-assisted metathesis mechanism is operative in these reactions. The addition of benzoic acid results in a mechanistic switch, so that cyclomanganation proceeds through a reversible AMLA-6 mechanism (kinetically and thermodynamically controlled). These stoichiometric findings are critical to catalysis, particularly subsequent insertion of a suitable acceptor substrate into the C–Mn bond of the regioisomeric cyclomanganated tetracarbonyl complex intermediates. The employment of time-resolved infrared spectroscopic analysis allowed for correlation of the rates of terminal acetylene insertion into the C–Mn bond with the relative thermodynamic stability of the regioisomeric complexes. Thus, more stable manganacycles, imparted by an ortho-fluorine substituent, exhibit a slower rate of terminal acetylene insertion, whereas a para-fluorine atom accelerates this step. A critical factor in governing C–H bond site selectivity under catalytic conditions is the generation of the regioisomeric cyclomanganated intermediates, rather than their subsequent reactivity toward alkyne insertion.

A side chain insertion method for the ring expansion of lactams into macrocyclic thiolactones is reported, that can also be incorporated into Successive Ring Expansion (SuRE) sequences. The reactions are less thermodynamically favourable than the analogous lactam- and lactone-forming ring expansion processes (with this notion supported by DFT data), but nonetheless, three complementary protecting group strategies have been developed to enable this challenging transformation to be achieved.

Emissive gold(iii) complexes of pincer 2,6-diphenylpyridines also bearing a phenylacetylide ligand have been modified at both the pincer and phenylacetylide to confer liquid crystalline properties, with most complexes showing a columnar hexagonal phase in the condensed phase. Solution NMR studies show a preferred orientation for self-association, consistent with structural parameters in the liquid crystal phase obtained by X-ray methods. While the pattern of substitution of the phenylacetylide has no discernible effect on the photophysics, when two alkoxy chains are attached to the pincer ligands, photoluminescence quantum yields (PLQY) of around 3% are found, whereas when four alkoxy chains are attached the PLQY increases significantly to 36%. Insight from computational chemistry indicated that the incorporation of the alkoxy donor groups raises the energy of the pincer-based HOMO−1 orbital, with a concomitant lowering of the LUMO ← HOMO−1 transition energy, consistent with an experimentally observed red shift. The gold complexes were fabricated into OLED devices using solution processing methods, being doped into the emissive layer at 5%, leading to external quantum efficiencies of up to 7.14%, values that compare well with those of related complexes in the literature.

The dual function and role of iron(0) pentacarbonyl [Fe(CO)5] has been identified in gaseous CO-free carbonylative Suzuki-Miyaura cross-couplings, in which Fe(CO)5supplied COin situ, leading to the propagation of catalytically active Pd-Fe nanoparticles. Compared with typical carbonylative reaction conditions, CO gas (at high pressures), specialised exogenous ligands and inert reaction conditions were avoided. Our developed reaction conditions are mild, do not require specialised CO high pressure equipment, and exhibit wide functional group tolerance, giving a library of biaryl ketones in good yields.

The ability of carboxylate groups to promote the direct functionalization of C-H bonds in organic compounds is unquestionably one of the most important discoveries in modern chemical synthesis. Extensive computational studies have indicated that this process proceeds through the deprotonation of a metal-coordinated C-H bond by the basic carboxylate, yet experimental validation of these predicted mechanistic pathways is limited and fraught with difficulty, mainly as rapid proton transfer is frequently obscured in ensemble measures in multistep reactions (i.e., a catalytic cycle consisting of several steps). In this paper, we describe a strategy to experimentally observe the microscopic reverse of the key C-H bond activation step underpinning functionalization processes (viz. M-C bond protonation). This has been achieved by utilizing photochemical activation of the thermally robust precursor [Mn(ppy)(CO)4] (ppy = metalated 2-phenylpyridine) in neat acetic acid. Time-resolved infrared spectroscopy on the picosecond-millisecond time scale allows direct observation of the states involved in the proton transfer from the acetic acid to the cyclometalated ligand, providing direct experimental evidence for the computationally predicted reaction pathways. The power of this approach to probe the mechanistic pathways in transition-metal-catalyzed reactions is demonstrated through experiments performed in toluene solution in the presence of PhC2H and HOAc. These allowed for the observation of sequential displacement of the metal-bound solvent by the alkyne, C-C bond formation though insertion in the Mn-C bond, and a slower protonation step by HOAc to generate the product of a Mn(I)-catalyzed C-H bond functionalization reaction.