A comprehensive understanding of carbon-carbon bond formation by alkyne migratory insertion into manganacycles

• Published in: Chemical science (Cambridge) 2022-08, Vol.13 (34), p.992-9913
• Main Authors: Hammarback, L. Anders, Eastwood, Jonathan B, Burden, Thomas J, Pearce, Callum J, Clark, Ian P, Towrie, Michael, Robinson, Alan, Fairlamb, Ian J. S, Lynam, Jason M
• Format: Article
• Language: English
• Subjects: Alkynes; Carbon monoxide; Chemistry; Computational chemistry; Covalent bonds; Insertion; Ligands; Photolysis; Rate constants; Solvents; Substitution reactions; Toluene; Transition metals
• ISSN: 2041-6520; 2041-6539
• DOI: 10.1039/d2sc02562k

Migratory insertion (MI) is one of the most important processes underpinning the transition metal-catalysed formation of C-C and C-X bonds. In this work, a comprehensive model of MI is presented, based on the direct observation of the states involved in the coupling of alkynes with cyclometallated ligands, augmented with insight from computational chemistry. Time-resolved spectroscopy demonstrates that photolysis of complexes [Mn(C^N)(CO) 4 ] (C^N = cyclometalated ligand) results in ultra-fast dissociation of a CO ligand. Performing the experiment in a toluene solution of an alkyne results in the initial formation of a solvent complex fac -[Mn(C^N)(toluene)(CO) 3 ]. Solvent substitution gives an η 2 -alkyne complex fac -[Mn(C^N)(η 2 -R 1 C 2 R 2 )(CO) 3 ] which undergoes MI of the unsaturated ligand into the Mn-C bond. These data allowed for the dependence of second order rate constants for solvent substitution and first order rate constants for C-C bond formation to be determined. A systematic investigation into the influence of the alkyne and C^N ligand on this process is reported. The experimental data enabled the development of a computational model for the MI reaction which demonstrated that a synergic interaction between the metal and the nascent C-C bond controls both the rate and regiochemical outcome of the reaction. The time-resolved spectroscopic method enabled the observation of a multi-step reaction occurring over 8 orders of magnitude in time, including the formation of solvent complexes, ligand substitution and two sequential C-C bond formation steps. Time-resolved spectroscopy and computational chemistry have informed a unified model of alkyne migratory insertion, an important processes underpinning the transition metal-catalysed formation of C-C and C-X bonds.