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Abstract
AbstractCross‐coupling reactions for C–C bond formation represent a cornerstone of organic synthesis. In most cases, they make use of transition metals, which has several downsides. Recently, metal‐free alternatives relying on electrochemistry have gained interest. One example of such a reaction is the oxidation of tetraorganoborate salts that initiates aryl–aryl and aryl–alkenyl couplings with promising selectivities. This work investigates the mechanism of this reaction computationally using density functional and coupled‐cluster theory. The calculations reveal a distinct difference between aryl–alkenyl and aryl–aryl couplings: While C–C bond formation occurs irreversibly and without an energy barrier if an alkenyl residue is involved, many intermediates can be identified in aryl–aryl couplings. In the latter case, intramolecular transitions between reaction paths leading to different products are possible. Based on the energy differences between these intermediates, a kinetic model to estimate product distributions for aryl–aryl couplings is developed.
Highlights
Many years after the development of transition metal catalyzed cross-coupling reactions, they are broadly used in organic synthesis
The coupling happens via a reductive elimination, which is facilitated by transition metals due to their unoccupied d orbitals
We have investigated the reaction mechanisms of aryl–aryl couplings and aryl–alkenyl couplings that follow the oxidation of quaternary borate salts
Summary
Many years after the development of transition metal catalyzed cross-coupling reactions, they are broadly used in organic synthesis. The coupling happens via a reductive elimination, which is facilitated by transition metals due to their unoccupied d orbitals. In metal-free compounds, as for example in the oxidized tetraphenylborate anion, which can produce biphenyl.[4,5] These reactions form the basis for an attractive synthetic route to C–C coupling products that avoids the use of transition metal catalysts. Following pathway (1), the three-membered ring B, which features a bond between two of the substituents, is formed by πstacking and subsequently opens to structure C, where the boron residue is bound to only one side of the coupling product. We model reaction paths for the elimination of the coupling product from intermediate C and investigate the role of the solvent in this reaction step
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