SET processes in Lewis acid–base reactions: the tritylation of N-heterocyclic carbenes

Abstract

Carbenes as single electron donors: the tritylation of N-heterocyclic carbenes proceeds via an initial SET step, giving highly reactive carbene radical cations and the trityl radical.

Trioxatriangulenium (TOTA+) as a robust carbon-based Lewis acid in frustrated Lewis pair chemistry

“… 35 Trapping experiments with TEMPO and benzoyl peroxide were also unsuccessful (see ESI † ). These observations suggest that a single electron transfer (SET) 36 between TOTA + and the NHC occurred, and then followed by radical recombination to form 11 or 11′ in non-polar solvent such as toluene, or a mixture of products in dissociative solvent such as acetonitrile. The calculated electron affinity and ionization potential of TOTA + (−7.41 eV) and I t Bu (+5.59 eV) suggest that SET process in the ground state is accessible (see Table S15 † ).…”

Frustrated Radical Pairs: Insights from EPR Spectroscopy

“…Instead of phosphines as the Lewis base, carbenes have also been employed in combination with the same Lewis acid. Recently, single-electron transfer reactions generating Cbased radicals in classical Lewis pairs have been observed and highlighted by Severin et al [26] In the reaction between the carbene 1,3-bis(2,6-diisopropylphenyl) imidazol-2-ylidene (IDipp) and [Ph 3 C][B(C 6 F 5 ) 4 ], a single-electron transfer from the carbene to the trityl borate salt afforded the persistent radical [Ph 3 C]C (12), as characterized by EPR spectroscopy. (Scheme 11, top).…”

Proton transfer vs. oligophosphine formation by P–C/P–H σ-bond metathesis: decoding the competing Brønsted and Lewis type reactivities of imidazolio-phosphines

“…The samples contained neither organic by-products traceable to imidazolio-substituents nor boron-or aluminium-containing impurities, thus confirming that the sequestering strategy succeeded, but comprised minor amounts (6-14%) of diphosphines. We envisage that these by-products originate from a dismutation between different phosphorus-containing species or from a SET reaction between a transient NHC and a phosphorus electrophile, 27 but further studies are needed to clarify this issue. Considering that the imidazolio-phosphides used can be produced directly from elemental phosphorus, 8c their conversion into secondary (di)phosphines marks a new approach to the preparation of simple organophosphorus compounds that bypasses the use of environmentally problematic 1d,e chlorophosphines.…”