Competitive solvent-molecule interactions govern primary processes of diphenylcarbene in solvent mixtures

Abstract

Photochemical reactions in solution often proceed via competing reaction pathways comprising intermediates that capture a solvent molecule. A disclosure of the underlying reaction mechanisms is challenging due to the rapid nature of these processes and the intricate identification of how many solvent molecules are involved. Here combining broadband femtosecond transient absorption and quantum mechanics/molecular mechanics simulations, we show for one of the most reactive species, diphenylcarbene, that the decision-maker is not the nearest solvent molecule but its neighbour. The hydrogen bonding dynamics determine which reaction channels are accessible in binary solvent mixtures at room temperature. In-depth analysis of the amount of nascent intermediates corroborates the importance of a hydrogen-bonded complex with a protic solvent molecule, in striking analogy to complexes found at cryogenic temperatures. Our results show that adjacent solvent molecules take the role of key abettors rather than bystanders for the fate of the reactive intermediate.

Activation of Molecular Hydrogen by Arylcarbenes

“…Thus, the ground state of 1 is switched from triplet to singlet. This is not only observed in cryogenic matrices at low temperature, but also in solution at room temperature . Carbene 2 shows a similar behavior, whereas in 3 the singlet‐triplet splitting is too large to be overcome by hydrogen bonding.…”

“…Thus, the ground state of 1 is switched from triplet to singlet.T his is not only observed in cryogenic matrices at low temperature, [10,11] but also in solution at room temperature. [12] Carbene 2 shows as imilar behavior, [13] whereasi n3 the singlet-triplet splitting is too large to be overcome by hydrogen bonding.H alogen bondings tabilizes the singlet stateso fc arbenes even more than hydrogen bonding, [14] and BF 3 formss tronglys tabilizedd onor-acceptor complexes, [15,16] similart oN H 3 -BF 3 .T hese resultss tress the high basicity of the closed-shell singlet stateso fc arbenes.…”

Treten in der N‐heterozyklischen Carben‐Organokatalyse wirklich Carbene auf?

“…[24] Ausdiesem Grund haben wir die Studie um eine Serie von Rechnungen mit einem impliziten Kontinuumsmodell (conductor-like polarizable continuum) erweitert, um so die Einflüsse von Toluol, THF und DMSO auf die Reaktionsbarrieren zu untersuchen. [24] Ausdiesem Grund haben wir die Studie um eine Serie von Rechnungen mit einem impliziten Kontinuumsmodell (conductor-like polarizable continuum) erweitert, um so die Einflüsse von Toluol, THF und DMSO auf die Reaktionsbarrieren zu untersuchen.…”

“…Es konnte bereits fürD iphenylcarbene -d ie sich strukturell allerdings sehr von NHCs unterscheiden -g ezeigt werden, dass Solvatation sehr starke Effekte auf die Reaktivitätvon Carbenen haben kann. [24] [25] Da Azolium-Kationen ebenfalls einen aromatischen Charakter aufweisen, [26,27] ist es angemessen, den assoziativen Mechanismus als eine elektrophile aromatische Substitutionsreaktion zu beschreiben.…”

Interaction of N‐Heterocyclic Carbenes and Simple Carbenes with Small Molecules (One to Three Atoms) Excluding Metals: Formation of Covalent C–X Bonds

“…Another aspect that we will not systematically discuss is the existence of carbenes in two forms, the singlet and the triplet. The singlet‐triplet gap , when not very large , can be modified by complexation a situation mainly studied by Sander . In this review we will specify the multiplicity when reported.…”