Triplet 1,3-Diphenylpropynylidene (Ph−C−C−C−Ph)

Abstract

Photolysis (lambda>571 nm) of 1,3-diphenyldiazopropyne (9) affords triplet 1,3-diphenylpropynylidene (3), as characterized by IR, UV/vis, and EPR spectroscopy in low-temperature matrices. Two conformational isomers of triplet 3 are spectroscopically distinguishable. The initially formed, non-relaxed conformer is believed to reflect the geometry of the diazo precursor, as enforced by the rigid matrix. Annealing the matrix permits the structure to relax to the equilibrium D2d geometry. The highly symmetric equilibrium structure of 3 is best envisioned as a 1,3-allenic diradical. Density functional theory calculations suggest that the equilibrium structure does not exhibit a bond-localized structure that would be characteristic of an acetylenic carbene. Chemical trapping with O2, however, affords products that are familiar as carbene trapping products: carbonyl oxide 10, ketone 11, and dioxirane 12. Irradiation (lambda>261 nm) of triplet 1,3-diphenylpropynylidene (3) results in cyclization to singlet diphenylcyclopropenylidene (6), a process that is photochemically reversible at lambda=232 nm. Diphenyl-1,2-propadienylidene (7) was not observed under any irradiation conditions.

Stable Cyclic Carbenes and Related Species beyond Diaminocarbenes

“…The spectroscopic data, especially the 13 C NMR chemical shift (d = 188 and 161 ppm), are very similar to those observed for CP 2 . Diphenylcyclopropenylidene CP 5 has recently been characterized by McMahon and co-workers [150] by infrared spectroscopy, but only in an argon matrix at 10 K. It was obtained by photochemical isomerization of the triplet diphenylpropynylidene (35), a process that is photochemically reversible at l = 232 nm (Scheme 39).…”

Carbocyclic carbene ligands in palladium-catalyzed C–N coupling reactions

“…Elemental analysis calcd for C 25 H 21 Br 2 PPd (M = 618.64 g/mol): C,48.54;H,3.42;Pd,17.20. Found: C,50.06;H,3.80;Pd,16.9% …”

Stabile cyclische Carbene und verwandte Spezies jenseits der Diaminocarbene

“…[ [9,155] Zudem wurde für Diphenylcyclopropenyliden (CP 5 ) eine Singulett-Triplett-Lücke von 43 kcal mol À1 berechnet, [150] die zwar kleiner ist als beim Bis(amino)cyclopropenyliden CP 3 (60 kcal mol À1 ), [148] aber wesentlich größer als beim bereits isolierten (Phosphanylsilylcarben A (27 kcal mol À1 ). Darüber hinaus wird vermutet, dass die nichtkatalysierte Dimerisierung [9,124,147,156,157] von SingulettCarbenen über einen "non-least motion"-Reaktionspfad verläuft, [158] Dieser umfasst den Angriff des freien s-Elektronenpaars des einen Singulett-Carbens in das leere p p -Orbital des zweiten Carbens, das wegen des 2p-Elektronensystems des Rings energetisch relativ hoch liegt.…”