Contrasting chemistry of diphenylcarbene and fluorenylidene in cyclohexane

Persistent Triplet Carbenes

“…Thus, intervention of the triplet state is also indicated in the C-H insertion reaction. 124 When 8a is generated in benzene by photolysis of the precursor diazomethane (8a-N 2 ), tetraphenylazine is formed, resulting from the reaction of 8a with unreacted 8a-N 2 . When the irradiation is carried out using an intense light (a laser lamp) on a dilute solution of 8a-N 2 in benzene, tetraphenylethene, the dimer of the carbene 8a, becomes the major product.…”

“…There is no crossover product present in the FL-cyclohexane adduct when generated in an equimolecular mixture of cyclohexane and cyclohexane-d 12 . 124 FL reacts with benzene to give the equilibrating norcaradiene-cycloheptatriene mixture. 179 All of those behaviors are completely different from that observed for DPC (vide supra).…”

Activation of Molecular Hydrogen by Arylcarbenes

“…The closed‐shell singlet states of carbenes 1 – 3 lie several kcal mol −1 above their triplet ground states, and therefore reactions from thermally populated excited singlet states can be excluded under the experimental conditions. Interestingly, coupled cluster calculations predict almost identical Δ E ST for 1 and 2 in the gas phase (3.3 and 3.4 kcal mol −1 , respectively, Table ), in contradiction to reactivity studies in solution at room temperature which conclude that Δ E ST for 2 is much smaller than that of 1 . The calculated Δ E ST for 3 is considerably larger than that of the other two carbenes.…”

“…Interestingly,c oupled cluster calculations predict almost identical DE ST for 1 and 2 in the gas phase (3.3 and 3.4 kcal mol À1 ,r espectively,T able 2), in contradiction to reactivity studies in solution at room temperature which concludet hat DE ST for 2 is much smallert han that of 1. [17] The calculated DE ST for 3 is considerably larger than that of the other two carbenes.…”

“…Thus, photolysis of diphenyldiazomethane 4 in cyclohexane at room temperature predominantly leads to the dimer of the benzhydryl radical together with small amountso ft he formal CÀHi nsertion product and diphenylmethane 5.I nc ontrast, photolysis of 9diazofluorene 6 in cyclohexane produces mainly (65 %) the CÀ Hi nsertion product 9-cyclohexylfluorene and only smaller amountso fr adical-recombination products. [17] This wast aken as evidencet hat 1 reacts with hydrocarbonsf rom its triplet ground state via hydrogen abstraction, whereas 2 with as maller S-T splitting undergoes direct CÀHi nsertions via its thermally populated singlet state. [18,19] Both singlet and triplet carbenes are in principle able to insert into the HÀHb ond, however,the mechanisms are entirely different.…”