Carbocation formation via carbene protonation studied by the technique of stopped-flow laser-flash photolysis

Visible‐Light Catalytic Photooxygenation of Monoterpene Indole Alkaloids: Access to Spirooxindole‐1,3‐oxazines

“…[11] If the photooxygenation of 5 was carried out under an atmosphere of 18 O 2 ,b oth the autoxidation reactioni nC HCl 3 (conditiona), and the reaction with catalyst C (3 mol %) in MeCN (condition b) produced 6 with ! 90 %i ncorporation of two 18 Ol abels (m/z 645 for doubly labeled [6 + H] + ), whereas hydroperoxide 7,i solable only from the catalyst-free photooxidation, was obtained as about a1 :1 mixture of the singly and doubly 18 O-labeled compounds (m/z 645 and 643 for [7 + H] + ;S cheme 4a and Figures S4-S8 in the Supporting Information). We attribute the latter observation to OÀOb ondp hotolysis of hydroperoxide 7 under the reaction conditions, followed by oxygen exchange with trace water in the reaction mixture.…”

“…Subsequent HAT to 25 generates reactive dienamine 26 , which is epoxidized, followed by ring opening of epoxide 27 to zwitterion 28 . At this stage, the key scission of the C2−C3 bond occurs, and we propose that intermediate 28 rearranges to give cyclic (alkyl)(amino) carbene 29 , which undergoes final ring closure by proton transfer and nucleophilic trapping of the iminium ion through chair‐like transition structure 30 , to yield the final product with the observed stereochemistry. Alternatively, carbene 29 may react through direct OH insertion with the C7‐hydroxy group.…”

Developments in the Photochemistry of Diazo Compounds

“…According to the authors, the 40 fold increase in the carbene protonation when para-chlorines were substituted by methoxy groups could only be explained if the O-H insertion occurred via a concerted pathway. [97] Recently, Kohler emphasised the mechanism where a carbocation is involved. Through femtosecond transient absorption spectroscopy and based on the isotopic effect of methanol, it was observed that the carbocation formation was extremely fast since the carbene is highly basic.…”

“…Though the 1 BpCCO 2 CH 3 decay in cyclohexane and cyclohexane-d 12 indicated that the C-H insertion product should be formed from the carbene singlet-triplet equilibrium mixture. [69] Based on the assumption that at low temperatures the triplet state of the carbene is favored, and together with several examples where increased yields of C-H insertion products were observed, a stepwise abstractionrecombination (a-r) mechanism was suggested for reactions performed in alcoholic 96 and alkenic 97 matrices for the phenyldiazomethane case and in alcoholic matrices for -diazobenzylphosphonates. [103,104] A matrix effect has been pointed as the decisive factor for the reactivity of the carbenic species.…”

Hydrogen bonds and halogen bonds in complexes of carbones L→C←L as electron donors to HF and ClF, for L = CO, N2, HNC, PH3, and SH2

“…2,3 These molecules contain a carbon atom with either two paired electrons in a single orbital giving rise to a singlet electronic configuration, or two unpaired electrons in two different orbitals in a triplet electronic configuration. [4][5][6] Singlet carbenes are very basic species, [7][8][9][10] are found in complexes with metals, 11 and can participate in a variety of non-covalent interactions 12 including hydrogen, [13][14][15] halogen, 16,17 chalcogen, 18 pnicogen [19][20][21] and tetrel bonds. 22,23 In recent years, Frenking and co-workers have explored the possibility of having carbon(0) compounds called carbones, which are stabilized by two electron pair donor molecules.…”