85990-32-5Relevant articles and documents
Production of Formamides from CO and Amines Induced by Porphyrin Rhodium(II) Metalloradical
Zhang, Jiajing,Zhang, Wentao,Xu, Minghui,Zhang, Yang,Fu, Xuefeng,Fang, Huayi
supporting information, p. 6656 - 6660 (2018/05/24)
It is of fundamental importance to transform carbon monoxide (CO) to petrochemical feedstocks and fine chemicals. Many strategies built on the activation of C≡O bond by π-back bonding from the transition metal center were developed during the past decades. Herein, a new CO activation method, in which the CO was converted to the active acyl-like metalloradical, [(por)Rh(CO)]? (por = porphyrin), was reported. The reactivity of [(por)Rh(CO)]? and other rhodium porphyrin compounds, such as (por)RhCHO and (por)RhC(O)NHnPr, and corresponding mechanism studies were conducted experimentally and computationally and inspired the design of a new conversion system featuring 100% atom economy that promotes carbonylation of amines to formamides using porphyrin rhodium(II) metalloradical. Following this radical based pathway, the carbonylations of a series of primary and secondary aliphatic amines were examined, and turnover numbers up to 224 were obtained.
Carbon-carbon bond activation of 2,2,6,6-tetramethyl-piperidine-1-oxyl by a RhII metalloradical: A combined experimental and theoretical study
Kin, Shing Chan,Xin, Zhu Li,Dzik, Wojciech I.,De Bruin, Bas
, p. 2051 - 2061 (2008/09/16)
Competitive major carbon-carbon bond activation (CCA) and minor carbon-hydrogen bond activation (CHA) channels are identified in the reaction between rhodium(II) meso-tetramesitylporphyrin [RhII(tmp)] (1) and 2,2,6,6-tetramethyl-piperidine-1-oxyl (TEMPO) (2). The CCA and CHA pathways lead to formation of [RhIII(tmp)Me] (3) and [RhIII(tmp)H] (5), respectively. In the presence of excess TEMPO, [RhII(tmp)] is regenerated from [RhIII(tmp)H] with formation of 2,2,6,6-tetramethyl- piperidine-1-ol (TEMPOH) (4) via a subsequent hydrogen atom abstraction pathway. The yield of the CCA product [RhIII(tmp)Me] increased with higher temperature at the cost of the CHA product TEMPOH in the temperature range 50-80°C. Both the CCA and CHA pathways follow second-order kinetics. The mechanism of the TEMPO carbon-carbon bond activation was studied by means of kinetic investigations and DFT calculations. Broken symmetry, unrestricted b3-lyp calculations along the open-shell singlet surface reveal a low-energy transition state (TS1) for direct TEMPO methyl radical abstraction by the RhII radical (SH2 type mechanism). An alternative ionic pathway, with a somewhat higher barrier, was identified along the closed-shell singlet surface. This ionic pathway proceeds in two sequential steps: Electron transfer from TEMPO to [RhII(por)] producing the [TEMPO] +[RhI(por)]- cation-anion pair, followed by net CH3+ transfer from TEMPO+ to RhI with formation of [RhIII(por)Me] and (DMPO-like) 2,2,6-trimethyl-2,3, 4,5-tetrahydro-1-pyridiniumolate. The transition state for this process (TS2) is best described as an SN2-like nucleophilic substitution involving attack of the dz2 orbital of [RhI(por)]- at one of the CMe-Cring σ* orbitale of [TEMPO] +. Although the calculated barrier of the open-shell radical pathway is somewhat lower than the barrier for the ionic pathway, R-DFT and U-DFT are not likely comparatively accurate enough to reliably distinguish between these possible pathways. Both the radical (SH2) and the ionic (S N2) pathway have barriers which are low enough to explain the experimental kinetic data.
Activation of methane and toluene by rhodium(II) porphyrin complexes
Wayland, Bradford B.,Ba, Sujuan,Sherry, Alan E.
, p. 5305 - 5311 (2007/10/02)
Thermodynamic and kinetic-mechanistic studies are reported for reactions of (tetramesitylporphyrinato)rhodium(II) monomer, (TMP)Rh., and (tetraxylylporphyrinato)rhodium(II) dimer, [(TXP)Rh]2, with methane that produce hydride and methyl derivatives. A RhII-RhII bond energy of ～ 12 kcal mol-1 in [(TXP)Rh]2 was determined by 1H NMR line broadening and found to dominate differences in the thermodynamic and kinetic parameters for reactions of methane with Rh(II) porphyrins. The sum of the Rh-H and Rh-CH3 energies is found to be ～ 117 kcal in both the (TMP)Rh and (TXP)Rh derivatives. Rate laws, activation parameters, and deuterium isotope effects suggest that a four-centered linear transition state (Rh-H3C-H-Rh) provides a relatively low activation enthalpy route for methane reacting with two metalloradicals. Comparative studies demonstrate that rhodium(II) porphyrins react with toluene exclusively at the benzylic C-H bond, and kinetic studies suggest that this reaction proceeds through a transition state related to that for the methane reactions. Aromatic C-H bond reactions are kinetically excluded for rhodium(II) porphyrins due to steric effects in the transition state.
