Carbon-carbon bond activation of 2,2,6,6-tetramethyl-piperidine-1-oxyl by a RhII metalloradical:: A combined experimental and theoretical study

被引:65
作者
Chan, Kin Shing [1 ]
Li, Xin Zhu [1 ]
Dzik, Wojciech I. [2 ]
de Bruin, Bas [2 ]
机构
[1] Chinese Univ Hong Kong, Dept Chem, Shatin, Hong Kong, Peoples R China
[2] Univ Amsterdam, Vant Hoff Inst Mol Sci, Dept Homogeneous & Supramol Catalysis, NL-1018 WV Amsterdam, Netherlands
关键词
D O I
10.1021/ja078157f
中图分类号
O6 [化学];
学科分类号
0703 ;
摘要
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 [Rh-II(tmp)] (1) and 2,2,6,6-tetramethyl-piperidine-1-oxyl (TEMPO) (2). The CCA and CHA pathways lead to formation of [Rh-III(tmp)Me] (3) and [Rh-III(tmp)H] (5), respectively. In the presence of excess TEMPO, [Rh-II(tmp)] is regenerated from [Rh-III(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 [Rh-III(tmp)Me] increased with higher temperature at the cost of the CHA product TEMPOH in the temperature range 50-80 degrees 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 Rh-II radical (S(H)2 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 [Rh-II(por)] producing the [TEMPO](+)[Rh-I(por)](-) cation-anion pair, followed by net CH3+ transfer from TEMPO+ to Rh-I with formation of [Rh-III(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 S(N)2-like nucleophilic substitution involving attack of the d(z)2 orbital of [Rh-I(por)](-) at one of the C-Me-C-ring alpha* orbitals 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 (S(H)2) and the ionic (S(N)2) pathway have barriers which are low enough to explain the experimental kinetic data.
引用
收藏
页码:2051 / 2061
页数:11
相关论文
共 80 条
  • [1] Contracted all-electron Gaussian basis sets for atoms Rb to Xe
    Ahlrichs, R
    May, K
    [J]. PHYSICAL CHEMISTRY CHEMICAL PHYSICS, 2000, 2 (05) : 943 - 945
  • [2] Ahlrichs R., 2002, TURBOMOLE VERSION 5
  • [3] AHLRICHS R, TURBOMOLE VERSION 5
  • [4] ENERGY-ADJUSTED ABINITIO PSEUDOPOTENTIALS FOR THE 2ND AND 3RD ROW TRANSITION-ELEMENTS
    ANDRAE, D
    HAUSSERMANN, U
    DOLG, M
    STOLL, H
    PREUSS, H
    [J]. THEORETICA CHIMICA ACTA, 1990, 77 (02): : 123 - 141
  • [5] [Anonymous], 2001, PQS VERSION 2 4
  • [6] Atkins P., 2014, Physical chemistry
  • [7] Aurich H. G., 1989, Nitrones, Nitronates and Nitroxides, P313, DOI 10.1002/9780470772195
  • [8] AN ALGORITHM FOR THE LOCATION OF TRANSITION-STATES
    BAKER, J
    [J]. JOURNAL OF COMPUTATIONAL CHEMISTRY, 1986, 7 (04) : 385 - 395
  • [9] REARRANGEMENT OF STRAINED DIPOLAR SPECIES, AZIRIDINE N-OXIDES .2.
    BALDWIN, JE
    BHATNAGA.AK
    CHOI, SC
    SHORTRID.TJ
    [J]. JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 1971, 93 (16) : 4082 - &
  • [10] Coordination complexes of a silicon-linked organic tetranitroxide
    Baskett, M
    Lahti, PM
    Palacio, F
    [J]. POLYHEDRON, 2003, 22 (14-17) : 2363 - 2374