The development of catalytic systems capable of oxygenating
unactivated
C–H bonds with excellent site-selectivity and functional group
tolerance under mild conditions remains a challenge. Inspired by the
secondary coordination sphere (SCS) hydrogen bonding in metallooxygenases,
reported herein is an SCS solvent hydrogen bonding strategy that employs
1,1,1,3,3,3-hexafluoroisopropanol (HFIP) as a strong hydrogen bond
donor solvent to enable remote C–H hydroxylation in the presence
of basic aza-heteroaromatic rings with a low loading of a readily
available and inexpensive manganese complex as a catalyst and hydrogen
peroxide as a terminal oxidant. We demonstrate that this strategy
represents a promising compliment to the current state-of-the-art
protection approaches that rely on precomplexation with strong Lewis
and/or Brønsted acids. Mechanistic studies with experimental
and theoretical approaches reveal the existence of a strong hydrogen
bonding between the nitrogen-containing substrate and HFIP, which
prevents the catalyst deactivation by nitrogen binding and deactivates
the basic nitrogen atom toward oxygen atom transfer and the α-C–H
bonds adjacent to the nitrogen center toward H-atom abstraction. Moreover,
the hydrogen bonding exerted by HFIP has also been demonstrated not
only to facilitate the O–O bond heterolytic cleavage of a putative
MnIII–OOH precursor to generate MnV(O)(OC(O)CH2Br) as an active oxidant but also to affect the stability
and the activity of MnV(O)(OC(O)CH2Br).
A new ring-shaped non-harmonic oscillator potential is proposed. The precise hound solution of Dirac equation with the potential is gained when the scalar potential is equal to the vector potential. The angular equation and radial equation are obtained through the variable separation method. The results indicate that the normalized angle wave function can be expressed with the generalized associated-Legendre polynomial, and the normalized radial wave function can be expressed with confluent hypergeometric function. And then the precise energy spectrum equations are obtained. The ground state and several low excited states of the system are solved. And those results are compared with the non-relativistic effect energy level in Phys. Lett. A 340 (2005) 94. The positive energy states of system are discussed and the conclusions are made properly.
Introduction of Brønsted acids into biomimetic nonheme reactions promotes the oxidative ability of metal−oxygen complexes significantly. However, the molecular machinery of the promoted effects is missing. Herein, a comprehensive investigation of styrene oxidation by a cobalt(III)−iodosylbenzene complex, [(TQA)-Co III (OIPh)(OH)] 2+ (1, TQA = tris(2-quinolylmethyl)amine), in the presence and absence of triflic acid (HOTf) was performed using density functional theory calculations. Results revealed for the first time that there is a low-barrier hydrogen bond (LBHB) between HOTf and the hydroxyl ligand of 1, which forms two valenceresonance structures [(TQA)Co III (OIPh)(HO − --HOTf)] 2+ (1 LBHB ) and [(TQA)Co III (OIPh)(H 2 O--OTf − )] 2+ (1′ LBHB ). Due to the oxo-wall, these complexes (1 LBHB and 1′ LBHB ) cannot convert to high-valent cobalt−oxyl species. Instead, styrene oxidation by these oxidants (1 LBHB and 1′ LBHB ) shows novel spin-state selectivity, i.e., on the ground closed-shell singlet state, styrene is oxidized to an epoxide, whereas on the excited triplet and quintet states, an aldehyde product, phenylacetaldehyde, is formed. The preferred pathway is styrene oxidation by 1′ LBHB , which is initiated by a rate-limiting bond-formation-coupled electron transfer process with an energy barrier of 12.2 kcal mol −1 . The nascent PhIO-styrene-radical-cation intermediate undergoes an intramolecular rearrangement to produce an aldehyde. The halogen bond between the OH − /H 2 O ligand and the iodine of PhIO modulates the activity of the cobalt−iodosylarene complexes 1 LBHB and 1′ LBHB . These new mechanistic findings enrich our knowledge of nonheme chemistry and hypervalent iodine chemistry and will play a positive role in the rational design of new catalysts.
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