ARTICLE IN PRESS
JID: CCLET
[m5G;September 17, 2021;4:23]
J. Lin, F. Wang, J. Tian et al.
Chinese Chemical Letters xxx (xxxx) xxx
Scheme 1. Possible intermediates for the MnII(R,R-PMCP)(OTf)2 with H2O2 in the presence of sulfuric acid.
ylating unactivated alkyl C-H bonds [17]. Interestingly, Nam and
co-worker have demonstrated that nonheme high-spin iron(III)-
acylperoxo complex with 13-TMC ligand (13-TMC
=
1,4,7,10-
tetramethyl-1,4,7,10-tetraazacyclotri-decane) is strong oxidant ca-
pable of oxygenating hydrocarbons prior to their conversion into
iron-oxo species via O-O bond cleavage [18]. Based on these
achievements, DFT calculations have definitely helped to predict
the possible active species and to address mechanistic issues,
thereby elucidating the multiple-oxidant nature in the catalytic cy-
cle of oxidation reactions in combination with the experimental
studies.
In parallel to iron systems, nonheme manganese complexes
bearing chiral N4 ligands have been shown to be effective cat-
alysts in the asymmetric oxidations [10–12]. In these manganese
catalytic systems, carboxylic acids (e.g., AcOH) still play a key role
in the activation of H2O2 together with the manganese complexes.
In the past decade, Costas [19,20], Bryliakov [16,21] and our group
[22–26] have developed a variety of nonheme manganese com-
plexes for efficient oxidation catalysis. Most of this class of effec-
tive iron and manganese catalysts thus far possesses same feature
that is supported by chiral N4 ligands, thus providing two labile
cis-binding sites for H2O2 activation. In these reactions, it has been
proposed with indirect experimental evidence that the active inter-
mediate is MnV-oxo species [16,27]. Compared to well documented
mechanism of nonheme iron catalysts, however, the mechanistic
aspects concerning the manganese catalysts are underdeveloped. In
this context, what kinds of manganese species will involve in the
manganese/H2O2 catalytic systems, MnV(O), MnIV(O) cation radi-
cal or other possible species [28]? An in-depth understanding of
the mechanism is a fundamental task for rational ligand design to-
ward tuning of enantio-control and reactivity. Previously, a variety
of manganese complexes with aryl-substituted MCP (MCP = N,N’-
dimethyl-N,N’-bis-(2-pyridinylmethylcyclo-hexane-1,2-diamine) N4
ligands by our group have been showed excellent enantiocontrol in
the asymmetric epoxidation of olefins, in which sulfuric acid could
significantly enhance the activity for olefin epoxidation [22,29,30].
Continuing on this line of research, we report herein the system-
atic theoretical calculations on the epoxidation mechanism cat-
alyzed by the manganese modeling catalyst, MnII(R,R-PMCP)(OTf)2
with H2O2 in the presence of sulfuric acid. Based on the DFT re-
sults, [MnV(O)(R,R-PMCP)(SO4)]+ with a triplet ground spin state
serves as the active species for the olefin epoxidation. Interestingly,
a MnIII-persulfate is likely involved in this catalytic system, how-
ever, DFT results suggest that the energy barrier of its C=C double
bond epoxidation (18.7 kcal/mol) is higher than that of its trans-
formation to a [MnV(O)(R,R-PMCP)(SO4)]+ species (9.5 kcal/mol).
In addition, the direct use of Oxone (potassium peroxymonosul-
fate, 2KHSO5·KHSO4·K2SO4) as oxidant also provides a comparable
enantioselectivity as that of H2O2/sulfuric acid system.
Fig. 1. Potential energy profiles for the conversion of MnIII-OOH to manganic per-
sulfate via MnV-oxo at B3LYP/Def2-TZVPP//TZVP(Mn, OOH, HSO4, N atoms in the
ligand)-6-31G∗ (C and H atoms in the ligand) level including zero-point energy and
solvation corrections.
those of nonheme iron catalytic systems. As previously proposed
by Bryliakov/Talsi and co-workers, still, the manganese catalytic
system started from an MnIII-hydroperoxide species, followed pro-
tonation of the hydroperoxide ligand by the coordinated acid in
MnIII-hydroperoxo intermediates facilitates the O-O bond cleav-
age, generating MnV-oxo species as reactive epoxidizing interme-
diates. As shown in Scheme 1, possibly, [MnV(O)(R,R-PMCP)(SO4)]+
3, MnIII-persulfate 4 can be generated from MnII(R,R-PMCP)(OTf)2
1
with H2O2 as the oxidant in the presence of sulfuric
acid. Previously, we have briefly studied the formation of
[MnV(O)(R,R-PMCP)(SO4)]+ 3 from its precursor, MnIII-OOH species
(2) by DFT calculations [29]. The results showed that the trans-
formation occurs reasonably via an O-O bond heterolysis by the
aid of coordinated sulphuric acid. For this process, as shown in
Fig. 1, the calculated ground state for complex 2 is the quin-
tet spin state. In addition, low-spin singlet 2 is involved, how-
ever, the singlet state is more than 33.0 kcal/mol higher in en-
ergy than quintet one. In the first step, cleavage of the O-OH
bond of 2 leads to the formation of Mn-oxo 3, in which a spin
change from the higher quintet spin to triplet spin state oc-
curs (two-state reactivity) [31,32]. In this case, the transition state
(TS) TS23 in triplet and quintet states are found to be 13.0 and
22.4 kcal/mol, respectively. Possibly, cyclic MnIII-persulfate is in-
volved as previously reported (S,S-PDP)FeIII(κ2-peracetate) species.
Actually, DFT calculations support the formation the MnIII
-
persulfate through the O-O bond coupling reaction with a barrier
of 11.2 kcal/mol. The potential energies of triplet MnV-oxo (33) and
quintet MnIII persalt (54) are equal, indicating that both species
can possibly exist in the reaction system and act as epoxidizing
reagent.
High-valent metal-oxo species are highly reactive intermediates
involved in the oxidation of organic substrates by heme and non-
heme enzymes and their model compounds. In the biomimetic
manganese catalyst/H2O2/acid catalytic system, MnV-oxo, MnIV-oxo
and MnIII-peroxo may be involved as the possible active species as
2 shows the key geometric parameters and spin-state
energies of MnV(O) 3, MnIII-persulfate 4 without the resulting
2