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A Journal of
Accepted Article
Title: C-H Insertions by Iron Porphyrin Carbene: Basic Mechanism and
Origin of Substrate Selectivity
Authors: Rahul L. Khade and Yong Zhang
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201704631
Link to VoR: http://dx.doi.org/10.1002/chem.201704631
Supported by
10.1002/chem.201704631
Chemistry - A European Journal
COMMUNICATION
C-H Insertions by Iron Porphyrin Carbene: Basic Mechanism and
Origin of Substrate Selectivity
Abstract: Recent experimental reports of heme carbene C-H
insertions show promising results for sustainable chemistry due to
good yield and selectivity, low cost of iron, and low/no toxicity of
hemes. But mechanistic details are mostly unknown. Despite
structural similarity and isoelectronic nature between heme carbene
IV
and Fe =O intermediate, our quantum chemical studies with
detailed geometric and electronic information for the first time
II
reveals an Fe -based, concerted, hydride transfer mechanism,
IV
different from the Fe -based stepwise hydrogen atom transfer
mechanism for C-H functionalization by native heme enzymes. A
trend of broad range experimental C-H insertion yields (0-88%) of
five different C-H bonds including mostly non-functionalized ones
was well reproduced. Results suggest that the substrate selectivity
originate from the hydride formation capability. The predicted
kinetic isotope effects were also in excellent agreement with
experiment. Useful geometry, charge, and energy parameters well
correlated with barriers were reported. Results provide the first
theoretical evidence that carbene formation is the overall ratelimiting step and suggest a key role of the formation of strong
electrophilic heme carbene in developing heme-based C-H
insertion catalysts and biocatalysts.
Catalytic C-H insertion by metal carbenoids is a powerful tool to
generate new C-C bond.[1] However, most transformations use
noble metals and/or non-biological ligands, which due to
potential issues in cost and toxicity may not be ideal for
sustainable chemistry. Inspired by the excellent catalytic
performance of cytochromes P450 for numerous biochemical
oxene transfer reactions,[2] biomimetic metalloporphyrins and
artificial heme proteins including cytochrome P450 and
myoglobin have been developed for carbenoid C-H
insertions.[1a, 1c, 1e, f, 1h, 1j, k, 1m, 3] In addition, cytochrome P450
carbenes have long been recognized as important
intermediates in natural P450 activities during the metabolism
of toxic polyhalogenated compounds and some drugs.[4] Iron
porphyrin or heme carbenes implied in many studies[1k, 5] were
directly shown to undergo C-H insertions of mostly nonfunctionalized C-H bonds with good substrate selectivity and up
to 88% yield.[1m] More work using iron porphyrin and other
heme-based carbenoids[1a, 1c, 1j, k, 3a, 3d] have resulted in even
[*]
Dr. R. L. Khade, Prof. Dr. Y. Zhang
Department of Chemistry and Chemical Biology
Stevens Institute of Technology
1 Castle Point on Hudson Hoboken, NJ 07030 (USA)
E-mail: yong.zhang@stevens.edu
Supporting information for this article is given via a link at the end of
the document.
better C-H insertion yields and selectivities. But their
mechanisms are largely unknown.[1h, 1k] In particular, for iron
porphyrin carbene (IPC) C-H insertions, which due to low cost
of iron and biocompatibility of hemes are highly appealing for
sustainable chemistry, no computational mechanistic studies
have been reported. So, many mechanistic questions remain to
be answered. For instance, does it involve hydrogen atom
transfer like in C-H functionalization by native heme enzyme
cytochrome P450,[2a] or hydride transfer, proton transfer, and
proton-coupled electron transfer as found for other transition
metal catalysts[1n, 6]? What are the key changes in energies,
charges, and geometries in the reaction pathway? What is the
origin of the experimentally observed substrate selectivity?[1k,
1m]
What factors are critical for catalyst development? Based on
successful predictions of various experimental spectroscopic
properties and reactivities of IPCs and other iron-containing
biocatalysts,[7] here we report the first quantum chemical study
of IPC C-H insertions, which not only provides data in excellent
agreement with experimental reactivity and selectivity results,
but also offers many previously unknown mechanistic details.
Scheme 1. C-H insertion pathway. Oval represents porphyrin.
In this first computational work to reveal the basic IPC C-H
insertion mechanism, reactions of the experimentally isolated
and well characterized [Fe(TPFPP)(C(Ph)CO2Et)] (TPFPP =
meso-tetrakis(penta-fluorophenyl)porphyrinato dianion)[1m] with
five different C-H bonds were studied: the activated α-C-H bond
of tetrahydrofuran (THF, 1), the non-functionalized benzylic C-H
bonds in cumene (2) and ethylbenzene (4), allylic C-H bond in
cyclohexene (3), and ring C-H bond in cumene (5), with
experimental yields from 0 to 88%.[1m] 2 and 5 are the same
substrate to directly compare regioselectivity. These
experimental results put a firm footing of metalloporphyrin
carbene’s role on heme-based C-H insertions in later
experimental studies.[1a, 1c, 1j, k, 3a, 3d] TPFPP was modelled as a
non-substituted porphyrin (Por) to focus on basic mechanism.[7a,
b]
All species were subject to full geometry optimization and
frequency analysis in experimentally used solvents,[1m] using a
This article is protected by copyright. All rights reserved.
Accepted Manuscript
Rahul L. Khade and Yong Zhang*
10.1002/chem.201704631
Chemistry - A European Journal
range-separated hybrid DFT method with dispersion correction,
based on its excellent performance on IPCs and other catalytic
systems,[7a, b, 8] see Supporting Information (SI) for details.
The electronic nature of catalytic intermediate is important
to its reactivity. Native P450s’ radical functionalizations of C-H
bonds originate from the radical feature of the major catalytic
intermediate, FeIV=O2- coupled with porphyrin radical cation.[2a]
Other FeIV=O2- species in native heme enzymes also have
radical feature as indicated by ~0.9 e spin density of oxygen.[9]
The resonance structure of FeIV={C(X)Y}2- (structurally similar
and isoelectronic to FeIV=O2- in P450 reactions) was recently
proposed in cyclopropanations via artificial heme enzymes[10]
and highlighted in iron porphyrin studies.[1m] But our recent
study[7a] based on accurate predictions of experimental
Mössbauer, X-ray, and NMR properties supports a dominant
FeIIß{:C(X)Y}0 feature instead, which is consistent with their
experimental electrophilicity.[1k, 1m, 5c, d, 10b] A more recent
computational work argues for an anti-ferromagnetically
coupled FeIII and negatively charged carbene radical open-shell
singlet,[11] based on computed energies. However, different
computational approaches can affect the calculated
energetically favourable state, as found in that report and also
our calculations in supporting information (SI), a more rigorous
comparison using many experimental properties as described
in SI shows that it is still the non-radical FeII carbene feature
consistent with all previous experimental results. This is further
supported by the latest experimental results using additional
experimental techniques reported in this year.[12]
However, whether transition state (TS) features of the heme
carbene C-H insertion are similar to those in native heme
enzymatic reactions remain to be disclosed. Because
conformation and spin state may influence the mechanism, we
studied such effects in details. Experimental spin states of ironcontaining reactant R1 and product P1 are singlet and triplet
respectively,[1m, 13] so both singlet and triplet states of R1, TS
and P1 were investigated. Our results reproduced the
experimental ground states for both R1 and P1 (see SI), further
supporting the calculations here. The best conformations of R1
reported previously[7b] and substrates (R2’s, see details in SI)
were also used here and in following TS and product
calculations. Due to the relatively simple structure of
cyclohexene, its TS was discussed first to show the detailed
conformation and spin state effect.
A
B
C
first examined. There are two chiral centers in P2: one is
carbene’s C, the other is the C in the inserted C-H bond (called
C’), so we examined two pro-chiral TS isomers, pro-SR and
pro-SS (the other two complementary isomers are their mirror
images). A concerted TS was first obtained for both isomers.
Due to relative orientations of cyclohexene and two carbene
substituents, there are two orientations of C=O bond with
respect to the new C-H bond, trans and cis (see Figure 1A and
1B), and there are two orientations of cyclohexene ring with
respect to carbene substituents, see Figure 1A and 1C. For
pro-SR transition state with the same toward Ph conformation,
the trans conformation is more stable than cis by ΔG of 0.94
kcal/mol, probably due to a stronger hydrogen bond (HB)
between carbonyl O and the closest phenyl H in the trans
isomer than the HB between ester O and the closest phenyl H
in the cis isomer (orange dashed lines in Figure 1), as indicated
by a shorter O…H distance in the former case by 0.051 Å. The
trend is the same in the pro-SS TS, with the trans isomer
having a lower ΔG by 0.53 kcal/mol and a shorter HB distance
by 0.025 Å. As shown in Figure 1A and 1C, for the two trans
pro-SR isomers, the conformation with cyclohexene toward Ph
is slightly more stable than the one toward CO2Et, by ΔG of
0.68 kcal/mol, due to more favourable van der Waals
interactions between the phenyl ring and cyclohexene ring.
Therefore, the trans and toward Ph conformation was used in
subsequent studies. Such first report of the conformation effect
on C-H insertion TS via metalloporphyrin carbenes may help
other studies in this field. The calculated Gibbs free energy of
activation (ΔG‡) for pro-SR TS of 21.61 kcal/mol is similar to
that for pro-SS TS of 20.67 kcal/mol, consistent with
experimentally found non-enantioselective reaction.[1m] Their
geometric parameters, charges, and spin densities are also
similar, see Tables S7-S9. So, the pro-SR TS was chosen to
describe basic mechanistic details below.
Table 1. Key Energy, Charge, and Geometry Results
Yield
a)
1
2
3
4
5
88
83
64
low
0
‡ b)
ΔG
(kcal/
mol)
18.45
19.40
21.61
23.73
33.10
QCT
(e)
-0.277
-0.278
-0.290
-0.283
-0.331
RCC’
T
S
(Å)
2.634
2.604
2.586
2.539
2.335
dashed line in A-C shows a hydrogen bond. Colour scheme: Fe – black, C –
cyan, N – blue, O – red, H – grey, Cyclohexene – purple, Hydride - pink.
Since P1 is iron-based triplet, the iron-based triplet TS was
1.503
1.460
1.449
1.408
1.277
ΔGf
(kcal/
mol)
222.40
223.28
230.69
231.59
280.35
c)
ΔG
(kcal/
mol)
-55.44
-36.03
-41.38
-37.26
-37.00
°
[a] Ref.1m. For 4, the experimental yield was described as insignificant. [b]
The TS’s are for pro-SR for substrates 1, 3, 4 based on above comparison
for 3 and additional comparisons in SI for 1 and 4; for 2 and 5, the pro-S
isomers were used as the pro-R isomers are mirror images. [c] The Gibbs
free energies of reaction are for corresponding chiral products in [b].
A large negative charge transfer (QCT) occurs from
cyclohexene to carbene from reactants to TS, see Table 1. In
this triplet TS, basically it is only Fe carrying the unpaired
electrons, since its spin density (ρ Fe) is 1.978 e, while the spin
densities of carbene’s C, substrate, and porphyrin moieties, are
all <0.03 e (see Table S9). These results show an FeII-based
hydride transfer feature, consistent with the FeII feature of
IPC.[7b, 12] It is interesting to note that other mechanistic trials
with initial setups as 1) FeII-based hydrogen atom transfer, 2)
FeII-based proton transfer, 3) FeIII-based hydrogen atom
αβ
Figure 1. (A,B,C) Optimized pro-SR TS conformations for 3. The orange
TS
RCH
(Å)
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COMMUNICATION
2
1
3
H insertion as the triplet FeII-based hydride transfer. Actually,
such transition states were successfully obtained for all five CH insertions here, indicating for the first time a general
concerted, non-radical, FeII-based hydride transfer mechanism
for IPC C-H insertion, similar to that for dirhodium catalysts,[1n]
but different from the step-wise, radical, hydrogen atom transfer
mechanism in C-H functionalization by native P450s.[2a]
4
5
Figure 2. Atomic charge changes from reactants to TS and charge transfers as indicated by arrows and numbers in parentheses.
Charge analysis in Figure 2 shows that in all reactions, a
significant QCT occurs from substrate to carbene. It has an
excellent correlation with ΔG‡ (see Figure 3A). Accordingly, the
carbon (C’) donating hydride has a charge change from R2 to
TS (see numbers in Figure 2) also well correlated with ΔG‡,
R2=0.9487, while charge changes of other atoms (e.g. Fe and
C) do not have good correlations (R2<0.5), further supporting
the critical role of hydride transfer.
A
There is also an excellent correlation between hydride
formation energy (ΔGf, see SI for computational details) and
ΔG‡ (see Fig.3D), which further substantiates the crucial role of
hydride transfer in heme carbene C-H insertion.
B
2
R =0.9277
2
R =0.9917
D
C
2
2
R =0.9840
R =0.9612
‡
TS
S7), the new C-C bond formation is relatively behind, showing a
non-synchronous feature, as proposed from experimental
kinetic isotope effects (KIEs) of similar reactions.[1k] In fact, our
calculated KIE data for 1-5 of 1.85, 1.84, 2.21, 1.99, 2.06 with
an average of 1.99 are close to experimental KIEs of ~1.97 for
several iron porphyrin catalysed C-H insertions.[1k] As seen
from Figure 3B and 3C, low barriers go with long new C-C/C-H
bonds, showing an early TS feature that is also unknown before.
TS
Figure 3. Plots of ΔG vs. (A) QCT, (B) RCC' , (C) RCH , and (D) ΔGf.
There are also excellent correlations (see Figure 3B and
3C) among ΔG‡ and distances between carbene’s C and both C’
and H in the inserted C-H bond at transition state, RCC’TS and
RCHTS. In contrast, the Fe-C bond length is not correlated with
ΔG‡ (R2<0.3), suggesting that breaking the old C-H bond (not
the Fe-C bond) to form hydride is a critical part of TS.
Compared to new C-C/C-H bond lengths in products (see Table
Because the experimentally reported main side reaction is
carbene dimerization[1m] and the same carbene is used in all
five reactions, the experimental stoichiometric C-H insertion
yields under same conditions may correlate with ΔG‡’s. The
calculated ΔG‡’s from using respective experimental conditions
in Table 1 and using the same conditions for 2 (20.39, 19.40,
21.64, 23.75, 33.10 kcal/mol for 1-5) show basically the same
trend: 1 ~ 2 (their difference is within typical computational
error) < 3 < 4 < 5. Interestingly, this reactivity trend is consistent
with the experimental C-H insertion yields in 24 h:[1m] 1 (88%) ~
2 (83%, this reaction time is 15 h, so its 24 h yield may be
closer to 1) > 3 (64%) > 4 (insignificant) > 5 (0%). These trends
and above correlation results for the first time show that
experimental substrate chem- and regio-selectivity may be due
to the C-H bond’s hydride formation capability.
Another interesting result is that C-H insertion ΔG‡’s for 1-4
(i.e. those can react) are all lower than heme carbene formation
ΔG‡ by 14.84, 13.89, 11.68, and 9.56 kcal/mol respectively,
based on the reported heme carbene formation ΔG‡ and Gibbs
free energy of reaction (ΔG°) using the same method.[7b] These
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transfer (see SI for details), all finished the same as FeII-based
hydride transfer. The singlet pathways were also examined.
Both FeII-based hydride transfer and FeIII-based hydrogen atom
transfer TS’s were successfully located. But their ΔG‡’s are of
higher energies by 5.13 and 4.70 kcal/mol respectively than the
triplet FeII-based hydride transfer TS. This systematic study
suggests the most favourable TS for [Fe(Por)(C(Ph)CO2Et)] C-
10.1002/chem.201704631
Chemistry - A European Journal
data for the first time directly show that, the heme carbene
formation is the overall rate-limiting step if the reaction starts
from iron porphyrin and diazo compound, as proposed
experimentally.[1k] In addition, all C-H insertion products are
thermodynamically favourable, as shown in Table 1. These
results suggest that the C-H insertion catalyst development
shall be focused on formation of strong electrophilic heme
carbene, for which our recent systematic study of carbene
substituent, porphyrin substituent, and axial ligand effects on
heme carbene formation may be helpful.[7b]
Overall, this work provides several important and novel
results for IPC C-H insertions: 1) the mechanism is FeII-based
hydride transfer with early TS feature, which is different from
that by high-valent ferryl species in native heme enzymes; 2)
the favourable TS conformation has more hydrogen bond and
van der Waals interaction and the reaction pathway involves
spin state change from reactant via TS to product; 3) several
geometry, charge, and energy parameters were found to well
correlated with ΔG‡; 4) a broad range of experimental substrate
selectivity was found to originate from its hydride formation
capability; 5) formation of strong electrophilic heme carbene is
important for C-H insertion catalyst development. Results
establish an important basis for future more detailed study of
effects of carbene substituent, porphyrin substituent,
porphyrin/heme protein axial ligand, and metal center on hemebased C-H insertion reactivity and selectivity to help understand
mechanisms in more recent experimental studies of C-H
insertions by other metalloporphyrins and engineered heme
proteins.[1a, 1c, 1h, 1j, k, 3a, b] Since iron is the most abundant and
inexpensive transition metal for catalysis and heme is biocompatible, these results will facilitate future chemical catalysis
and biocatalysis.
Acknowledgements
[2]
[3]
[4]
[5]
[6]
[7]
[8]
This work was supported by an NSF grant CHE-1300912 to YZ.
YZ thanks Timothy Garvey, Sparkle Springfield, Marauo Davis,
Brian Minevich, and Matthew Lovell for their participation in the
preliminary studies.
Keywords: C-H activation • carbenoids • heme proteins •
porphyrinoids • density functional calculations
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
Heme carbene C-H insertion is good
for sustainable chemistry. Our results
for the first time show 1) the FeIIbased, hydride transfer, early TS; 2)
the conformation and spin state
effects on TS; 3) the useful geometry,
charge, and energy parameters to well
correlate with ΔG‡; 4) the theoretical
explanation of a broad range
experimental C-H insertion yields; 5)
the significance of strong electrophilic
heme carbene with low formation
barrier for catalyst development.
Rahul L. Khade and Yong Zhang*
Page No. – Page No.
C-H Insertions by Iron Porphyrin
Carbene: Basic Mechanism and
Origin of Substrate Selectivity
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