Functionalization of rhenium aryl bonds by O-atom transfer

Article abstract of DOI:10.1021/om2002365

Aryltrioxorhenium (ArReO₃) has been demonstrated to show rapid oxy-functionalization upon reaction with O-atom donors, YO, to selectively generate the corresponding phenols in near quantitative yields. ¹⁸O-Labeling experiments show that the oxygen in the products is exclusively from YO. DFT studies reveal a 10.7 kcal/mol barrier (Ar = Ph) for oxy-functionalization with H₂O₂ via a Baeyer-Villiger type mechanism involving nucleophilic attack of the aryl group on an electrophilic oxygen of YO coordinated to rhenium.

Full text of DOI:10.1021/om2002365

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Functionalization of Rhenium Aryl Bonds by O-Atom Transfer
Steven M. Bischof,† Mu-Jeng Cheng,‡ Robert J. Nielsen,‡ T. Brent Gunnoe,§ William A. Goddard, III,‡
and Roy A. Periana*,†
†The Scripps Energy & Materials Center, Chemistry Department, The Scripps Research Institute, Jupiter, Florida 33458, United States
‡Materials and Process Simulation Center, Beckman Institute, Division of Chemistry and Chemical Engineering,
California Institute of Technology, Pasadena, California 91125, United States
§Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904, United States
S Supporting Information
b
ABSTRACT: Aryltrioxorhenium (ArReO3) has been demonstrated to
show rapid oxy-functionalization upon reaction with O-atom donors, YO,
to selectively generate the corresponding phenols in near quantitative
yields. 18O-Labeling experiments show that the oxygen in the products is
exclusively from YO. DFT studies reveal a 10.7 kcal/mol barrier (Ar = Ph)
for oxy-functionalization with H2O2 via a BaeyerÀVilliger type mechanism involving nucleophilic attack of the aryl group on an
electrophilic oxygen of YO coordinated to rhenium.
fundamental challenge in the development of hydrocarbon
A
hydroxylation catalysts based on CÀH bond activation1 is
the requirement for efficient and compatible oxy-functionalization of
the LnMÀR intermediates to generate RÀOH.2,3 The most effective
catalysts for methane hydroxylation are based on strongly electro-
philic and oxidizing cations such as Hg(II),4 Pt(II),5 Pd(II),6 and
Au(I/III)7 in strongly acidic media where both CÀH activation and
oxy-functionalization of the electrophilic LnMÀCH3 intermediates
are facile. However, well-defined oxy-functionalization catalysts for
hydrocarbons based on less electrophilic and less oxidizing metals
such as Re, Ru, Ir, and Os have not yet been well developed.8 This is
likely because, while CÀH activation is known with these systems,9
oxy-functionalization reactions of less electrophilic LnMÀR inter-
mediates are not well developed and have only recently been reported
when R = alkyl group.10 For electrophilic late transition metals, CÀO
bond formation proceeds via nucleophilic attack of an O-based
nucelophile (e.g., water) on the electrophilic hydrocarbyl group.
Not surprisingly, the functionalization reactions of LnMÀR com-
plexes with more electropositive metals operate by very different
mechanisms, where the hydrocarbyl group is the nucleophile. For
example, Brown and Mayer reported oxy insertion into a phenyl
ligand from a Re(VII) bis-oxo system.11 However, examples of oxo
ligands that are sufficiently electrophilic for migration of a nucleophilic
hydrocarbyl ligand are rare.11,12 More recently, some of us have
reported the reaction of methyl rhenium trioxide (CH3ReO3) with
various O-atom donors, YO, such as IO4À, H2O2, or PhIO, in
aqueous solvents that proceeds at room temperature (RT) to rapidly
and cleanly generate methanol and [ReO4]À presumably via the
rhenium methoxy species (Figure 1, R = Me).10a
Figure 1. Oxy-functionalization of a MÀR complex proposed to
proceed via a BaeyerÀVilliger-type oxidation.
mechanisms. To begin addressing this using the RÀReO3 motif
as a well-behaved system with a nucleophilic hydrocarbyl, we have
begun to systematically explore the scope of oxy-functionalization
chemistry where R = aryl. Since the bonds of metalÀaryls are
typically stronger than metalÀalkyls, it is plausible that higher
barriers could be possible and reactions proceed through different
mechanisms. However, in organic BV reactions, aryl migration is
more facile than alkyl, and it will be interesting to contrast the
organic and organometallic systems based on this precedent.13
The aryl complexes of rhenium trioxide, ArReO3, have been
previously reported.14 Our initial studies involved reaction of the
phenyl (PhReO3) and para-tolyl trioxorhenium (p-TolReO3)
derivatives of ArReO3 with 3 equiv of H2O2 in a THF-D8/D2O
(5:1) mixture. A color change was observed instantaneously upon
shaking at RT. Subsequent analysis by NMR and GC-MS showed
formation of phenol and only para-cresol. However, full quantitative
and kinetic analyses of these and other known monosubstituted
ArReO3 complexes were complicated by their characteristic thermal
instabilities.14b
Since the 2,4,6-trimethylphenyl trioxorhenium (MesReO3)
complex14b was reported to be thermally stable, we focused our
studies on this system. Significantly, similar to the parent phenyl
A BaeyerÀVilliger (BV) type mechanism has been proposed for
this reaction based on DFT studies and could provide the basis for
the design of new catalysts that operate via CÀH activation for
hydrocarbon oxidation reactions. However, relatively little is known
about the scope and details of oxy-functionalization reactions of
poorly electrophilic LnMÀR systems by this or other reaction
Received: March 17, 2011
Published: March 29, 2011
r
2011 American Chemical Society
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|

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analogues, treatment of a THF-D8/D2O (5:1) solution of MesReO3
with 3 equiv of H2O2 led to an instantaneous color change at RT and
selective formation of only 2,4,6-trimethylphenol on the basis of 1H
NMR and GC-MS analysis. Examination of this reaction using an
external standard showed that the reaction is essentially quantitative
with no evidence of MesReO3 decomposition. Also, the functiona-
lization reaction proceeds efficiently under both air and argon.
To determine whether the oxygen in the 2,4,6-trimethylphenol
was derived from H2O2, the ReO3 motif, or D2O solvent, we
examined the reaction with >95% 18O-labeled hydrogen peroxide
(which does not undergo O-atom exchange on the time scale of the
reaction with water or Re). Confirming that the oxygen atom was
derived from H2O2, analysis of the reaction mixture by GC-MS
showed that 2,4,6-trimethylphenol from the reaction with 18O-labeled
H2O2 was >95% 18O-labeled. A similar result was obtained from our
previous reports involving the reaction of CH3ReO3 with NaI18O4.10a
To examine the scope of the reaction with respect to O-atom
donors, we examined the reaction with NaIO4, DMSO, and PyO.
We were particularly interested in O-atom donors such as PyO
and DMSO, as these can be generated from O2.15 As can be seen
in Table 1, all of these O-donors generated 2,4,6-trimethylphenol
but with different efficiencies. Detailed kinetic studies were not
carried out, but as shown in column 3, the reactions with H2O2
and NaIO4 were fast and selective with essentially quantitative
yields of the 2,4,6-trimethylphenol after 1 h at RT. In contrast,
reactions with PyO and DMSO were much slower, and these
O-atom donors showed no reaction under our standard condi-
tions. However, heating the PyO and DMSO reaction mixtures at
75 °C for 24 h and 100 °C for 72 h, respectively, showed clean
formation of 2,4,6-trimethylphenol, albeit in low yields (column
4, Table 1). These reactions are selective, as the phenol and
unreacted starting material accounted for >90% mass balance.
To compare the relative reactivity of alkyl and aryl derivatives of
RÀReO3 complexes, we investigated competitive reactions between
CH3ReO3 and MesReO3. The study was carried out under pseudo-
first-order conditions by treatment of a 5:1 THF-D8/D2O solution
containing 33 mM of both CH3ReO3 and MesReO3 with 0.05 equiv
of H2O2 (based on total added Re).16 After shaking at RT, in situ
1
analysis of the crude reaction mixture by H NMR showed
formation of 2,4,6-trimethylphenol and CH3OH in a 3.8:1 ratio
along with unreacted CH3ReO3 and MesReO3 with >90% mass
balance. These results suggest that the aryl complex is ∼4 times
more reactive than the alkyl complex. Examination of the compe-
titive oxidation in THF by DFT confirmed this result to give ΔG‡
barriers of 25.5 and 26.9 kcal/mol for MesReO3 and CH3ReO3,
respectively, for aryl and alkyl migration (ΔΔG‡ ∼1 kcal/mol for
∼4:1 ratio of rates).17
Table 1. Functionalization Results of MesReO3 and Pre-
dicted DFT Barriers for the BV TSa
complex
YO std cond % arylÀOH max yield % arylÀOH BV TSd
MesReO3 H2O2
MesReO3 NaIO4
MesReO3 PyO
MesReO3 DMSO
>90
>90
0
>90
10.7
>90
∼20b
0
∼15c
35.1
In order to gain further insight into the functionalization of the
ArReO3 complexes, we examined the reactions of PhReO3, 1, with
H2O2 by DFT and continuum solvation representing water. These
studies show that, as for organic BV-type mechanisms, on the most
favorable pathway the reaction proceeds in two steps: (1) YO
coordination to the Re center and (2) concerted migration of the
aryl group to the oxygen of the coordinated YO, Figure 2. Other
a Reaction conditions: A screw cap NMR tube with PTFE insert was
loaded with 5 mg of MesReO3. The solid was dissolved in 0.6 mL of a
THF-D8/D2O (5:1) solution to which 3 equiv of YO was added, and the
NMR tube was shaken at RT and allowed to sit for 1 h before
monitoring. b 24 h, 75 °C. 72 h, 100 °C. d Calculated ΔG‡ between
c
PhReO3 and phenyl migration barrier in water and in DMSO,
respectively.
Figure 2. Standard Gibbs free energy surfaces (kcal/mol, including aqueous solvation using M06) of the BaeyerÀVilliger oxidation at pH = 7 and
298 K.
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Scheme 1. Oxidation of PhReO3 with DMSO Reporting
Gibbs Free Energy Surfaces (kcal/mol, including solvation) at
pH = 7
known class of phenyl 4-substituted trioxorhenium complexes
were thermally unstable. The 2,6-dimethylphenyl trioxorhenium
complexes can be utilized in such a study, but the required family
of para-substituted complexes are not known and are currently
being synthesized in our laboratories.
These results show that the functionalization of LnMÀR
complexes where M is not of the class of strongly electrophilic
metals can undergo facile oxy-functionalization when R = alkyl or
aryl. As in the case of the alkyl systems, theoretical calculations
for when R = Ar are consistent with the reactions proceeding via a
BV-type reaction mechanism, which would be consistent with
the observed faster rates for the aryl systems. Importantly, the
results disclosed herein suggest similarity between the organic
and organometallic BV transformation and showcase the broad
potential for facile oxy-functionalization of more electropositive
organometallic intermediates by a BV-type reaction mechanism.
Given the facile characteristics of these oxy-functionalization
reactions, we are exploring the possibility of coupling with CÀH
activation to design new hydroxylation catalysts. It should be
noted that the Re center in the RÀReO3 motif is formally d0, and
thus competitive oxidation by YO of the metal center versus the
R group is not possible. Since lower oxidation state metal centers
are typically more active for CÀH activation, we anticipate that
this undesirable oxidation of the metal center versus the R group
could be a key challenge in utilizing this type of functionalization
reaction to design new hydrocarbon functionalization catalysts
based on a CÀH activation/MÀR functionalization cycle. Con-
sequently, an important focus of our ongoing work will be to
determine if the BV-type mechanism is possible with non-d0,
more electropositive organometallics.
pathways involving a migration of the aryl group to either oxo or
μ2-peroxo groups result in much higher barriers (>23 kcal/mol).
Since coordination of H2O2 to the Re center is expected to be
pH dependent, we examined the reaction profile at pH = 7 to
mimic the reaction conditions. Neutral adducts between 1 and
H2O2 were found to be less stable than monodeprotonated
adducts with no stable adduct between 1 and neutral H2O2
located in our survey of the reaction surface. With a pKa of 11.6
for H2O2,18 proton dissociation is uphill by 6.3 kcal/mol at pH =
7 and coordination of HOOÀ to the Re center to generate the
monodeprotonated adduct, 3, is stabilizing, À0.2 kcal/mol, and
is slightly favorable relative to 1. As shown in Figure 1, the overall
reaction to generate the O-atom insertion product, PhOReO3, 4,
is highly favorable, À78.4 kcal/mol downhill. The lowest calcu-
lated pathway (10.9 kcal/mol) was found from the monodepro-
tonated species, 3, proceeding through a BV-type aryl migration
to coordinated hydroperoxide (TS1).
These values are consistent with the observed high functionaliza-
tion yields and rapid reaction upon mixing at RT. We have assumed
that the hydrolysis of the ArReO3 intermediate to the corresponding
aryl alcohol is fast. Electronically the reaction proceeds as a
concerted displacement of the OÀO σ-bonding pair of electrons
by the attacking carbon σ-bond. It is interesting that the mono-
deprotonated TS1, where OHÀ is the leaving group, is substantially
lower in energy than the neutral TS2, where the leaving group is
H2O. This would seem to suggest that with these more electropos-
itive organometallics, inducing nucleophilic character on the aryl
group by operating under basic conditions could be more beneficial
than stabilizing the leaving group through protonation under more
acidic conditions. It will be interesting to examine the pH depen-
dency of these reactions.
Given the experimental observation that potentially air recyclable
oxidants such as PyO and DMSO can functionalize MesReO3, we
examined the oxidation of PhReO3 with DMSO by DFT. Interest-
ingly, unlike the case for H2O2, calculations predicted that no stable
DMSO adduct exists. Instead, consistent with the observed slow
reaction, a concerted insertion transition state was located with an
activation free energy of 35.1 kcal/mol, leading to the formation of
PhOReO3 and dimethylsulfide (Scheme 1).
These results suggest a parallel to the BV reactions observed in
organic chemistry, where conversion of aryl ketones to the
corresponding esters by treatment with O-atom donors such as
H2O2 are more facile than with alkyl ketones.13 This is typically
explained by the greater stabilization of charge by delocalization
in the transition state that is possible in aryl versus alkyl
migration. To address this, a Hammett study of substituted
ArReO3 complexes would be ideal. However, our attempts at
Hammett studies were hampered since, as noted above, the
’ ASSOCIATED CONTENT
S
Supporting Information. Synthetic procedures, charac-
b
terization data, experimental details, and DFT coordinates are
available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: rperiana@scripps.edu.
’ ACKNOWLEDGMENT
The authors acknowledge the Chevron Corporation, the
Center for Catalytic Hydrocarbon Functionalization, a DOE
Energy Frontier Research Center (DOE DE-SC000-1298), and
The Scripps Research Institute for financial support of this
research.
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