Mechanistic Insights into the ReIO2(PPh3)2-Promoted Reductive Coupling of Alcohols

Article abstract of DOI:10.1021/acs.organomet.8b00285

A mechanistic investigation of the reductive coupling of benzylic and allylic alcohols by triphenylphosphine catalyzed by ReIO₂(PPh₃)₂ (1) is disclosed utilizing (1) stoichiometric reaction studies of 1 with alcohols, with PPh₃ and with OPPh₃; (2) rate law determination of the reaction of benzhydrol with PPh₃ catalyzed by 1; (3) substrate structure-dependent reactivity/selectivity studies; and (4) DFT computational analysis of various potential reaction pathways in the benzyl alcohol/PPh₃ reaction. In situ NMR monitoring of reactions of 1 with PPh₃ and various alcohols demonstrate (a) facile, reversible PPh₃ dissociation from 1; (b) association of various alcohols to form Re-alcohol/alkoxide complexes, (Ph₃P)IReO₂(ROH) and (Ph₃P)IReO(OH)(OR); and (c) thermal conversion of these alcohol(ate)-rhenium complexes to Ph₂CH-CHPh₂ and OPPh₃ at >50 °C. Under pseudo-first-order conditions, the initial rate kinetics of reductive coupling of Ph₂CHOH/PPh₃ catalyzed by 1 shows (a) a reaction rate that is first-order each in ROH, catalyst and first-order (or higher) in PPh₃ and (b) the reaction is inhibited by OPPh₃. Alcohol structure effects show (a) relative reactivity of sec-, tert-benzylic = allylic > prim-benzylic/allylic sec-, prim-alkyl and (b) low regioselectivity of the dimers from unsymmetrical allylic alcohols. A DFT computational study of the reaction of benzyl alcohol/PPh₃ with 1 reveals a preferred pathway involving: (a) formation of rhenium-alcohol and alkoxide intermediates, (Ph₃P)IReO₂(ROH) and (Ph₃P)IReO(OH)(OR); (b) reduction of the latter by PPh₃ to form (OPPh₃)(Ph₃P)IRe(OH)(OBn) (E); (c) association of a second BnOH with E to give (Ph₃P)IRe(OBn)₂ (K); (d) facile dissociation of a benzyl radical from K by C-O homolysis; and (e) a second rhenium-O-Bn homolysis from (PPh₃)IRe(H₂O)(OBn) (O), giving bibenzyl via benzyl radical recombination and regenerating (PPh₃)ReIO₂.

Full text of DOI:10.1021/acs.organomet.8b00285

Article
Cite This: Organometallics XXXX, XXX, XXX−XXX
Mechanistic Insights into the ReIO2(PPh3)2‑Promoted Reductive
Coupling of Alcohols
Camille Boucher-Jacobs,† Peng Liu,‡ and Kenneth M. Nicholas*,†
†Department of Chemistry and Biochemistry, University of Oklahoma, Norman, Oklahoma 73019, United States
‡Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States
S
* Supporting Information
ABSTRACT: A mechanistic investigation of the reductive coupling of
benzylic and allylic alcohols by triphenylphosphine catalyzed by
ReIO2(PPh3)2 (1) is disclosed utilizing (1) stoichiometric reaction studies
of 1 with alcohols, with PPh3 and with OPPh3; (2) rate law determination of
the reaction of benzhydrol with PPh3 catalyzed by 1; (3) substrate structure-
dependent reactivity/selectivity studies; and (4) DFT computational analysis of various potential reaction pathways in the
benzyl alcohol/PPh3 reaction. In situ NMR monitoring of reactions of 1 with PPh3 and various alcohols demonstrate (a) facile,
reversible PPh3 dissociation from 1; (b) association of various alcohols to form Re-alcohol/alkoxide complexes,
(Ph3P)IReO2(ROH) and (Ph3P)IReO(OH)(OR); and (c) thermal conversion of these alcohol(ate)-rhenium complexes to
Ph2CH−CHPh2 and OPPh3 at >50 °C. Under pseudo-first-order conditions, the initial rate kinetics of reductive coupling of
Ph2CHOH/PPh3 catalyzed by 1 shows (a) a reaction rate that is first-order each in ROH, catalyst and first-order (or higher) in
PPh3 and (b) the reaction is inhibited by OPPh3. Alcohol structure effects show (a) relative reactivity of sec-, tert-benzylic =
allylic > prim-benzylic/allylic ≫ sec-, prim-alkyl and (b) low regioselectivity of the dimers from unsymmetrical allylic alcohols. A
DFT computational study of the reaction of benzyl alcohol/PPh3 with 1 reveals a preferred pathway involving: (a) formation of
rhenium−alcohol and −alkoxide intermediates, (Ph3P)IReO2(ROH) and (Ph3P)IReO(OH)(OR); (b) reduction of the latter
by PPh3 to form (OPPh3)(Ph3P)IRe(OH)(OBn) (E); (c) association of a second BnOH with E to give (Ph3P)IRe(OBn)2 (K);
(d) facile dissociation of a benzyl radical from K by C−O homolysis; and (e) a second rhenium−O−Bn homolysis from
(PPh3)IRe(H2O)(OBn) (O), giving bibenzyl via benzyl radical recombination and regenerating (PPh3)ReIO2.
6
7
2
ethers and phenols, having strong Csp −O bonds, has been
disclosed, catalyzed by nickel and iridium complexes.
INTRODUCTION AND BACKGROUND
■
The search for efficient processes to convert abundant,
renewable resources to chemicals and fuels has spurred recent
efforts to develop selective chemical transformations of
biomass-derived, oxygen-rich feedstocks by refunctionalizing
C−O bonds.1 Besides dehydration and oxidation, reductive
processes that give products with reduced oxygen (and
increased energy) content and/or different functionality are
receiving particular attention. Hydrodeoxygenation (HDO,
Scheme 1) is one such reaction that is practiced in chemical
An alternative reductive process, deoxydehydration
(DODH), involving vicinal hydroxyl group elimination to
give unsaturated products (Scheme 1), has been developed
more recently8 and has been catalyzed primarily by oxo-
rhenium compounds and an expanding set of reductants
including PPh3,9 H2,10 secondary and activated alcohols,11
sulfite,12 elements,13 and hydroaromatics.14 We reported the
first DODH reactions catalyzed by nonprecious oxo-vanadium
complexes,15 while others have recently disclosed oxo-
molybdenum catalyzed DODH with alcohol reductants.16
A
Scheme 1. Hydrodeoxygenation (HDO) and
Deoxydehydration (DODH) of Alcohols
catalytic cycle involving a reduced metalloglycolate species that
undergoes olefin extrusion by concerted C−O bond cleavage
appears to be a common feature of the DODH reactions.17
The ability of oxo-metal complexes to activate C−O bonds
in the dehydration18 and deoxydehydration8 of alcohols
prompted us to investigate their potential for the deoxygena-
tion of monoalcohols. In our initial study we discovered that
activated alcohols such as benzylic, allylic and α-keto are
deoxygenated by reaction with PPh3 catalyzed by
(PPh3)2ReIO2 (1) (Scheme 2).19 Remarkably, however, for
most of the alcohols tested, alkane dimers were the exclusive
industry and in the laboratory,2 typically with activated
(benzylic, alylic) alcohols using H2/Pd−C,3 Lewis acid/
silanes,4 and for unactivated alcohols with heterogeneous
metal and metal oxide catalysts,5 but HDO of unactivated
alcohols typically require severe conditions and often have
limited product selectivity. Recently, the selective HDO of aryl
Received: May 3, 2018
© XXXX American Chemical Society
A
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combined with 5 equiv of triphenylphosphine. Additionally,
adding excess PPh3 to a 3 day old yellow benzene solution of
ReIO2(PPh3)1,2 caused the sample to return to the purple color
of 1; the 31P NMR spectrum of the final solution showed the
signal of 1 at 4.6 ppm to be predominant again and the −12.0
ppm signal to have nearly vanished. These observations are
indicative of a room temperature equilibrium between 1 and a
monophosphine derivative, tentatively formulated as (PPh3)-
ReIO2 (2) (Scheme 3). As suggested by a reviewer,
monophosphine derivative 2 could exist in part or entirely as
an iodide-bridged dimer.
Scheme 2. Re-Catalyzed Deoxygenation (DO) and
Reductive Coupling (RC) of Alcohols
reduction products, the result of reductive coupling (RC).
Very recently, we have reported that oxo-vanadium complexes
catalyze a related reductive coupling of alcohols in which the
alcohol (rather than phosphine) serves as the reductant
producing ketones as coproducts.20
Scheme 3. Ligand Dissociation Equilibrium for Precatalyst 1
The deoxygenation and particularly the reductive coupling
(RC) of alcohols promoted by soluble transition metal
compounds is little known. The deoxygenation of activated
alcohols (to alkanes) has been effected stoichiometrically using
Cp2TiCl21 and WCl2(PMe3)422 as reagents and catalytically by
polymethylhydrosiloxane/PdCl2.23 Stoichiometric reductive
coupling of activated alcohols by reduced Ti compounds24
and La/Me3SiCl,25 and a Cu-catalyzed La/Me3SiCl variant has
also been reported.26 The formation of bibenzyl as a minor
product in the recently reported molybdate-catalyzed HDO of
benzyl alcohol by isopropanol is also noted.16 The Re-
catalyzed, phosphine-mediated catalytic RC system that we
discovered recently provides a means to convert alcohols to
higher hydrocarbons under neutral and relatively mild
conditions, while avoiding prefunctionalization of the alcohol.
It thus has potential use for conversion of biomass-derived
alcohols to chemicals as an alternative to the unselective
Guerbet reaction.27
Our initial report of oxo-rhenium-promoted reductive
coupling of alcohols was suggestive but inconclusive as to
the nature of reactive intermediates involved in the process.
The RC reaction was observed only with activated alcohols,
i.e., benzylic and allylic alcohols which have weaker C−O
bonds,28 suggesting that C−O bond breaking is likely the rate-
limiting step. The reaction showed little regioselectivity in the
coupling of unsymmetrical allylic alcohols, which could be
explained by the involvement of a nearly symmetrical
intermediate, e.g., carbocation/radical, but oxo-metal-catalyzed
isomerization of the allyl alcohol substrate29 also could explain
this result. While the formation of hydrocarbon dimers and
alkane C−H reduction products are suggestive of carbon free
radical intermediates, attempted trapping of radicals with H
atom donating hydroaromatic compounds failed to produce H-
donor derived products19 and thus could not conclusively
implicate free radical intermediates. These mechanistic
ambiguities, together with the novelty and potential synthetic
utility of the RC reactions prompted a follow-up study to
address these issues, which is reported herein.
Interaction of 1 with Alcohols. A series of stoichiometric
reactivity experiments was conducted to elucidate the trans-
formations of 1 in the presence of various alcohols at room
1
temperature. H NMR monitoring of a room temperature
solution of 1 in benzene-d6 containing 5 equiv of benzhydrol
showed after 1−2 days two new low intensity singlets shifted
downfield from those of the free alcohol at 6.3 and 5.5 ppm
(approximate ratio 2:1; Figure 1). HSQC NMR spectra
indicated that these signals correlate with 13C peaks at 35 and
75 ppm, indicative of H−CPh2−ORe moieties. In the 31P
NMR spectrum of the mixture, free PPh3 (−8.0 ppm) and
additional signals at −9.0 and at −11.5 ppm (ReO2IPPh3, 2)
were detected. The new species were tentatively assigned as
Re-alcohol and Re−alkoxo-hydroxo species, e.g., A−D
(Scheme 4). This assignment was supported by NMR
monitoring of the reaction of ReO2I(PPh3)1,2 with other
alcohols and computational reaction modeling (vide infra).
We also examined the NMR spectra of 1:5 mixtures of
complex 1 with other alcohols, including ethanol, benzyl
alcohol, 1-phenylethanol, and cinnamyl alcohol (PhCH
CHCH2OH). All of these solutions changed color over 1−2
1
days and exhibited changes in their NMR spectra. The H
NMR spectra of the aged samples with the first three alcohols
showed, besides the signals of the free alcohol, two new sets of
signals appearing substantially upfield from the free alcohol,
Δ(ppm) from 1.1 to 1.7. The spectrum of the reaction mixture
with ethanol is shown in Figure 2; see the Supporting
1
Information for others. The integrated H NMR signals for
these two pairs of peaks were about 1:2 (and about 1:5 with
cinnamyl alcohol). The two unequal intensity sets of alcohol-
derived proton signals correlated with the presence of a large
31P NMR signal at −10.5 to −10.9 ppm and a smaller one at
−7 to −6 ppm in the region for coordinated PPh3, indicating
the formation of two major PPh3-containing product
complexes.
Up to four alcohol/alkoxide adduct structures arising from
the interaction of 1 with excess alcohol can be envisioned: A−
D in Scheme 4, mono- or bis-adducts, as either alcohol
complexes, M(ROH) (A, C) or metal hydroxo−alkoxides
(HO)MOR (B, D). To distinguish among these possibilities
we reasoned that the mono-/bis-adduct equilibrium should be
affected by the ROH/Re ratio, so the ratio of ethanol to
rhenium complex 1 was varied at 2:1 and 10:1 in two samples.
RESULTS AND DISCUSSION
■
Stoichiometric Reactivity Studies of IReO2(PPh3)2 (1).
Solution Behavior of 1. A benzene-d6 solution of IRe-
O2(PPh3)2 (1)30 undergoes color changes from purple to
crimson to yellow over 1−2 days. When monitoring this
process by 31P NMR spectroscopy, the signal for 1 at 4.6 ppm
diminished over time with concomitant growth of peaks at
−5.0 (for free PPh3) and at −12.0 ppm. These color and
spectroscopic changes were completely suppressed when 1 was
B
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1
Figure 1. H (left) and 31P (right) NMR spectra of 5:1 Ph2CHOH/1 at room temperature over 35 days in CDCl3, then at 50 °C for 1 day.
Scheme 4. Potential Alcohol Adducts from the Reaction with ReO2I(PPh3)2 (1)
been well-characterized;32 hence, NMR data comparable to the
present results are insufficient to definitively assign structures
to the detected rhenium alcohol/alkoxide species. In our DFT
computational modeling of the Re-catalyzed reductive
coupling pathways, we find support for the energetic viability
of both types of complexes.
A role of the alcohol/alkoxide adducts in the catalytic
reductive coupling is implicated by the result of heating
solutions containing these species. When a 35 day aged sample
derived from mixing 5 equiv of Ph2CHOH with 1 was warmed
to 50 °C, the NMR signals from the Re-OCHPh2 species
gradually disappeared and Ph2CH−CHPh2 was detected after
about 24 h (Figure 1), along with a 31P NMR signal at ∼27−
24 ppm, assigned to the coproduct, OPPh3 (Scheme 5).
Formation of a (PPh3)Re(OPPh3) Species under Ambient
Conditions. Another rhenium−phosphine complex was
1
Figure 2. H NMR spectrum of 5:1 CH3CH2OH + 1 (CDCl3, 20
detected in the NMR experiments that could be relevant to
the catalytic mechanism. After many days at room temperature,
solutions of 1 and Ph2CHOH displayed a pair of equal
intensity doublets in the 31P NMR spectrum at 50.7 and −11.5
ppm (J = 15 Hz), which is typical of a two- or three-bond
phosphorus−phosphorus coupling (see the Supporting In-
formation).33 The 50 ppm signal is downfield of that of free
OPPh3 (25 ppm), in the region of reported phosphine oxide
metal complexes.34 These features suggest that the species has
a Ph3P−O−Re−PPh3 subunit. To further clarify its composi-
tion an NMR sample was prepared with a 1:5 ratio of
IReO2(PPh3)2 (1) to OPPh3. After 24 h at room
temperature, this solution was yellow and exhibited the same
°C).
After 24 h, the integrated ratio of the two new CH2 quartet
signals was not appreciably different for these samples, showing
the independence of the product ratio to the ROH/1 ratio.
This result is best explained in terms of a coordinated alcohol/
alkoxide mixture, e.g., A/B or C/D, rather than a mono/bis-
equilibrium mixture (Scheme 5). The electronically different
nature of alcohol vs alkoxide coordination is also consistent
with the very different chemical shifts for the two sets of signals
in the H and 13C NMR spectra.
1
Although a number of rhenium−alkoxide complexes have
been reported,31 very few alcohol complexes of any metal have
Scheme 5. Formation and Conversion of Alcohol-Alkoxide Adducts to Products
C
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pair of doublets in its 31P NMR spectrum as produced after
several hours in the reaction of Ph2CHOH with 1. We
therefore identify this species as (PPh3)ReO2I(OPPh3) (3)
(Scheme 6). The formation of 3 from the reaction of 1 with
A substrate stereochemical probe was also considered,
homochiral PhCH(OH)CH3 (5), for which the question was
whether the resulting chiral anti hydrocarbon dimer would be
nonracemic. If so, then it would indicate a concerted or
stereodefined replacement of the C−O bond by the C−C
bond of the dimer. Unfortunately, the decisiveness of this test
was also undermined by the discovery that the homochiral
alcohol was substantially racemized under the reaction
conditions (150 °C), indicating that C−O bond cleavage is
reversible with this substrate. These substrate-based probes
thus failed to identify the character of the C−O bond cleavage.
Kinetics Study. We sought to determine the reaction order
in each component, i.e., the precatalyst, ReO2I(PPh3)2 (1), the
alcohol substrate and PPh3 in the reductive coupling of
benzhydrol (Scheme 7). NMR spectroscopy was used to
Scheme 6. Formation of Mixed Phosphine Oxide−
Rhenium−Phosphine Complex 3
OPPh3 shows the rapid ligand substitution of 1 as indicated in
the earlier NMR studies. The generation of 3 during long-term
interaction of 1 with benzhydrol may be the result of Re(V) to
Re(III) reduction by dissociated PPh3 (or Ph2CHOH, see
below) and subsequent rhenium reoxidation by adventitious
oxygen or during slow reductive coupling of the alcohol.
PPh3/ReO2I(PPh3)2 Reactivity. To further evaluate the
viability of generating a Re(III) species by phosphine
reduction of ReO2I(PPh3)1,2 in the catalyzed RC reaction, a
solution of 1 and PPh3 in C6D6 was monitored by 31P NMR at
r.t. for 6 days, then at 150 °C for 30 min. The NMR spectra
(see the Supporting Information) showed gradual formation of
OPPh3 and, eventually, the complete disappearance of 1; the
Re-containing product was not identified (Scheme 6). This
result demonstrates the ready reducibility of 1 by PPh3 and
thus the viability of producing a reduced presumably Re(II or
III) species at the higher temperature of the catalyzed
reactions. The reducibility of other LRe(V)O2 species by
phosphines has been noted in a number of reports,35 and the
reduction of a Re(V)−diolate to a Re(III)−diolate by sec-
alcohols has been suggested in the MeReO3-catalyzed
deoxydehydration of glycols.36
Scheme 7. Reductive Coupling of Benzhydrol for Kinetic
Analysis
collect concentration/time data for the product dimer,
Ph2CH−CHPh2; with chlorobenzene as the solvent at 125
°C, dimer yields of 75−80% were obtained. Reactions were
monitored with hexamethylbenzene as an internal integration
reference, sampling every minute to determine initial reaction
rates (conversion ≤15%). Sets of reactions varying only the
concentration of one component were run under pseudo-first-
order conditions. Keeping the alcohol and phosphine at a fixed
concentration (0.10 M) the catalyst concentration was varied
at 0.010, 0.005, and 0.0010 M, and the initial reaction rates
were determined (Figure 3). Plotting the initial rate versus the
catalyst concentrations gave a value of the reaction order (n) in
catalyst of approximately one (Figure 4).
Substrate Structure/Reactivity Effects. A few substrate
structure-based reactivity probes were also evaluated to gain
additional insights into the nature of the C−O bond cleavage
step. From our preliminary study, we observed selective
formation of dimeric hydrocarbon products from the most
activated alcohols; less activated substrates, e.g., BnOH and
PhCHOH−C(O)Ph, gave significant amounts of H-transfer
reduction products, i.e., toluene and PhCH2COPh, respec-
tively.19 Additionally, in our initial study the reductive coupling
of unsymmetrical allylic substrates, e.g., cinnamyl alcohol and
its isomer, produced essentially the same mixture of isomeric
diene dimers.19 Although this could be taken as evidence for
the formation of a common isomerizing or symmetrical
intermediate, e.g., allylic radicals, the proven ability of oxo-
metal species to catalyze the facile isomerization of the
substrate allylic alcohols,29 renders this result inconclusive.
An alcohol substrate that could implicate a radical
intermediate by the formation of ring-opened products,
cyclopropyl carbinol 4,37 also proved inconclusive because
this compound was quite unreactive under our typical
conditions, achieving little conversion after 5 days and failing
to provide an adequate amount of a characterizable product.
Figure 3. Initial rate data for Ph2CHCHPh2 formation at various
concentrations of precatalyst 1.
1H NMR sampling of the Ph2CHOH/PPh3/ReO2I(PPh3)2
reaction was also carried out with various concentrations of
benzhydrol at a fixed concentration of catalyst (0.01 M) and
excess PPh3 (0.50 M). Under these conditions the reaction rate
had an apparent first-order dependence on the alcohol
concentration (Figure 5), but at 0.10 M PPh3 (typical of
catalytic conditions) increasing the benzhydrol concentration
from 0.050 to 0.2 M actually resulted in a lower reaction rate
(Table 1, entries 1 and 2). This apparent substrate inhibition
D
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mechanism39 in which one substrate, e.g., PPh3, is converted to
product (OPPh3) while generating a different catalyst species,
e.g., Re(III), which then turns over the other substrate
(alcohol) to the dimeric hydrocarbon, i.e., ROH to R−R.
Considering that triphenylphosphine oxide, OPPh3, is a
coproduct of the reductive coupling, that phosphine oxides are
established ligands for higher oxidation state metals,34,40 and
the evidence for a mixed phosphine oxide−rhenium−
phosphine complex species in the stoichiometric reactivity
study (op cit), we sought to determine if OPPh3 is also a
product inhibitor of the RC reaction. An experiment was
conducted in which 5 equiv of triphenylphosphine oxide was
added to the standard reaction mixture (Ph2CHOH/PPh3/1);
dimer formation was monitored by NMR at 125 °C. A 60%
decrease of the reaction rate was observed (Figure 6),
Figure 4. Reductive coupling rate dependence on the catalyst 1
concentration.
Figure 5. Reductive coupling rate dependence on the benzhydrol
concentration at 0.50 M PPh3.
has been observed in enzyme kinetics and suggests the
Figure 6. Effect of added OPPh3 on the initial rate of Ph2CH−CHPh2
formation at 125 °C. squares: [PPh3] = [Ph2CHOH] = 0.10 M, [1] =
0.010 M, no added OPPh3; triangles: 0.50 M OPPh3, [PPh3] =
[Ph2CHOH] = 0.10 M, [1] = 0.010 M.
formation of a “dead-end” (inactive) intermediate.38
Table 1. Initial Rates of Reductive Coupling Showing
Substrate Inhibition at High Concentrations of PPh3 and
Ph2CHOH
demonstrating an inhibitory effect of triphenylphosphine
oxide in the reaction. The presence of high concentrations of
the phosphine oxide may disfavor an equilibrium step in which
it decoordinates or its coordination forms a less reactive
complex.
entry
[Ph2CHOH] (M)
[PPh3] (M)
initial rate (μM/min)
1
2
3
4
0.05
0.20
0.10
0.10
0.10
0.10
0.05
0.20
0.0026
0.0021
0.0029
0.0024
Summarizing, under pseudo-first-order conditions the
approximate empirical rate law derived is rate =
k[cat]1[Ph2CHOH]1[PPh3]≥1. However, at concentrations
used for the catalytic reaction runs ([ROH] = [PPh3] =
10[1]), we see departures from this rate law, apparently the
result of catalyst saturation and/or competitive substrate
binding and inhibition. With these provisos in mind the
kinetics results are consistent with a catalytic pathway in which
a monometallic catalyst reacts with at least one alcohol and
one phosphine molecule in or before the turnover-limiting
step. The inhibitory effect of phosphine oxide suggests that
OPPh3 production via coordinated OPPh3 is reversible.
Computational Reaction Modeling. In our initial report
of the ReIO2(PPh3)2-catalyzed reductive coupling19 a prelimi-
nary DFT study of the deoxygenation of PhCH2OH by the
model catalyst MeReO3 supported the viability of a
mechanistic pathway involving PPh3 reduction of MTO to
MeReO2, condensation of the latter with BnOH to give
MeReO(OCH2Ph)(OH), ReO−CH2Ph bond homolysis to
benzyl radical, and radical dimerization. The PhCH2−ORe
bond was suggested to be much weaker than typical, with a
calculated dissociation energy of only 35 kcal for the
intermediate, PhCH2−ORe(O)Me(OH)(PMe3), compared
to that for PhCH2−OH homolysis (81 kcal/mol, B3LYP-
calculated; 82 kcal/mol, experimental).27
To determine the reaction order in the reductant the PPh3
concentration was varied between 0.05−0.2 M at a fixed
concentration of Ph2CHOH (0.1 M) and the typical catalyst
concentration of 1 (0.01 M). These conditions led to a
substantial increase in the formation of the ether Ph2CH−O−
CHPh2 from alcohol dehydration and more scatter of the data
points in the concentration versus time plots. The rate versus
[PPh3] plot showed a positive phosphine concentration
dependency (see Supporting Information) but gave only a
moderate fit with a first-order dependence (R2 = 0.82).
Additionally, at high [PPh3] (0.2 M) the overall reaction rate
was slower (Table 1, entries 3 and 4), similar to that observed
with increasing [Ph2CHOH] at moderate [PPh3]. As for a
possible explanation in the present system, high PPh3
concentrations favor the coordinatively saturated
(PPh3)2ReIO2 (1) (Scheme 3), limiting binding and activation
of the alcohol substrate. Conversely, an excess of alcohol would
favor the formation of bis-alcohol/alcoholate-rhenium species
that could be less reactive toward reduction by PPh3 for steric
or electronic reasons. These concentration-dependent effects
on the kinetic behavior provide evidence of mutual substrate
inhibition and are consistent with a ping-pong type reaction
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Figure 7. B3LYP-optimized, M06(benzene) single point free energy profile of the ReIO2(PPh3)2-catalyzed reductive coupling of PhCH2OH via a
monoalkoxide pathway A; transition states are shown in blue.
In light of the observations in the stoichiometric reactivity
and kinetics studies, we carried out a more thorough and
experimentally more realistic DFT computational analysis of
the ReO2I(PPh3)2-catalyzed reductive coupling reaction,
selecting PPh3 as the ligand and reductant and benzyl alcohol
as the substrate. As a starting point and benchmark, the B3LYP
calculated structure of ReO2I(PPh3)2 was compared to its X-
ray structure.30 There is good overall agreement between the
two trigonal bipyramidal structures with trans phosphine
ligands, and the bond lengths and angles ( 0−4%), with the
largest variances in the Re-heavy atom distances, e.g., Re−O:
X-ray 1.74 Å, B3LYP calcd 1.74 Å; Re−P: X-ray 2.49 Å, B3LYP
calcd 2.54 Å; Re−I: X-ray 2.66 Å, B3LYP calcd 2.77 Å.
The favorable PPh3 dissociation from 1, established
experimentally, was also modeled computationally. We note
that the energy for dissociation of PMe3 from ReO2I(PMe3)2
has been estimated at 7.3 kcal/mol.41 On the basis of B3LYP-
calculated electronic energies (0 K, vacuum) a substantial
energy penalty of 16.9 kcal/mol for the PPh3 ligand
dissociation from 1 (→ ReIO2(PPh3) (A) + PPh3) was
found. When corrected for solvation (benzene) and temper-
ature (293 K), the free energy of reaction was approximately
thermoneutral, ΔG ∼0.2 kcal/mol. The ΔGrxn using the M06
functional (single point with solvation and temperature
correction) gave ΔG = 10.6 kcal, a poorer agreement with
the experimentally observed favorable dissociation. The
inaccuracies of these DFT methods for calculating metal-
phosphine dissociation energies are acknowledged.42 In light of
the generally better accuracy of the M06 method for TS
energies43 and our experimental observation of substantial
PPh3 dissociation and alcohol coordination at room temper-
ature, we have set ΔG to zero for the monophosphine complex
ReO2I(PPh3) (A) in the calculated relative energy profiles.
The coordination of benzyl alcohol with ReIO2(PPh3) to
form (PPh3)ReIO2(BnOH) (C) is calculated (M06/benzene,
298 K) to be nearly thermoneutral, ΔG = −0.7 kcal. Alcohol
complex C is found to be of similar energy to the
corresponding alkoxo-hydroxo-complex (PPh3)ReIO(OH)-
(OBn) (D), ΔG = −0.4 kcal. The isomeric alcohol and
alkoxo-hydroxo adduct pairs of other aliphatic alcohols, e.g.,
ethanol, methanol, and benzyhydrol, also have comparable
thermodynamic stabilities (see Supporting Information) and
thus should exist in similar amounts in a tautomeric
equilibrium. The energetic viability of such rhenium−
alcohol/alkoxide species is demonstrated by their NMR
detection from the interaction of various alcohols with
ReIO2(PPh3)2 (1).
We have considered two subsequent reaction pathways from
the monoalkoxide complex (PPh3)ReIO(OH)(OBn) (D): (1)
O-transfer reduction of D by PPh3, followed by R-ORe
cleavage processes (pathway A) or (2) association of a second
ROH to give a dialkoxo-rhenium species, followed by
reduction by PPh3 and then R−ORe cleavage (pathways B
and C). The calculated energy profile for the first of these is
presented as scheme A in Figure 7. Oxo-rhenium attack by
PPh3 on the alkoxo-hydroxo species D has a substantial free
energy of activation barrier (28.2 kcal/mol) via TSD−E to
produce the Re(III)−phosphine oxide species E in a nearly
thermoneutral step in a singlet manifold. The d4-Re(III)
species, e.g., E, could potentially exist as a low- or high-spin
species, and examples of both spin states are documented,
depending on the ligand set.44 B3LYP calculations for the
closed shell singlet and open shell triplet for the model
ReIPPh3(OMe)2 found the singlet species to be approximately
30 kcal more stable. On this basis, the other Re(III) species
evaluated in our study were also optimized as closed shell
singlets. The substantial activation barrier in the phosphine
reduction of B is derived from the counterbalancing enthalpic
contributions of P−O bond-making and ReO bond-
breaking, and the considerable entropic cost of an associative
process. In transition state TSD−E (Figure 8), the P−O and
Re−O distances are both much longer than for typical O−PR3
and ReO lengths.45 The electronic character of the
transformation may be simply viewed as an electron pair
transfer from the HOMO of the nucleophile PPh3 to the
LUMO of the electrophile ORe(V)I(OH)(OBn)(PPh3)
(D). This picture is in accordance with other computational
F
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the activation energy range estimated by the Arrhenius
equation for an experimental reaction rate of approximately 1
turnover/h at 150 °C. In this pathway step H−I is marginally
rate-limiting, so a second-order rate dependence in the alcohol
would be expected. However, the experimental kinetics study
was conducted with benzhydrol as the substrate. Although we
did not model the complete reaction pathway with benzhydrol,
the B3LYP-calculated electronic dissociation energies for the
conversion of (PPh3)(OH)(O)IRe−O−R to (PPh3)(OH)-
(O)IRe• and R•, for both R = CH2Ph and CHPh2 were
determined. The dissociation energy was 9.8 kcal lower for the
benzhydryl relative to the benzyl derivative. One can estimate
then that the activation free energies for benzhydryl radical
dissociation from a Re−alkoxy species would be at least a few
kcal lower than for the benzyl species, which in turn would
make the phosphine-O-transfer step (D−E) rate-limiting,
consistent with the experimentally determined first-order
dependence on the benzhydrol concentration.
Since in the catalytic reductive coupling reaction there is an
excess of alcohol substrate relative to catalyst, it is reasonable
to consider the formation of bis-alcohol/alkoxide species prior
to O−C bond cleavage events (Figure 10, pathway B). From
monoalkoxide complex E a second alcohol could coordinate,
tautomerize to the bis-alkoxo aquo species, and dehydrate to
provide dialkoxy complex K; overall this process is nearly
thermoneutral. Dialkoxo complex K could dissociate to benzyl
radical and Re(IV) monobenzyloxy complex L. This trans-
formation is calculated to have an activation free energy of only
20.2 kcal/mol via the open-shell singlet transition state and is
only mildly endoergic, once again indicating an extraordinarily
weak O−C bond in a rhenium(III)−alkoxide species. The
transition state structure TSK−Ldissoc (not shown) features a
very long O---C distance, indicative of a late transition state
and extensive spin delocalization onto primarily the Re−O and
benzylic fragments. After the first R−O homolysis, (K−L)
monobenzyloxy species L was considered to undergo the
second C−O cleavage by either of two processes, dissociative
or associative. The transition state for the dissociative process
TSL−Mdissoc (Figure 11) features a very long O---C distance,
indicative of a late transition state and extensive spin
delocalization onto primarily the Re−O and benzylic frag-
ments. A rebound reaction between Re−alkoxide L and the
benzyl radical could occur via associative transition state
TSK−Lassoc to produce bibenzyl and the original catalyst species
ReO2I(PPh3) (B); overall, this is a highly exoergic trans-
formation. The very long O−C and C−C distances in the
cleaving and forming bonds are noteworthy. The electron spin
densities in TSL−Massoc are also quite delocalized, being
distributed primarily at the Re−O unit, both benzylic carbons,
and the aryl rings. This reaction step is a bimolecular radical
substitution at carbon, of which there are relatively few verified
examples.48 The calculated activation energy for the associative
process via TSL−Massoc (29.5 kcal) is somewhat higher than that
for the dissociative step via TSL−Massoc (24.5), which points to
the primary operation of the dissociative process for O−C
cleavage. The total calculated energy barrier for the dissociative
pathway B is 24.5 kcal, significantly lower than that for
pathway A, and thus is considered to be more viable for the
catalytic process.
Figure 8. Transition state for the PPh3 reaction with D.
and experimental studies of phosphine O-transfer reactions of
oxo-metal complexes.46
Considering the most likely options for O−C bond cleavage
in a nonpolar medium (reaction solvent = benzene), the
energetics of homolytic processes were evaluated. A transition
state search for the dissociation of E to the spin doublets
benzyl radical and (PPh3)Re(IV)IO(OH) (F), yielded a saddle
point TSE−F with open shell singlet having an activation energy
of only 26.4 kcal (Figure 9). The O−C bond energy for
Figure 9. Transition state (left) and product (right) structures for C−
O homolysis of E and their Mulliken spin densities.
dissociation of benzyl radical from E is remarkably low
compared to the B3LYP-calculated and experimental C−O
dissociation energy of 80−82 kcal for PhCH2−OH itself.28,47
Product oxo-rhenium(IV) species F is calculated to have its
unpaired electron delocalized primarily on the Re and oxo
centers. The origin of the extraordinarily weak rhenium O−R
bond is discussed in more detail subsequently.
Doublet Re(IV) species F thus generated can then undergo
a nearly thermoneutral dissociative ligand substitution of
OPPh3 by benzyl alcohol via species G to give benzyl alcohol
complex H. Dissociation of benzyl radical from H to give
Re(V)−dihydroxo species I is once again found to be facile,
with an activation free energy for the open-shell singlet
transition state of 26.0 kcal/mol, nearly equal to that for the
corresponding dissociation from E. The highly exothermic
combination of two benzyl radicals generated from F and H,
with expected low barrier, would give the bibenzyl product.
Finally, dihydroxo species I can isomerize to modestly less
stable aquo complex J, from which water dissociation would
regenerate ReIO2(PPh3) (B) to re-enter the catalytic cycle.
For the complete reaction sequence of pathway A (Figure 7)
the total free energy barrier is 29.2 kcal at TSH−I, which is in
A variation in the dialkoxide pathway B was also considered
and is shown in the energy profile of Figure 12 (pathway C).
This pathway diverges from Re(IV)−alkoxide species L, which
could undergo nearly thermoneutral substitution of phosphine
G
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Figure 10. M06-solvent free energy profile for the ReO2I(PPh3)2-catalyzed reductive coupling of PhCH2OH via a dialkoxide pathway B; shows
dissociative and associative TSs for ReO−C bond cleavage, TSL−M; transition states are shown in blue.
calculation shows this as a viable step, but with a substantially
higher activation barrier (36.0 kcal) than reduction of
(PPh3)ReOI(OH)OBn) D (28.2 kcal). The experimental
detection of room-temperature phosphine dissociation and
alcohol association with ReO2I(PPh3)2 (1) to produce
detectable alcohol/alkoxide intermediates D and E supports
the conclusion that alcohol association is more facile than the
reduction of the dioxo species (A or B) by phosphine.
Could ReIO(OBn)2(PPh3) undergo benzyl radical dissoci-
ation before reduction? Comparing activation energies
(B3LYP, vac, 0 K) for Re(V)IO(OBn)2(PPh3) → Bn-rad +
Re(VI)O2I(OBn)(PPh3) (28.3 kcal) versus Re(III)I-
(OBn)2(PPh3) → Bn-rad + Re(IV)IO(OBn)(PPh3) (23.4
kcal) indicates that benzyl radical dissociates more easily from
the more reduced Re(III)−alkoxo species. Since the homolytic
dissociation is formally an oxidation of the rhenium center, the
more facile homolysis of the reduced rhenium alkoxo species
Figure 11. Dissociative and associative transition state structures for
C−O homolysis of L and Mulliken spin densities.
may reflect a lower energy requirement for the Re(III) to
Re(IV) versus Re(V) to Re(VI) oxidations.49
A few nonradical pathways to the hydrocarbon dimer were
also evaluated as shown in Scheme 8. These include concerted
oxidative elimination from a bis-alkoxide species (P) and
reductive elimination from a bis-alkyl complex (S).
oxide by water to give the Re(IV)−alkoxo species O.
Dissociation of O into benzyl radical and dihydroxo species I
has an activation barrier of only 20.7 kcal via the open shell
singlet transition state TSO−I. In pathway C, the total activation
barrier, peaking at TSO−I, is 23.8 kcal, the lowest among
pathways A−C for the second ReO−R cleaving step. We thus
consider pathway C to be the most favorable for the reductive
coupling reaction. Although pathway C still has the radical-
dissociation step (O−I) with a slightly higher energy barrier
than the reduction of D by phosphine (D−E), the expected
lower barrier for the corresponding radical-forming steps from
benzhydrol relative to benzyl alcohol, and the limits of
computational accuracy inherent in the B3LYP, M06, and
solvation modeling would reconcile the computational and
kinetics results.
We sought to find a transition state for concerted “oxidative
elimination” of the hydrocarbon dimer Bn−Bn from a
ReI(OBn)2(PPh3) species, e.g., P, analogous to the metal−
glycolate intermediate verified in the oxo-metal promoted
deoxydehydration of glycols.8,9,17 However, attempts to
approach such a transition state via constrained geometry
scans failed to show an inflection point over a wide range of
R−R distances (R = Me or PhCH2) or a tendency toward C−
C bonding or significant O−C lengthening even at close
contact and at high energy cost. Such a process leading to
dimer formation thus appears energetically unfavorable.
Could dialkyl-rhenium complexes, e.g., (R)2ReO2I (T),
arising from O-to-Re alkyl group migration (deinsertion) form
the dimeric hydrocarbon by reductive elimination (T−A)? We
Alternative Pathways to Reductive Coupling. The
possibility of initial reduction of ReIO2(PPh3)1,2 before the
alcohol coordination was also considered. B3LYP-level
H
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Figure 12. Free energy profile for ReO2I(PPh3)2-catalyzed reductive coupling of PhCH2OH via a dialkoxide pathway (M06-benzene); transition
states are shown in blue.
Scheme 8. Alternative Pathways to Reductive Coupling
found that reductive elimination of ethane from the model
species ReO2IMe2 could proceed via a concerted transition
state, but with a rather high barrier of 50.4 kcal (B3LYP level).
Furthermore, we could not locate a transition state for a
concerted migratory deinsertion process to form a dialkyl-
rhenium species, e.g., (PhCH2)2ReO2IPPh3 (S), from ReI-
(PhCH2O)2PPh3 (P). We conclude, therefore, that the
migratory deinsertion also has a substantial activation barrier
probably greater than for homolytic O-R dissociation. Indeed,
to our knowledge, proven examples of a metal−alkoxide to
oxo-metal alkyl transformation have not been reported.50 This
conversion has been suggested to account for the formation of
small amounts of coupling product in molybdate-catalyzed
dehydration of benzylic alcohols.16
Our study of the oxo-rhenium promoted reductive coupling
reaction supports the idea that lower oxidation state metal−
alkoxides can be precursors to carbon radicals. There is rather
little direct experimental evidence for this concept, however,
from prior reports.49 Peters and co-workers provided evidence
for the generation of t-butyl radicals in the room-temperature
decomposition of (t-BuO)3Mo(μ-N)Ti(NRAr)3.51 In the
Cp2Ti(III)Cl-promoted alcohol deoxygenation and coupling
reactions, C−O cleavage was suggested to take place via the
generation of organic radicals from a reduced Ti−alkoxide
complex,21 and this hypothesis was supported computation-
ally.52 Our recent discovery of oxo-vanadium catalyzed
oxidation-reductive coupling of activated alcohols provides
additional evidence that reduced vanadium alkoxide species are
precursors to C-centered radicals.20 It is also noteworthy that
metal−alkoxides have been used as thermolytic precursors to
nanoparticle metal oxides,53 but the resulting organic products
usually have not been identified. These experimental and
computational results raise the question of whether weak C−O
bonds are typical of reduced metal−alkoxides. Although
additional studies are needed to more fully address this
fundamental question, the homolytic bond dissociation energy
of the MO−R species should be diminished when the one
electron oxidation potential for the metal species is favorable
and a delocalized metallo-radical species is formed.
CONCLUSIONS
■
The key findings of the experimental and computational
studies of the oxorhenium-promoted reductive coupling
reaction of alcohols include: (1) alcohols that have weaker
C−O bonds, i.e., allylic and benzylic, undergo reductive
coupling most readily; (2) unsymmetrical allylic alcohols
produce regioisomeric dimers; (3) a room temperature
equilibrium exists between ReIO2(PPh3)2 (1) and Re-
IO2(PPh3) by dissociation of a triphenylphosphine ligand;
(4) rhenium−alcohol and −alkoxide adducts can be detected
by NMR under ambient conditions; (5) these species
disappear upon heating with formation of the hydrocarbon
dimer and OPPh3; (6) PPh3 can deoxygenate Re(V)-
IO2(PPh3)1,2; (7) under pseudo-first-order conditions the
I
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Kinetics Study. In a 1 dram vial were weighed quantities of
complex 1, benzhydrol, and triphenylphosphine, to which was added
0.90 mL of distilled chlorobenzene (stored over molecular sieves 4 Å)
and 0.10 mL of hexamethylbenzene (0.1 M). The vial was capped and
vortexed for 20 s. The sample was then split and dispensed into 10
NMR tubes (0.10 mL in each tube). After capping the tubes they
were placed in a preheated oil bath at 125 °C and a timer was started.
An NMR tube was removed from the oil bath every minute starting at
2 min. As soon as the tube was removed it was placed in an ice bath,
and 0.50 mL of CDCl3 was added. The NMR tubes remained in ice or
in the freezer until the NMR spectra were acquired.
empirical rate law is approximately first-order in both the
catalyst and the alcohol concentrations and first-order or
higher in PPh3; (8) DFT calculations support the viability of
both (PPh3)IRe(V)− and −Re(III)−alkoxide intermediates;
and (9) homolytic cleavage of Re(III)−benzyloxide species to
benzyl and rhenium-centered radical species is calculated to be
facile, with activation energies of 20−25 kcal/mol and
dissociation energies of only about 15 kcal/mol.
Taken together, the results from the experimental and
computational probes lead us to suggest a preferred reaction
pathway for the phosphine-driven reductive coupling of
activated alcohols that begins with alcohol coordination to
form monoalcohol/alkoxide complex, then reduction of the
latter by PPh3 to give Re(III)−monoalkoxide species E.
Association of a second alcohol molecule produces a Re(III)−
bis-alkoxide which can undergo sequential, facile C−O bond
homolysis to generate the hydrocarbon radicals and one-
electron oxidized rhenium oxo-species. The former recombine
to form the dimeric hydrocarbon and regenerate the original
dioxo−Re(V) species. With less activated alcohols, e.g., benzyl
alcohol, benzoin, which lead to more reactive radicals, a greater
amount of H atom abstraction product relative to dimer is
observed.54 Our current efforts seek to expand the range of
alcohol substrates for reductive coupling and deoxygenation
reactions and to develop more active and economical catalysts
and reductants.
Computational Methods. The B3LYP57 and M0658 methods
resident in Gaussian 09S1 were used to determine the energy-
minimized structures, vibrational frequencies and electronic energies.
For the B3LYP optimizations, the 6-31G(d) basis set was used for H,
C, and O atoms, 6-311+G(d,p) was used for P, and LANL2DZ was
used for Re and I. For single-point energy (M06) calculations, the
basis sets were as follows: 6-311++G(d,p) for H, C, O, and P and
SDD for Re and I and the SMD solvent continuum method59 was
included for modeling the species in the experimental reaction
medium (benzene). The Gibbs free energies and enthalpies include
zero-point vibrational energies and thermal corrections at 298 K.
Transition states were approached by mod-redundant scans and
characterized by single imaginary frequencies with displacement along
the reaction coordinate. Energy values and Cartesian (x,y,z)
coordinates for each of the species are provided in the Supporting
Information.
ASSOCIATED CONTENT
* Supporting Information
■
S
EXPERIMENTAL SECTION
■
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00285.
Materials and Methods. All reactants and solvents were obtained
commercially and used without further purification. H, 31P, and 13C
1
NMR spectra were collected on Varian V-NMRS at 400 or 500 MHz.
The NMR data were processed using MestReNova55 and ACD56
software. Gas chromatograms were collected on a Shimadzu GC-2014
equipped with an AOC 20i+s auto sampler, with a 3% SE-54 packed
column, and a FID detector.
NMR spectra for the interaction of ReO2I(PPh3)2 (1)
with alcohols, kinetics data, DFT-calculated energies for
the species in the putative reaction pathways (PDF)
Cartesian coordinates (XYZ)
Preparation of ReIO2(PPh3)2 (1). Complex 1 was prepared in
two steps following a literature procedure.30 Ammonium perrhenate
(1.0 g, 3.7 mmol) and triphenylphosphine (5.0 g, 19 mmol) were
added to a mixture of 56% hydroiodic acid solution (5 mL) and
ethanol (30 mL). The reaction was brought to reflux for 15 min.
Green crystals of I2ReO(OEt)(PPh3)2 formed in the mixture. After
cooling to room temperature the crystals were filtered off, washed
with ethanol, and dried under high vacuum. To a mixture of acetone
(50 mL) and water (2 mL) was added ReI2O(OEt)(PPh3)2 (1.00 g,
0.97 mmol). The green suspension was magnetically stirred at room
temperature. After an hour the suspended crystals had changed color
to violet. The crystals were filtered off and washed with cold acetone.
The product was recrystallized from hot 1:1 benzene/hexanes, giving
an 80% yield of ReIO2(PPh3)2 which was stored in a desiccator over
CaCl2.
AUTHOR INFORMATION
Corresponding Author
*E-mail: knicholas@ou.edu.
ORCID
Peng Liu: 0000-0002-8188-632X
Kenneth M. Nicholas: 0000-0002-5180-1671
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
K.N. thanks the U.S. Department of Energy, Basic Energy
Sciences (DE-11ER16276) and the National Science Founda-
tion (CHE1566213) for support. DFT calculations at the
University of Oklahoma were performed at the OU Super-
computing Center for Education and Research (OSCER) and
at the University of Pittsburgh Center for Research Computing
and the Extreme Science and Engineering Discovery Environ-
ment (XSEDE) supported by the National Science Founda-
tion.
Stoichiometric NMR Experiments Involving 1. Ligand
Dissociation. Each sample was prepared with about 1 mL of
benzene-d6 and 4.0 mg of ReIO2(PPh3)2 under nitrogen (5 mM). For
the samples in which triphenylphosphine was added, 26 mg was used.
Interaction with Alcohols. ReIO2(PPh3)2 (1) (0.0060 mmol, 5.0
mg) and benzhydrol (60 μL of 0.06 M solution in benzene-d6) were
added to 0.74 mL of benzene-d6 in a thick-walled NMR tube. The
NMR tube was capped and purged (evacuated, then backfilled) with
nitrogen three times. The tube was left at room temperature for
1
several weeks. The changes over time were followed by H and 31P
NMR. After a month, the sample was placed in an oil bath at 50 °C;
REFERENCES
1
the changes were followed by H and 31P NMR about every 16 h.
■
(1) (a) Boucher-Jacobs, C.; Nicholas, K. M. Selective Catalysis for
Renewable Feedstocks and Chemicals. Top. Curr. Chem. 2014, 353,
163. (b) Chheda, J. N.; Huber, G. W.; Dumesic, J. A. Liquid-Phase
Catalytic Processing of Biomass-Derived Oxygenated Hydrocarbons
to Fuels and Chemicals. Angew. Chem., Int. Ed. 2007, 46, 7164−7183.
Reaction of 1 with PPh3. A solution of 1 and PPh3 (1:5 equiv) in
C6D6 was monitored by 31P NMR at 20 °C for 6 days, then at 150 °C
for 30 min. The NMR spectra showed gradual disappearance of the
signal from 1 (4 ppm) and increasing formation of OPPh3 (25 ppm;
see the Supporting Information).
J
DOI: 10.1021/acs.organomet.8b00285
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(2) (a) Dutta, S. Deoxygenation of Biomass-Derived Feedstocks:
Hurdles and Opportunities. ChemSusChem 2012, 5, 2125−2127.
(b) Zacher, A. H.; Olarte, M. V.; Santosa, D. M.; Elliott, D. C.; Jones,
S. B. A Review and Perspective of Recent Bio-Oil Hydrotreating
Research. Green Chem. 2014, 16, 491−515. (c) Ruppert, A. M.;
Weinberg, K.; Palkovits, R. Hydrogenolysis Goes Bio: From
Carbohydrates and Sugar Alcohols to Platform Chemicals. Angew.
Chem., Int. Ed. 2012, 51, 2564−2601.
Hydrogen-Driven, Vanadium-Catalyzed Deoxydehydration of Gly-
cols. ACS Catal. 2016, 6, 1901−1904.
(11) (a) Arceo, E.; Ellman, J. A.; Bergman, R. G. Rhenium-Catalyzed
Didehydroxylation of Vicinal Diols to Alkenes Using a Simple Alcohol
as a Reducing Agent. J. Am. Chem. Soc. 2010, 132, 11408−11409.
(b) Shiramizu, M.; Toste, F. D. Expanding the Scope of Biomass-
Derived Chemicals through Tandem Reactions Based on Oxorhe-
nium-Catalyzed Deoxydehydration. Angew. Chem., Int. Ed. 2013, 52,
12905−12909. (c) Boucher-Jacobs, C.; Nicholas, K. M. Catalytic
Deoxydehydration of Glycols with Alcohol Reductants. ChemSusChem
2013, 6, 597−599.
(3) (a) Kieboom, A. P. G.; De Kreuk, J. F.; Van Bekkum, H.
Substituent Effects in the Hydrogenolysis of Benzyl Alcohol
Derivatives over Palladium. J. Catal. 1971, 20, 58−66. (b) Sawadjoon,
S.; Lundstedt, A.; Samec, J. S. M. Pd-Catalyzed Transfer Hydro-
genolysis of Primary, Secondary, and Tertiary Benzylic Alcohols by
Formic Acid: A Mechanistic Study. ACS Catal. 2013, 3, 635−642.
(c) Tsuji, J.; Mandai, T. Palladium-Catalyzed Hydrogenolysis of
Allylic and Propargylic Compounds with Various Hydrides. Synthesis
1996, 1996, 1−24.
(12) (a) Vkuturi, S.; Chapman, G.; Ahmad, I.; Nicholas, K. M.
Rhenium-Catalyzed Deoxydehydration of Glycols by Sulfite. Inorg.
Chem. 2010, 49, 4744−4746. (b) Ahmad, I.; Chapman, G.; Nicholas,
K. M. Sulfite-Driven, Oxorhenium-Catalyzed Deoxydehydration of
Glycols. Organometallics 2011, 30, 2810−2818.
(13) McClain, J. M.; Nicholas, K. M. Elemental Reductants for the
Deoxydehydration of Glycols. ACS Catal. 2014, 4, 2109−2112.
(14) Boucher-Jacobs, C.; Nicholas, K. M. Oxo-Rhenium-Catalyzed
Deoxydehydration of Polyols with Hydroaromatic Reductants.
Organometallics 2015, 34, 1985−1990.
(4) (a) Larson, G.; Fry, J. Ionic and Organometallic-Catalyzed
Organosilane Reductions. In Organic Reactions; John Wiley & Sons,
Inc.: Hoboken, NJ, 2008; pp 1−737. (b) McLaughlin, M. P.; Adduci,
́
L. L.; Becker, J. J.; Gagne, M. R. Iridium-Catalyzed Hydrosilylative
Reduction of Glucose to Hexane. J. Am. Chem. Soc. 2013, 135, 1225−
1227. (c) Yasuda, M.; Onishi, Y.; Ueba, M.; Miyai, T.; Baba, A. Direct
Reduction of Alcohols: Highly Chemoselective Reducing System for
Secondary or Tertiary Alcohols Using Chlorodiphenylsilane with a
Catalytic Amount of Indium Trichloride. J. Org. Chem. 2001, 66,
7741−7744. (d) Mayr, H.; Dogan, B. Selectivities in Ionic Reductions
of Alcohols and Ketones with Triethylsilane/Trifluoroacetic Acid.
Tetrahedron Lett. 1997, 38, 1013−1016. (e) Adlington, M. G.;
Orfanopoulos, M.; Fry, J. L. A Convenient One-Step Synthesis of
Hydrocarbons from Alcohols through Use of the Organosilane-Boron
Trifluoride Reducing System. Tetrahedron Lett. 1976, 17, 2955−2958.
(5) (a) Hong, Y.-K.; Lee, D.-W.; Eom, H.-J.; Lee, K.-Y. The
Catalytic Activity of Pd/WOx/γ-Al2O3 for Hydrodeoxygenation of
Guaiacol. Appl. Catal., B 2014, 150−151, 438−445. (b) de Souza, P.
M.; Rabelo-Neto, R. C.; Borges, L. E. P.; Jacobs, G.; Davis, B. H.;
Sooknoi, T.; Resasco, D. E.; Noronha, F. B. Role of Keto
Intermediates in the Hydrodeoxygenation of Phenol over Pd on
Oxophilic Supports. ACS Catal. 2015, 5, 1318−1329. (c) Martin, A.;
Armbruster, U.; Gandarias, I.; Arias, P. L. Glycerol Hydrogenolysis
into Propanediols Using in Situ Generated Hydrogen − A Critical
Review. Eur. J. Lipid Sci. Technol. 2013, 115, 9−27. (e) Murru, S.;
Nicholas, K. M.; Srivastava, R. S. Ruthenium (II) Sulfoxides-Catalyzed
Hydrogenolysis of Glycols and Epoxides. J. Mol. Catal. A: Chem. 2012,
363−364, 460−464. (f) Schlaf, M.; Ghosh, P.; Fagan, P. J.;
Hauptman, E.; Bullock, R. M. Catalytic Deoxygenation of 1,2-
Propanediol to Give n-Propanol. Adv. Synth. Catal. 2009, 351, 789−
800. (d) Tomishige, K.; Nakagawa, Y.; Tamura, M. Selective
Hydrogenolysis of C−O Bonds Using the Interaction of the Catalyst
Surface and OH Groups. Top. Curr. Chem. 2014, 353, 127−162.
(6) Sergeev, A. G.; Webb, J. D.; Hartwig, J. F. A Heterogeneous
Nickel Catalyst for the Hydrogenolysis of Aryl Ethers without Arene
Hydrogenation. J. Am. Chem. Soc. 2012, 134, 20226−20229.
(15) Chapman, G.; Nicholas, K. M. Vanadium-Catalyzed Deoxy-
dehydration of Glycols. Chem. Commun. 2013, 49, 8199.
(16) (a) Dethlefsen, J. R.; Lupp, D.; Teshome, A.; Nielsen, L. B.;
Fristrup, P. Molybdenum-Catalyzed Conversion of Diols and
Biomass-Derived Polyols to Alkenes Using Isopropyl Alcohol as
Reductant and Solvent. ACS Catal. 2015, 5, 3638−3647. (b) Larsen,
D. B.; Petersen, A. R.; Dethlefsen, J. R.; Teshome, A.; Fristrup, P.
Mechanistic Investigation of Molybdate-Catalysed Transfer Hydro-
deoxygenation. Chem. - Eur. J. 2016, 22, 16621−16631.
(17) (a) Gable, K. P.; Zhuravlev, F. A. Kinetic Isotope Effects in
Cycloreversion of Rhenium (V) Diolates. J. Am. Chem. Soc. 2002, 124,
̂
3970−3979. (b) Nunes, D.; Domingos, A.; Paulo, A.; Patrício, L.;
Santos, I.; Fermanda, M.; Carvalho, N. N.; Pombeiro, A. J. L.
Synthesis, Characterization and Study of the Redox Properties of
Rhenium (V) Diolates. Inorg. Chim. Acta 1998, 271, 65−74.
(18) (b) Zhu, Z.; Espenson, J. H. Organic Reactions Catalyzed by
Methylrhenium Trioxide: Dehydration, Amination, and Disproportio-
nation of Alcohols. J. Org. Chem. 1996, 61, 324−328. (c) Korstanje,
T. J.; Jastrzebski, J. T. B. H.; Klein-Gebbink, R. J. M. Mechanistic
Insights into the Rhenium-Catalyzed Alcohol-to-Olefin Dehydration
Reaction. Chem. - Eur. J. 2013, 19, 13224−13234.
(19) Kasner, G. R.; Boucher-Jacobs, C.; McClain, J. M.; Nicholas, K.
M. Oxo-Rhenium Catalyzed Reductive Coupling and Deoxygenation
of Alcohols. Chem. Commun. 2016, 52, 7257−7260.
(20) Steffensmeier, E.; Nicholas, K. M. Oxidation−reductive
Coupling of Alcohols Catalyzed by Oxo-Vanadium Complexes.
Chem. Commun. 2018, 54, 790−793.
́
́
(21) Dieguez, H. R.; Lopez, A.; Domingo, V.; Arteaga, J. F.; Dobado,
J. A.; Herrador, M. M.; Quílez del Moral, J. F.; Barrero, A. F.
Weakening C−O Bonds: Ti(III), a New Reagent for Alcohol
Deoxygenation and Carbonyl Coupling Olefination. J. Am. Chem.
Soc. 2010, 132, 254−259.
(22) Crevier, T. J.; Mayer, J. M. C−O Bond Homolysis in a
Tungsten Alkoxide: The Mechanism of Alcohol Deoxygenation by
WCl2(PMe3)4 and WH2Cl2(PMe3)4. J. Am. Chem. Soc. 1997, 119,
8485−8491.
(23) Wang, H.; Li, L.; Bai, X.-F.; Shang, J.-Y.; Yang, K.-F.; Xu, L.-W.
Efficient Palladium-Catalyzed C-O Hydrogenolysis of Benzylic
Alcohols and Aromatic Ketones with Polymethylhydrosiloxane. Adv.
Synth. Catal. 2013, 355, 341−347.
(7) Kusumoto, S.; Nozaki, K. Direct and Selective Hydrogenolysis of
Arenols and Aryl Methyl Ethers. Nat. Commun. 2015, 6, 6296.
(8) (a) Boucher-Jacobs, C.; Nicholas, K. M. Deoxydehydration of
Polyols. Top. Curr. Chem. 2014, 353, 163−184. (b) Petersen, A. R.;
Fristrup, P. New Motifs in Deoxydehydration: Beyond the Realms of
Rhenium. Chem. - Eur. J. 2017, 23, 10235−10243. (c) Mao, G.; Jia,
B.; Wang, C. Recent Progress in Re-Catalyzed Dehydroxylation
Reactions. Youji Huaxue 2015, 35, 284−293.
(24) (a) van Tamelen, E. E.; Schwartz, M. A. Reductive Coupling of
Alcohols to Hydrocarbons. J. Am. Chem. Soc. 1965, 87, 3277−3278.
(b) McMurry, J. E.; Silvestri, M. A Simplified Method for the
Titanium(II)-Induced Coupling of Allylic and Benzylic Alcohols. J.
Org. Chem. 1975, 40, 2687−2688. (c) Isakov, V. E.; Kulinkovich, O.
G. The Head-to-Head ReductiveCoupling of Homoallylic Alcohols
Promoted by Titanium(II)-Olefin Complexes. Synlett 2003, 967−970.
(d) Karunakar, G. V.; Periasamy, M. A Simple Method for the
(9) Cook, G. K.; Andrews, M. A. Toward Nonoxidative Routes to
Oxygenated Organics: Stereospecific Deoxydehydration of Diols and
Polyols to Alkenes and Allylic Alcohols Catalyzed by the Metal Oxo
Complex (C5Me5)ReO3. J. Am. Chem. Soc. 1996, 118, 9448−9449.
(10) (a) Ziegler, J. E.; Zdilla, M. J.; Evans, A. J.; Abu-Omar, M. M.
H2-Driven Deoxygenation of Epoxides and Diols to Alkenes
Catalyzed by Methyltrioxorhenium. Inorg. Chem. 2009, 48, 9998−
10000. (b) Gopaladasu, T. V.; Nicholas, K. M. Carbon Monoxide and
K
DOI: 10.1021/acs.organomet.8b00285
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Conversion of Propargyl Alcohols to Symmetrical 1,5-Diynes Using
Low Valent Titanium Reagents. Tetrahedron Lett. 2006, 47, 3549−
3552.
(33) Ku
̈
hl, O., Ed. The Range of Chemical Shifts, Coupling
Constants, and What Influences Each. In Phosphorus-31 NMR
Spectroscopy; Springer: Berlin Heidelberg, 2008; pp 7−23.
(34) (a) Dean, P. A. W.; Phillips, D. D.; Polensek, L. A 31P Nmr
Spectroscopic Study of Complexation of Tin(II) and Lead(II) by
Some Phosphines, Phosphine Oxides, and Related Ligands, with the
31P Nuclear Magnetic Resonance Spectra of Two Tetratertiary
Phosphine Tetraoxides and the Analogous Tetra-Sulfides An. Can. J.
Chem. 1981, 59, 50−61. (b) Hursthouse, M. B.; Levason, W.;
Ratnani, R.; Reid, G. Synthesis and Spectroscopic Properties of
Mo(VI) Complexes with Phosphine Oxide Ligands and the Crystal
Structures of [MoO2X2(OPMe3)2] (X = Cl or Br) and [MoO2Br2{o-
C6H4(P(O)Ph2)2}] · 2CH2Cl2. Polyhedron 2004, 23, 1915−1921.
(35) (a) Saucedo Anaya, S. A.; Hagenbach, A.; Abram, U. Reactions
of Phenylbis(2-Pyridyl)Phosphine with Oxorhenium(v) Complexes.
Inorg. Chem. Commun. 2009, 12, 128−130. (b) Machura, B.;
(25) Nishino, T.; Nishiyama, Y.; Sonoda, N. Deoxygenative
Dimerization of Benzylic and Allylic Alcohols, and Their Ethers and
Esters Using Lanthanum Metal and Chlorotrimethylsilane in the
Presence of a Catalytic Amount of Iodine and Copper(I) Iodide. Bull.
Chem. Soc. Jpn. 2003, 76, 635−641.
(26) Nishino, T.; Nishiyama, Y.; Sonoda, N. Lanthanum Metal-
Assisted Deoxygenative Coupling of Alcohols. Tetrahedron Lett. 2002,
43, 3689−3691.
́
̈
(27) Gabriels, D.; Hernandez, W. Y.; Sels, B.; Van Der Voort, P.;
Verberckmoes, A. Review of Catalytic Systems and Thermodynamics
for the Guerbet Condensation Reaction and Challenges for Biomass
Valorization. Catal. Sci. Technol. 2015, 5, 3876−3902. (b) Aitchison,
H.; Wingad, R. L.; Wass, D. F. Homogeneous Ethanol to Butanol
Catalysis - Guerbet Renewed. ACS Catal. 2016, 6, 7125−7132.
(c) Kozlowski, J. T.; Davis, R. J. Heterogeneous Catalysts for the
Guerbet Coupling of Alcohols. ACS Catal. 2013, 3, 1588−1600.
(28) Luo, Y.-R. Handbook of Bond Dissociation Energies in Organic
Compounds; CRC Press, 2003.
́
Jankowska, A.; Kruszynski, R.; Kłak, J.; Mrozinski, J. Structural and
Spectroscopic Studies on Rhenium(III) Diphenyl(2-Pyridyl)-
Phosphine Oxide Complexes. Polyhedron 2006, 25, 2663−2672.
(36) Liu, S.; Senocak, A.; Smeltz, J. L.; Yang, L.; Wegenhart, B.; Yi,
̈
J.; Kenttamaa, H. I.; Ison, E. A.; Abu-Omar, M. M. Mechanism of
MTO-Catalyzed Deoxydehydration of Diols to Alkenes Using
Sacrificial Alcohols. Organometallics 2013, 32, 3210−3219.
(29) (a) Bellemin-Laponnaz, S.; Gisie, H.; Le Ny, J. P.; Osborn, J. A.
Mechanistic Insights into the Very Efficient [ReO3OSiR3]-Catalyzed
Isomerization of Allyl Alcohols. Angew. Chem., Int. Ed. Engl. 1997, 36,
976−978. (b) Herrmann, A. T.; Saito, T.; Stivala, C. E.; Tom, J.;
Zakarian, A. Regio- and Stereocontrol in Rhenium-Catalyzed
Transposition of Allylic Alcohols. J. Am. Chem. Soc. 2010, 132,
5962−5963. (c) Jacob, J.; Espenson, J. H.; Jensen, J. H.; Gordon, M.
S. 1,3-Transposition of Allylic Alcohols Catalyzed by Methyltrioxo-
rhenium. Organometallics 1998, 17, 1835−1840.
(30) Ciani, G. F.; D’Alfonso, G.; Romiti, P. F.; Sironi, A.; Freni, M.
Rhenium(V) Oxide Complexes. Crystal and Molecular Structures of
the Compounds trans-ReI2O(OR)(PPh3)2 (R = Et, Me) and of Their
Hydrolysis Derivative ReIO2(PPh3)2. Inorg. Chim. Acta 1983, 72, 29−
37.
(31) (a) Shcheglov, P. A.; Drobot, D. V. Rhenium Alkoxides. Russ.
Chem. Bull. 2005, 54, 2247−2258. (b) DuMez, D. D.; Mayer, J. M.
Rhenium(V) Oxo-Alkoxide Complexes: Syntheses and Oxidation to
Aldehydes. Inorg. Chem. 1995, 34, 6396−6401. (c) Paulo, A.;
Domingos, A.; Marcalo, J.; Pires de Matos, A.; Santos, I. Reactivity
of a Tetrakis(Pyrazolyl)Borate Oxorhenium Complex. Inorg. Chem.
1995, 34, 2113−2120. (d) Seisenbaeva, G. A.; Shevelkov, A. V.;
Tegenfeldt, J.; Kloo, L.; Drobot, D. V.; Kessler, V. G. Homo- and
Hetero-Metallic Rhenium Oxomethoxide Complexes with a M4(μ-
O)2(μ-OMe)4 Planar Corea New Family of Metal Alkoxides
Displaying a Peculiar Structural Disorder. Preparation and X-Ray
Single Crystal Study. J. Chem. Soc. Dalt. Trans. 2001, 2762−2768.
(e) Erikson, T. K. G.; Bryan, J. C.; Mayer, J. M. Low-Valent Oxo
Compounds. 5. Low-Valent Rhenium Oxo Alkoxide Complexes.
Synthesis, Characterization, Structure, and Ligand Exchange and
Carbon Monoxide Insertion Reactions. Organometallics 1988, 7,
1930−1938. (f) Matano, Y.; Northcutt, T. O.; Brugman, J.; Bennett,
B. K.; Lovell, S.; Mayer, J. M. Tp* Rhenium(V) Oxo-Halide,
-Hydride, -Alkyl, -Phenyl, and -Alkoxide Complexes: Syntheses and
Oxidations. Organometallics 2000, 19, 2781−2790.
(32) (a) Casey, C. P.; Guan, H. Cyclopentadienone Iron Alcohol
Complexes: Synthesis, Reactivity, and Implications for the Mechanism
of Iron-Catalyzed Hydrogenation of Aldehydes. J. Am. Chem. Soc.
2009, 131 (7), 2499−2507. (b) Appelt, A.; Ariaratnam, V.; Willis, A.
C.; Wild, S. B. Alcohol Complexes of Platinum(II). Syntheses and
Crystal Structures of [S-(R*,R*)]-(+)589-[Pt(OSO2CF3)2[1,2-
C6H4(PMePh)2] and [S-(R*,R*)]-(+)589-[Pt(HOCH2CH2OH)
[1,2-C6H4(PMePh)2](CF3SO3)2. Tetrahedron: Asymmetry 1990, 1,
9−12. (c) Bakhmutov, V. I.; Vorontsov, E. V.; Antonov, D. Y. NMR
Evidence for Formation of New Alcohol Rhenium Complexes as
Intermediates in Ionic Hydrogenations of Carbonyl Groups with
Systems Composed of ReH2(NO)(CO)(PR3)2 (R = Pr-i, CH3, OPr-
i) and CF3COOH. Inorg. Chim. Acta 1998, 278, 122−126.
(37) (a) Newcomb, M. Radical Kinetics and Clocks. In Encyclopedia
of Radicals in Chemistry, Biology and Materials; John Wiley & Sons,
Ltd: Chichester, U.K., 2012. (b) Griller, D.; Ingold, K. U. Free-
Radical Clocks. Acc. Chem. Res. 1980, 13, 317−323.
(38) (a) Reed, M. C.; Lieb, A.; Nijhout, H. F. The Biological
Significance of Substrate Inhibition: A Mechanism with Diverse
Functions. In BioEssays; Wiley-Blackwell, 2010; pp 422−429.
(b) Jones, L. Substrate Inhibition. http://what-when-how.com/
molecular-biology/substrate-inhibition-molecular-biology/.
(39) (a) Shearer, J. Insight into the Structure and Mechanism of
Nickel-Containing Superoxide Dismutase Derived from Peptide-
Based Mimics. Acc. Chem. Res. 2014, 47, 2332−2341. (b) Boone, C.
D.; Pinard, M.; McKenna, R.; Silverman, D. Catalytic Mechanism of
α-Class Carbonic Anhydrases: CO2 Hydration and Proton Transfer.
Subcell. Biochem. 2014, 75, 31−52. (c) Cook, P.; Cleland, W. W.
Enzyme Kinetics and Mechanism; Garland Science, 2007.
(40) Walker, B. J., Hobbs, J. B., Walker, B. J., Eds.Phosphines Oxides
and Related Compounds. In Organophosphorus Chemistry; Royal
Society of Chemistry: Cambridge, 1989; pp 72−86.
(41) Chung, L. W.; Lee, H. G.; Lin, Z.; Wu, Y.-D. Computational
Study on the Reaction Mechanism of Hydrosilylation of Carbonyls
Catalyzed by High-Valent Rhenium(V)−Di-Oxo Complexes. J. Org.
Chem. 2006, 71, 6000−6009.
(42) (a) Zhang, S.; Katz, A.; Gates, B. C.; Dixon, D. A. Structures,
Relative Energies, and Ligand Dissociation Energies of Iridium
Carbonyl Phosphine Clusters. Comput. Theor. Chem. 2015, 1069, 18−
35. (b) Chen, M.; Craciun, R.; Hoffman, N.; Dixon, D. A. Ligand
Bond Energies in Cis- and Trans-[LPd(PH3)2Cl]+ Complexes from
Coupled Cluster Theory (CCSD(T)) and Density Functional
Theory. Inorg. Chem. 2012, 51, 13195−13203.
(43) Wang, Y.; Jin, X.; Yu, H. S.; Truhlar, D. G.; He, X. Revised
M06-L Functional for Improved Accuracy on Chemical Reaction
Barrier Heights, Noncovalent Interactions, and Solid-State Physics.
Proc. Natl. Acad. Sci. U. S. A. 2017, 114, 8487−8492.
(44) (a) Erikson, T. K. G.; Bryan, J. C.; Mayer, J. M. Low-Valent
Oxo Compounds. 5. Low-Valent Rhenium Oxo Alkoxide Complexes.
Synthesis, Characterization, Structure, and Ligand Exchange and
Carbon Monoxide Insertion Reactions. Organometallics 1988, 7,
1930−1938. (b) Tang, H.; Brothers, E. N.; Hall, M. B. The
Distinctive Electronic Structures of Rhenium Tris(Thiolate) Com-
plexes, an Unexpected Contrast to the Valence Isoelectronic
Ruthenium Tris(Thiolate) Complexes. Inorg. Chem. 2017, 56, 583−
593. (c) Palion-Gazda, J.; Gryca, I.; Machura, B.; Lloret, F.; Julve, M.
Synthesis, Crystal Structure and Magnetic Properties of the Complex
[ReCl3(Tppz)]·MeCN. RSC Adv. 2015, 5, 101616−101622.
́
(d) MacHura, B.; Wolff, M.; Gryca, I.; Mrozinski, J. Two Novel
L
DOI: 10.1021/acs.organomet.8b00285
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Rhenium Complexes Derived from [ReO(OMe)Cl2(dpphen)] -
Synthesis, Crystal Structure, Spectroscopic and Magnetic Properties.
Polyhedron 2011, 30, 354−363. (e) Machura, B.; Kruszynski, R.;
(56) ACD/Structure Elucidator; Advanced Chemistry Development,
Inc., 2014.
(57) Becke, A. D. Density-Functional Thermochemistry. III. The
Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648−5652.
(58) Zhao, Y.; Truhlar, D. G. The M06 Suite of Density Functionals
for Main Group Thermochemistry, Thermochemical Kinetics,
Noncovalent Interactions, Excited States, and Transition Elements:
Two New Functionals and Systematic Testing of Four M06-Class
Functionals and 12 Other Functionals. Theor. Chem. Acc. 2008, 120,
215−241.
(59) Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. Universal
Solvation Model Based on Solute Electron Density and on a
Continuum Model of the Solvent Defined by the Bulk Dielectric
Constant and Atomic Surface Tensions. J. Phys. Chem. B 2009, 113,
6378−6396.
́
Mrozinski, J.; Kusz, J. Synthesis, Spectroscopic Characterization, X-
Ray Structure and DFT Calculations of Rhenium(III) Complex with
1-Isoquinolinyl Phenyl Ketone. Polyhedron 2008, 27, 1739−1746.
(45) (a) Machura, B.; Wolff, M.; Gryca, I.; Kruszynski, R. Syntheses,
Structures, Spectroscopic Properties and DFT Calculations of Re(V)-
Benzothiazole and 2-(2-Aminophenyl)Benzothiazole Complexes.
Polyhedron 2012, 40, 93−104. (b) Machura, B.; Wolff, M.;
́
́
Switlicka, A.; Kruszynski, R.; Mrozinski, J. Synthesis, X-Ray Studies,
Spectroscopic Investigation, and DFT Calculations of [ReBr3(dppt)-
(OPPh3)]. Struct. Chem. 2010, 21, 761−769. (c) Machura, B.;
́
Swietlicka, A.; Wolff, M.; Kruszynski, R. Novel Rhenium Oxocom-
plexes of Indazole-3-Carboxylic Acid - Synthesis, X-Ray Studies,
Spectroscopic Characterization and DFT Calculations. Polyhedron
2010, 29, 2061−2069. (d) MacHura, B.; Kusz, J.; Tabak, D.
Synthesis, Spectroscopic Characterization, X-Ray Structure, and
DFT Calculations of [ReOBr2(Hmquin-7-COOH)(AsPh3)]. Struct.
Chem. 2009, 20, 361−368.
́
́
(46) (a) Kail, B. W.; Perez, L. M.; Zaric, S. D.; Millar, A. J.; Young,
C. G.; Hall, M. B.; Basu, P. Mechanistic Investigation of the Oxygen-
Atom-Transfer Reactivity of Dioxo-Molybdenum(VI) Complexes.
Chem. - Eur. J. 2006, 12, 7501−7509. (b) Millar, A. J.; Doonan, C. J.;
Smith, P. D.; Nemykin, V. N.; Basu, P.; Young, C. G. Oxygen Atom
Transfer in Models for Molybdenum Enzymes: Isolation and
Structural, Spectroscopic, and Computational Studies of Intermedi-
ates in Oxygen Atom Transfer from Molybdenum(VI) to
Phosphorus(III). Chem. - Eur. J. 2005, 11, 3255−3267.
(47) Li, X.-H.; Yin, G.-X.; Zhang, X.-Z. Theoretical studies on bond
dissociation energies for some aliphatic alcohol compounds by density
functional theory and cbs-q method. J. Theor. Comput. Chem. 2011,
10, 179−189.
́
̀
(48) Walton, J. C. Homolytic Substitution: A Molecular Menage a
Trois. Acc. Chem. Res. 1998, 31, 99−107.
(49) (a) Sugimoto, H.; Sasaki, Y. Oxorhenium(V) Complex of
Amidophenolate: Structure and Redox Characteristics. Chem. Lett.
1998, 27, 197−198. (b) Bennett, B. K.; Crevier, T. J.; DuMez, D. D.;
Matano, Y.; McNeil, W. S.; Mayer, J. M. Electrochemical and
Reactivity Comparisons among Isoelectronic Oxo, Imido, and Nitrido
Complexes of Rhenium and Osmium. J. Organomet. Chem. 1999, 591,
96−103.
(50) Mayer, J. M. Not Solely a Spectator: Exploring the Chemistry
of the CO Bond in Alkoxide and Related Ligands. Polyhedron 1995,
14, 3273−3292.
(51) Peters, J. C.; Johnson, A. R.; Odom, A. L.; Wanandi, P. W.;
Davis, W. M.; Cummins, C. C. Assembly of Molybdenum/Titanium
μ-Oxo Complexes via Radical Alkoxide C−O Cleavage. J. Am. Chem.
Soc. 1996, 118, 10175−10188.
́
(52) Prieto, C.; Gonzalez Delgado, J. A.; Arteaga, J. F.; Jaraíz, M.;
́
́
Lopez-Perez, J. L.; Barrero, A. F. Homocoupling versus Reduction of
Radicals: An Experimental and Theoretical Study of Ti(III)-Mediated
Deoxygenation of Activated Alcohols. Org. Biomol. Chem. 2015, 13,
3462−3469.
(53) (a) Niederberger, M.; Garnweitner, G. Organic Reaction
Pathways in the Nonaqueous Synthesis of Metal Oxide Nanoparticles.
Chem. - Eur. J. 2006, 12, 7282−7302. (b) Chandran, K.; Kamruddin,
M.; Muralidaran, P.; Ganesan, V. Thermal Decomposition of Sodium
Propoxides. J. Therm. Anal. Calorim. 2013, 112, 63−71. (c) Rupich,
M. W.; Liu, Y. P.; Ibechem, J.; Hachey, J. P. Synthesis of
Superconductors from Soluble Metal Oxo Alkoxide Precursors. J.
Mater. Res. 1993, 8, 1487−1496.
(54) (a) Huang, R. L.; Lee, H. H.; Malhotra, M. S. Abstraction of
benzylic hydrogen atoms from dibenzyl ethers by benzyl radicals. J.
Chem. Soc. 1964, 5951−6. (b) Bass, K. C. Abstraction by benzyl
radicals. Nature 1964, 201, 700−1.
(55) MestReNova Software, V10.0.2−15465; Mestrelab Research,
S.L.: Santiago de Compostela, Spain, 2015.
M
DOI: 10.1021/acs.organomet.8b00285
Organometallics XXXX, XXX, XXX−XXX