Influence of the pendant arm in deoxydehydration catalyzed by dioxomolybdenum complexes supported by amine bisphenolate ligands

Article abstract of DOI:10.1039/d0nj02151b

Dioxomolybdenum complexes supported by aminebisphenolate ligands were evaluated for their potential in catalyzing the deoxydehydration (DODH) reaction to establish structure-activity relationships. The nature of the pendant arm in these aminebisphenolate ligands was found to be crucial in determining reactivity in the deoxydehydration of styrene glycol (1-phenyl-1,2-ethanediol) to styrene. Pendant arms bearing strongly coordinating N-based groups such as pyridyl or amino substituents were found to hinder activity while those bearing non-coordinating pendant arms (benzyl) or even weakly coordinating groups (an ether) resulted in up to 6 fold enhancement in catalytic activity. A dioxomolybdenum complex featuring an aminemonophenolate ligand derived from the aminebisphenolate skeleton also resulted in similar yield enhancements. Although aromatic solvents were found to be ideal for performing these catalytic reactions, polar solvents such as N,N-dimethylformamide (DMF) and N,N′-dimethylpropyleneurea (DMPU) were also suitable. The catalyst was found to maintain its structural integrity under the optimized conditions and could be recycled for a second catalytic run without loss of activity. With the activated substrate meso-hydrobenzoin, trans-stilbene was obtained in a 56% yield at 220 °C along with benzaldehyde (71%) suggesting that the diol is a competing reductant under these conditions. This journal is

Full text of DOI:10.1039/d0nj02151b

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Influence of the pendant arm in deoxydehydration
catalyzed by dioxomolybdenum complexes
supported by amine bisphenolate ligands†
Cite this:DOI: 10.1039/d0nj02151b
Timothy C. Siu, ‡ Israel Silva Jr, ‡ Maiko J. Lunn
and Alex John
*
Dioxomolybdenum complexes supported by aminebisphenolate ligands were evaluated for their
potential in catalyzing the deoxydehydration (DODH) reaction to establish structure–activity relation-
ships. The nature of the pendant arm in these aminebisphenolate ligands was found to be crucial in
determining reactivity in the deoxydehydration of styrene glycol (1-phenyl-1,2-ethanediol) to styrene.
Pendant arms bearing strongly coordinating N-based groups such as pyridyl or amino substituents were
found to hinder activity while those bearing non-coordinating pendant arms (benzyl) or even weakly
coordinating groups (an ether) resulted in up to 6 fold enhancement in catalytic activity. A dioxo-
molybdenum complex featuring an aminemonophenolate ligand derived from the aminebisphenolate
skeleton also resulted in similar yield enhancements. Although aromatic solvents were found to be ideal
for performing these catalytic reactions, polar solvents such as N,N-dimethylformamide (DMF) and
N,N0-dimethylpropyleneurea (DMPU) were also suitable. The catalyst was found to maintain its structural
integrity under the optimized conditions and could be recycled for a second catalytic run without loss
of activity. With the activated substrate meso-hydrobenzoin, trans-stilbene was obtained in a 56% yield
at 220 1C along with benzaldehyde (71%) suggesting that the diol is a competing reductant under these
conditions.
Received 29th April 2020,
Accepted 29th May 2020
DOI: 10.1039/d0nj02151b
rsc.li/njc
Introduction
Deoxygenation of biomass is gaining momentum as a process
to produce, and complement bulk chemicals currently manu-
factured by petrochemical industries.1 The highly oxygenated
nature of biomass has proven to be an obstacle when attempting
to reduce these molecules into platform chemicals. This challenge
Scheme 1 Deoxydehydration of vicinal diol to an olefin.
in reduction reactions has prompted chemists to develop new alkene yields well over 90%.5 Cook and Andrews documented
methods in the removal of oxygen from biomass.2 In this respect, early findings of the deoxydehydration reaction with the use
the deoxydehydration (DODH) reaction has proven to be a favor- of Cp*ReO3 for the conversion of styrene glycol to styrene
able reaction for the production of olefins from vicinal diols, a (Fig. 1(a)).6 Other research groups have studied heterogeneous
functionality abundantly present in lignocellulosic biomass rhenium catalysts that are often coupled with precious metals
(Scheme 1).3 The reaction typically employs high oxidation to act as promoters.7 Alternatively, researchers have turned to
state oxometal complexes as catalysts and a reducing agent in vanadium and molybdenum as an economically viable option
an aromatic or alcoholic solvent.4
for catalysts.8 However, these readily available catalysts were
The most effective catalysts reported thus far for the deoxy- found to have relatively low DODH activity when compared
dehydration reaction are rhenium-based that have achieved to rhenium-based catalysts. In order to increase reactivity,
complexes where the metal is supported by different ancillary
Chemistry and Biochemistry Department, California State Polytechnic University,
Pomona, 3801 West Temple Ave., Pomona, CA 91768, USA. E-mail: ajohn@cpp.edu
† Electronic supplementary information (ESI) available: Characterization data for
new compounds, protocol for catalytic reactions, and NMR spectra for unknown
compounds. See DOI: 10.1039/d0nj02151b
ligands have been evaluated.8 The choice of reductant has been
demonstrated to be crucial in achieving effective catalysis using
molybdenum complexes.9
In the initial phases of using molybdenum complexes for
DODH catalysis, most reactions involved molybdate salts, though,
further research has been conducted on molybdenum complexes
‡ These authors contributed equally to the work, and should be regarded as
co-first authors.
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Fig. 1 DODH of styrene glycol catalyzed by selected metal complexes.
which feature supporting ligands.9 Molybdenum complexes eval-
Our research group is interested in developing catalytic
uated thus far for DODH reactivity are based on a wide range of processes based on molybdenum catalysts for converting
ancillary ligands. Acylpyrazolonate ligand based molybdenum vicinal diols to olefins. We recently reported on deoxydehydration
complexes were the first to be tested for their reactivity in DODH of glycols using catalytic amounts of ammonium heptamolybdate
reactions by Hills and coworkers in 2013 (Fig. 1(c)).9a These (AHM) in the presence of Na2SO3 as a benign reductant.11 Our
complexes were found to be moderately effective in reducing interest in evaluating the role of ancillary ligands in supporting
1-phenylethane-1,2-diol to styrene (10–13%) and cyclooctane-1,2- dioxomolybdenum centers in facilitating the deoxydehydration
diol to cyclooctene (55%) using a 2 mol% loading of the catalyst at reaction, prompted us to explore the use of amine bisphenolate
110 1C over 18 h. The higher yield obtained with an aliphatic diol ligands. These ligands have been used extensively in stabilizing
(1,2-octanediol) compared to the activated diol (1-phenylethane- metal centers from across the periodic table for various catalytic
1,2-diol) is notable. The Okuda group used bis(phenolate)molyb- applications.12 We were particularly attracted by the modular
denum complexes as catalysts for the DODH of anhydroerythritol synthesis of this class of ligands which allows incorporating subtle
at 200 1C employing 3-octanol as solvent and reductant.9d The changes to sterics and electronics as well as the nature of the
olefin product (2,5-dihydrofuran) was obtained in a 57% yield pendant arm. All of these features can be favorably tuned to study
with conventional heating while a 49% yield was obtained under the effect of the ligand backbone on catalytic activity, and thus
microwave irradiation using their best catalyst. In 2018, De Vos develop structure–activity relationships (SAR). Similar SARs have
and Stalpaert investigated the stabilizing effects of bulky been previously reported in olefin polymerization studies cata-
b-diketone ligands (Fig. 1(d) and Table S1, ESI†), and demon- lyzed using complexes stabilized by amine bisphenolate ligands.
strated that the bulky b-diketone 2,2,6,6-tetramethylheptane-3,5- Based on these studies, the presence of a pendant arm featuring
dione (4 equiv.) gave a significant increase in alkene yield (up to an amine donor in the ligands was suggested to yield thermally
93%) with several different substrates (at 170–200 1C) using stable and reactive complexes.13 Other studies have demonstrated
10 mol% of MoO2(acac)2.9f The addition of the bulky ligand a correlation between the M–X (where X is the donor atom in
was demonstrated to inhibit catalyst oligomerization under the the pendant arm) bond length in such complexes and olefin
reaction conditions, which was proposed to prevent catalyst polymerization activity.14 Dioxomolybdenum complexes based
precipitation and hence increase catalytic activity. More recently, on aminebisphenol ligands have previously found utility in
Kilyanek and Tran demonstrated the reactivity of a five-coordinate various applications including olefin epoxidation, and oxo-
dioxomolybdenum complex and its OPPh3 adduct (six-coordinate) transfer catalysis.15
in the DODH reaction (Fig. 1(e) and Table S1, ESI†).10 The five-
In this study, we have evaluated the catalytic promise of
coordinate complex (up to 46% alkene yield) was found to be dioxomolybdenum complexes supported by amine bisphenolate
marginally inferior to its OPPh3 adduct (up to 62% alkene yield) (NxOy; x = 1 or 2; y = 2 or 3) ligands in effecting the DODH
for several substrates using various reductants in temperature reaction (Fig. 1(f)). Catalytic reactions were conducted using
range from 150–190 1C.
styrene glycol as a model substrate at 170 1C for 24 hours.
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Catalytic activity was evaluated as a function of ligand features to yield of styrene was obtained (Fig. 2). This modest increase in
establish structure–activity relationships. Based on these studies, styrene yield with 2b suggested that the pyridyl pendant arm in
the nature of the pendant arm was found to be critical in complex 1b was perhaps impeding its catalytic activity. This is
determining catalytic potential of these complexes and yield presumably due to the stronger binding ability of the rigid
enhancements up to 6 times were observed.
pyridine N to the molybdenum center compared to the amino N
in complex 2b. Alternatively, the hemilabile nature of the
pendant arm in 2b could result from the sterically hindered
nature of the tertiary amine donor in the dialkylamino pendant
group. To further explore the potential of hemilabile pendant
Results and discussion
At the outset we synthesized a dioxomolybdenum complex 1b, arms in facilitating the deoxydehydration reaction, we synthesized
based on an aminebisphenolate ligand 1a featuring a pyridyl complex 3b featuring an ether group in the pendant arm. The O
pendant arm (Scheme 2). The molybdenum complex was donor in the pendant arm should be an even more weakly
synthesized in a low yield (26%) by the reaction of the ligand coordinating group given the higher electronegativity (lower basicity)
with MoO2(acac)2 in THF at 50 1C, and was characterized by of O compared to N. Quite significantly, this resulted in a styrene
1H & 13C{1H} NMR, IR spectroscopy and elemental analysis.
yield of 31% (Fig. 2) under the reaction conditions which is
Complex 1b was then evaluated for its catalytic potential in significantly higher compared to the yield (5%) observed with
the deoxydehydration of 1-phenyl-1,2-ethanediol (styrene diol/ catalyst 1b. Expanding on this result, when complex 4b featuring
glycol) using PPh3 as a reductant in toluene as a solvent at a non-coordinating benzyl arm was used in the deoxydehydration
170 1C; only a 5% yield of styrene was detected under these reaction, gratifyingly, styrene was also obtained in 34% yield (Fig. 2).
conditions (Fig. 2). Styrene yield was quantified by 1H NMR This result further confirms the importance of the coordinating
spectroscopy using 1,3,5-trimethoxybenzene as an internal nature of the pendant arm in controlling catalytic activity.
standard. Since a low styrene yield was obtained with catalyst
To understand the importance of the phenol arms in amine
1b we became curious about the feature in the amine bispheno- bisphenolate ligands in controlling the catalytic potential of
late ligand which was responsible for impeding catalytic activity. these molybdenum complexes, we synthesized an amine mono-
We sought to understand this undesirable effect of the ligand on phenol ligand (5a) through a reductive amination reaction
catalysis by making systematic changes to the ligand backbone (Scheme 2). The corresponding dioxomolybdenum complex 5b
and evaluating the resulting molybdenum complexes in the above was obtained as a m-oxo dimer under the reaction conditions.16
deoxydehydration reaction.
When the catalysis reaction was performed using 5 mol% 5b
To uncover the ligand feature which is responsible for the (10 mol% Mo since it is a dimeric complex), styrene was produced
low catalytic activity of complex 1b, we began by modifying the in 32% yield suggesting that removal of a phenol arm has the
pendant arm (Scheme 2). We hypothesized that the strength same effect as the transition from a strongly coordinating to a
of interaction between the pendant arm and the molybdenum hemilabile or weakly coordinating pendant arm (Fig. 2). It must
center could potentially influence catalytic activity. A strongly be noted that the dinuclear nature of the complex 5b could result
coordinating pendant arm could resist dissociation from the in a change in mechanism of the catalytic reaction as compared to
molybdenum center resulting in reduced catalytic activity while the mononuclear complexes 1b–4b. When the catalyst loading of
a hemilabile pendant arm would create a free coordination-site 5b was increased to 15 mol%, the reaction resulted in a modest
around the metal when required during the catalytic cycle. increase of styrene yield to 48%.
When complex 2b featuring a dialkylamino substituent (NMe2)
was evaluated in the deoxydehydration of styrene diol, a 9% (4b) in H NMR spectra of reactions at the end a catalytic run,
The detection of resonances characteristic of the catalyst
1
Scheme 2 Synthesis of ligands and molybdenum complexes used in this study.
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Fig. 2 Molybdenum catalysts supported over amine bisphenolate ligands used in this study.
Table 1 Solvent screen for DODH of styrene glycol using 4ba
prompted us to test the reuse of the catalyst. Hence, a second
equivalent of the substrate was added to the reaction flask after
the first 24 hour period (37% yield of styrene from first run)
and the catalytic reaction allowed to run for an additional
16–24 hours. The catalyst in solution was found to be active,
and an additional 44% of styrene was formed during the second
run. Attempts to further reuse the catalyst were unsuccessful. This
loss of activity after the second run could presumably be indica-
tive of the gradual formation of catalytically inactive (hetero-
geneous) molybdenum species during the catalysis.
Yield (%)
Entry
Solvent
CQCb
CQOe
1.
2.
3.
4.
5.
6.
7.d
8.
Mesitylene
Xylene
Toluene
DMF
DMPU
H2O
31
5
4
1
nd
13
0
1
5
30
34 (33)c
18
22
0
5
33
3-Octanol
Chlorobenzene
The choice of solvent is crucial to realizing the full potential
of DODH reaction in converting biobased vicinal diols to
corresponding alkenes. Previous studies using rhenium catalysts
used alcohols as solvent and sacrificial reductant for effecting
conversion of sugar alcohols.17 A screen was performed to identify
solvent compatibility for DODH catalyzed by complex 4b (Table 1).
Aromatic solvents including mesitylene, xylene, chlorobenzene,
and toluene were found to be equally effective (30–34%) in
a
Reaction conditions: styrene glycol (0.5 mmol), PPh3 (0.75 mmol) and
4b (10 mol%) were combined in the solvent (ca. 2.5 mL) and heated at
170 1C for 16–24 hours. DMF = N,N-dimethylformamide, DMPU = N,N0-
dimethylpropyleneurea. Yield of styrene was determined by 1H NMR
b
spectroscopy using 1,3,5-trimethoxybenzene as an internal standard.
c
Dry toluene was used as solvent, and reagents added inside a glove-
d
box. Using 3-octanol as solvent and sacrificial reductant; no PPh3
added. Yield of benzaldehyde.
e
promoting catalysis at 170 1C. A catalytic reaction carried out in (Table 1; CQO) for these solvents was in the range of 1–13%; the
dry toluene as solvent where reagents were loaded into the reactor highest yield was obtained in DMPU, with styrene and benzalde-
inside a glovebox did not offer any improvement, and produced hyde obtained in a 2 : 1 ratio. The benzaldehyde side-product is
styrene in 33% yield (Table 1, entry 3). Although polar solvents obtained by oxidative cleavage of styrene glycol and is indicative of
such as DMF and DMPU furnished styrene in comparatively lower the diol acting as a reductant under our catalytic conditions.9b,10
yields (B20%), these results are worth noting as further develop-
In efforts to improve the catalytic efficiency of 4b, we
ments in this direction may be imperative to realize the full explored the effect of temperature and catalyst loading on the
substrate scope of sugar-based polyols in the DODH reaction. DODH of styrene glycol. No significant enhancements in styrene
No reaction was observed when H2O was used as solvent. yield were noted in reactions performed at a higher temperature
Although alcohols have been used as a sacrificial reductant and (B190–200 1C) or at a higher catalyst loading (20% at 170 1C).
solvent in a variety of prior studies, a low yield of 5% styrene was As noted earlier, a marginal improvement in styrene yield (48%)
obtained when 3-octanol was employed under our conditions. was observed when a higher catalyst loading (15 mol%) of 5b was
Although higher temperatures (200 1C) have been typically used. However, styrene was formed in 36% yield even when the
employed in molybdenum catalyzed reactions involving alcohol catalysis was performed at 150 1C (Table 2, entry 1). We also
reductants,9d,g we did not explore such conditions using 3-octanol. explored the possibility of generating the active catalyst in situ
The yield of benzaldehyde formed under the reaction conditions under the catalytic conditions from the molybdenum precursor
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Table 2 Reductant and substrate scope for DODH using 4ba
reduction from the diol substrate under our catalytic conditions.
When meso-hydrobenzoin was used as the substrate at an elevated
temperature (220 1C), trans-stilbene was produced in a high yield
of 56% along with 71% of benzaldehyde (95% conversion).
Although inefficient at converting aliphatic diols, the yields of
styrene and stilbene obtained with catalyst 4b are comparable to
other reported molybdenum complexes (Table S1, ESI†).9a,f,10
Studies based on vanadium complexes exhibit relatively higher
catalytic activities for the conversion of styrene glycol.8c,d
In contrast, rhenium based catalytic systems developed so far
display much higher conversions and yields under relatively
milder reaction conditions. Generally, quantitative diol conver-
sion as well as high alkene yields (490%) have been achieved
starting from both styrene glycol and hydrobenzoin substrates
using various Re catalysts.5a,18f,19
Yield (%)
Entry
Substrate
Reductant
CQCb
CQOc
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.f
Styrene glycol
PPh3
34 (36)d
5
37
31
31
5
3
1
2
18
10
12
3-Octanol
Na2SO3
Carbon
Zn granules
PPh3
PPh3
PPh3
1,2-Decanediol
1,2-Cyclohexanediol
Diethyl tartrate
(R,R)-(+)-Hydrobenzoin
meso-Hydrobenzoin
18
PPh3
PPh3
42 (5)e
56 (4)e
68
71
a
Reaction conditions: styrene glycol (0.5 mmol), reductant (0.75 mmol)
and 4b (10 mol%) were combined in toluene (ca. 2.5 mL) and heated at
b
170 1C in an oil bath for 16–24 hours. Yield of styrene was determined
by 1H NMR spectroscopy using 1,3,5-trimethoxybenzene as an internal
c
d
standard. Yield of benzaldehyde. Reaction carried out at 150 1C.
e
The major product is trans-stilbene; yield of cis-stilbene in parenth-
eses. Reaction carried out at 220 1C in a heating block.
f
Conclusion
To summarize, we have explored the potential of dioxomolyb-
denum complexes supported over aminebisphenolate ligands
in catalyzing the deoxydehydration reaction. A series of amine-
bisphenol ligands were evaluated, and the coordinating ability
of the pendant arm was found to be central in determining
catalytic potential with weakly-coordinating or non-coordinating
arms exhibiting the highest catalytic activity. The removal of one
of the phenol arms also resulted in a similar yield enhancement.
The solution of catalyst 4b featuring a non-coordinating benzyl
pendant arm could be recycled for a second catalytic run without
loss of activity. Aromatic solvents were found to facilitate the
deoxydehydration reaction and inorganic reductants were found
to be as effective as PPh3. The detection of benzaldehyde in our
catalytic reactions is supportive of a competing reduction by the
diol substrate through an oxidative cleavage pathway. Although
the catalyst 4b was ineffective at catalyzing DODH of aliphatic
diols, higher yields were obtained with activated substrates such
as (R,R)-(+)-hydrobenzoin and meso-hydrobenzoin especially at
elevated temperatures. Mechanistic investigations are currently
underway in our laboratory to understand these ligand effects.
MoO2(acac)2 and stoichiometric equivalents of the ligand. Thus, a
catalytic reaction carried out using 10 mol% MoO2(acac)2 and
10 mol% of ligand 4a produced styrene in 30% yield. A higher
loading of the ligand (20 mol%) did not offer any improvements
under the standard reaction conditions.
Practical limitations to large-scale use of PPh3 as a reducing
agent in DODH reactions prompted us to evaluate alternate
reductants (Table 2, entries 1–5). Although a variety of reduc-
tant classes support rhenium catalyzed DODH reactions,18
alcohols represent the most explored category of reductants
in molybdenum catalyzed reactions apart from PPh3.9b,c,d However,
most reported studies based on alcohols as reducing agents in
DODH reactions catalyzed by molybdenum catalyzed reactions
were performed at temperatures around or above 200 1C. Only a
low yield of styrene (5%) was detected when 3-octanol was used the
reductant under our standard conditions at 170 1C. The inorganic
reductant Na2SO3 (37%) as well as elemental reductants C and Zn
(31% each) produced styrene in yields at par with PPh3. The
relatively inexpensive nature of these reductants as well as the
nature of their oxidized byproducts offer advantages from an
environmental as well as process standpoint compared to PPh3.
The inorganic reductants (Na2SO3, C and Zn) resulted in the
formation of higher amounts of benzaldehyde (10–18%) compared
to PPh3 (1%).
Experimental section
General procedures
A brief survey of the substrate scope using dioxomolybdenum All air and water sensitive manipulations were carried out under a
catalyst 4b was also undertaken (Table 2, entries 6–10). The nitrogen atmosphere by using standard Schlenk line techniques
catalyst was found to be ineffective for aliphatic diols such as or using an MBraun Labstar pro glovebox. All 1H, 13C{1H} and 31
P
1,2-decanediol and 1,2-cyclohexanediol producing the corres- NMR spectra were collected on a Varian 400-MR spectrometer.
1
ponding olefins in low yields (o10%) (Table 2, entries 6 & 7). Chemical shifts (d) for H NMR spectra were referenced to the
With the bio-derived substrate diethyltartarate, the corresponding residual protons on deuterated chloroform (7.26 ppm) and
olefin product diethyl fumarate was obtained in 18% yield. 13C{1H} NMR spectra were referenced to the residual chloroform
Starting from highly activated diol (R,R)-(+)-hydrobenzoin, trans- (77.1 ppm). Infra-red spectra were recorded on a Thermo Scien-
stilbene was obtained in a 42% yield along with cis-stilbene (5%) tific NICOLET iS10 Spectrophotometer equipped with a SMART
at a high diol conversion of 81%. Benzaldehyde, the product iTR. Gas Chromatography-Mass Spectrometric (GC-MS) analysis
resulting from oxidative cleavage of the diol substrate, was also was performed on an Agilent 6850 series GC system connected
produced in a 68% yield along with the olefin products. This to an Agilent 5973N Mass Selective Detector equipped with a
observation is again supportive of the possibility of competitive HP-5MS column (30 m ꢀ 0.25 mm ꢀ 0.25 mm). Elemental analysis
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was performed at Robertson Microlit Laboratories (New Jersey, then added to the reaction mixture and the solution turned
USA). Precursors for ligand and complex synthesis were used from clear to orange upon this addition. An orange precipitate
as received. CDCl3 was used as received from Sigma. Solvents is collected through gravity filtered and washed with cold
(methanol, toluene, and hexanes) were purchased from Fisher methanol (2.96 g, 72% yield). 1H NMR (DMSO-d6, 28 1C,
Scientific and used as received. All starting materials were 400 MHz) d 7.35–7.33 (m, 3H), 7.02 (m, 4H), 6.73 (s, 2H), 4.05
2
2
procured from commercial sources and used without further (d, 2H, JHH = 12 Hz), 3.96 (s, 3H), 3.23 (d, 2H, JHH = 12 Hz),
purification. The ligands (1a–5a) and complex 5b used in 2.25 (s, 6H), 1.38 (s, 18H). 13C{1H} NMR (CDCl3, 26 1C, 100
this study were synthesized by modifications of literature MHz) d 152.3, 137.6, 136.8, 129.6, 129.1, 129.0, 128.0, 128.0,
protocols.16,20
127.5, 122.1, 58.5, 56.6, 34.7, 29.6, 20.8. Selected IR (cmꢁ1) 928,
908. Anal. calcd for C31H19NO4Mo: C, 63.58%; H, 6.71%;
N, 2.39%. Found: C, 63.64%; H, 6.57%; N, 2.39%.
Synthesis of dioxomolybdenum complexes
Synthesis of 1b. To an oven dried round bottom flask was
Synthesis of 5b. To a round bottom flask was added
added ligand 1a (0.1 g, 0.217 mmol) and MoO2(acac)2 (0.071 g, MoO2(acac)2 (0.492 g, 1.50 mmol) and ligand (5a) (0.491 g,
0.217 mmol) in THF (ca. 6 mL). The round bottom flask was 1.50 mmol) in methanol (ca. 5 mL). The reaction was main-
stirred in an inert nitrogen atmosphere for 26 hours and gravity tained at 50 1C for 2 hours during which a bright orange
filtered to produce an orange precipitate (0.032 g, 26%). crystalline solid formed. The precipitate was then gravity
3
1H NMR (CDCl3, 400 MHz, 28 1C) d 9.16 (d, 1H, JHH = 8 Hz), filtered and washed with methanol to obtain the product as
7.55(t, 1H, 3JHH = 8), 7.16 (t, 1H, 3JHH = 8 Hz), 6.89 (s, 2H), 6.80 an orange solid (0.365 g, 26%). 1H NMR (CDCl3, 400 MHz,
(d, 1H, 3JHH = 8 Hz), 6.75 (s, 2H), 4.96 (d, 2H, 2JHH = 16 Hz), 3.95 28 1C) d, 8.69 (d, 1H, 3JHH = 4 Hz), 7.48 (t, 1H, 3JHH = 8 Hz), 6.99
2
3
(s, 2H), 3.68 (d, 2H, JHH = 12 Hz), 2.21 (s, 6H), 1.25 (s, 18H). (t, 1H, JHH = 8 Hz), 6.94 (s, 1H), 6.90 (d, 1H, 3JHH = 8 Hz), 6.83
13C{1H} NMR (CDCl3, 26 1C, 100 MHz) d 159.3, 155.2, 151.5, (s, 1H), 5.47 (m, 1H), 5.18–5.12 (m, 1H), 4.72 (d, 1H, JHH
=
2
139.3, 138.4, 128.7, 127.7, 127.6, 122.7, 122.1, 121. 1, 63.5, 58.9, 12 Hz), 3.91 (d, 2H, 2JHH = 16 Hz), 3.82 (d, 1H, 2JHH = 12 Hz) 1.22
34.6, 29.8, 20.8. Selected IR (cmꢁ1) 922, 898. Anal. calcd for (s, 9H), 1.17 (s, 9H).
C
30H38N2O4Mo: C, 61.43%; H, 6.53%; N, 4.78%. Found: C,
Representative procedure for deoxydehydration reactions
61.03%; H, 6.52%; N, 4.66%.
Synthesis of 2b. To a round bottom flask was added ligand
2a (0.257 g, 0.6 mmol) and methanol (ca. 2.5 mL). The resulting
mixture was heated to 50 1C before addition of MoO2(acac)2
(0.197 g, 0.6 mmol) and then maintained at 50 1C for 2 hours.
The mixture was then gravity filtered and methanol washed to
A pressure tube reactor was charged with diol (0.500 mmol),
reductant (0.750 mmol), molybdenum complex (0.050 mmol)
and 2.5 mL of solvent. The reactor tube was sealed and the reaction
mixture was stirred at 170 1C for 24 hours. The reactions were
cooled to room temperature before adding the internal standard
for analysis. 1,3,5-Trimethoxybenzene (0.010 g, 0.059 mmol) was
added after the 24 hours in heat as an internal standard. An aliquot
of the sample of the reaction mixture was analyzed by 1H NMR in
CDCl3. The integration of peaks corresponding to the internal
standard to diol and alkene product was used to determine yield
of alkene product and carbonyl compound.
1
yield bright yellow precipitate (0.330 g, 97%). H NMR (CDCl3,
2
400 MHz, 28 1C) d 7.05 (s, 2H), 6.79 (s, 2H), 4.08 (d, 2H, JHH
=
12 Hz), 3.92 (d, 2H, 2JHH = 12 Hz) 2.90 (s, 6H), 2.87–2.80 (m, 4H),
2.26 (s, 6H), 1.42 (s, 18H). 13C{1H} NMR (CDCl3, 26 1C,
100 MHz) d 161.3, 136.8, 129.3, 127.3, 127.2, 123.3, 60.9, 56.6,
53.5, 50.5, 34.7, 30.3, 30.2, 20.8. Selected IR (cmꢁ1) 914, 877.
Anal. calcd for C28H42N2O4Mo: C, 59.35%; H, 7.47%; N, 4.94%.
Found: C, 59.50%; H, 7.36%; N, 4.93%.
Conflicts of interest
Synthesis of 3b. To a round bottom flask, ligand 3a (1.00 g,
2.34 mmol) and methanol (ca. B10 mL) were added together
The authors declare no conflicts of interest.
and refluxed for about
3 hours. MoO2(acac)2 (0.762 g,
2.34 mmol) was then added to the reaction mixture and the
solution turned from clear to yellow upon this addition.
A yellow precipitate is collected through gravity filtration and
washed with cold methanol (1.10 g, 86% yield). 1H NMR
(CDCl3, 28 1C, 400 MHz) d 7.10 (s, 2H), 6.78 (s, 2H), 4.68
Acknowledgements
Funding for this work was provided by the US National Science
Foundation under award no. CHE-1800605 (A. J.). Experimental
work was carried out at Chemistry & Biochemistry Department,
College of Science at California State Polytechnic University,
Pomona. AJ would like to thank Prof. Francis X. Flores, Cal Poly
Pomona & Prof. Kenneth M. Nicholas, University of Oklahoma
for insightful discussions and comments.
2
2
(d, 2H, JHH = 16 Hz), 3.78 (s, 3H), 3.68 (d, 2H, JHH = 14 Hz),
3
3
3.30 (t, 2H, JHH = 8 Hz), 2.84 (t, 2H, JHH = 8 Hz), 2.28 (s, 6H),
1.42 (s, 18H). 13C{1H} NMR (CDCl3, 26 1C, 100 MHz) d 158.0,
138.3, 129.2, 128.0, 127.5, 122.4, 69.4, 63.6, 63.1, 52.7, 34.8,
29.9, 20.9. Selected IR (cmꢁ1) 929, 909. Anal. calcd for
C
27H39NO5Mo: C, 58.58%; H, 7.10%; N, 2.53%. Found: C,
58.42%; H, 7.02%; N, 2.51%.
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