A. Jefferson, R.S. Srivastava / Polyhedron 160 (2019) 268–271
269
2.2. Representative procedure for DODH reactions
OH
R'
R
LMOx
OH
R
Red
+
1-Phenyl-2-ethanediol (0.32 mmol, 44 mg), MTO (0.032 mmol,
8 mg), indoline (0.36 mmol, 34 L), toluene (1 mL), and dodecane
(5 L) were added to a thick-walled Ace glass tube reactor. The
+
H2O
Red-O
+
l
R'
l
reactor was placed in a heating mantle bath at 190 °C for 2.5 h
while stirring magnetically. After cooling to room temperature,
the reaction mixture was filtered and analyzed by GC–MS. For
NMR of Diprophylline (7-(2,3-Dihydroxypropyl)-3,7-dihydro-1,3-
dimethyl-1H-purine-2,6-dione), mucic acid (tetrahydroxyadipic
Scheme 1. General Deoxydehydration reaction of glycols.
and Lewis acids (Zn, Fe, Mn, C) [15] have been extensively
explored. In addition to oxorhenium compounds, oxo-molybde-
num and oxovanadium compounds have also been utilized as
DODH catalysts in combination with phosphines and sulfites as
reductants. Moreover, sacrificial alcohols as potential H-transfer
reductants have also received much attention [7,11,16–18].
In the search for a better hydrogen transfer reductant for metal-
catalyzed DODH reactions, we initiated a project for the develop-
ment of new DODH reaction procedures. The new DODH system
acid), and inositol, a 100
lL aliquot of reaction mixture was with-
drawn. To this mixture CDCl3 and 2
lL DMSO as internal standard
for NMR analysis was added. The product was identified and quan-
tified by 1H NMR spectroscopy and by comparison with an authen-
tic sample.
2.3. Isolation
must have
a practical reductant, which would generate H2
MeReO3 (28 mg, 0.1 mmol), (R,R)-(+)-hydrobenzoin (214 mg,
in situ. Hydroaromatics H-transfer reagents have been used as liq-
uid organic hydrogen carriers for hydrogenation reactions and for
hydrogen storage [19]. Various hydroaromatics, are abundantly
found in fossil resources. The catalytic activity of redox-active solid
metal oxides has been explored for the dehydrogenation of alkyl
aromatics [20]. This indicates the potential use of these reductants
in homogeneous oxo-metal-catalyzed reactions. Nonetheless, very
few hydroaromatics are known for their reactivity with soluble
oxo-metal species [21]. Hydroaromatics are visualized as hydrogen
carriers and could carry hydrogen safely to the reaction mixture.
Sacrificial alcohols including 1-butanol have been used as reduc-
tants in the biomass reduction. We report here the MTO catalyzed
DODH reactions with a small amount of indoline as reductant and
in the absence of 1-butanol (acts as a reducing agent) compared to
earlier reports [22], where two reductants, indoline and 1-butanol
were used. We revisited the chemistry of MeReO3 and developed a
modified process that requires a small amount (3.0 Â 10À4 mol)
one reductant (indoline) compared to earlier reports. In addition,
four different examples (1, 2, 3, 9, 10, Table 1), a comparative study
of solvent/catalyst and various catalyst loading compared to ref 22
were studied.
1 mmol), indoline (119 lL), and anhydrous toluene (5 mL) were
added to a thick-walled Ace glass reactor tube. The reactor was
placed in a heating mantle bath in the range of 190 °C for 24 h
while stirring magnetically. The reaction mixture was cooled and
filtered over silica gel. Solvents were removed using a rotary evap-
orator under reduced pressure. The product was separated by col-
umn chromatography using ethyl acetate/hexane eluent (1: 2.5).
Product yield was 121 mg, 67%.
3. Result and discussion
To test the viability of a dihydroaromatic compound as a reduc-
tant for the DODH of glycols, an exploratory reaction was carried
out with styrene diol, indoline and MTO (10 mol %) in anhydrous
toluene, which produced styrene quantitatively in 2.5 h at 190 °C.
We began the survey with several Re-catalysts, polar and nonpolar
solvents at various temperatures. Among the four-rhenium cata-
lysts examined, MTO was for superior than the others, which pro-
duces
a quantitative yield of alkene. NH4ReO4, Re2O7, and
Re2(CO)10 also exhibited modest activity and good selectivity to
styrene (Table 1, entries 1–4). Catalyst loading is one of the crucial
steps in the catalytic cycle. Consequently, to know the efficacy of
MTO, we examined MTO loading (10 mol%, 5 mol% and 2.5 mol%)
and found that 10 mol% yielded better result of alkene (Table 1,
run 4,11,12). The addition of 4 Å molecular sieve did not improve
product yield. The coordinating solvent such as THF and acetoni-
trile also produced significantly lower yield of styrene, probably
due to the coordination of solvent to Re (Table 1). Our next step
was to screen the effect of temperature and we found that 190 °C
was the optimum temperature for the model substrate for opti-
mum yield of alkene. We performed the reaction to test the effi-
2. Experimental
2.1. General information
All reactants were purchased commercially and used without
further purification. All solvents were ACS grade and were used
directly. GC–MS analyses were obtained on an Agilent instrument
using a Stabilwax capillary column.
cacy
of
hydroaromatic
reductants
such
as
1,2,3,4-
Table 1
Optimization.
tetrahydronaphthalene (tetralin) and indoline under identical
reaction conditions. The results demonstrated that indoline proved
to be a better hydrogen transfer reagent than 1,2,3,4-tetrahydron-
aphthalene for DODH of the model substrates. In addition to the
excellent yield of olefin produced by indoline, its co-product is
indole, which is easily detected by GC. The olefin to indole ratio
were found to be close to 1:1.
The results of the conversion of various diols to olefins are
shown in Table 2. The olefin formation in the DODH reaction is
regioselective since no other isomers were detected. The DODH
reaction of styrene diol at 150 °C and 170 °C produced 48% and
58% styrene respectively in 24 h. However, the similar reaction at
190 °C produced a quantitative yield of styrene in 2.5 h (Table 2,
entry 1). A long chain glycol, 1,2-tetradecanediol at optimized
Entry Catalyst
Solvent
Styrene (%) Time (h)
1
2
3
4
5
6
7
8
NH4ReO4
Re2O7
Re2(CO)10
Toluene
Toluene
Toluene
67
29, 49
9
24
2.5, 10
5
MeReO3 (MTO) 10 mol% Toluene
99
17
55
9
2.5
2.5
2.5
2.5
2.5
2.5
24
MTO, 10 mol%
MTO, 10 mol%
MTO, 10 mol%
MTO, 10 mol%
MTO, 10 mol%
MTO, 10 mol%
MTO 5 mol%
Benzene
THF
Acetonitrile
Dichloroethane 34
9
Indoline
Isopropanol
Toluene
33
7
10
11
12
67
35
2.5
2.5
MTO 2.5 mol%
Toluene