Ruthenium (II) sulfoxides-catalyzed hydrogenolysis of glycols and epoxides

Article abstract of DOI:10.1016/j.molcata.2012.07.025

New selective deoxygenation reactions are needed for the efficient conversion of biomass-derived oxygenates to useful chemicals, including fuels. A new catalytic system is reported here for the selective hydrogenolysis of glycols to hydrocarbons. We find that cis-[RuCl₂(sulfoxide) ₄] {sulfoxides: TMSO = tetramethylene sulfoxide; DMSO = dimethyl sulfoxide} catalyze the hydrogenolysis of glycols to alcohols and hydrocarbons by molecular hydrogen at 190-200 °C and 6.8-26 atm; the product yields range from moderate to excellent. The acid generated by catalysts in situ serves the purpose of dehydration step, hence added Bronstead acid as co-catalyst is not a prerequisite. Under similar conditions epoxides are converted primarily to mono-alcohols and hydrocarbons.

Full text of DOI:10.1016/j.molcata.2012.07.025

Journal of Molecular Catalysis A: Chemical 363–364 (2012) 460–464
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Ruthenium (II) sulfoxides-catalyzed hydrogenolysis of glycols and epoxides
Siva Murrua, Kenneth M. Nicholasb, Radhey S. Srivastavaa,∗
a Department of Chemistry, University of Louisiana at Lafayette, LA 70504, United States
b Department of Chemistry & Biochemistry, University of Oklahoma, Norman, OK 73019, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
New selective deoxygenation reactions are needed for the efficient conversion of biomass-derived
oxygenates to useful chemicals, including fuels. A new catalytic system is reported here for the
selective hydrogenolysis of glycols to hydrocarbons. We find that cis-[RuCl2(sulfoxide)4] {sulfoxides:
TMSO = tetramethylene sulfoxide; DMSO = dimethyl sulfoxide} catalyze the hydrogenolysis of glycols to
alcohols and hydrocarbons by molecular hydrogen at 190–200 ◦C and 6.8–26 atm; the product yields
range from moderate to excellent. The acid generated by catalysts in situ serves the purpose of dehy-
dration step, hence added Bronstead acid as co-catalyst is not a prerequisite. Under similar conditions
epoxides are converted primarily to mono-alcohols and hydrocarbons.
Received 30 April 2012
Received in revised form 17 July 2012
Accepted 22 July 2012
Available online 31 July 2012
Keywords:
Ruthenium catalysis
Hydrogenolysis
Glycols
© 2012 Elsevier B.V. All rights reserved.
Epoxides
1. Introduction
unsaturated and saturated hydrocarbons (Scheme 1). Several
multi-step stoichiometric methods for glycol DODH are known
The search for new, efficient processes for the conversion of
reduced oxygen-content products from renewable resources to
value-added chemicals and fuels has stimulated new interest in
the discovery of selective chemical transformation of biomass-
derived carbohydrates and polyols [1–4]. Metal-catalyzed biomass
conversion to fuels and organic chemicals is vital to efficiently use
ample, renewable resources [2–6]. Metal-catalyzed deoxygenation
of glycol and epoxides are less developed than the epoxidation and
dihydroxylation of alkenes which is extensively applied in organic
synthesis and industry [7–9]. The use of biomass substantially
reduces net carbon dioxide emissions because the latter is recycled
in biomass production. Stoichiometric methods for the deoxyde-
hydration of diols are abundant [10,11] but catalytic methods are
limited [12–16]. Hydrogenolysis of polyols by various heteroge-
neous catalytic systems has been recently reported [17,18]. Bullock
[17,18], but one step [19–21] and catalytic methods are few
and the latter are almost entirely based on oxo-rhenium com-
plexes. The Bergman group reported the efficient conversion of
polyols to olefins by formic acid at high temperature [24]. The
potential of catalyzing DODH reactions by oxo-metal complexes
(Cp*ReO3) was demonstrated by Andrews and Cook [12] in the
presence of PPh3 as reductant. Gable and coworkers, with the
help of experimental and computational studies, provided mech-
anistic insight of the reaction involving LReO3 complexes as
catalysts, especially the olefin-producing metallo-glycolate frag-
mentation step [25–30]. Abu-Omar et al. recently [13] achieved
deoxygenation of epoxides and deoxydehydration of glycols to
alkenes and alkanes using MeReO3 as catalyst with the more
practical reductant H2. The Nicholas group [15] has demon-
strated sulfite-driven DODH catalyzed by Me3ReO3 and Z+ReO4
−
+
and Fagan found that {[Cp*Ru(CO)2]2(␮-H)} OTf− is a precursor
and examined its scope and catalytic pathway under moderately
mild reaction conditions. More recently the Bergman group [31]
used rhenium-carbonyl as a pre-catalyst for the deoxydehydra-
tion of glycols to olefins using secondary alcohols as reducing
agents.
The efficacy of ruthenium catalysts in hydrogenation and
hydrogenolysis reactions is well established by us and others
[32,33]. This prompted us to study the readily available and inex-
pensive catalysts for deoxydehydration with the more practical
reductant H2. Molecular hydrogen is an attractive, robust, and inex-
pensive reductant due to its strong reducing potential and the
production of environmentally benign water as a byproduct. In
for the selective hydrogenolysis of 1,2-propanediol to 1-propanol
(Scheme 1) [19–21]. The total deoxygenation of glycerol and 1,2-
hexanediol to saturated hydrocarbons by [LRu(H2O)x(diimine)]2+
and [Cp*Ru(OH2)(N N)]+ was reported by Schlaf and coworkers
[22,23].
Removal of two adjacent hydroxyl groups in a polyol by deoxy-
dehydration (DODH) is a new attractive process for producing
∗
Corresponding author.
E-mail address: rss1805@louisiana.edu (R.S. Srivastava).
1381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2012.07.025

S. Murru et al. / Journal of Molecular Catalysis A: Chemical 363–364 (2012) 460–464
461
LMHx
LMOx
Thus, styrene oxide produced 2-phenethyl alcohol (34%), phenyl
acetaldehyde (20%) and ethyl benzene (15%) (entry 6). Similarly,
1,2-hexene oxide was converted to 1-hexanol (40%) and hexanal
(11%) (entry 7). On the other hand, cyclohexene oxide afforded
cyclohexene (64%) and cyclohexanone (11%) (entry 8). In a separate
experiment cyclohexene was found to be inert to hydrogenation
catalyzed by 1a.
R
R
OH
+
Reductant
OH
HO
R
R
+
Scheme 1. Metal hydrides or metal oxides catalyzed deoxygenation of glycols.
The presence of carbonyl and mono-alcohol products in many
of the glycol and epoxide reactions led us to consider them as
possible reaction intermediates and hence to ascertain this we
evaluated their hydrogenation in the present system. When the
activated ketone, acetophenone, was subjected to our standard
reaction conditions, ethyl benzene, from complete hydrogenol-
ysis, was produced quantitatively (entry 9) in 24 h, signifying
the intermediacy of ketones in alkane formation. In contrast to
this, its constitutional isomer, phenyl acetaldehyde, gave 2-phenyl
ethanol in 72% yield (run 11), confirms the aldehyde as inter-
mediate for mono-alcohol formation. The unactivated aldehyde
hexanal is slowly converted to the corresponding alcohol, hex-
anol, in 19% yield while leaving 42% unreacted hexanal (entry 10).
The similar observations were made in case of epoxides, where
carbonyl compounds are being intermediates during hydrogenol-
ysis process. Interestingly, aliphatic mono alcohols deoxygenated
under the present reaction conditions to yield hydrocarbons
(entries 12 and 13). In contrast, 2-phenethyl alcohol under the
same reaction conditions was unchanged. This indicates that
further reduction of phenethyl alcohol is unlikely under the
present reaction conditions. Our observation of the hydrogenol-
ysis reactions of glycols and epoxides catalyzed by 1a,b have little
precedent in homogeneous Ru-catalysis except for the glycol to
alkane hydrogenation catalyzed by [LRu(H2O)x(diimine)]2+ com-
plexes [19–21]. In fact we can find no reports of complex 1a,b
serving as a hydrogenolysis pre-catalyst for any class of sub-
strates.
The catalytic pathways for hydrogenolysis promoted by 1a,b
are presently unclear. However, the similar products formed from
both diols and epoxides, suggests the participation of common
organic intermediates i.e. carbonyl compounds (Scheme 3). Exist-
ing precedents [19–21,38–44] and our initial findings suggest that
two cyclic pathways (I and II) are viable, as shown in Scheme 3.
We suggest the hydrogenative loss of one chloride ligand to gen-
erate L4RuHCl + HCl and the initial reaction proceeds through an
ionic mechanism. Cycle I involves in formation of mono-alcohol
from diol. In path A, acid catalyzed dehydration generates alde-
hyde; whereas in path B, epoxide rearranged to aldehyde. The
this report we reveal the first examples of selective conversion
of glycols and epoxides to hydrocarbons and alcohols catalyzed
by RuCl2(sulfoxide)4 1a,b. The catalysts, cis-[RuCl2(TMSO)4] (1a)
(TMSO = tetramethylene sulfoxide) and cis-[RuCl2(DMSO)4] (1b),
are easily prepared by the reaction of hydrated RuCl3 with the
respective sulfoxides [34,35].
2. Results and discussion
We started our investigation using 2-phenyl-1,2-ethanediol as
a model glycol substrate (Scheme 2, R = Ph) and cis-[RuCl2(TMSO)4]
(1a) or cis-[RuCl2(DMSO)4] (1b) as pre-catalysts. The reaction
between the glycol and H2 (6.8 atm) with 10 mol% 1a or 1b was
conducted in benzene in a Parr steel autoclave at 190 ◦C. The GC
and GC–MS analysis of the reaction mixture indicated the selec-
tive formation of 2-phenethyl alcohol (83% yields) and a small
amount of phenyl acetaldehyde after 8 h (entry 1, Table 1). We
also examined the reaction in the absence of catalyst under the
similar reaction condition, only a trace amount of aldehyde was
detected.
We extended the hydrogenolysis reactions promoted by 1a to
representative aliphatic glycols, better models for carbohydrate-
derived polyols, to determine the products, selectivity and
efficiency (Table 1). When 1,2-hexanediol was subjected to
optimized reaction conditions (190 ◦C, 6.8 atm H2, 10 mol% 1a)
hexane was produced in 58% yield after 8 h (entry 2) as the
only detected product; at 120 ◦C hexane was still produced
exclusively in 62% yield in 3 h (entry 3). 1,2-Tetradecanediol,
a long chain glycol, was less reactive, and no hydrocarbons
were detected at 200 ◦C under 6.8 atm H2 in 48 h. However,
when the reaction was conducted at 200 ◦C with 26 atm H2,
tetradecane (36%), tetradecanol (9%) and tetradecanal (trace)
were produced in 96 h (entry 4). In contrast, the cyclic glycol
cis-1,2-cyclohexanediol was unreactive under these conditions.
To validate the utility of the Ru-catalyzed deoxygenation of
a biomass-derived glycol, we investigated the reaction of 1,4-
anhydroerythritol, derived from starch-based erythritol [36,37].
When 1,4-anhydroerythritol was hydrogenated by 1a under our
reaction conditions, it afforded THF in modest yield (23%, entry
5).
C
O bond of the intermediate aldehyde would be hydrogenated
via an ionic hydrogenation mechanism, which involve protona-
tion of the aldehyde by L4RuHCl·HCl followed by hydride transfer
from L4RuHCl to give mono-alcohol. Similar ionic hydrogena-
tion of ketones promoted by Mo- and W are known [45–50].
Formation of alkene or alkane from diols (entries 2–6, 8, 9)
can be explained as shown in cycle II (Scheme 3), involves in
dehydration followed by reduction sequence. The mono-alcohol
formed is further dehydrating to the alkene, which in turn get
reduced to alkane (cycle II, Scheme 3). In another experiment,
hydrogenation of 1-hexene produced hexane in 30% yield in
22 h under standard condition, which supports this hypothe-
sis.
The hydrogenolysis of cyclohexene oxide mostly to cyclo-
hexene may be the result of easier dehydration of the
intermediate cyclohexanol (a trace amount of cyclohexanol
was detected, entry 6). With respect to intervening Ru-
complexes, the prevalence of alcohols promoted by 1a may
suggest the involvement of ionic intermediates, implicated
in the Bullock–Fagan studies of glycol mono-hydrogenolysis
[19–21].
The hydrogenolysis of epoxides catalyzed by 1a and 1b,
proceeded smoothly under standard reaction conditions, gener-
ating various reduction products depending on the substrate.
R
R
R
OH
(or)
HO
OH
O
H2
+
[RuCl2(L)4]
R
R
R
O
R = Aryl, Alkyl; L = DMSO or TMSO
Scheme 2. Ru-sulfoxide catalyzed hydrogenolysis of glycols and epoxides.

462
S. Murru et al. / Journal of Molecular Catalysis A: Chemical 363–364 (2012) 460–464
Table 1
Ru-sulfoxide catalyzed hydrogenolysis of glycols and epoxides.
Entry
1
Substrate
Product(s)
Yield (%)
83 (78)d
Time (h)
8
OH
OH
Ph
OH
OH
Ph
2
3
58
8
OH
”
”
62
9
3b
OH
OH
( )11
( )11
4
96c
H3C
OH
O
Trace
36
( )11
( )11
HO
O
OH
5
6
23
48
30
O
34 (35)d
20 (18)d
15 (17)d
OH
O
Ph
O
Ph
Ph
Ph
40
18
OH
O
O
7
8
64
11
O
8
9
8
O
O
Ph
Ph
99
24
Ph
Ph
O
OH
10
11
12
19
72
43
24
52
24
O
OH
OH
OH
13
18
24
a0.14 mmol glycol or epoxide in benzene (6–10 mL), 0.014 mmol cis-[Ru(TMSO/DMSO)4], 190 ◦C, 6.8 atm H2; yields determined by GC and GC–MS with n-dodecane as internal
standard.
b
Reaction at 120 ◦C.
c
Reaction at 200 ◦C, 28 atm H2.
d
Yields in parenthesis represents the reactions used cis-[Ru(DMSO)4] catalyst.

S. Murru et al. / Journal of Molecular Catalysis A: Chemical 363–364 (2012) 460–464
463
RuCl2L4 (1)
H2
A
OH
OH
[L4RuHCl] + HCl
(2)
OH
R
R
(1)
H2
B
H
H
R
R
O
[L4RuHCl]
H+
(3)
(2)
O
RuCl2L4 (1)
R
+
(3)
OH
R
H
O
OH
O
H
R
H2O
[L4RuHCl] +
H2O
R
H
(3)
+
(3)
R
R = Aryl, Alkyl; L = DMSO or TMSO
Cycle I
Cycle II
Scheme 3. Plausible catalytic cycle for hydrogenolysis of glycols and epoxides.
3. Conclusion
Appendix A. Supplementary data
In summary, we have developed a new reaction system for
the selective deoxygenation of glycols and epoxides, to alcohols
and/or alkanes, using H2 as reductant and easily prepared cis-
[RuCl2(sulfoxide)4] as catalyst. It is remarkable to note that the
catalysts generate acid in situ; our system does not require any
acid co-catalyst for dehydration. The product yields are moderate to
excellent. Future investigations will focus on elucidating the impor-
tant catalytic species and mechanisms of these reactions and their
application to biomass-derived polyols.
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.
molcata.2012.07.025.
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