Guerbet Alcohols: From Processes under Harsh Conditions to Synthesis at Room Temperature under Ambient Pressure

Article abstract of DOI:10.1002/cctc.201501241

A novel synthetic approach towards Guerbet alcohols, which are important intermediates in the production of plasticizers, lubricants, and surfactants, was developed. In contrast to the harsh reaction conditions of Guerbet alcohols produced today, which include high temperatures, the new developed process runs at room temperature. The key feature of this alternative process is the combination of organocatalytic and enzymatic steps towards a chemoenzymatic synthesis. In detail, the piperidinyloxyl-catalyzed oxidation of 1-hexanol by using hypochlorite and the lysine-catalyzed homoaldol condensation of the resulting aldehyde were combined with two subsequent enzymatic reductions of the C=C and C=O bonds of the in situ formed 2-branched α,β-unsaturated aldehyde by means of an ene reductase from Gluconobacter oxydans and an alcohol dehydrogenase from Rhodococcus sp. under in situ cofactor regeneration. The desired 2-branched aliphatic primary alcohol was obtained with high conversion and selectivity and without the need for intermediate purifications. Fantastic four: The synthesis of Guerbet alcohols at room temperature under ambient pressure is reported; this is in contrast to today's industrial production, which is performed under harsh reaction conditions. The presented process is based on the combination of two organocatalytic steps with a biocatalytic two-step tandem process starting from readily available aliphatic alcohols. High conversions and selectivities are found for all steps.

Full text of DOI:10.1002/cctc.201501241

DOI: 10.1002/cctc.201501241
Communications
Guerbet Alcohols: From Processes under Harsh Conditions
to Synthesis at Room Temperature under Ambient
Pressure
Marc Biermann, Hendrik Gruß, Werner Hummel, and Harald Grçger*[a]
Dedicated to Professor Dr. Albrecht Berkessel on the occasion of his 60th birthday
A novel synthetic approach towards Guerbet alcohols, which
are important intermediates in the production of plasticizers,
lubricants, and surfactants, was developed. In contrast to the
harsh reaction conditions of Guerbet alcohols produced today,
which include high temperatures, the new developed process
runs at room temperature. The key feature of this alternative
process is the combination of organocatalytic and enzymatic
steps towards a chemoenzymatic synthesis. In detail, the piper-
idinyloxyl-catalyzed oxidation of 1-hexanol by using hypochlor-
ite and the lysine-catalyzed homoaldol condensation of the re-
sulting aldehyde were combined with two subsequent enzy-
matic reductions of the C=C and C=O bonds of the in situ
formed 2-branched a,b-unsaturated aldehyde by means of an
ene reductase from Gluconobacter oxydans and an alcohol de-
hydrogenase from Rhodococcus sp. under in situ cofactor re-
generation. The desired 2-branched aliphatic primary alcohol
Scheme 1. Original and alternative routes for the Guerbet reaction of 1-hex-
was obtained with high conversion and selectivity and without
the need for intermediate purifications.
anol (1) to 2-butyl-1-octanol (2).
products. Modern improvements of the “classic” Guerbet reac-
tion include the application of transition-metal-based hydroge-
nation catalysts[6] such as Rh,[7] Ir,[8] Pd,[9] and Cu[10] as well as
metal oxides[11] to work at lower temperatures and atmospher-
ic pressure. However, owing to various limitations for their use
at the industrial scale, the “classic” process at high temperature
and pressure is still used. Thus, the development of sustainable
alternatives enabling the synthesis of Guerbet alcohols under
smooth reaction conditions and avoiding the Cannizzaro[12]
and Tishchenko[13] reactions as two major side reactions based
on disproportionation of the aldehyde is a current challenge in
organic chemistry.
Guerbet alcohols, the first synthesis of which dates back to
1899, are 2-branched primary aliphatic alcohols[1,2] that have
a distinctly lower melting point than their straight-chain homo-
logues owing to their sterically demanding structures. This
property and their amphiphilic character make them attractive
intermediates, for example, for plasticizers, lubricants, and sur-
factants.[3] The original Guerbet reaction is particularly used for
the production of higher homologues of 2-branched aliphatic
alcohols on the industrial scale[4] and formally represents
a “self-condensation reaction” of primary aliphatic alcohols
(Scheme 1). A selected example (and a model reaction for our
studies) is the conversion of 1-hexanol (1) into 2-butyl-1-octa-
nol (2).
In spite of the broad industrial application range of Guerbet
alcohols, their preparation still has drawbacks, including elevat-
ed reaction temperatures and high-pressure conditions, both
of which are required even if alkali hydroxides and Raney-Ni
are used as catalysts.[1,5] Such harsh reaction conditions also
cause selectivity concerns and the formation of undesired by-
In continuation of our studies on chemoenzymatic one-pot
processes,[14] we became interested in integrating biocatalytic
reactions in multistep syntheses of bulk chemicals. Biocatalysis
already plays an important role in today’s industrial production
of fine chemicals, for which stereoselectivity is often required.
However, only a few industrial processes for bulk chemicals in-
volve enzymatic steps.[15,16] On the other hand, biocatalysts
provide unique advantages such as high selectivity and mild
reaction conditions, which thus also makes them attractive for
the field of bulk chemicals. Addressing this issue, in the follow-
ing, a biointegrated cascade synthesis of Guerbet alcohols is
reported.
[a] M. Biermann, H. Gruß, Prof. Dr. W. Hummel, Prof. Dr. H. Grçger
Faculty of Chemistry
Bielefeld University
Universitätsstrasse 25, 33615 Bielefeld (Germany)
E-mail: harald.groeger@uni-bielefeld.de
In detail, our concept follows the mechanism of the “classic”
Guerbet reaction (Scheme 1) but at the same time allows these
reaction steps to be conducted at room temperature under
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201501241.
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ambient pressure conditions instead of under harsh reaction
conditions. Extensive studies have provided insight into the
mechanism of the Guerbet reaction,[17] which can be divided
into four sequential steps: one, oxidation of 1 to hexanal (3);
two, aldol condensation to 2-butyl-2-octenal (5); three, C=C
bond reduction to 2-butyloctanal (4); four, aldehyde reduction
to 2. Our goal was to develop highly efficient reaction steps
catalyzed by chemo- or biocatalysts that run at room tempera-
ture and ambient pressure and that are compatible with each
other so that they can be integrated within a one-pot-like cas-
cade sequence without the need to isolate any intermediate.
Starting with the initial step of the process, namely, the oxi-
dation of the primary alcohol to the aldehyde, extensive stud-
ies were made by using model substrate 1 by evaluating differ-
ent “green” oxidation catalysts. One of the challenges in work-
ing with aldehydes, especially in an aqueous environment, is
their oxidation sensitivity, as documented by Shapiro and Viga-
lok.[18] In agreement with their results, we found aldehyde 3 (as
well as the other aldehydes studied) to be rapidly oxidized in
air. To overcome this undesired oxidation, all steps with alde-
hydes as substrates and/or products were performed under an
argon atmosphere and by using degassed solvents for the re-
actions. Upon evaluating an enzymatic oxidation by screening
several alcohol dehydrogenases, only poor oxidation properties
were identified, which thus made an efficient oxidation process
difficult (data not shown). In addition, oxygen-consuming alco-
hol oxidases were tested and showed moderate activity, but
unfortunately, 3 was further oxidized to the corresponding
acid (data not shown). Next, 2,2,6,6-tetramethylpiperidin-1-oxyl
(TEMPO) and related polymer-immobilized piperidinyloxyl
(PIPO)[19] jointly with bromide as a catalytic system was used
for the oxidation of 1 owing to the known[20] prevention of
overoxidation of aldehydes in air in the case of TEMPO. In this
process, cheap hypochlorite serves as a stoichiometric oxidant
in an aqueous/organic biphasic solvent mixture. With both
TEMPO and PIPO, high conversions were obtained after only
90 min at a low catalyst loading of 1 mol% (Figure 1). Under
optimized conditions (i.e., PIPO, dosage of hypochlorite), an
excellent conversion of >99% and selectivity of 98% were ob-
tained for the formation of aldehyde 3. Notably, only a minor
amount of hexyl hexanoate (2%) was found, which thus indi-
cates that byproduct formation was nearly prevented. In addi-
tion, no overoxidation to the corresponding acid was found
with PIPO despite the fact that an excess amount of hypo-
chlorite (1.5 equiv.) was used.
Figure 1. Comparison of TEMPO and PIPO as catalysts for the oxidation of
1 in batch and under dosage of hypochlorite. Products are given as percent-
age of the sum of all products. [a] Overall consumption of substrate.
[b] Hexyl hexanoate. [c] Hexanal dihexyl acetal.
Figure 2. Screening of catalysts for the homoaldol condensation of hexanal
(3). [a] Confirmed in CH2Cl2 at 50 gLÀ1 by GC analysis.
The second step consists of a self-aldol condensation of al-
dehyde 3. A screening of potential catalysts for this reaction
revealed l-lysine to be the most suitable candidate upon con-
ducting this aldol condensation in an organic solvent. In the
presence of l-lysine (20 mol%), quantitative conversion to de-
sired product 5 was obtained after 6 h (Figure 2). The use of
an organic solvent offers the advantage of the direct use of
the organic phase from step 1 containing aldehyde 3, which
avoids workup of aldehyde 3 prior to step 2. Furthermore, the
amino acid l-lysine represents an attractive catalyst for various
reasons: first, it is produced on >1000000 metric-ton scale
and thus is commercially available on large scale; second,
l-lysine is insoluble under the reaction conditions, which thus
enables simple catalyst separation from the reaction mixture
and provides an opportunity for catalyst recycling.
The third and fourth steps consist of reduction of the C=C
bond in 5 by means of an ene reductase (ER), followed by re-
duction of formed aldehyde 4 catalyzed by an alcohol dehy-
drogenase (ADH). Both the ER and ADH are NAD(P)H depen-
dent and therefore require an in situ cofactor for regeneration.
For these enzymatic ene and aldehyde reductions, 12 ERs and
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11 ADHs were tested with respect to their activity by means of
spectrophotometric screening; this revealed an ER from Gluco-
nobacter oxydans (GOx-ER)[21] and an ADH from Rhodococcus
sp. (Rsp-ADH)[22] to be suitable and the most promising candi-
dates (for data of this screening, see the Supporting Informa-
tion). Both enzymes show complementary cofactor specificities,
as GOx-ER prefers NADPH and Rsp-ADH prefers NADH. Hence,
for in situ cofactor regeneration, a glucose dehydrogenase
(GDH) accepting both cofactors and d-glucose were chosen,
which thus enabled efficient in situ recycling of both cofactors
although with the use of only one glucose dehydrogenase
(which would be beneficial for subsequent combination of
steps 3 and 4).
lysine residues, we assume that this loss is caused by adduct
formation through Schiff base formation with enzymes, as pre-
viously described.[23] A well-known option to address this issue
is to keep the apparent concentration of the aldehyde low by
dosing the substrate and by applying a fast aldehyde-consum-
ing reaction. Thus, from this perspective we also considered an
increase in the efficiency and conversion of step 3 and in par-
ticular step 4 in buffer as a task of highest priority. Towards
this end, an organic cosolvent was added to increase the solu-
bility of the substrates. In this study, water-immiscible and
water-miscible solvents were used; 2-methyltetrahydrofuran (2-
MeTHF) and methanol (MeOH) were chosen as representatives
because of their preferred use in terms of sustainability
(Figure 3).
With these two efficient biocatalysts in hand, we next con-
ducted biotransformations on a preparative scale (Figure 3).
First, enzymatic reduction of the C=C and C=O bonds was per-
formed separately in pure aqueous buffer as the solvent; mod-
erate conversions of 70 and 78% were obtained, respectively,
with high selectivities of >99%. However, during downstream
processing and product-isolation steps, a significant amount of
material was lost (substrate 5 and intermediate 4) and precipi-
tation of the protein was found. As both compounds bear al-
dehyde groups and aldehydes are likely to react with surface
In the presence of 20%v/v 2-MeTHF, quantitative conversion
for the reduction of enal 5 was obtained (step 3), but a low
conversion (36%) was found for the ADH-catalyzed reduction
of aldehyde 4 in step 4. However, upon changing the organic
cosolvent to a water-miscible one, we were pleased to find ex-
cellent conversions (>99%) and selectivities (99%) for both
steps 3 and 4 in the presence of 20%v/v MeOH. Notably, recy-
cling of the in situ cofactor proceeded fully through the GDH-
catalyzed transformation of d-glucose into d-gluconolactone.
Alternative consumption of MeOH through the ADH and its
possible oxidation to formaldehyde was not an issue, because
the activity of Rsp-ADH for MeOH oxidation is very low
(<0.1 UmgÀ1).
We next focused on the combination of steps 3 and 4 to-
wards a tandem process; this was challenging, as 5 is also a po-
tential substrate for the ADH, which would then lead to an un-
desired side reaction. Resulting 2-butyl-2-octenol in turn is not
a substrate for the GOx-ER. However, we were pleased to find
that upon combining reactions steps 3 and 4 in a tandem-type
one-pot process, quantitative conversion and a high selectivity
of 94% (showing only 6% of 2-butyl-2-octenol as a byproduct)
were obtained (Scheme 2).
After establishing the individual reactions in steps 1 to 4 suc-
cessfully and already obtaining a tandem-type combination of
steps 3 and 4, a “proof of concept” for a combined process of
all steps with minimized required workup steps was demon-
strated (Scheme 3).
In such
a process, initial PIPO-catalyzed oxidation of
1 (step 1) led to>99% conversion and 98% selectivity for al-
dehyde 3. After simple, but careful, phase separation (to
ensure complete absence of water) and without further
workup, l-lysine (in total 40 mol%, added in two portions) was
added to catalyze the desired aldol condensation to 5 (step 2).
Both high conversion (92%) and high selectivity (>99%) were
Figure 3. Preparative-scale enzymatic reactions in different solvent systems.
Reaction conditions: GOx-ER or Rsp-ADH, NAD(P)+, GDH, d-glucose, potassi-
um phosphate buffer (200 mm, pH 7), 20%v/v organic cosolvent, RT, 24 h.
Scheme 2. Preparative-scale tandem-type one-pot process. Reaction conditions: GOx-ER and Rsp-ADH, NAD(P)+, GDH, d-glucose, potassium phosphate buffer
(200 mm, pH 7), 20%v/v MeOH, RT, 24 h.
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Experimental Section
Combined chemoenzymatic cascade for the synthesis of
Guerbet alcohol 2 (according to Scheme 4)
After dissolving 1-hexanol (1; 1 mL, 8 mmol) in CH2Cl2 (20 mL),
NaBr (82.3 mg, 0.8 mmol, 10 mol%) and PIPO (25 mg, 0.08 mmol
aminoxyl, 1 mol%) were added, and the resulting mixture was
cooled to 58C. Then, a precooled solution of 0.35m NaOCl
(12 mmol, 1.5 equiv.) and NaHCO3 (0.53m, 17.7 mmol, 2.2 equiv.) in
H2O (33.7 mL) was added over 60 min, and the mixture was stirred
for another 30 min at 58C. The organic phase, showing >99% con-
version and 98% selectivity, was passed through a phase-separat-
ing filter. Subsequently, l-lysine (229 mg, 1.6 mmol, 20 mol%) was
added, and the mixture was stirred for 16 h at room temperature
prior to the addition of an additional portion of l-lysine (229 mg,
1.6 mmol, 20 mol%). After a total reaction time of 48 h, 92% con-
version and >99% selectivity were obtained for aldol condensa-
tion product 5. The mixture was washed with 0.5m HCl (3”20 mL)
and dried (MgSO4) prior to evaporation of the solvent under re-
duced pressure to obtain 5 as a crude product (80% yield). The en-
zymes GOx-ER (10 U, 6.6 mg purified by N-terminal His-tag,
1.5 UmgÀ1, 100 UmmolÀ1) and Rsp-ADH (20 U, 2.73 mL crude ex-
Scheme 3. Workup illustrating the flow chart of the process unit-operation
steps to 2-butyl-1-octanol (2) starting from 1-hexanol (1).
obtained in this step. The reaction mixture was washed with
dilute aqueous hydrochloric acid, dried with magnesium sul-
fate, and concentrated under reduced pressure to yield 80%
crude 5 over both steps. Using crude product 5 for the subse-
quent two-step enzymatic reduction process (steps 3 and 4)
then gave the desired target product, 2-butyl-1-octanol (2),
with 97% conversion and 97% selectivity, which thus indicated
that residual components of the previous steps (such as PIPO
and dichloromethane) had no significant negative impact on
the two-step tandem biotransformation (steps 3 and 4). A yield
of 78% was achieved for the last two steps, which led to
a combined overall yield of 62% (Scheme 4).
tract, 7.3 UmLÀ1
,
200 UmmolÀ1), GDH2 (Amano, 100 U,
1000 UmmolÀ1), cofactor NADP+ sodium salt (2.3 mg, 3 mmol,
3 mol%), cofactor NAD+ (2.0 mg, 3 mmol, 3 mol%), and d-glucose
(0.6 mmol, 108.1 mg, 6 equiv.) were dissolved in degassed potassi-
um phosphate buffer (pH 7, 200 mm) to a total volume of 4 mL.
Crude product 5 (18.1 mg, 0.1 mmol) obtained as described above
was dissolved in MeOH (1 mL) and was added to the solution. The
mixture was stirred at room temperature for 24 h, acidified with
0.5m HCl (1 mL), and extracted with methyl tert-butyl ether (3”
5 mL), thus as a step in which water-soluble glucose was also sepa-
rated. A conversion of 97%, a selectivity of 97%, and an overall
yield of 62% were obtained after evaporation of the or-
ganic solvent under reduced pressure.
Acknowledgements
The authors gratefully acknowledge generous support
Scheme 4. Biointegrated four-step cascade to synthesize 2-butyl-1-octanol (2) starting
from 1-hexanol (1).
from the German Federal Ministry of Education and Re-
search (Bundesministerium für Bildung und Forschung,
BMBF) within the project “Biotechnologie 2020+,
In conclusion, a “proof of concept” for the synthesis of Guer-
Nächste Generation biotechnologischer Verfahren” (grant number
031A184A).
bet alcohols at room temperature under ambient pressure
conditions was established (in contrast to today’s industrial
production for which harsh reaction conditions, e.g., high tem-
perature, are needed). The process concept was based on
a chemoenzymatic synthesis and consisted of a combination
of two organocatalytic steps with a biocatalytic two-step
tandem process. In detail, this biointegrated cascade to Guer-
bet alcohols started from 1-hexanol as a readily available ali-
phatic primary alcohol and led to both high conversions and
selectivities for all four steps. The combination of the four reac-
tion steps towards a cascade process was done with mini-
mized required workup steps for the intermediates and gave
conversions of 92 to >99% and selectivities of 97 to >99%
for steps 1 to 4. Among the challenges for future work are fur-
ther optimization of the reaction and workup steps as well as
an increase in the substrate loading and overall yield of the de-
sired Guerbet alcohol product.
Keywords: biocatalysis · chemoenzymatic synthesis · domino
reactions · Guerbet alcohols · organocatalysis
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Received: November 11, 2015
Published online on January 26, 2016
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