Is the tungsten(IV) complex (NEt4)2[WO(mnt)2] a functional analogue of acetylene hydratase?

Article abstract of DOI:10.3762/bjoc.13.230

The tungsten(IV) complex (Et₄N)₂[W(O)(mnt)2] (1; mnt = maleonitriledithiolate) was proposed (Sarkar et al., J. Am. Chem. Soc. 1997, 119, 4315) to be a functional analogue of the active center of the enzyme acetylene hydratase from Pelobacter acetylenicus, which hydrates acetylene (ethyne; 2) to acetaldehyde (ethanal; 3). In the absence of a satisfactory mechanistic proposal for the hydration reaction, we considered the possibility of a metal-vinylidene type activation mode, as it is well established for rutheniumbased alkyne hydration catalysts with anti-Markovnikov regioselectivity. To validate the hypothesis, the regioselectivity of tungsten- catalyzed alkyne hydration of a terminal, higher alkyne had to be determined. However, complex 1 was not a competent catalyst for the hydration of 1-octyne under the conditions tested. Furthermore, we could not observe the earlier reported hydration activity of complex 1 towards acetylene. A critical assessment of, and a possible explanation for the earlier reported results are offered. The title question is answered with "no".

Full text of DOI:10.3762/bjoc.13.230

Is the tungsten(IV) complex (NEt4)2[WO(mnt)2] a functional
analogue of acetylene hydratase?
Matthias Schreyer1,2 and Lukas Hintermann*1,2
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2017, 13, 2332–2339.
1Department Chemie, Technische Universität München,
Lichtenbergstr. 4, 85748 Garching bei München, Germany and 2TUM
Catalysis Research Center, Ernst-Otto-Fischer-Str. 1, 85748 Garching
bei München, Germany
doi:10.3762/bjoc.13.230
Received: 28 July 2017
Accepted: 04 October 2017
Published: 02 November 2017
Email:
Lukas Hintermann* - lukas.hintermann@tum.de
Associate Editor: I. Marek
* Corresponding author
© 2017 Schreyer and Hintermann; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
acetylene hydratase; alkynes; catalytic hydration; enzyme models;
tungsten complexes
Abstract
The tungsten(IV) complex (Et4N)2[W(O)(mnt)2] (1; mnt = maleonitriledithiolate) was proposed (Sarkar et al., J. Am. Chem. Soc.
1997, 119, 4315) to be a functional analogue of the active center of the enzyme acetylene hydratase from Pelobacter acetylenicus,
which hydrates acetylene (ethyne; 2) to acetaldehyde (ethanal; 3). In the absence of a satisfactory mechanistic proposal for the
hydration reaction, we considered the possibility of a metal–vinylidene type activation mode, as it is well established for ruthenium-
based alkyne hydration catalysts with anti-Markovnikov regioselectivity. To validate the hypothesis, the regioselectivity of tung-
sten-catalyzed alkyne hydration of a terminal, higher alkyne had to be determined. However, complex 1 was not a competent cata-
lyst for the hydration of 1-octyne under the conditions tested. Furthermore, we could not observe the earlier reported hydration ac-
tivity of complex 1 towards acetylene. A critical assessment of, and a possible explanation for the earlier reported results are
offered. The title question is answered with "no".
Introduction
In 1985, the enzyme acetylene hydratase (classification: hydro- structural data of the enzyme [4], the catalytic reaction mecha-
lyases, EC 4.2.1) was isolated from the bacterium Pelobacter nism was not immediately obvious [4,7]. Several mechanisms
acetylenicus [1], which feeds anaerobically on acetylene as sole have so far been considered and investigated in silico [8-12].
carbon source [2]. The enzyme is a tungsten iron–sulfur protein The most recent works favor nucleophilic addition of water to
requiring a strongly reducing environment for converting acety- tungsten-coordinated 2 with assistance of a catalytic carboxyl-
lene (ethyne; 2) to acetaldehyde (ethanal; 3) by redox-neutral ate as key-step (Scheme 1b), followed by protonation of
addition of water (Scheme 1a) [2-6]. Even based on X-ray the intermediary 2-hydroxyethenyltungstate to release vinyl
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Beilstein J. Org. Chem. 2017, 13, 2332–2339.
alcohol [10-12]. In 1997, Sarkar et al. reported that the rearrangement to a tungsten vinylidene complex and addition of
oxidation-sensitive but water-stable tungsten(IV) complex the alcohol hydroxy group to the vinylidene α-carbon [18].
(Et4N)2[W(O)(mnt)2] (1) (mnt = maleonitriledithiolate) is a
catalyst for hydration of acetylene to acetaldehyde, with 9
turnovers over 4 h at ambient temperature (Scheme 1c) [13].
Scheme 2: a) π-Activation pathway in Markovnikov selective alkyne
hydration, e.g., with mercury catalysts. b) Ruthenium-catalyzed anti-
Markovnikov hydration via key vinylidene intermediate.
The vinylidene mechanism is related to that of ruthenium-cata-
Scheme 1: a) Acetylene hydratase catalyzes the hydration of acety-
lene to ethanal. b) Currently favored key-steps for the reaction mecha-
lyzed anti-Markovnikov hydration of terminal alkynes to alde-
hydes (Scheme 2b) [21-23]. Thus, we wondered if tungsten
complex 1, and by analogy acetylene hydratase, is an alkyne
hydration catalysts that follows a vinylidene–metal mechanism.
This idea has also been considered by others [7-10] and was in-
vestigated in silico by Hillier and co-workers [9]. Experimental-
ly, the vinylidene mechanism is revealed in the hydration of a
terminal alkyne by producing an aldehyde (anti-Markovnikov
type addition) as opposed to a methyl ketone (Markovnikov
type addition; typical for π-activation mechanisms) [18]. Hydra-
tion reactions of 1 involving higher alkynes have not been re-
ported [13] and substrate scope tests for acetylene hydratase
have so far failed with higher alkynes [6]. We wished to test the
potential activity and regioselectivity of complex 1 for hydra-
tion of higher terminal alkynes, as an extension to our studies of
ruthenium-catalyzed anti-Markovnikov hydration [18,24-28].
nism of acetylene hydratase [10-12]. c) Tungsten complex
(NEt4)2[WO(mnt)2] (1), which was reported as acetylene hydration
catalyst [13].
Tungsten(IV) complex 1 with its two dithiolate ligands that
resemble the natural pyranopterindithiolate cofactor ligand [14-
16] was suggested to be a functional mimic of the enzyme and a
tool to experimentally investigate the mechanism of tungsten-
catalyzed acetylene hydration, and by extension the enzymatic
reaction mechanism [13]. A theoretical study has considered
water addition to coordinated ethyne in [W(η1-OH)(mnt)2(η2-
C2H2)]– with general base activation through the hydroxo
ligand and found a reaction pathway with an energy barrier of
20 kcal/mol [17]. Prior to 1, no molecular tungsten compound
had been reported to catalyze alkyne hydration [18], but
W(CO)6 catalyzes the related cycloisomerization of alkynols, in
which the alcohol adds to the alkyne [19]. The reaction of Results and Discussion
[W(CO)5(THF)] with ortho-ethynylacetophenone and excess Tungsten complex (NEt4)2[WO(mnt)2] (1) was prepared ac-
water gives 1,2-diacetylbenzene via neighboring group attack to cording to the literature procedure from Na2WO4, Na2mnt and
complexed alkyne, and hydrolysis [20]. The latter pathway buffered aqueous dithionite, followed by precipitation with
represents the π-activation pathway of alkyne hydration Et4NBr (Scheme 3a) [29]. The compound was characterized by
(Scheme 2a), whereas alkynol cycloisomerization proceeds via 1H and 13C NMR spectroscopy, IR spectroscopy, and by its
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dark purple color. A diagnostic analytical property in solution is potential catalysts. The test system is based on heating sub-
δC of C-2/3 in the mnt ligand (δC = 140.4 ppm for 1) that strate 10-undecyn-1-ol (4) together with a potential catalyst in
depends on changes in the oxidation state, particularly oxida- degassed water–solvent mixtures to 160 °C for 15 min in a
tion to [WO2(mnt)2]2− (δC = 123.3 ppm).
microwave reactor (Table 1). Analysis of the reaction mixture
by 1H NMR against internal standard reveals conversion and
product selectivity (5 vs 6), or points to important side-reac-
tions through spectroscopic identification and quantification of
side-products. Extensive catalyst screening studies that also
included established alkyne hydration catalysts have shown that
every single of the known catalyst shows significant activity
under the conditions of this test [30]. The first two entries in
Table 1 exemplify the performance of typical alkyne hydration
catalysts with Markovnikov ([AuPPh3]+, entry 1; product 5) or
anti-Markovnikov selectivity (entry 2; product 6) in the
screening setup.
More specifically, gold complex AuCl(PPh3) is not usually
considered an alkyne hydration catalyst itself, but turns into a
catalytically active gold(I) cation after activation with silver salt
or Lewis acid [31-34]. Under the forcing microwave reaction
Scheme 3: a) Synthesis of complex (NEt4)2[WO(mnt)2] (1) [29].
b) Attempted catalytic hydration reaction with a terminal alkyne.
In a preliminary experiment, 1-octyne and complex 1 (2 mol %) conditions in aqueous methanol, ionization is brought about
were heated at 50 °C in aqueous acetone. In situ analysis of the without an added reagent, and catalytic activity towards
reaction mixture after 20 hours by GC–MS failed to reveal any Markovnikov product 5 revealed, even if the conversion is low
new product next to unchanged 1-octyne.
(Table 1, entry 1). A cyclopentadienylruthenium(II) catalyst
with the ambifunctional steering ligand ISIPHOS (2-(diphenyl-
We have recently developed standardized screening procedures phosphino)-6-(2,4,6-triisopropylphenyl)pyridine) [24-26,28]
for detecting alkyne hydration activity and regioselectivity of expectedly brings about anti-Markovnikov hydration, with alde-
Table 1: Hydration experiments with 10-undecyn-1-ol (4).a
Entry Catalyst (mol %)
Solventb
MeOH
4 (%)
5 (%)
6 (%)
7 (%)
8c (%)
Recovery (%)
1
AuClPPh3 (2)
57.7
0.0
40.7
0.1
n.d.f
0.8
0.0
0.0
0.0
0.0
0.4
91.2
0.7
1.1
0.3
0.1
0.2
0.2
0.8
0
0
100.0d
92.8
2
CpRuCl(PPh3)2–ISIPHOS (2) acetone
1.5
n.d.f
0
3e
4e
5
1 (20)
–
acetone
acetone
acetone
acetone
MeCNg
acetone
96.8
88.4
97.1
97.8
97.1
97.8
n.d.f
0.8
0.8
0.7
0.5
0.8
97.5
91.1
–
1.2
1.4
1.5
1.7
99.4
6
1 (20)
1 (20)
1 (20)
100.0
99.3
7
8h
100.5
aReaction conditions: microwave heating, 160 °C, 15 min. Composition of crude product is given in mol % relative to initial 4, as determined by qNMR
against internal standard. Recovery is the sum of analytically detected 4 and products derived from it. bSolvent and water were applied in a 4:1
volume ratio, unless otherwise mentioned. cMay include other alkanols, like undecan-1,11-diol or 1-decanol. dIncludes 0.4 mol % of the dimethyl-
acetal of 5. eNon-distilled starting material 4 was used. fA low signal-to-noise ratio prevented reliable detection of 5, 7 and 8, which in any case must
have been low. gSolvent–H2O volume ratio 2:1. hReaction performed in a Schlenk tube at 60 °C for 24 h. The higher than 100% recovery reflects ex-
perimental error.
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hyde 6 as major product. Table 1, entry 3 represents a first test
of tungsten complex 1 in the microwave hydration protocol, at a
fairly high catalyst loading of 20 mol %. Besides unchanged 4,
the reaction mixture contained 0.7% of aldehyde 6. Could this
mean that 1 is indeed an anti-Markovnikov hydration catalyst?
The low turnover number of 0.035 implies that this is unlikely.
A blank experiment revealed that even higher amounts of 6 are
generated from 4 in the absence of catalyst (Table 1, entry 4).
Aldehyde 6 is an impurity in the starting alkyne 4, but may also
be formed under the reaction conditions if the latter contains
autoxidation impurities. Kugelrohr vacuum distillation of 4
reduced the aldehyde level to below 0.3% (Table 1, entries
5–8). Even if technically unsatisfactory, entries 3 and 4 are
included in Table 1 to illustrate a potential pitfall in the study of
catalytic oxyfunctionalization of unsaturated hydrocarbons,
where autoxidation products may feign false positive results
[35]. The effect is most problematic at low mol % loadings, and
it is thus necessary to substantiate a presumed catalytic activity
by increasing conversion to higher levels through increasing the
catalyst loading [36]. Tests of complex 1 with purified 4 in ace-
Scheme 4: a) Unexpected isolation of acetone 2,4-dinitrophenyl-
hydrazone (10) from an attempted catalytic hydration of ethyne (acety-
lene gas) in the presence of 1. b) Acetaldehyde 2,4-dinitrophenyl-
hydrazone (9) exists as two geometrical isomers. DNPH = 2,4-dinitro-
phenylhydrazine.
tone–water or acetonitrile–water failed to show catalytic activi- drazones is a well-established identification method for carbon-
ty (Table 1, entries 6 and 7). The recovery of 4 was excellent yl compounds that recommends itself for small amounts of vol-
(97–98%) and the side products allenol 7 and alkenol 8 are atile products [38,39]. However, aldehyde dinitrophenylhydra-
impurities already present in distilled starting material. Thus, zones often exist as mixtures of E and Z-isomers (Scheme 4b),
complex 1 does not show hydration activity against higher ter- which interconvert in the presence of co-precipitated acid, and
minal alkynes. To further validate those negative results, we stable melting points cannot be achieved, unless special purifi-
wished to demonstrate a positive activity of complex 1, namely cation protocols are followed [40,41]. The inadequacy of identi-
the reported hydration of ethyne to acetaldehdye [13]. A com- fying 9 by standard melting point measurements is emphasized
mercial acetylene pressure bottle (purity grade 2.6, i.e., 99.6%, by the histogram in Figure 1 which was created from melting
containing ≤5 ppm sulfur or phosphorus compounds) was avail- point data in the Reaxys database and shows scattering over
able for the experiment. Gas was first bubbled through a solu- 30 °C.
tion of 1 in acetonitrile–water (2:1) at 40 °C, then the vessel
was closed and the reaction mixture incubated at room tempera- Consequently, measuring a single or even a mixed melting point
ture. Derivatization of the reaction solution with 2,4-dinitro- with a reference sample is not a reliable identification criterion
phenylhydrazine (DNPH) precipitated a yellow substance. The for 9. An analysis of the compound by HPLC had also been per-
original report had identified the precipitate as acetaldehyde formed, where a peak retention time (3.072 min) was given for
2,4-dinitrophenylhydrazone (9) by recording a melting point the product, together with some specifics of the analytical setup,
(147 °C) and determining an HPLC peak retention time against but without identifying the analytical column. In the absence of
reference material. Since neither analytical method provides proof that mixed injections of 9 and 10 would give rise to sepa-
structural information, we analyzed the product by 1H NMR rate peaks, the HPLC test cannot be considered to identify 9 or
spectroscopy instead and found to our initial surprise that the differentiate it from 10. In short, we find that the analytical evi-
precipitated material was the dinitrophenylhydrazone 10 dence presented in ref. [13] to identify 9 was insufficient, and
derived from acetone, with no acetaldehyde hydrazone 9 thus the catalytic hydration of 2 to 3 by complex 1 is not
present (Scheme 4a)!
proven. In particular, the possibility that acetone was mistaken
for 3 cannot be excluded.
Retrospectively, this result could have been expected, since ace-
tone is present as stabilizer in commercial acetylene pressure Continued interest in functional models of acetylene hydratase
bottles [37]. The original report on 1-mediated acetylene hydra- [14-16,42] and a theoretical study on the mechanisms of acety-
tion did not consider (and thus did not exclude) the generation lene hydration by complex 1 [17] have motivated us to scruti-
of 10, and the source and purification method for substrate 2 nize the claimed biomimetic catalytic activity by performing the
were not indicated [13]. Derivatization to 2,4-dinitrophenylhy- hydration experiment with acetone-free 2. A stream of gas was
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Beilstein J. Org. Chem. 2017, 13, 2332–2339.
Figure 1: Frequency of reported melting points for acetaldehyde 2,4-dinitrophenylhydrazone (9) from the Reaxys database (June 2017). Where a
melting-range had been reported, the upper limit was chosen for the analysis.
generated by dropping water on calcium carbide (CaC2) and Oxygen was carefully excluded from the reaction system to
passed through two washing bottles with concentrated sulfuric prevent oxidation of 1 to inactive the tungsten(VI) species [13].
acid to remove polar impurities (Figure 2).
The experiment was carried out either by incubating catalyst 1
Figure 2: Experimental setup for the study of catalytic acetylene hydration. Red arrows indicate the direction of the gas flow.
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Beilstein J. Org. Chem. 2017, 13, 2332–2339.
in acetylene-saturated aqueous acetonitrile (static conditions), or Since the catalysis had given a negative result, we felt it
by prolonged bubbling of a stream of 2 through the catalyst important to ascertain that we would have detected acetalde-
solution (dynamic conditions) [13]. In the latter case, the hyde, had it formed under reaction conditions. As a test of the
exhaust gas was bubbled through an acidic solution of DNPH to analytical procedures, a hydration experiment with acetylene
absorb volatile carbonyl compounds. Emphasis was placed both gas was performed in the same setup (Figure 2), but with
on unequivocal and direct analysis of acetaldehyde (3) in the CpRuCl(PPh3)2–ISIPHOS as established anti-Markovnikov
reaction solution, and on identification of all major species in alkyne hydration catalyst (cf. Table 1) [24]. The catalyst solu-
the reaction solution or the DNPH solution. For this purpose, tion was prepared in aqueous triethyleneglycol dimethyl ether
1H and 13C NMR analyses of the reaction (catalyst) solutions (triglyme), because acetonitrile is a competitive inhibitor of the
were performed with addition of DMSO-d6 for locking. The ruthenium catalyst [24], and acetone would have disturbed the
components detected in the reaction solution were catalyst 1 by DNPH-test for acetaldehyde. Bubbling acetylene through the
δH 1.14 and 3.15 for the tetraethylammonium cation, and δC solution for 4 h at 35 °C produced plenty of precipitate in the
119.5, 141.0 for the mnt ligand, which is characteristic for the DNPH-exhaust solution, which was shown to be acetaldehyde
tungsten complex and proves that 1 was intact throughout the dinitrophenylhydrazone 9 by 1H and 13C NMR spectroscopy.
reaction. Signals for 2 were detected at δH 2.66 and δC 75.1; the The NMR spectrum of the catalyst solution contained distinct
identity as acetylene was proven beyond doubt by analyzing the signals for acetaldehyde (δH 2.11, 9.64; 3J(H,H) = 9.6 Hz), but
13C,1H coupling pattern from the 13C satellites in the 1H NMR their rather low intensity indicates that product vapors are effi-
spectrum (Figure 3) [43,44].
ciently transferred further into the DNPH exhaust solution by
the acetylene stream.
Conclusion
Acetylene hydratase is a fascinating enzyme that catalyzes the
hydration of ethyne to ethanal (acetaldehyde) by what appears
to be a nucleophilic mechanism with assistance by carboxylate
base. As such, the mechanism is related to Reppe's alkali-medi-
ated addition of alcohols to acetylene that gives vinyl ethers
[45]. Alkali-mediated hydration of alkynes is not known for
regular alkynes, since the carbonyl reaction products are
unstable to the strongly alkaline reaction medium [18]. Base-
mediated hydration is possible for π-acceptor substituted
alkynes, but nucleophilic addition of secondary amines fol-
lowed by acidic hydrolysis to the carbonyl compound is usually
preferred [18]. The discovery of a tungsten-based enzyme with
ethyne hydration activity was quite surprising and the enzy-
matic reaction mechanism was not immediately evident, for
simple tungsten complexes had not been known to bring about
alkyne hydration. The situation changed 1997 by a report of
Sarkar et al. who described the activity of complex 1 for acety-
lene hydration [13]. This observation was at the same time
remarkable in terms of a new reactivity and appeared to imme-
diately "explain" the enzymatic reaction as primarily metal-
Figure 3: Identification of ethyne (2) in the reaction solution by cou-
pling pattern analysis of 13C-satellite signals.
The concentration of 2 in the final reaction solution was deter- based. Through our writing of a review article on alkyne hydra-
mined to 0.1 mol/L. No new compounds could be detected in tion chemistry [18] as well as our work in ruthenium-catalyzed
appreciable amounts. Acetaldehyde in particular was absent, anti-Markovnikov hydration of terminal alkynes [24-28] we be-
with an estimated limit of detection corresponding to 0.05 came aware of this chemistry and considered the fascinating
turnovers. The result was the same in the dynamic (bubbling) or possibility of an enzymatic vinylidene-type alkyne hydration
static (incubation) experiment. No carbonyl hydrazones were mechanism. If applicable, this would have opened new possibil-
detected in the DNPH absorption solutions or the reaction solu- ities for catalytic anti-Markovnikov hydration of alkynes too,
tions after treating with acidic DNPH by recording 1H NMR which is currently limited to ruthenium(II) complexes [18].
spectra of the precipitates or filtrates after evaporation to Unfortunately, neither was complex 1 active in the hydration of
dryness. Only unchanged DNPH was detected.
higher, terminal alkynes, nor could we reproduce the originally
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Beilstein J. Org. Chem. 2017, 13, 2332–2339.
16.Schulzke, C.; Ghosh, A. C. Molybdenum and Tungsten Oxidoreductase
Models. In Bioinspired Catalysis; Weigand, P.; Schollhammer, P., Eds.;
Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2014;
pp 349–382. doi:10.1002/9783527664160.ch13
17.Liu, Y.-F.; Liao, R.-Z.; Ding, W.-J.; Yu, J.-G.; Liu, R.-Z.
J. Biol. Inorg. Chem. 2011, 16, 745–752.
reported hydration of acetylene. The experiments presented here
exclude generation of acetaldehyde from ethyne under the reac-
tion conditions, even in trace amounts. They also highlight
potential difficulties in the analytical detection of acetaldehyde
and emphasize the problem of potential contamination of acety-
lene by acetone, which is used as stabilizer in commercial
acetylene pressure gas bottles. Conclusively, the title question
can be answered with "no".
doi:10.1007/s00775-011-0775-x
18.Hintermann, L.; Labonne, A. Synthesis 2007, 1121–1150.
doi:10.1055/s-2007-966002
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20.Iwasawa, N.; Shido, M.; Kusama, H. J. Am. Chem. Soc. 2001, 123,
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Supporting Information
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2013, 32, 6867–6870. doi:10.1021/om400445n
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Supporting Information File 1
Experimental procedures and NMR spectra.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-13-230-S1.pdf]
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ORCID® iDs
Lukas Hintermann - https://orcid.org/0000-0003-2622-8663
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