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

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 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".


General experimental procedure for catalytic microwave hydrations
A 10 mL microwave vial with magnetic stirring bar was thrice evacuated and argon backfilled.
Under argon, degassed water (0.5 mL), degassed acetone (2.0 mL), catalyst and 10-undecyn-1-ol (20.0 µL, 104 µmol) were added and the vial was capped. The reaction mixture was microwaveheated to 160 °C and stirred for 15 min at target temperature. After cooling, the reaction mixture was transferred into a vial containing a solution of a known amount of 1,3-dinitrobenzene in Et 2 O (0.5 mL). The reaction vessel was repeatedly rinsed with Et 2 O (3 × 2 mL) and the resulting solution thoroughly homogenized. An aliquot (0.5 mL) was diluted with Et 2 O (2 mL), washed with water (3 × 2 mL) and brine (2 mL) and dried over anhydrous MgSO 4 . After evaporation of the solvent (water jet pump), CDCl 3 (0.6 mL) was added to the residue. The homogenized solution was transferred into an NMR tube and analyzed by NMR, using a pulse repetition delay of 20 seconds.

Product analysis by quantitative 1 H NMR spectroscopy
The 1 H NMR spectrum was Fourier-transformed, phase-corrected and baseline-corrected. The chemical shift was referenced to TMS ( 0.00). 3 The peak integral of the signal for internal standard 1,3-dinitrobenzene (DNB) ( 9.08; t, 4 J(H,H) = 2.2 Hz, 1 H; integration excluding 13 C-satellites) was set to the molar ratio n( DNB )/n( alkyne ).

Attempted catalytic hydration of 1-octyne with tungsten complex 1
Acetone (4 mL) and water (1 mL) were placed in a Schlenk vessel (10 mL) and degassed by argon sparging at 0 °C; tungsten complex (1; 18 mg, 24 mol) and 1-octyne (160 mg, 1.45 mmol) were added and the resulting purple solution placed into an oil-bath at 50 °C and stirred for 20 h. A sample was removed for GC-MS analysis and only revealed the presence of 1-octyne ( Figure S1).

S8
For the "incubation" reaction, the catalyst solution was placed on the bottom of a 250 mL Schlenk vessel which was filled with a pure ethyne atmosphere prior to incubation at 35 °C; since there was no gas-flow during the experiment, no DNPH-absorber vessel for exhaust gas was used.
After charging the apparatus with CaC 2 and degassed H 2 O, it was flushed with argon. The reaction flask was then charged with acetonitrile and water, placed in an ice-bath and flushed with argon for ca 20 min. Finally, the catalyst was added to the reactor in an argon counter-stream and acetylene generation was started by very careful, slow dropping of water on CaC 2 . The argon stream was stopped and replaced by a steady flow of acetylene.

a) Experiment with acetylene bubbling
The reaction flask was charged with complex 1 (100 mg, 135 mol), acetonitrile (10 mL) and water (10 mL Compared to the initial amount of solvents (10 mL each), the losses are readily explained by evaporation into the gas flow during the experiment.

Experiments using acetylene gas from a commercial pressure gas bottle
a) Blank experiment without catalyst: Acetylene gas from a pressure bottle was slowly bubbled through a mixture of water (10 mL) and acetonitrile (5 mL) in a 25 mL Schlenk vessel. The exhaust gas was bubbled through an acidic (2 M HCl aq) solution of DNPH. After 3 h, the precipitate, which had formed in the DNPH-solution, was isolated by filtration and washed with water to give small amounts of a dark yellow solid identified as acetone dinitrophenylhydrazone (10)