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

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 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".

Discovery and Comparison of Homogeneous Catalysts in a Standardized HOT‐CAT Screen with Microwave‐Heating and qNMR Analysis: Exploring Catalytic Hydration of Alkynes

“…Neither would group VI complex K 2 [Cr(oxalate) 3 ] (Table S1, entries 24–26) or Mo(CO) 6 (Table , entry 1); the latter displayed minor activity for alkyne hydrogenation and ‐trimerization. We have recently disproven the earlier claimed acetylene hydration activity of tungsten(IV) complex (NEt 4 ) 2 [WO(mnt) 2 ] (mnt=maleonitrile dithiolate) . The complex was also inactive in the HOT‐CAT screen with 1 (Table , entry 2).…”

“…We have recently disproven the earlier claimed acetylene hydration activity of tungsten(IV) complex (NEt 4 ) 2 [WO(mnt) 2 ] (mnt = maleonitrile dithiolate). [13,39] The complex was also inactive in the HOT-CAT screen with 1 (Table 2, entry 2). Group 7 metals.…”

“…Triisopropylphosphite (entries 10, 11) was supposed to give an approximation of Ishii's alkyne hydration catalyst, [56] but the system performed similar to [IrCl(COD)] 2 alone (entry 1), except showing higher levels of acetal 4 a. A PyBOX ligand suppressed most catalytic activity except polymerization (entry 12), whereas bipyridines restored the Markovnikov hydration and stimulated transfer hydrogenation (entries [13][14]. Fluorinated bipyridine with silver triflate activation slightly increases Markovnikov hydration (entry 15).…”

“…17 elements . Over the past 12 years, alkyne hydration activity has been reported for additional elements (Ca, Co, Ga, In, Tm, Yb, Sc, Bi, Y, Eu, La, Sn) while one reported activity has been refuted (W) . Many more new catalyst systems have been described for elements with previously established activity, the frequency of reports following the order Au≫Cu, Ag>Pt, Ru, Fe, Pd.…”

“…[6] Over the past 12 years, alkyne hydration activity has been reported for additional elements (Ca, [9] Co, [10,11] Ga, [8c] In, [12] Tm, [8d] Yb, [8c] Sc, [8c] Bi, [8a] Y, [8a] Eu, [8a] La, [8a] Sn [8e] ) while one reported activity has been refuted (W). [13] Many more new catalyst systems have been described for elements with previously established activity, the frequency of reports following the order Au @ Cu, Ag > Pt, Ru, Fe, Pd. We now wish to explore a generalized, systematic activity screening that can be applied to any type of potential alkyne hydration catalysts, and which will provide information on the relative activity and selectivity of each catalyst.…”