A highly negatively charged γ-Keggin germanodecatungstate efficient for Knoevenagel condensation

Abstract

A tetra-n-butylammonium (TBA) salt of a γ-Keggin -6-charged germanodecatungstate, [γ-H(2)GeW(10)O(36)](6-) (I), could act as an efficient homogeneous catalyst for Knoevenagel condensation of active methylene compounds with carbonyl compounds.

Base Catalysis by Mono- and Polyoxometalates

“…Thus, Knoevenagel condensation is typically utilized as a test reaction to estimate the basic strength of the catalyst because there are various active methylene compounds with different pK a values. [39,46,56], which indicates that highly negative charges of POMs play an important role in the Knoevenagel condensation. In particular, TBA 6 [γ-H 2 GeW 10 O 36 ] could be applied to the Knoevenagel condensation of a wide range of substrates including unreactive phenylacetonitrile (as a donor) and ketones (as acceptors) (Scheme 9) [39].…”

“…Thus, the terminal lacunary oxygen atoms and the bridging oxygen atoms would be possible active sites for [γ-XW10O34(OH)2] 6− (X = Si and Ge) and [γ-XW10O35(OH)] 7− (X = Si and Ge), respectively ( Figure 6). The di-and mono-protonated germanodecatungstates [γ-GeW10O34(OH)2] 6− and [γ-GeW10O35(OH)] 7− could act as homogeneous catalysts for Knoevenagel condensation and the chemoselective acylation of alcohols, respectively [39,46].…”

“…A typical reaction for the preparation of phosphododecatungstate is given in Equation (2) [39,46]. assignment of the four protons on the lacunary sites remains controversial.…”

Classical Keggin Intercalated into Layered Double Hydroxides: Facile Preparation and Catalytic Efficiency in Knoevenagel Condensation Reactions

“…Interestingly, the TON can reach as high as 47980 (Table 4, Entry 1), which is the highest reported so far in comparison with other reported catalysts such as VAp in water (TON = 39583, Entry 2), [44] DMAN/SiO2 in ethanol (TON = 10000, Entry 3), [45] polystyrene-supported poly(amidoamine)dendrimer in ethanol (TON = 198, Entry 4), [46] amine-grafted PE-MCM-41 in cyclohexane (TON = 198, Entry 5), [47] and others listed in Table 4. [48][49][50][51][52][53][54][55][56][57][58][59][60] In an effort to confirm the fact that the catalysis is truly heterogeneous, the Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate has been selected as an example. After 120 min of reaction, we removed the Mg3Al-PW12 catalyst from the reaction mixture of benzaldehyde and ethyl cyanoacetate by filtration, and the reaction mixture left to proceed under the same conditions (60 o C, Vi-propanol:Vwater = 2 : 1).…”

Lewis Base Catalytic Properties of [Nb10O28]6− for CO2 Fixation to Epoxide: Kinetic and Theoretical Studies

“…[6][7][8] It was also reported that [gHGeW 10 O 36 ] 7À showedB rønsted base catalysis for Knoevenagel condensation reactions. [9,10] The base catalysis was ascribed to the negativity of the Oa toms. Group Ve lements (V,N b, Ta ) form more negatively charged POMs than Group VI elements (Mo, W) due to the difference in their maximum oxidation states.…”