Highly Efficient C−C Bond-Forming Reactions in Aqueous Media Catalyzed by Monomeric Vanadate Species in an Apatite Framework

Abstract

A calcium vanadate apatite (VAp), in which PO4(3-) of hydroxyapatite (HAP), Ca10(PO4)6(OH)2, is completely substituted by VO4(3-) in the apatite framework, was synthesized. Physicochemical analysis of the VAp reveals the presence of isolated VO4 tetrahedron units with a pentavalent oxidation state. The VAp acts as a high-performance heterogeneous base catalyst for various carbon-carbon bond-forming reactions such as Michael and aldol reactions in aqueous media and the H-D exchange reactions using deuterium oxide. For example, a 200-mmol-scale Michael reaction under triphasic conditions proceeded rapidly, with an extremely high turnover number of up to 260 400 and an excellent turnover frequency of 48 s(-1). No vanadium leaching was detected during the above reactions, and the catalyst was readily recycled with no loss of activity.

Designing bifunctional acid–base mesoporous hybrid catalysts for cascade reactions

“…The presence of a weight loss 50 at this high temperature in the bifunctionalized hybrids confirms the complete oxidation of thiols into sulfonic groups. Moreover, in the case of SO 3 H/OMS-5 (Section C, curve d), a weight loss at around 350ºC is also visible, that can be associated with the decomposition of the residual of HDA molecules that were not 55 completely removed after the surfactant extraction. The DTA curves of grafted-DMAN hybrids (Section B, curves c and d) showed a minor weight loss at around 80-150° suggesting, a lower amount of physisorbed water with respect the one-pot bifunctional hybrids.…”

“…[17][18][19][20] Another strategy to produce acidbase bifunctional catalysts focuses on the use of acid sites derived from the inorganic support surfaces such as weak acid silanols in 50 silica, 21 Brønsted acid sites in silica-alumina 22 or aluminium tetrahedrally coordinated in the framework of the MMW layers in which the base moieties are also incorporated into the inorganic matrix. 23 Among the different synthetic strategies available to covalently 55 incorporate organic functional groups into the silica network, post-synthetic grafting and co-condensation processes are the most suitable. [24][25][26] The later approach, also referred as one-pot synthesis, has been the preferred route because of the easier single-pot synthetic protocol and better control of organosilane 60 loading joined to a more homogeneous distribution of active sites; in fact, the grafting method presents several drawbacks such as the partial blockage of the pores and undesired site-site interactions due to the supramolecular association of the silylated organic precursors.…”

“…Synthesis of the silylated base precursor 1,8-bis(dimethylamino)naphthalene (DMAN) was functionalized with terminal silyl-alcoxide reactive groups, 34,35 (see Fig.1SI for NMR characterisation of the silyl derived DMAN), obtaining 50 mono and bi-silylated DMAN precursors. In fact, to graft DMAN onto the wall of mesoporous silica, a mono-silylated precursor (DMAN-1Sil) is necessary, whilst two terminal reactive silyl groups (DMAN-2Sil) are needed to integrate the base organocatalysts into the framework of mesoporous material 55 through one-pot synthesis with conventional monosilane precursors (TMOS or TEOS), used as additional silicon sources, through sol-gel or micellar routes.…”

“…External surface area and micropore volume were estimated with the t-plot method in the t range from 3.5 to 5. The pore diameter and the pore size distribution were obtained 55 following the Barret-Joyner-Halenda (BJH) method 39 on the adsorption branch of the isotherms. Transmission electron microscopy (TEM) micrographs were obtained with Philips CM10 with capacity to operate at 100KV.…”

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

“…It is worth noting that the Knoevenagel condensation of benzaldehyde and ethyl cyanoacetate gave us a yield of >99.9% using a reaction mixture ratio of benzaldehyde : ethyl cyanoacetate : Mg3Al-PW12 = 47980 : 71970 : 1 at 60 o C in Vipropanol:Vwater = 2 : 1. 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.…”