Heterogeneized Brönsted base catalysts for fine chemicals production: grafted quaternary organic ammonium hydroxides as catalyst for the production of chromenes and coumarins

Designing bifunctional acid–base mesoporous hybrid catalysts for cascade reactions

“…500-550ºC (III). 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.…”

“…In scheme 2, a representation of the two 30 processes used to synthesize the bifunctional hybrids are reported, the last step for both processes being the oxidation of the thiol groups in strong sulfonic acid sites by using a H 2 O 2 aqueous solution, without the use of strong acid solution that also would negatively affect the base properties of DMAN. 35 40 45 Scheme 2-One-pot synthesis of bifunctional mesoporous hybrids Additionally, DMAN base molecules were grafted through post-synthesis treatments, pending from the walls of ordered and non-ordered mesoporous silicas channels, after the transformation 50 of thiols into sulfonic acids, as reported in scheme 3. In this case, more post-synthesis steps (anchoring and oxidation processes) are involved in the preparation of the bifunctional hybrid mesoporous materials.…”

“…This approach has been verified through the use of disilane organosilyl precursors which contain ionic liquids as functional building units. [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.…”

“…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.…”

“…Volumetric analyses were performed by N 2 adsorption isotherms at 77K with a Micromeritics ASAP2010. Before the measurements, the samples were outgassed for 12 hours at 100 º C. The BET specific surface area 37 was calculated from the nitrogen adsorption data in a 50 relative pressure range from 0.04 to 0.2. The total pore volume 38 was obtained from the amount of N 2 adsorbed at a relative pressure of ~ 0.99.…”