Nano-isocyanurate-Periodic mesoporous organosilica ( PMO ) : a heterogeneous catalyst for three-component synthesis of tetrahydrobenzo [ b ] pyrans in water

Isocyanurate containing periodic mesoporous organosilicas (ICS-PMO) without any post-modification was used as a novel heterogeneous catalyst for synthesis of tetrahydrobenzo[b]pyran derivatives via one‐pot three‐ component condensation of aromatic aldehydes with malononitrile (or ethyl cyanoacetate) and dimedone. The catalyst was characterized by FT-IR Spectroscopy, TEM, SEM and BET techniques. Products were obtained in short reaction times with excellent yields in water under reflux conditions. The catalyst could be recycled and reused for several times without noticeably decreasing the catalytic activity.


Introduction
Green chemistry can be recognized as a pioneering approach, which widely reports intrinsic atom economy, energy savings, waste reduction, easy workups, and the avoidance of hazardous chemicals [1].In this context, the efficiency of heterogeneous catalysis in organic synthesis could be considerably improved by employing nano-sized catalysts due to their extremely small size and large surface to volume ratio [2].The development of environmentally benign, efficient, and economical methods for the synthesis of biologically interesting compounds remains a significant challenge in synthetic chemistry [3].Along this line, PMOs have received great attention as heterogeneous catalysts.A PMO is a special type of hybrid organic-inorganic materials, in which organic moieties are integrated into the silica framework.PMOs can be synthesized by using various structure directing agents such as ionic surfactants, oligomeric surfactants, and nonionic block copolymers.Because of uniform distribution of organic bridging groups inside mesopore walls, PMOs become very promising materials for potential applications ranging from highly selective adsorbents and catalysts to sensing devices and hosts for biomolecules.
On the hand, water has been applied in organic reaction as solvent, and it has several advantages such as low cost, safety, non-polluting nature, and operational simplicity [4].Furthermore, tetrahydrobenzo [b] pyran derivatives are an important class of heterocyclic compounds having important pharmaceutical and biological activity.These compounds are used as anticancer, anticoagulant, diuretic, spasmolytic, and, etc [5].Realizing the importance of 4H-pyran derivatives, several synthetic methods have been reported with the aim of obtaining more biologically potent heterocyclic systems using different catalysts including nano-sized magnesium oxide [6], silica-bonded 1,4-diazabicyclo [2.2.2]octane [7], silica nanoparticles [8], electro generated base, baker's yeast, and aminofunctionalized ionic liquid.Other synthetic methods contain of microwaves, ultrasonic radiation, and utilizing additives like hexadecyltrimethylammonium bromide, triethylbenzylammonium chloride, other alkylammonium salts, 4-dodecylbenzenesulfonic acid, and (S)-proline.However, limitations of the above methods include poor yields, difficult work-up, and toxic elements.We report herein the synthesis of isocyanurate-PMO (PMO-ICS) as a new catalysis and its application in the synthesis of pyran derivatives (Scheme 1).Scheme 1 Three-component reaction of malononitrile (2a), 4-chlorobenzaldehyde (3a), and dimedone (4a)

Preparation of isocyanurate-PMO
1 mmol of 1,3,5-tris(2-hydroxy ethyl)isocyanurate and 2 mmol of 4-toluenesulfonyl chloride were poured in a test tube and then dry acetone was added as solvent.This tube was placed in an ultrasonic bath at 60 C for one hour.In the next stage, MCM-41 (261 mg) was added to test tube and then the reaction mixture was sonicated at 60 C for 1 hour.The obtained white solid was filtered and dried at 60 C over night

Preparation of 4H-benzo[b]pyran derivatives
A mixture of aromatic aldehydes (1.0 mmol), malononitrile (1.0 mmol), dimedone (1.0 mmol), and 0.01g isocyanurate containing PMO was stirred under reflux conditions in water.Progress of the reaction was monitored by TLC.After completion of the reaction, mixture was filtered for separation of heterogeneous catalyst.

Results and Discussion
The IR spectrum of isocyanurate-PMO indicates the CH-stretching mode at 2854 and 2926 cm -1 and the mode of amidic carbonyl groups related to incorporation of isocyanurate substitution at 1689 cm - 1 as well comparatively long peak of Si-O-Si bindings at 1078cm -1 .
Only a trace amount of the favorable product was obtained under solvent-free conditions even at 100 °C (Table 1 entries1-2).We began to detect the catalytic activity of different amounts of 1, 3, 5-Tris (2hydroxy ethyl) isocyanurate.An excellent yield was obtained with 8 mol%.The presence of higher amounts of catalyst did not produce higher yields (entries 3-5).Then we investigated the performance of 8 mol% of 1, 3, 5-Tris (2-hydroxy ethyl) isocyanurate to catalyze the reaction in different organic solvents (Table 1 entries 6-11).It is noteworthy to mention that H2O showed the best result.Both aromatic carbocyclic and heterocyclic aldehydes containing electron-withdrawing and electrondonating groups underwent this condensation to furnish tetrahydrobenzo [b]pyrans in high to excellent yields ( Table 2).

Fig 2
Fig 2 BET analysis of PMO-ICS
Fig 1 IR spectrum of isocyanurate containing PMO The surface area and pore diameter obtained 1545.96 and 2.6 nm respectively, using BET analysis was shown in fig 2.