Solid‐Supported Green Synthesis of Substituted Hydrocinnamic Esters by Focused Microwave Irradiation

Abstract

An efficient chemoselective hydrogenation protocol for substituted cinnamic esters is developed for the synthesis in quantitative yield of corresponding bioactive dihydrocinnamic esters with solid-supported palladium chloride/ammonium formate (cat.) in HCOOH/H 2 O 1 : 2 as a hydrogenating agent under focused-microwave irradiation for 10 min.Introduction. -Environmental constrains [1] have led to the development of new chemical processes employing microwave, ultrasound, ionic liquids, recyclable catalysts and reagents, and biotransformations, etc. Catalysts and reagents on solid support under solventless (solvent-free) conditions is an especially important development of 'green' technologies which have made landmark contributions to preserve environment due to elimination of complicated workup procedures and reduction in waste effluent (more than 15 billion kg of solvent consumed worldwide annually!) [2]. Further, use of microwave irradiation on solid-supported reactions has enhanced the impact of the latter as the high heating efficiency of microwave results in remarkable rate enhancement, higher yields, greater selectivity, and ease of manipulation of such reactions [3]. In addition, the limitation of the microwave-assisted reactions in solvents enforcing development of high pressure and special sealed vessels are circumvented by this solid-support strategy [4], which enables organic reactions such as oxidation, hydrogenation, etc., to occur rapidly at atmospheric pressure and to upscale the reaction on a preparative scale.A lot of hydrogenation protocols are available for the selective hydrogenation of C=C bonds of unsaturated esters which either require a special apparatus and H 2 gas (potentially explosive) [5]

Pd/C Catalysis under Microwave Dielectric Heating

“…some examples of a rapid cinnamate hydrogenation [56] (ii) the preparation of a key intermediate (57) The use of Pd/C with formates under MWs also led to the double bond reduction of an intermediate for the synthesis of enantiopure 7-substituted azepane-2-carboxylic acids [60]. An interesting application of the formate hydrogen transfer process has been described in the synthesis of pyrrole derivatives with multiple aryl substituents (such as 64 in Scheme 19).…”

“…(i) some examples of a rapid cinnamate hydrogenation [56] (ii) the preparation of a key intermediate (57) for the first synthesis of the unusual nitrogen-containing N-(3-carboxylpropyl)-5-amino-2-hydroxy-3-tridecyl-1,4-benzoquinone isolated from Embelia ribes [57]; (iii) the synthesis of novel selective σ1 opioid ligands (59) [58]; (iv) the preparation of an intermediate (61) for synthesis of fluorinated cyclic β-amino acid [59] (Scheme 18).…”

Chitosan as biosupport for the MW-assisted synthesis of palladium catalysts and their use in the hydrogenation of ethyl cinnamate

“…As an example, EHC is used for the synthesis of the HIV-1 protease inhibitor and lastly it also finds several employments in food and flavor industries [33,34]. This reaction has been carried out by other researches under different reaction conditions [14,[35][36][37][38][39][40][41], including also microwave irradiation [38][39][40][41]. Irfan studied and claimed the complete conversion of ethyl cinnamate on a 0.1 mmol scale after 5 min of MW irradiation at 80 • C in ethyl acetate as solvent, using 2 mol% of Pd/C catalyst (10 wt%), providing the expected ester in 86% yield with a TON of 50 [38].…”

“…The authors moreover compared the catalytic performances of the same system conducting the reaction both under domestic-microwave and under conventional heating. In the first case, EHC was obtained in 79% of yield after 12 min working at 960 W, whereas when traditional heating was adopted and the mixture was refluxed at 100 • C, only a 15% of yield was ascertained after 12 h of reaction [41]. Now, the selective hydrogenation of ethyl cinnamate to ethyl hydrocinnamate in the presence of MW-synthesized palladium nanoparticles supported on chitosan adopting traditional heating and MW irradiation has been investigated and, to the best of our knowledge, this is the first study on this hydrogenation where this type of catalyst is employed.…”

Bi(OTf)3-catalysed prenylation of electron-rich aryl ethers and phenols with isoprene: a direct route to prenylated derivatives

“…3b Unfortunately direct prenylation under various conditions gave the O-prenyl derivative as the major product. 45 The synthesis of derrubone 1 was achieved indirectly by; 1) selective MOM-protection of the isoflavone core at 7-OH, 2) O-allylation at 5-OH, 3) Claisen rearrangement, 4) crossmetathesis using Grubbs' second-generation catalyst with 2methyl-2-butene to provide the prenyl group and 5) MOM- 50 deprotection in 27% yield over 5 steps (Scheme 1).…”