Can Molecular Sieves be Used as Water Scavengers in Microwave Chemistry?

Abstract

The use of sealed-vessel microwave-assisted organic synthesis in combination with 4 Å molecular sieves as water scavengers is investigated. Two classical model transformations, namely acetal formation and imine formation, are evaluated under both conventional heating and microwave dielectric heating. The results obtained in these studies indicate that molecular sieves cannot be used effectively as water scavengers in microwave-assisted transformations owing to the fact that these zeolites are generally most effective in adsorbing water at room temperature. For both acetal and imine formation, performing these reversible reactions at a higher temperature in the presence of molecular sieves leads to significantly reduced conversions compared with experiments at room temperature.

Metal Free, Microwave Assisted Preparation of N-Sulfonyl and N-Sulfinyl Imines and Imidates

“…The addition of solid dehydrating agents such as MgSO 4 , or the more acidic CuSO 4 (Table , entry 2,3), did not improve the result: both the inhomogeneity of the reaction medium and the strong absorption of the microwave by the desiccant led to local overheating causing the partial degradation of the substrates or products. Employing molecular sieves is often a solution for trapping water in classically heated reaction; however, it has been shown to be inefficient under microwave irradiation, because of its strong microwave absorption causing overheating and limiting the capture of water . In order to circumvent this obstacle, it was envisioned to sequester the molecular sieves at the top of the tube, where the water vapors would be trapped, without overheating.…”

“…Employing molecular sieves is often a solution for trapping water in classically heated reaction; however, it has been shown to be inefficient under microwave irradiation, because of its strong microwave absorption causing overheating and limiting the capture of water. 23 In order to circumvent this obstacle, it was envisioned to sequester the molecular sieves at the top of the tube, where the water vapors would be trapped, without overheating. A perforated septum filled with 4 Å molecular sieves was thus placed in the tube (the septum being half way down) (Figure 1), and the reaction mixture was heated at 180 °C for 20 min (Table 1, entry 4).…”

Acyloxylation of Cyclic Enones: Synthesis of Densely Oxygenated Guaianolides

“…We considered five variables to study: (i) time prior to the substrate addition, needed for the formation of the radical Mn(III) carboxylate, (ii) temperature, (iii) equivalents of KMnO 4 , (iv) equivalents of acetic acid, and (v) amount of 4 Å molecular sieves. As illustrated in Table (entries 1–6), the use of 4 Å molecular sieves resulted in poor yields presumably due to the high temperature . Other additives were also evaluated for the same role, among them acetic anhydride.…”

“…8,7.8,5.2,2.6 Hz,1H), 2.09 (dddd, J = 17. 1, 12.7, 12.7, 5.2 Hz, 1H), 1.97 (s, 3H), 1.70 (tq,J = 7.4,7.4 Hz,2H), 0.97 (t, J = 7.4 Hz, 3H); 13 C NMR (126 MHz,CDCl 3 ) δ 193.7,173.0,162.3,125.6,72.9,36.2,30.6,28.4,24.3,18.6,13.7; IR (film) ν max 2966,2936,2876,1743,1689,1633,1381,1177,1104 7, 173.2, 162.3, 125.6, 72.9, 34.3, 31.4, 30.5, 28.4, 24.7, 24.2, 22.4, 14.0; IR (film) ν max 2956,2933,2872,1743,1690,1633,1380,1208,1167,1103 3-Methyl-6-octanoyloxycyclohex-2-enone (11). Yellow oil (47 mg, 74%); 1 H NMR (400 MHz, CDCl 3 ) δ 5.90 (br s, 1H), 5.31 (dd, J = 13.3, 5.4 Hz, 1H), 2.55 (m, 1H), 2.39 (m, 3H), 2.22 (dddd, J = 7.…”

Dehydration of Benzyl Alcohols in Phosphonium Ionic Liquids: Synthesis of Ethers and Alkenes

“…Kappe et al. have previously noted that molecular sieves are not efficient water scavengers under microwave‐assisted reaction conditions 26…”