Monomode reactor

Until 2004, Biotage (formerly Personal Chemistry) offered the Emrys monomode reactor series of instruments (Fig. 3.19). Although no longer commercially available, many instruments are currently still in use. Therefore, this line of products is discussed in detail in this chapter [15]. 

Consequently, which strategy is utilized in reaction optimization experiments is highly dependent on the type of instrument used. Whilst multimode reactors employ powerful magnetrons with up to 1500 W microwave output power, monomode reactors apply a maximum of only 300 W. This is due to the high density microwave field in a single-mode set-up and the smaller sample volumes that need to be heated. In principle, it is possible to translate optimized protocols from monomode to multimode instruments and to increase the scale by a factor of 100 without a loss of efficiency (see Section 4.5). 

To reduce the risk of container failure, the pressure vessels are equipped with several safety features. These can include an effective self-venting system where unforeseen overpressure is released by a quick open-resealing step, or the use of safety disks which rupture when their pressure limit is reached. The small vials (0.2-20 mL) of some monomode reactors are protected by the pressure limit (20 bar) of the caps used, which is significantly lower than the operating limit of the vials themselves (40-50 bar). 

Nucleophilic substitutions of benzyl chloride or benzyltrimethylammonium chloride as electrophiles with Ph3P or Bu3P as nucleophiles have been performed with accurate control of the power and temperature by use of a monomode reactor. The results were carefully compared under similar conditions with microwave or A activation [85] (Eq. (30) and Tab. 3.11). 

Tab. 3.19 Transesterification in 15 min at 160°C in basic medium with methyl benzoate and laurate (monomode reactor, relative amounts 1 2 2).

Fig. 5.1 Thermal behavior induced by microwave irradiation of PhC02l< under different conditions (monomode reactor, 180W).

R R Multimode oven (250 W) Monomode reactor (90 W) Classical heating... 

The advantage of using a monomode reactor rather than a domestic oven is clearly apparent. 

Tab 5.22 -Elimination from bromoacetals in a monomode reactor (75 W) comparison with sonochemical conditions and classical heating. 

Tab. 5.26 1,3-Dipolar cycloaddition of DNPI to chalcones in a monomode reactor (30 W).

MW monomode reactor Ultrasound (cleaning bath) Conventional heating (oil bath)... 

Reactions involving a solvent under reflux require the use of modified commercial microwave ovens. In these modified systems the oven is perforated on the top to accommodate a reflux condenser and a 10 cm pipe is used to avoid microwave leakage the turnable dish is replaced by a magnetic stirrer or by monomode reactors especially designed for chemical synthesis [15]. 

Deshayes described the cydoaddition of 1-amino-1,3-butadienes 63 with ethyl acrylate (34) under the action of microwave irradiation in a monomode reactor with temperature control [59]. Irradiation for 30 min at 70 °C in the absence of solvent afforded a 60 40 ratio of an inseparable mixture of the endo and exo isomers in 90% yield. Classical heating under the same conditions did not affect the selectivity but the yield was lower (Scheme 9.17). 

Garrigues described the reaction of the 1-azadiene 84 with dimethyl acetylenedi-carboxylate (26b) on a graphite support [35], After sequential microwave irradiation for 10 min in a monomode reactor pyridine 85 was obtained in 60% yield. This azadiene does not react when conventional heating is used (Scheme 9.25). 

Diaz-Ortiz described the cycloaddition of 4,6-dimethyl-l,2,3-triazine (105) with en-amines to give condensed pyridine systems [82]. These reactions were performed in a monomode reactor at a power of 270 W for 20 min at 15 °C. The reactions can also be performed with pyrrolidine, with cyclic ketones used as precursors of the enam-... 

Monomode reactors Prolabo, Synfhe-wave S402 and S1000 (actually not on the market). CEM, STAR system 2 and 6 and Discover. Personal Chemistry, Smith Synthesizer and Smith Creator. Temperature measurement is one of the main problems in microwave-assisted reactions. See Ref. [2] for temperature-measurement systems. 

Another example of microwave-assisted PSR chemistry involves the rapid conversion of amides to thioamides by use of a polystyrene-supported Lawesson-type thio-nating reagent. By use of microwave irradiation at 200 °C in sealed vessels (monomode reactor), a range of secondary and tertiary amides was converted within... 

Fluoren-9-one In a Pyrex matrix adapted to a Synthewave 402 monomode reactor, fluorene 1 (2 mmol, 0.332 g) was added to the KMn04-alumina mixture (6 mmol, 4.74 g). After 5 min of mechanical stirring, the mixture was irradiated (under stirring) at 150 W for 10 min. At the end of exposure to microwaves, the mixture was cooled to room temperature and eluted with diethyl ether (50 mL). After filtration and solvent removal, the crude product was identified by comparison (GC and NMR) with an authentic sample. 

All reactions were performed in a cylindrical Pyrex vessel using 10 mmol of nitrone 1 and 20 mmol of dipolarophiles 2 or 4. The mixtures were introduced into the monomode reactor (Maxidigest MX 350 Prolabo) at the powers and times indicated in Table 1. Temperatures were recorded throughout the reaction using an IR detector connected to the reactor. At the end of the reaction, after cooling down and extraction with CH2CI2, products 3, 5a and 5b were analyzed by GC methods using an internal standard and authentic samples. 

A mixture of. sy/i-benzaldoxime 1 (10 mmol, 1.21 g) or 2-hydroxyacetophenone oxime 4 (10 mmol, 1.52 g) and one equivalent of 98% anhydrous zinc chloride (10 mmol, 1.36 g) was introduced in a Pyrex tube and submitted to microwave irradiation for 20 min in the Synthewave S402 monomode reactor at 140 °C. The mixture was cooled to room temperature and dissolved in methanol and then filtered through silica gel. Products were analyzed by capillary gas chromatography using methyl benzoate as an internal standard and compared by NMR with authentic samples. 

Solvent-free conditions can be employed according to three main methods (a) using only neat reactants (b) reactants adsorbed onto solid supports or (c) reactants in the presence of phase transfer catalysts (in the case of anionic reactions). Besides the apparent potential benefits in solvent usage, reactions can be conducted conveniently and rapidly, often without temperature measurement in domestic microwave ovens. However, they now are often carried out under more precisely controlled conditions using monomode reactors initially introduced by the former French manufacturer Prolabo. Nowadays, several systems are available that provide facilities for the accurate measurement and monitoring of temperature throughout the reaction by modulation of emitted power with an infrared detector or an optical fibre. 

The simplest solvent-free method involves irradiation of neat reactants as interfacial reactions in an open container, either in a domestic oven or in a monomode reactor [34,55]. In the absence of reagents or supports, the scope for such processes appears to be limited to relatively straightforward condensations that can be conducted without added catalysts, to nucleophilic additions, Sn2 alkylations using neutral nucleophiles as amines or phosphines or to intramolecular thermolytic processes such as rearrangement or elimination. 

Bougrin et al. (1995) reported the first practical utilization of microwave irradiation with nitrile imine as 1,3-dipole using solvent-free conditions. In this work, a comparative study of the reactivity of diphenylnitrilimine (DPNI) (1) with some dipola-rophiles was made in dry media using microwave irradiation. The good yields were obtained on mineral support in a monomode reactor. 

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