Microwave irradiation, and

In 2000, it was proposed that the regioselectivity of the [3 + 2] cycloaddition of fullerenes could be modified under microwave irradiation. Under conventional heating, N-methylazomethine yhde and fullerene-(C7o) gave three different isomeric cycloadducts because of the low symmetry of C70 vs. Ceo. Using microwave irradiation and o-dichlorobenzene as a solvent, only two isomers were obtained, the major cycloadduct 114 being kinetically favored (Scheme 39) [75]. The same authors had previously reported the 1,3-dipolar cyclo addition of pyrazole nitrile oxides, generated in situ, to Geo under either conventional heating or microwave irradiation. The electrochemical characteristics of the cycloadduct obtained with this method made this product a candidate for photophysical apphcations [76]. 

Oxazolidines 130 were obtained by reaction of (-)ephedrine with aldehydes under microwave irradiation and in the presence of molecular sieves in order to remove the water formed in the reaction (Scheme 46) [84,85]. As expected. 

When cyanogen bromide was used instead of CS2, the corresponding guanidines 169 were obtained under analogous conditions [108]. Moreover, differently substituted guanidines 171 could be obtained in very good yields when the isothiourea 168 was alkylated with Mel under microwave irradiation and the product treated with a primary amine. An intramolecular version of this reaction was also described for the preparation of structure 172 present in several important natural products (Scheme 61). 

A new approach to indoloquinohne alkaloids from Cryptolepis sanguino-lenta has been reported based on the cyclization of an o-substituted vinyl isocyanate 244 imder microwave irradiation and further additional cyclization based on an Aza-Wittig reaction carried out in the presence of microwaves [159]. The apphcation of this synthetic scheme to the synthesis of Cryptotackienine 247 is reported in Scheme 91. 

A very detailed comparison for every single step of the whole sequence (linking of a pyrazinone to the solid support, Diels-Alder reaction with acetylenic dienophile and cleavage of a formed pyridinone from the resin) was made between microwave irradiation and conventional heating conditions [115]. 

Under microwave irradiation and applying MCM-41-immobilized nano-iron oxide higher activity is observed [148]. In this case also, primary aliphatic alcohols could be oxidized. The TON for the selective oxidation of 1-octanol to 1-octanal reached to 46 with 99% selectivity. Hou and coworkers reported in 2006 an iron coordination polymer [Fe(fcz)2Cl2]-2CH30H with fez = l-(2,4-difluorophenyl)-l,l-bis[(l//-l,2,4-triazol-l-yl)methyl]ethanol which catalyzed the oxidation of benzyl alcohol to benzaldehyde with hydrogen peroxide as oxidant in 87% yield and up to 100% selectivity [149]. An alternative approach is based on the use of heteropoly acids, whereby the incorporation of vanadium and iron into a molybdo-phosphoric acid catalyst led to high yields for the oxidation of various alcohols (up to 94%) with molecular oxygen [150]. 

Using a conventional dorm-room quality microwave oven, we have successfully performed Suzuki-Miyaura cross-coupling reactions catalyzed by Pd/C. Shorter reaction times are obtained (13 min of irradiation) using microwave irradiation compared to conventional methods of heating (> 1 h). Yields with relatively non-volatile aryl bromides range from 65 to 83%. Lower yields (15 - 27%) are obtained with relatively volatile aryl bromides substrates which may be evaporating during the course of the reaction. Ease of reaction set-up, rate enhancement from the microwave irradiation and facile work-up provided by the use of Pd/C makes this a very efficient procedure to mn. 

A series of 1,4-dithiocarbonyl piperazines has been synthesized from aldehydes, piperazine, and elemental sulfur under the action of microwave irradiation and solvent-free conditions. An important nonthermal effect of the radiation was revealed [72] (Eq. 19). 

Fusel oil basically comprises a mixture of alcohols such as isopentanol and isobutanol. The synthesis of isopentyl stearate has been performed using both microwave irradiation and conventional heating under solvent-free conditions (Eq. 20) [73],... 

A nonthermal microwave effect was not observed when identical temperature gradients were produced by classical heating and microwave irradiation and if the reaction temperature was strictly controlled. 

Xu et al. have obtained similar results with n-butyl bromide using TBAB (10 mol%) and alumina (4 1 ivjxv) as the catalyst [13]. Benzyl acetate was also conveniently prepared from sodium acetate and benzyl halide by use of microwave irradiation and PTC in synergy [14]. 

A series of new ethers has been obtained by alkylation of dianhydrohexitols (i. e. iso-sorbide, isomannide, isoidide) under the action of microwave irradiation and PTC conditions. Yields exceeded 90%, a dramatically improvement compared with those from conventional heating, despite similar temperature profiles. The best yields, for example from isosorbide, were obtained in the presence of a small amount of xylene and TBAB as catalyst at 140 °C (Eq. 9 and Tab. 5.6) [20],... 

Reaction times were improved by microwave irradiation, and the same conditions were extrapolated to the synthesis of a series of new furanic diethers by alkylation of furfuryl alcohol by dihalides [83],... 

The Diels-Alder reaction between azonianthracene (61) and bis-substituted olefins to give 6,ll-ethanobenzo-[fo]-quinolizinium salts 62 was greatly accelerated by the action of microwave irradiation and, consequently, the reaction might be applicable to... 

Similar results were obtained in the esterification of acetic acid with 1-propanol performed in the presence of a heterogeneous silica catalyst [39]. The results showed that for this reaction microwave irradiation and conventional heating had similar effects on the reaction rate. 

Partial oxidation of methane to syngas over Ni and Co catalysts was effected by use of microwave irradiation, and compared with conventional heating [73]. Although the same conversion levels and H2/CO ratio (2.0 0.2) were observed, the temperature of the catalyst bed was much lower (200 K) when microwave irradiation was used than with conventional heating. Under both activation modes the Ni-based... 

Very recently a novel one-pot three-step Wittig reaction using microwave irradiation and polymer-supported triphenylphosphine has been reported [57]. By use of... 

A recent method for the synthesis of the indolizine skeleton is represented by a three-component reaction between a-bromo ketones 16, pyridine 17, and ethyl propiolate or diethyl acetylenedicarboxylate. These three reagents, under microwave irradiation and catalysis by basic alumina, afforded a good variety of 3-aroyl indolizines 18 <20030L435> (Scheme 4). 

The application of phase-transfer catalysis to the Williamson synthesis of ethers has been exploited widely and is far superior to any classical method for the synthesis of aliphatic ethers. Probably the first example of the use of a quaternary ammonium salt to promote a nucleophilic substitution reaction is the formation of a benzyl ether using a stoichiometric amount of tetraethylammonium hydroxide [1]. Starks mentions the potential value of the quaternary ammonium catalyst for Williamson synthesis of ethers [2] and its versatility in the synthesis of methyl ethers and other alkyl ethers was soon established [3-5]. The procedure has considerable advantages over the classical Williamson synthesis both in reaction time and yields and is certainly more convenient than the use of diazomethane for the preparation of methyl ethers. Under liquidrliquid two-phase conditions, tertiary and secondary alcohols react less readily than do primary alcohols, and secondary alkyl halides tend to be ineffective. However, reactions which one might expect to be sterically inhibited are successful under phase-transfer catalytic conditions [e.g. 6]. Microwave irradiation and solidrliquid phase-transfer catalytic conditions reduce reaction times considerably [7]. 

Solid-phase [2-i-2-i-2] cycloaddihon was also catalyzed by the Ir complex (Scheme 11.6) [14]. Here, the reachon of a resin-bound dipropargylamine and various monoalkynes proceeded under heating by microwave-irradiation, and isoindoline derivatives were obtained in moderate yield after trifluoracetic acid (TFA)-cleavage of the resin-bound intermediates. 

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