Organic solvents, absence cycloaddition

In the absence of organic solvents, Sc(OTf)3 catalysed the cycloaddition of aziridines to nitriles to produce substituted imidazolines in good to excellent yields at room temperature and in an air atmosphere. The reaction is believed to progress through a highly reactive cationic intermediate in which the aziridine nitrogen is coordinated to Sc(OTf)3.65 The phosphine-catalysed enantioselective 3 + 2-cycloaddition of buta-2,3-dienoates with arylimines yielded 2-aryl-3-pyrrolidines with 64% ee.66... 

Baker s yeast (By) catalyzed the Diels-Alder reactions of maleic acid and its ethyl ester with cyclopentadiene (5) (Scheme 5.8) at pH 7.2 in H20/Et0H 70 30 at 37°C. After an incubation period of 48 h the cycloadditions gave exo adducts exclusively while, in the absence of biocatalyst, the endo adducts were prevalent in both water and organic solvent. 

We have found that azodicarboxylates possess a unique level of reactivity in their reactions with various imsaturated hydrocarbons under on water conditions. We first noticed this phenomenon in the context of our study on strained olefins. In particular, 1,2-diazetidines such as 40 were accessed via the 2cr - - 2cr -F 2tt cycloaddition of quadricyclane (38) with dimethyl azodicarboxylate (39 Fig. 11.10). This reaction typically requires prolonged reaction times and/or elevated temperatures when carried out in an organic solvent or in the absence of solvent. However, it proceeds rapidly at room temperature or below when performed on water . In this case, the on water reaction appears to be 2—3 orders... 

Product formation was postulated to arise from the spontaneous rearrangement of the initially formed hetero Diels-Alder adducts 127. The similar reaction when performed with an alkyl glyoxylate in the absence of water (neat) or in organic solvent (toluene) inevitably leads to cyclopentadiene dimerization instead of the expected cycloaddition. 

The HDA reaction allows for rapid access to chiral six-membered heterocyclic structures that serve as valuable intermediates in organic synthesis. The first highly enantioselective HDA reaction promoted by a chiral hydrogen bond donor was reported from the Rawal laboratory. While investigating the cycloaddition reactions of amino-siloxy diene 115, it was observed that this diene was exceptionally reactive to heterodienophiles, and underwent HDA reactions with various aldehydes at room temperature, even in the absence of any added catalyst (Scheme 6.14). Subsequent treatment of the intermediate cycloadducts (116) with acetyl chloride afforded the corresponding dihydro-4-pyrones (117) in good overall yields [101]. Further studies of this reaction revealed a pronounced solvent effect,... 

Organic reactions under microwave conditions, including cycloadditions, frequently involve solvent-free dry media procedures. The reaction is performed between neat reagents, at least one of which must be a liquid and polar substance. These are liquid-liquid or liquid-solid systems the latter implies the solid is soluble in the liquid phase or at least that the liquid counterpart is adsorbed on the solid, so reaction occurs at the interface [3e, 32a]. Because of the absence of solvent, the radiation is absorbed directly by the reagents, so the effect of the microwaves is more marked. As a result, high overall yields and purity of the desired products are usually obtained more quickly. 

We also monitored the progress of many of these reactions by NMR in order to get a more accurate measure of rate acceleration. Accordingly, the cycloaddition of quadricyclane with DEAD (49% solution in toluene) was examined both on water and in the absence of any other solvent (Fig. 11.12). A clear rate difference is observed between the two reactions, indicating that the on water effect is operative even when a nonpolar solvent comprises part or most of the organic phase. ... 

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