ISSN:
0947-3440
Keywords:
Diels-Alder cycloaddition
;
Electron transfer
;
Chemistry
;
Organic Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
(E,E)-1 and (E,Z)-1,4-bis(dimethylamino)-1,3-butadiene (2) as model systems for highly electron-rich dienes were allowed to react with olefins of increasing electron-acceptor capacity under standard concentrations (0.4 M solutions of diene and dienophile) and at reaction temperatures from -50°C to a maximum of +60°C (slow decomposition of the diene). Methyl acrylate (3), acrylonitrile (4), and dimethyl methylfumarate (6) did not form cycloadducts with 1 at 35°C in six weeks. Isopropylidenemalononitrile (5) undergoes dimerization. Maleo- (9) and fumaronitrile (10) add to 1 with retention of stereochemistry ( ≥98%). Dimethyl maleate (7) isomerizes under the reaction conditions to dimethyl fumarate (8) and reacts with 1 to the cycloadduct of 8. N-Methylmaleimide (11) gave the expected cycloadduct. The monosubstituted C atoms of the C—C double bond of 1,1,2-tris(methoxycarbonyl)ethene (12) and 2-cyano-1,1-bis(methoxycarbonyl)-ethene (13) add to C-2 of 1 to form zwitterions which stabilize themselves by a 1,3-hydrogen shift to 24 and 25. The reactions of 1 with 1,1-dicyano-2-methoxycarbonylethene (14), 1,1-dicyano-2,2-bis(methoxycarbonyl)ethene (15), and tetracyanoethene (18) provided, though very reactive, no isolable cycloadducts or other identifiable products. Dimethyl dicyanomaleate (16) and dimethyl dicyanofumarate (17) lead in non-stereospecific reactions with 1 to the same cycloadduct even at -50°C in dichloromethane. The same cycloadduct is obtained from 2 and 17. For the reactions of 16 and 17 with 1 electron transfer from diene to dienophile could be demonstrated. Intermediates were detected 1H-NMR-spectroscopically at -95°C which transformed into the cycloadduct when warmed to-50°C. A comparison of the preparative results with oxidation potentials of 1 and 2, and the reduction potentials of the olefins shows, that electron transfer is possible when oxidation and reduction potential are close together. It is concluded from the loss of stereochemistry during the cycloadduct formation in those cases (16 and 17) where electron transfer is observed, and furthermore from retention of stereochemistry (9 and 10) when no electron transfer could be detected, that electron transfer may be a possibility in [4 + 2] cycloadditions, but that it is not the general mechanism of the Diels-Alder reaction.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/jlac.1995199507154
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