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Studies on electron transfer as mechanistic concept for [4 + 2] cycloadditions, 1. Cycloadditions of 1,4‐bis(dimethylamino)‐1,3‐butadiene to electron‐deficient olefins
Author(s) -
Lücking Karin,
Rese Michael,
Sustmann Reiner
Publication year - 1995
Publication title -
liebigs annalen
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 0947-3440
DOI - 10.1002/jlac.1995199507154
Subject(s) - chemistry , electron , electron transfer , 1,3 butadiene , computational chemistry , photochemistry , organic chemistry , catalysis , quantum mechanics , physics
( 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 CC 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 1 H‐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.