For a long time, and particularly following the presentation of the Woodward-Hoffmann rules, it was assumed that there is one universally valid mechanism for [4 + 21-cycloadditions. Recent investigations, however, indicate that the mechanism is dependent on the pattern of substitution of the diene and the dienophile.[" Thus, some cycloadditions proceed via diradicals,". ' 1 while others are reported to proceed via polar intermediate~. [~] Even an electron transfer with formation of radical ions has been proposed as primary step.I4' The observation of charge-transfer (CT) complexes and the analysis of their significance in Diels-Alder reactions have stimulated further discussion on the degree of charge transfer during these cycl~additions.~~ Herein we report on cycloadditions of the stereoisomeric dienes 1 and 2 and of the bicyclic diene 3 with dienophiles of different acceptor strength. A combination of stereochemical studies with electrochemical measurements and ESR investigations should provide information on the significance of electron transfer in the reactions. For 1, this result can be explained in terms of a thermal isomerization of 1 to 2 prior to the cycloaddition and a rapid reaction of 2, whereas for 2, product formation can be rationalized in terms of concerted cycloaddition. This explanation is consistent with the observation that Z-I-substituted dienes react considerably slower than the Eisomer.['ol It has been shown independently that 1 isomerizes quantitatively to 2 within 5d at 35 "C. If the cycloaddition of 1 and fumaric dinitrile is carried out at 50 "C using the 16-fold higher concentration described in the l i t e r a t~r e ,~~] the isomerization is suppressed and up to 96% of 5 is formed. [''] In each case it was demonstrated by mixing that 1.5 YO of the other isomer could have been detected by 'H-NMR spectroscopy. The stereoselectivity of the reactions is accordingly 2 98.5 %.Maleic dinitrile reacts with 2 in benzene (c = 0.2 M for both components, T = 35 "C) within 23 d to give a mixture of 6 a and 6 b (96%, ratio 86:14, determined by 'H-NMR spectroscopy). A CT complex is formed initially and absorbs at i,,,, = 486 nm. The products could not be separated without decomposition and were characterized spectroscopical-IY.['~] In the reaction of maleic dinitrile with I the same product was formed as in the reaction with 2. Isomerization of 1 could not be suppressed upon reaction with the less reactive maleic dinitrile. 6a/6bThe reactions of the respective 1,2-dicyano-substituted dimethyl esters of fumaric acid and maleic acid with 1 and 2 were carried out (0.2 M solution of the components in methylene chloride) at -50 "C, since a higher temperature led to unidentifiable products. The reaction was complete within 5 min-the minimum time required for the recording of a 'H-NMR spectrum after mixing the compounds-and a single adduct was always isolated (97 %) with all four combinations of the reaction partners. At -95°C formation of a bluish-green CT complex (A,,, = 540 nm) is briefly observed. ...

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The radical ions that have been postulated as intermediates in the Diels‐Alder reaction could be detected spectroscopically in the reaction of the diene 1 with tetracyanoethylene (TCNE). The radical ion pair 2 is also formed when the adduct 3, which precipitates as colorless crystals from a deep‐colored solution, is redissolved in tetrahydrofuran.

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Hochvakuum liefern 4 als r o t a Pulver, das nach Umkristallisieren BUS Toluol (O'C) dunkelrote Nadeln gibt. Ausbeute 190 mg (87%). Zersetzung a b 110°C. Die Komplexe 1 (farblose Nadeln. Ausbeute 65 O h ) . 2 und 3 (orangegelbe Kristalle, Ausbeute 40%) werden analog rnit leicht modifizierter Aufarbeitung erhalten. Eingegangen am 3. Juli. erginzte Fassung am 29. August 1989 [Z 34201 Angew. Chem. 101 (1989) Nr. 12 VCH Ver/ug.~gesrllsch~J~ mhH, 0-6940 Weinhrim. 1989 1~044-8249/89/1212-1701 S 02.S0/0