E,E)-1 and (E,Z)-l,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 miximum 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 (a98 YO). 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(methoxycarbo-ny1)ethene (12) and 2-cyano-1, I-bis(methoxycarbony1)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, I-dicyano-2-methoxycarbonylethene (14), 1,l-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 'H-NMR-spectroscopically at -95 "C which transformed into the cycloadduct when warmed to -5OoC. 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 stereochemisty (9 and 10) when no electron transfer could b e detected, that electron transfer may be a possibility in [4 + 21 cycloadditions, but that it is not the general mechanism of the Diels-Alder reaction.The formation of Diels-Alder cycloadducts proceeds by different routes [']. Whereas the majority of known reactions are obviously in accord with the rules of the conservation of orbital symmetry[2] multistep reactions proceeding via biradicalsr31 or dipolar interm e d i a t e~[ '~~-~~] are also known. The limits between concerted and multistep routes are being investigated actively. Zwitterions could be isolated both in Diels-Alder reactions with inverse electron demand [14] and in reactions of electron-rich diene~['~,l~I with electrondeficient dienophiles, although in the former case their involvement in product formation has not been proven so far and in the latter case decomposition rather than conversion of the zwitt...

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. ...

Electron transfer as initiating step in 14 + 21 cycloadditions was studied with 2,3-bis(dimethylaminomethylene)bicyclo-[ 2.2.11 heptane (1) and 2,3-bis(dimethylaminomethylene)bicyclo[2.2.2]octane (2) as models. These dienes, having the necessary synperiplanar configuration for a concerted [4 + 21 cycloaddition were allowed to react with dienophiles of increasing acceptor capacity. Slightly activated alkenes like methyl acrylate and acrylonitrile undergo smooth cycloaddition in contrast to the corresponding open-chain dienes where cycloaddition with these dienophiles could not be observed. The use of dimethyl fumarate and dimethyl maleate lead to identical cycloaddition products due to a n isomerization of dimethyl maleate catalyzed by 1 and 2. Fumaronitrile and maleonitrile proved to be stable under the reaction conditions and lead to different stereoisomers in the cycloaddition to 2. The introduction of three and four acceptor groups, different combinations of cyano and methoxycarbonyl groups, leads to a sudden increase in reactivity towards 1 and 2. At the same time the cycloadditions are no longer stereospecific in those cases where they could be checked, and radical ions could be detected either by ESR spectroscopy or by stopped-flow visible spectroscopy. This applies in particular to dichloromethane as solvent. Stopped-flow measurements in combination with a rapid-scan unit reveals a transient absorption at h = 420 nm in acetonitrile for 1,l-dicyano-2,2-bis(methoxycarbonyl)ethene and dimethyl dicyanofumarate as dienophiles which is ascribed to zwitterionic intermediates by a comparison with related systems. The cycloadditions of TCNE to 1 and 2 lead to thermally unstable adducts in both cases. Electron transfer was not only observed in solution but could also be detected in the isolated solid products, in particular if they were placed in a polar environment like KBr where complete dissociation in radical ions seems to take place. The relevance of these studies for the mechanism of Diels-Alder reactions is that suitable model systems can be constructed where electron transfer can be observed. Due to the loss of stereospecificity in these cases it is concluded that complete electron transfer is not the principal mechanism of these [4 + 21 cycloadditions which in their majority occur with retention of stereochemistry in the reactants.

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