[4 + 2]‐Cycloadditions with 1,4‐Bis(N,N‐dimethylamino)‐1,3‐dienes: Stereochemical Studies and Observation of Radical Ions

Abstract

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. ...

Asymmetry in Symmetric Cycloadditions

“…This species may quench a third polar reactant or solvent (e.g., methanol). Otherwise, its intervention leads to stereochemical scrambling of the cyclohexene products . Thus, the asymmetry in [4 + 2] cycloaddition paths may give the product distribution; formation of the stereospecific cyclohexene is not exclusive.…”

Acid Catalysis via Acid‐Promoted Electron Transfer

“…ET processes play very important roles as the activation step for some Diels-Alder reactions of electron-rich dienes having high-lying highest occupied molecular orbitals with electron-deficient dienophiles having low-lying lowest unoccupied molecular orbitals. [66][67][68][69] The photodriven Diels-Alder reactions via photoinduced ET and charge-transfer of the D-A complexes formed between dienes and dienophiles by irradiation have also been well recognized. [70][71][72] Lewis acids including redoxinactive metal ions act as effective catalysts to accelerate Diels-Alder reactions of anthracenes with p-benzoquinone (Q) derivatives, which have been considered as weak or inert dienophiles.…”

“…ET processes play very important roles as the activation step for some Diels‐Alder reactions of electron‐rich dienes having high‐lying highest occupied molecular orbitals with electron‐deficient dienophiles having low‐lying lowest unoccupied molecular orbitals 66–69 . The photodriven Diels‐Alder reactions via photoinduced ET and charge‐transfer of the D–A complexes formed between dienes and dienophiles by irradiation have also been well recognized 70–72 .…”

Dialkyl dicyanofumarates and dicyanomaleates as versatile building blocks for synthetic organic chemistry and mechanistic studies

“…−50 °C) to avoid the formation of complex product mixtures. Based on the 1 H NMR analysis, only one product, identical in reactions with E - and with Z - 1b , was formed [ 47 ]. Also bis(dimethylamino)-substituted 1,3-dienes including bicyclic representatives were used for reactions with E - and Z - 1b [ 48 – 49 ].…”