trans-l-(Dimethylamino)-l,3-butadiene is allowed to react with methyl acrylate, acrylonitrile, maleo-and fumaronitrile, dimethyl maleate and fumarate, as well as with dimethyl dicyanomaleate and dicyanofumarate. In all cases except the two last ones where only a single isomer is obtained mixtures (endolexo) of cycloadducts are formed. Structures are determined by a combination of one-and two-dimensional NMR spectroscopy. The E / Z isomeric maleo-and fumaronitrile add stereospecifically to the diene. Cycloaddition of dimethyl maleate and fumarate leads to the same mixture of isomers in almost the same ratio. The reaction of dimethyl dicyanomaleate and dimethyl dicyanofumarate yields a cycloadduct which seems to be the thermodynamically most stable one. The results are interpreted in such a manner that a concerted cycloaddition takes place with maleo-and fumaronitrile, while stepwise reactions, presumably proceeding via zwitterions, are observed with dimethyl maleate, fumarate, the strongly electrophilic dimethyl dicyanomaleate and dicyanofumarate. With increasing capability of the dienophile to stabilize a negative charge the mechanism changes from a concerted to a stepwise reaction. described reactions with acrolein and crotonaldehyde and derived the cyclic structure from the compounds which were formed after elimination of dimethylamine. Hunig and Kahanekt3] carried out a careful study on cycloadditions of trans-l-(dimethylamino)-1,3-butadiene to methyl acrylate, methyl vinyl ketone, acrolein, and acrylonitrile. Both investigations concluded that "ortho"-substituted cyclohexenes are produced. A thorough analysis of the configuration of the adducts was not possible at the time due to the lack of modern spectroscopic techniques. Thus, the question whether endo or exo addition occurs, or whether a mixture of both isomers is encountered could not be answered. Some time after this early work a few more cycloadditions of l-(dialkylamino)-l,3-butadienes were described by Sat~inger [~].It may be asked what kind of new information one may obtain from further studies on this diene system. Presently, we investigate the influence of one and several dialkylamino substituents in different positions of a diene on the reactivity, the regioselectivity, the stereochemistry, and the mechanism of cycloaddition. Preliminary results on reactions of &,trans-and trans,trans-l,4-bis(dimethylamino)-1,3-butadiene have already been reported[''. A full account on the behavior of l,l-bis(dimethylamino)-l,3-butadiene of which a part has been described earlierr6] is given in an accompanying publication 1' 1. The aspects being studied in this work are whether a common mechanism is followed in all cases and whether a polar reaction path can be induced by suitable substitution. Preparation and properties of trans-(l-dimethylamino)-l,3-butadiene (1)Dieneamine 1 can be easily prepared by condensation of crotonaldehyde with dimethylamine. The NMR spectral data, the photoelectron (PE) spectrum, and electrochemical measurements support the nucleophili...

Key Words: Zwitterion formation and decay, kinetics of J Cycloadditions trans-1-(Dimethylamino)-1,bbutadiene and dimethyl dicy-by a first-order process. The analysis of the kinetic data (E, = anofumarate react in acetonitrile and dichloromethane at 14.5 f 0.1 kcal mol-l, 1gA = 14.71 f 0.05, AH* = 14.1 f 0.1 -40°C with a second-order rate constant of 1 0 ' to lo8 1 mol-' kcal mol-', AS* = 7.2 f 0.2 cal mol-' K-', AG* = 11.95 f s-'. This was determined by following the decay of the UV 0.01 kcal mol-') in combination with stereochemical studies absorption of diene and dienophile with a stopped-flow spec-on the cycloaddition of the E/Z isomeric dienophiles dimethyl trofluorimeter. After disapperance of the reagents a new ab-dicyanofumarate and dicyanomaleate leads to the interpretasorption (h,,, = 400 nm, lg& = 2.514) is recorded which decays tion that the new species is a zwitterion.The systematic study of (4 + 2) cycloadditions between a diene and a dienophile as a function of substitution has shown that these reactions do not occur via one single Evidence has been presented that biradical and zwitterionic structures are the most plausible intermediate^ [^-^]. These conclusions derive from indirect evidence, most often from stereochemical studies. If a cycloaddition takes place with loss of stereochemistry in the reactants an intermediate is indicated. If further the rate of reaction is measured as a function of the solvent polarity a distinction between a biradical and zwitterionic pathway is possible, Hui~gen'~.'] has shown this in a very elegant manner for (2 + 2) cycloadditions.The measurement of activation volumes can also support the discrimination between different pathways, even allowing the recognition of concurrent concerted and two-step mechanisms[31. Direct observation of zwitterions has become possible in recent years. Hartmann and Heuschmann[*] have established the structure of a zwitterion which is formed from a tetrazine and a ketene N,N-acetal by X-ray crystallography and have discussed its significance in the cycloaddition process. We have characterized zwitterions formed from bis(l,l-dimethylamino)-1,3-butadiene and TCNEf9' or dimethyl dicyanofumaraterlol which, however, do not cyclize to (4 + 2) adducts. Quast et al. ["] have carried out similar studies on 1,3-dipolar cycloadditions of electrophilic azides to a ketene aminal. Direct kinetic measurements on zwitterions have not been reported so far. It has also not been known what the kinetics of disappearance of such species should be like. Here we present results of measurements obtained by stopped-flow techniques which suggest that we have observed for the first time a transient zwitterion and determined the activation parameters for its disappearance in the cycloaddition of trans-l-(dimethylamino)-l,3-butadiene to dimethyl dicyanofumarate.trans-I -(Dimethylamino)-l,3-butadiene (1) reacts with dimethyl dicyanofumarate (2) or dimethyl dicyanomaleate in high yield to give a (4 + 2) cycloadduct 3 the structure of which has been determined by NMR...

The Diels-Alder reaction of 1,2-dimethylenecyclopentane (1) and tetracyanoethylene (TCNE) is kinetically studied by conventional and stopped-flow UV/Vis absorption-spectroscopic techniques. A transient charge-transfer (CT) absorption band with h, , , = 490 nm (half-life at 20°C ca. 3.4 x lo-' s) is detected in dichloromethane solution. For the molar extinction coefficient a value of E~~~ = 310 M -I cm-I has been determined from measurements of the optical densitity at zero time as a function of excess diene concentration. With the aid of computer simulation the combined kinetic analysis of the disappearance of reactants and CT complex reveals that the data obtained from experiments at equimolar concentration and at a single temperature (20°C) can be interpreted equally well by both the model where the CT complex is an intermediate on the reaction path and where it is in a nonreactive side equilibrium with the reactants. Activation parameters have been determined for the disappearance of the starting compounds and of the CT complex. Contrary to what is expected for a CT complex being an intermediate in a concerted Diels-Alder reaction a more negative activation entropy for the decay of the CT complex than for the disappearance of the reactants is found. This suggests that the CT complex is unlikely to exhibit a transition-state-like structure as is required for a concerted cycloaddition. Experiments under pseudo-first-order conditions have led to an extension of the reaction scheme by a second-order reaction of the CT complex with the excess diene, the rate constant of which is virtually identical to that for the direct bimolecular cycloaddition. The conclusion is drawn for our case that there are more arguments against than arguments for the CT complex being a preorientation complex in the [4 + 21 cycloaddition.Soon after the discovery of the general principle of the Diels-Alder cycloaddition in 1928 I), Kuhn and WagnerJauregg') reported the observation of transient colors during the course of these reactions. This phenomenon has been observed many times in later years and its significance for the mechanism of the Diels-Alder reaction has been under discussion ever since'). Mulliken's theory of charge-transfer (CT) complexes4) provided an explanation for the origin of the new absorption band which occurred on mixing solutions of an electron donor diene and an electron acceptor olefin. It did not, however, provide any information about the role of these species in the cycloaddition. from rate measurements for the disappearance of the starting compounds at a single temperature. On the other hand, activation parameters may provide more definite information. Kiselev and Miller6) postulated that the negative activation enthalpy they determined for the cycloaddition reaction of 9,lO-dimethylanthracene with tetracyanoethylene (TCNE) in some solvents is evidence for a CT complex as an intermediate. However, a negative activation enthalpy only demonstrates that an intermediate is involved in the reaction. The nature of th...

1,1‐Bis(dimethylamino)‐1,3‐butadiene (1) as a strong donor diene (E1/2 = 0.03 V vs. SCE, 1st IPv = 6.94 eV) is treated with acrylonitrile, dimethyl dicyanofumarate, and tetracyanoethylene. Cycloaddition with acrylonitrile is slow and requires elevated temperatures at which elimination of dimethylamine with formation of 1‐cyano‐2‐(dimethylamino)‐1,3‐cyclohexadiene occurs. The reaction of 1 with TCNE at −40°C in acetonitrile is very fast and leads to the zwitterion 5. At T ⩾ −20°C, 5 eliminates hydrogen cyanide to give a highly colored (λmax = 489 nm, lgϵ = 4.716) merocyanine. A zwitterion 9 generated from 1 and dimethyl dicyanofumarate can be isolated in crystalline form. The structural analysis shows that the zwitterions are produced by attack of the olefin at the antiperiplanar conformation of 1. Both zwitterions which can be trapped as crystalline picrates cannot be converted to cycloadducts. These results suggest that cycloaddition is observed if zwitterion formation becomes energetically unfavorable.

The rearrangement of matrix‐isolated organic radicals with bicyclo[3.2.0]heptadienyl structure (2, 5, 11a,b), 3‐quadricyclanyl structure (16, 26, 27), and of 7‐norbornadienyl radical 15 is studied. Final rearrangement products are radicals with tropylium structure. 16, 26, and 27 isomerize to radicals with bicyclo[3.2.0]heptadienyl skeleton before electrocyclic ring opening to tropylium radicals takes place. 7‐Norbornadienyl radical 15 is the least stable radical on the C7‐hypersurface. Sigmatropic 1,2‐vinyl shifts in bicyclo[3.2.0]heptadienyl radicals could not be observed. Substituents do not influence the rearrangement behaviour.