Dienophiles surfactant

Jaeger and co-workers studied the regioselectivity of the reaction of a surfactant diene with a surfactant dienophile in micellar media" ". The orientational effects in the aggregates could result in an increase in the regjoselectivity in aqueous solutions of these compounds as compared to the reaction in organic media. 

In all surfactant solutions 5.2 can be expected to prefer the nonpolar micellar environment over the aqueous phase. Consequently, those surfactant/dienophile combinations where the dienophile resides primarily in the aqueous phase show inhibition. This is the case for 5.If and S.lg in C12E7 solution and for S.lg in CTAB solution. On the other hand, when diene, dienophile and copper ion simultaneously bind to the micelle, as is the case for Cu(DS)2 solutions with all three dienophiles, efficient micellar catalysis is observed. An intermediate situation exists for 5.1c in CTAB or C12E7 solutions and particularly for 5.If in CTAB solution. Now the dienophile binds to the micelle and is slid elded from the copper ions that apparently prefer the aqueous phase. Tliis results in an overall retardation, despite the possible locally increased concentration of 5.2 in the micelle. 

Assuming complete binding of the dienophile to the micelle and making use of the pseudophase model, an expression can be derived relating the observed pseudo-first-order rate constant koi . to the concentration of surfactant, [S]. Assumirg a negligible contribution of the reaction in the aqueous phase to the overall rate, the second-order rate constant in the micellar pseudophase lq is given by ... 

The surfactant dienes 90 and 91 (Figure 4.3), analogs to commercially available sodium dodecyl sulfate (SDS) and dodecyl maltoside, react rapidly with highly hydrophilic and reactive triazoline dione 92 in water at 25 °C forming, quantitatively, the corresponding adducts. The Diels-Alder reactions with less potent dienophile 93 gave, similarly, quantitative yields in 0.5 h and 3 h with 90 and 91, respectively. 

Parallel studies on the cycloadditions of non-surfactant dienes 106 and 107 and the dienophile 108 (Figure 4.4), analogs of 97,103 and 98-100, respectively, show that the regioisomer adducts were, in this case, obtained in equal amounts, supporting the idea that orientational effects in micelles promote the regioselectivity of a Diels-Alder reaction of a surfactant diene and a surfactant dienophile. 

Figure 12.2 Preferred orientation of diene and dienophile at a surfactant aggregate-water interface.

As for surfactants, they have uncertain, sometimes contradictory, consequences on reaction rates [45], but the main advantage of using surfactants as additives lies in their solubilizing effect. Special attention has been paid to the rate-accelerating effect of Lewis acid catalysts. The first study deals with the Diels-Alder reaction between cyclopentadiene and a bidentate dienophile a large acceleration can be achieved by the combined use of copper(II) nitrate as catalyst and water as solvent. The rate enhancement imposed on the catalyzed Diels-Alder reaction is much less pronounced than that for the uncatalyzed reaction... 

Jaeger, D.A., Su, D., Zafar, A., Piknova, B. and Hall, S.B. (2000) Regioselectivity control in Diels-Alder reactions of surfactant 1,3-dienes with surfactant dienophiles. /. Am. Chem. Soc., 122, 2749-2757. 

The work of Keana and co-workers must also be mentioned in the context of structural modification of the diene and dienophile components in aqueous Diels-Alder reactions [25]. They found that the surfactant dienes 2.40 and 2.43 react very cleanly with hydrophilic dienophiles 2.41 and 2.45 in water to give the corresponding adducts, which no longer exhibit detergentlike behavior ... 

Jaeger and co-workers have reported the results of experiments which demonstrate that expected orientational effects in micelles or related aggregates cannot always be counted upon to override the intrinsic FMO regiocontrol of the Diels-Alder reaction [38]. Thus, the surfactant diene 3.12 reacted with a four-fold excess of (non-orientational) dienophile 3.13a (R = H) in water to give a mixture of ortho adducts 3.14a/3.15a in 93% combined yield ... 

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