Diels-Alder reaction olefin substituent effects

Styrenes may act as 2n and 4n components of the Diels-Alder reaction depending on the substitution site and the electronic effects of the substituent. Electron-donating groups at the a-carbon of the olefinic double bond enhance the dienic reactivity of styrenes [30]. 

The asymmetric hetero-Diels-Alder reaction of aldehydes with Danishefsky s diene catalyzed by Ti catalysts generated from a library of 13 chiral ligands or activators has also been reported (Scheme 8.18). The catalyst library contains 104 members. The Ti catalysts bearing L, L , L, and J are found to have a remarkable effect on both enantioselectivity (76.7-95.7% ee) and yield (63-100%). On the other hand, ligands bearing sterically demanding substituents at the 3,3 -positions are found to be detrimental to the reaction. The optimized catalysts, both L /Ti/L and L /Ti/L , are the most efficient for the reaction of a variety of aldehydes, including aromatic, olefinic, and aliphatic derivatives. 

This obstacle can be overcome by moving electron withdrawing substituents away from the double bond and increasing the reactivity of double bond by positioning it in a strained ring. This is achieved using bicyclic monomers. The monomers are readily obtained from the Diels-Alder reactions of substituted olefins with cyclopentadiene. This route is effective also for fluorinated monomers. These types of monomers undergo a ROMP with a variety of one component and two-component initiator systems. 

Azodicarboxylates are best recognized for their ability to participate as 2w components in normal (HOMOdiene controlled) Diels-Alder reactions with dienes (Chapter 6)5 71 113 and for their effective participation in ene reactions with reactive olefins.113 In addition, electron-rich or reactive olefins that do not contain a reactive allylic hydrogen atom and consequently cannot enter into an ene reaction do possess the ability to participate in competing [2 + 2] and [4 + 2] cycloadditions with azodicarboxylates.5 71 113 The Diels-Alder [4 + 2] cycloaddition of azodicarboxylates with olefins provides 1,3,4-oxadiazines (Scheme 9-VI) and is sensitive to solvent and substituent effects.114-120 In general, the competing [2 + 2] cycloaddition to provide 1,2-diazetidines intervenes and predominates as the olefin nucleophilicity and the reaction solvent polarity are increased [Eqs. (46) and (47)].113... 

Analysis of olefin substituent effects relevant to the Diels-Alder reaction. A. The familiar mixing diagram for the formation of the tt and ir orbitals of ethylene, which serves as a reference for the rest of the figure. 

Maleimide. Maleimide and maleic anhydride are simple olefins and very useful molecules in organic syntheses for introducing various substituents. Addition reactions, cyclodimerization, Diels-Alder reactions and polymerization reactions are as well-known as their photochemical reactions. They are also known as electron acceptors in photoinduced electron-transfer reactions. The photosensitized reaction of maleimide with xanthone in 2-propanol has been investigated by TR EPR. The emissive CIDEP spectrum observed in 2-propanol is predominantly assigned to two kinds of maleimide alkyl-type radicals. On the other hand, the absorptive spectrum of the maleimide radical-anion was observed in 2-propanol in the presence of hydrochloric acid. The hydrochloric acid addition effect on the CIDEP patterns indicates the existence of two mechanisms for the photosensitization of maleimide by xanthone. One is a T-T energy transfer to maleimide followed by hydrogen abstraction by maleimide... 

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