Are there any examples of Diels-Alder reaction in which carboxyl group (in acids or esters) acts as a part of a dien or a dienophile? What are the reasons of its inactivity in this reaction?
You need to analyze FMO interactions and - subsequently - AO coefficients for reactions centers. Alternatively, analysis of global and local electrophilicity indices may be performed. Probably, reactivity coefficients for centers from C=O group are significantly lower than (for example) typical ethylenic systems. If you need help in the calculation of these indices please send me email > [email protected]
Addition across the C=C is energetically favourable and that across C=O is not.Therefore the carboxyl group is inactive in Diels-Alder reaction.
Yes, there are examples of methylene barbituric acid derivatives and methylene meldrum's acid derivatives undergoing hetero Diels Alder reactions as a diene component. These systems are strongly electronically activated and sterically fixed in cisoid conformation.
Cinnamates and similar systems have also been used but often require an electron rich dienophile and sometimes occur via step-wise rather than concerted process.
The major limitation for the use of esters and carboxyl groups is that they are not electrophilic enough to make good dienophiles and possess poor orbital overlap on the carbon atom in addition to suffering from higher steric requirements.
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