By Марч Дж.

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Depending on the specific components, either carboxylic or heterocyclic products can be obtained. The stereospecificity of the reaction was firmly established even before the importance of orbital symmetry was recognized. In terms of orbital symmetry classification, the Diels-Alder reaction is a π4s + π2s cycloaddition, an allowed process. SCOPE OF DIELS ALDER REACTION The great advantage of Diels Alder reaction in organic synthesis lies in its high stereoselectivity. Examples of Diels-Alder reactions are known for almost all types of π bonds acting as dienophiles.

18 MECHANISMS IN ADVANCED ORGANIC CHEMISTRY 1. O2 R 2. Sn 2+ – CN R R CN R OH –HCN R R 1O2 – C NO2 O R R – C O2, – H2O R O2 – – CO2 R R COOH R OOH (ii) Oxidative Couplings The simple coupling reaction works well in certain cases R (– ) [O ] RR One of the most useful coupling reactions is the Eglintion reaction in which the oxidation of acetylene anions with ferric ion proceeds very satisfactorily. RC C – Fe 3 + RC C C R (iii) Dianion Oxidations Oxidation of dianions, in which both the electrons are oxidized gives products which are biologically important: O 2 – O [O ] O O O O C 2 ″ – C O O O 2 – O O B r– C H 2 –C H 2 – B r O THE CARBANION CHEMISTRY 19 When the product of two electron oxidation is a diradical, the isolated product can be thought of as being formed by intra or intermolecular coupling of this diradical.

R 3C — H + B : – ⊕ R 3 C + BH The benzyl carbanion has been found to be most stable because of the extensive delocalization of the negative charge over the various resonating structures. – CH2 CH2 CH2 CH2 – – – The stability becomes greater due to the presence of electron attracting groups like NO2, – C≡N or carbonyl group in the molecule and it decreases due to electron releasing groups. – CH2 CH2 – O O C N C N O O The stability of the carbanions has been found to be in the order benzyl > vinyl > phenyl > –C2H5 > n-propyl > isobutyl > neopentyl.

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