Hetero-Diels-Alder and Cheletropic Additions of Sulfur Dioxide to Conjugated Dienes. Experimental Facts and Theoretical Analysis

The recent developments of a new chemistry of sulfur dioxide in which this substance adds to 1,3-dienes, acting as a dienophile, are reviewed. Experimental data and theoretical results are analyzed and discussed in order to elucidate the mechanisms through which hetero-Diels-Alder and cheletropic cycloaddition reactions proceed. Discussions about the concerted vs. multi-step nature of the mechanism, the regio- and stereoselectivity aspects, the role played by Lewis acids as catalysts, solvent and substituent effects, and conformational preferences of the resulting sultines and sulfolenes are carried out. The important finding that sulfur dioxide promotes its hetero-Diels-Alder and cheletropic additions to dienes is analyzed in details. Structural, thermodynamic and kinetic features affecting the competition between the two reactions are presented and discussed. Particularly, the suitability of a mechanism in which, by an appropriate rearrangement, sultines might give rise to sulfolenes is analyzed in the light of experimental and computational studies. It is shown that the combined experimental/theoretical research carried out to develop this new chemistry of sulfur dioxide constitutes a synergistic approach bearing excellent fruits.