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Substituent Effect on the Competition between Hetero‐ Diels‐Alder and Cheletropic Additions of Sulfur Dioxide to 1‐Substituted Buta‐1,3‐dienes
Author(s) -
Roversi Elena,
Monnat Frédéric,
Vogel Pierre,
Schenk Kurt,
Roversi Pietro
Publication year - 2002
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/1522-2675(200203)85:3<733::aid-hlca733>3.0.co;2-u
Subject(s) - chemistry , substituent , cycloaddition , reactivity (psychology) , medicinal chemistry , diels–alder reaction , alkyl , stereochemistry , bicyclic molecule , catalysis , organic chemistry , medicine , alternative medicine , pathology
The reactivity of sulfur dioxide toward variously substituted butadienes was explored in an effort to define the factors affecting the competition between the hetero‐ Diels‐Alder and cheletropic additions. At low temperature (<−70°), 1‐alkyl‐substituted 1,3‐dienes 1 that can adopt s‐cis ‐conformations add to SO 2 in the hetero‐ Diels‐Alder mode in the presence of CF 3 COOH as promoter. In the case of ( E )‐1‐ethylidene‐2‐methylidenecyclohexane (( E )‐ 4a ), the [4+2] cycloaddition of SO 2 is fast at −90° without acid catalyst. ( E )‐1‐(Acyloxy)buta‐1,3‐dienes ( E )‐ 1c , ( E )‐ 1y , and ( E )‐ 1z with AcO, BzO, and naphthalene‐2‐(carbonyloxy) substituents, respectively also undergo the hetero‐ Diels‐Alder addition with SO 2 +CF 3 COOH at low temperatures, giving a 1 : 10 mixture of the corresponding cis ‐ and trans ‐6‐(acyloxy)sultines c ‐ 2c,y,z and t ‐ 2c,y,z , respectively). Above −50°, the sultines undergo complete cycloreversion to the corresponding dienes and SO 2 , which that add in the cheletropic mode at higher temperature to give the corresponding 2‐substituted sulfolenes (=2,5‐dihydrothiophene 1,1‐dioxides) 3 . The hetero‐ Diels‐Alder additions of SO 2 follow the Alder endo rule, giving first the 6‐substituted cis ‐sultines that equilibrate then with the more stable trans ‐isomers. This statement is based on the assumption that the S=O group in the sultine prefers a pseudo‐axial rather than a pseudo‐equatorial position, as predicted by quantum calculations. The most striking observation is that electron‐rich dienes such as 1‐cyclopropyl‐, 1‐phenyl‐, 1‐(4‐methoxyphenyl)‐, 1‐(trimethylsilyl)‐, 1‐phenoxy‐, 1‐(4‐chlorophenoxy)‐, 1‐(4‐methoxyphenoxy)‐, 1‐(4‐nitrophenoxy)‐, 1‐(naphthalen‐2‐yloxy)‐, 1‐(methylthio)‐, 1‐(phenylthio)‐, 1‐[(4‐chlorophenyl)thio]‐, 1‐[(4‐methoxyphenyl)thio]‐, 1‐[(4‐nitrophenyl)thio]‐, and 1‐(phenylseleno)buta‐1,3‐diene, as well as 1‐(methoxymethylidene)‐2‐methylidenecyclohexane ( 4f ) do not equilibrate with the corresponding sultines between −100 and −10°, in the presence of a large excess of SO 2 , with or without acidic promoter. The hetero‐ Diels‐Alder additions of SO 2 to 1‐substituted ( E )‐buta‐1,3‐dienes are highly regioselective, giving exclusively the corresponding 6‐substituted sultines. The 1‐substituted ( Z )‐buta‐1,3‐dienes do not undergo the hetero‐ Diels‐Alder additions with sulfur dioxide.