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The Hetero‐Diels–Alder Addition of Sulfur Dioxide to 1‐Fluorobuta‐1,3‐dienes: The Sofa Conformations Preferred by 6‐Fluorosultines (6‐Fluoro‐3,6‐dihydro‐1,2‐oxathiin‐2‐oxides) Enjoy Enthalpic and Conformational Anomeric Effects
Author(s) -
Roversi Elena,
Scopelliti Rosario,
Solari Euro,
Estoppey Raphaël,
Vogel Pierre,
Braña Pedro,
Menéndez Bibiana,
Sordo José A.
Publication year - 2002
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/1521-3765(20020315)8:6<1336::aid-chem1336>3.0.co;2-r
Subject(s) - chemistry , cycloaddition , diene , moiety , isomerization , double bond , diels–alder reaction , stereochemistry , reactivity (psychology) , crystallography , medicinal chemistry , polymer chemistry , organic chemistry , catalysis , medicine , natural rubber , alternative medicine , pathology
Abstract The reactivity of ( E )‐ and ( Z )‐1‐fluorobuta‐1,3‐diene (( E )‐ and ( Z )‐ 11 ), 2‐fluorobutadiene ( 12 ), ( E )‐ and ( Z )‐1‐(fluoromethylidene)‐2‐methylidenecyclohexane (( E )‐ and ( Z )‐ 13 ) toward SO 2 has been explored and compared with that of ( Z )‐ and ( E )‐1‐(fluoromethylidene)‐2‐methylidene‐3,4‐dihydronaphthalene (( Z )‐ 8 and ( E )‐ 8 ). In agreement with quantum calculations, 12 is unreactive toward SO 2 (no cycloaddition, only polymerization), whereas ( E )‐1‐fluoro‐1,3‐dienes react more rapidly than their ( Z )‐isomers to give the corresponding 6‐fluorosultines following the endo (Alder rule) mode of hetero‐Diels–Alder addition. No sulfolene has been observed following the cheletropic mode of addition with the fluorodienes, in contrast to other substituted dienes. In agreement with the calculations, cis ‐2‐fluoro‐3,4‐oxathiabenzobicyclo[4.4.0]dec‐1(6),9‐diene‐4‐oxide ( cis ‐ 9 , the sultine obtained by SO 2 addition to ( Z )‐ 8 under conditions of kinetic control) adopts a sofa conformation with the oxygen atom of the ring lying in the average plane of the four carbon atoms of its sultine moiety when it is in the crystalline state at −100 °C. A similar sofa conformation was found for its trans ‐isomer, trans ‐ 9 , obtained by isomerization of cis ‐ 9 or by hetero‐Diels–Alder addition of SO 2 to ( E )‐ 8 . Experiments (equilibrium constant for hetero‐Diels–Alder additions, bond lengths, and bond angles in crystalline fluorosultines cis ‐ 9 and trans ‐ 9 ) and high‐level quantum calculations on cis ‐ and trans ‐6‐fluoro‐3,6‐dihydro‐1,2‐oxathiin‐2‐oxide ( cis ‐ and trans ‐ 20 ) confirm the existence of a stabilizing, enthalpic, anomeric ( gem ‐disubstitution by sulfinyloxy and fluoro groups) effect, which is interpreted in terms of (lone pair) n(O1)→σ*(C−F) hyperconjugative interactions. This effect is strongest in the sofa conformers with a gauche arrangement of the σ(O1,S2) and σ(C6,F) bonds. The calculations suggest also that n(O1)→σ*(S2,O2′), π*(S=O), and n(S2)→σ*(O1,C6) interactions intervene and affect the relative stability of the conformers (sofa, boat, pseudo‐chair) found for 6‐fluorosultines cis ‐ and trans ‐ 20 .