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Radical Anions of Sterically Protected Polyenes: An ESR and ENDOR Study
Author(s) -
Gerson Fabian,
Merstetter Pascal,
Fischer Dagmar,
Hopf Henning,
Kiliçkiran Pinar
Publication year - 1999
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/(sici)1522-2675(19990804)82:8<1266::aid-hlca1266>3.0.co;2-h
Subject(s) - chemistry , steric effects , tetrahydrofuran , resonance (particle physics) , radical ion , stereochemistry , hyperfine structure , medicinal chemistry , methylene , crystallography , ion , organic chemistry , physics , particle physics , quantum mechanics , solvent
Owing to the steric protection by four bulky substituents in the terminal positions 1 and n , several conjugated polyenes could be reduced with K or Cs metal in 1,2‐dimethoxyethane (DME) or tetrahydrofuran (THF) to fairly persistent radical anions. These compounds, denoted here as 2 , 3 ,… 7 (which corresponds to the number, \2 n =2, 3, …7, of their formal double bonds) are 1,1, n , n ‐tetra( tert ‐butyl) derivatives of buta‐1,3‐diene, hexa‐1,3,5‐triene, octa‐1,3,5,7‐tetraene, deca‐1,3,5,7,9‐pentaene, dodeca‐1,3,5,7,9,11‐hexaene, and tetradeca‐1,3,5,7,9,11,13‐heptaene. In addition to the six polyenes 2 – 7 with all‐ trans ‐configuration, the studies comprised an isomer of 3 , the trans , cis , trans ‐triene, c ‐3 . The radical anions 2 . − – 7 . − and c ‐3 . − were characterized by their hyperfine data acquired with ESR, ENDOR, and TRIPLE‐resonance spectroscopy. The 1 H‐coupling constants comply with the spin distribution predicted for the radical anions of such `linear' π ‐systems by simple MO models. Ion pairs formed with K + in DME were loose but became tighter with Cs + in THF. Propensity to ion pairing decreased with the lengthening of the π ‐system on going from 2 . − to 3 . − – 7 . − . Hyperfine data are likewise reported for the radical anions of all‐ trans ‐polyenes 8 and 9 , in which two tert ‐butyl substituents in one terminal position of 2 and 3 , respectively, were replaced by CN groups.