Radical Anions of Sterically Protected Polyenes: An ESR and ENDOR Study

Abstract

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 1H-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.