Radical anions of sterically protected polyenes:: An ESR and ENDOR study

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 12-dimethoxyethane (DME)or tetrahydrofuran (THF) to fairly persistent radical anions. These compounds denoted here as 2, 3,...7 (which corresponds to the number. 1/2 n=2. 3,...7. of their formal double bonds) are 1,1,1,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,Sr,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-trienc. 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 H-1-coupling constants comply with the spin distribution predicted for the radical anions of such 'linear' pi-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 pi-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, respectively, were replaced by CN groups.