UNUSUALLY TIGHT ION-PAIRING OF THE 1,4-DI-TERT-BUTYLBUTA-1,3-DIENE AND 2,3-DI-TERT-BUTYLBUTA-1,3-DIENE RADICAL-ANIONS WITH ALKALI-METAL CATIONS - AN ESR AND ENDOR STUDY

Radical anions of several alkyl-substituted buta-1,3-dienes, 4-9, and of 1,4-di-tert-butylcyclohexa-1,3-diene (11) were characterized by the ir hyperfine data with the use of ESR and ENDOR spectroscopy. Some of these radical anions, which are protected by bulky alkyl substitutents , could be generated by reaction of the corresponding neutral compound s with a potassium, rubidium, or cesium mirror in 1,2-dimethoxyethane (DME) and, mostly, tetrahydrofuran (THF), In particular, the radical a nions of 1,4- and 2,3-di-tert-butylbuta-1,3-dienes (6 and 7, respectiv ely) prepared by this method proved to be fairly persistent, and their ESR and ENDOR spectra were thoroughly studied in a wide temperature r ange. Characteristic of the hyperfine patterns of 6(.-) and 7(.-) are very large coupling constants of the alkali-metal nuclei in the counte rion: a(K-39) = 0.12-0.15, a(Rb-85) = 0.40-0.84, a(Rb-87) = 1.4-2.8, a nd a(Cs-133) = 0.70-2.6 mT. Values of this size, unusual for counterio ns of hydrocarbon radical anions, point to a tight or contact ion pair ing of 6(.-) and 7(.-) with the alkali-metal cations M+ (M = K, Rb, Cs ). Whereas for 6(.-)/M+ in DME and THF only such tight ion pairs were observed, their coexistence with loose or solvent-separated ion pairs was noticed for 7(.-)/K+ in DME at very low temperatures. Apart from t he large hyperfine splittings due to the alkali-metal nuclei, a striki ng feature of the tight ion pairs 7(.-)/M+ is the coupling constant of two protons in the 1,4-positions; its absolute value (ca. 0.4 mT) is much smaller than that (ca. 0.7 mT) of the corresponding protons in th e radical anions of buta-1,3-dienes. Assignment of this reduced value to the endo-protons is compatible with the ESR and ENDOR spectra of 7- d(2)(.-)/M+ dideuterated in the 1-position. The structures of 6(.-)/M and 7(.-)/M+ were discussed with the aid of theoretical calculations. Undoubtfully, the planar trans-conformation with the bulky 1,4-tert-b utyl substituents in the sterically unhindered exo-positions is genera lly favored for 6(.-)M(+). The counterion M+ should be situated on the 2-fold axis above or below the molecular plane of 6(.-). In 7(.-), th e steric overcrowding by the 2,3-tert-butyl substituents is relieved b y a strong twist about the C(2)-C(3) bond; loose and tight ion pairs 7 (.-)M(+) may have different conformations. For the tight ion pairs 7(. -)/M+, a conformation should be preferred in which the counterion M+ i s situated on the 2-fold axis with the two tert-butyl substituents ''o n the other side'' of the molecule. The close contact of the alkali-me tal cation M+ with either 6(.-) or 7(.-) must be promoted by the spati al arrangement of these substituents.