Unusually Tight Ion Pairing of the 1,4- and 2,3-Di-tert-butylbuta-1,3-diene Radical Anions with Alkali-Metal Cations:  An ESR and ENDOR Study

Abstract

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 their 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 compounds with a potassium, rubidium, or cesium mirror in 1,2-dimethoxyethane (DME) and, mostly, tetrahydrofuran (THF). In particular, the radical anions of 1,4- and 2,3-di-tert-butylbuta-1,3-dienes (6 and 7, respectively) prepared by this method proved to be fairly persistent, and their ESR and ENDOR spectra were thoroughly studied in a wide temperature range. Characteristic of the hyperfine patterns of 6 •- and 7•- are very large coupling constants of the alkali-metal nuclei in the counterion: a(39K) = 0.12−0.15, a(85Rb) = 0.40−0.84, a(87Rb) = 1.4−2.8, and a(133Cs) = 0.70−2.6 mT. Values of this size, unusual for counterions of hydrocarbon radical anions, point to a tight or contact ion pairing 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 the large hyperfine splittings due to the alkali-metal nuclei, a striking 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 the 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-butyl substituents in the sterically unhindered exo-positions is generally 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 •-, the steric overcrowding by the 2,3-tert-butyl substituents is relieved by 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+ is situated on the 2-fold axis with the two tert-butyl substituents “on the other side” of the molecule. The close contact of the alkali-metal cation M+ with either 6 •- or 7 •- must be promoted by the spatial arrangement of these substituents.

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

“…The finding that association of the radical anions with their alkali-metal counterions becomes weaker upon lengthening of the p-system is indicated by the occurrence of the loose ion pairs even with Cs in THF, as well as by the lesser changes in the 1 H-coupling constants and in the g factor and by the decreasing j a Cs j value. Although the 133 Cs-coupling constants observed for 2 .À ± 4 .À and c-3 .À are larger than those generally found for the ion pairs of hydrocarbon p-radical anions, they do not reach by far the j a Cs j values attained for the radical anions of 1,4-and 2,3-di(tertbutyl)buta-1,3-dienes [4]. Obviously, as stated in [4], the geometry of these two species represents an exceptionally favorable case for the formation of unusually tight ion pairs.…”

“…In general, reaction of these compounds with an alkali metal in an ethereal solvent fails to yield a persistent radical anion, because of rapid polymerization to rubber-like products (see, e.g., [3]). However, as has recently been shown [4], this process can be hindered by bulky substituents. Introduction of two tertBu groups into each of the terminal positions of 1 should endow its radical anion with an even greater persistence, and an analogous statement may also hold for longer conjugated polyenes.…”

“…Apart from the spin distribution in the radical anions of 2 ± 7 and c-3, the interest is focused on whether these species form unusually tight ion pairs with their alkali-metal counterions in an ethereal solvent, as do the radical anions of 1,4-and 2,3-di(tertbutyl)buta-1,3-dienes [4]. The occurrence of such ion pairs in DME and THF was strikingly demonstrated by the extremely large coupling constants of 39 K and 133 Cs nuclei in the associated cations K and Cs , a K 0.12 to 0.16 and a Cs 0.7 to 2.6 mT [4].…”

“…The occurrence of such ion pairs in DME and THF was strikingly demonstrated by the extremely large coupling constants of 39 K and 133 Cs nuclei in the associated cations K and Cs , a K 0.12 to 0.16 and a Cs 0.7 to 2.6 mT [4]. These values exceed by an order of magnitude those generally found for ion pairs of hydrocarbon p-radical anions in DME and THF, j a K j% 0.01 and j a Cs j% 0.1 mT [8a] [9].…”

“…The tetra(tert-butyl)-substituted polyenes 2 ± 7 and c-3 were reduced to their radical anions with a K or a Cs mirror in DME or THF. Li and Na were not employed to this aim, because they proved to have insufficient reducing power for alkyl-substituted buta-1,3-dienes [4]. Although the ease of formation (thermodynamic stability) of the radical anions gradually increased on going from 2 .À to its longer counterparts, their persistence (kinetic stability) decreased in the same order.…”