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.
