NJC
Paper
in line with the most stable configuration of the direduced than that of biquinoxaline, the previously reported starting
species where the p-electron density is paired on the central reagent. This provides exciting prospects for simple access to
0
5
0
0
0
C –C bond. In the case of x,x = 2,2 and 2,3 the fact that a new members of the biquinoxen family bearing various electron
3
3
more sterically hindered C2 atom is involved can strongly withdrawing/donating and possibly chiral groups as well as
2
2,23
disfavour the coupling reaction
and/or decrease the radical coordination functions.
stability of the direduced species by inducing a significant twist
0
0
0
24
of the molecule around the central C
2
–C
2
(C
3
) bond.
0
Acknowledgements
Furthermore, the insolubility of the x,x = 3,3 isomers
0
+
Rbqn -H in MeOH greatly enhances the yield of the reaction
compared to other solvents. As mentioned above, EPR confirmed
the observation of the N-alkyl-4-hydroquinoxalinyl radical species
The authors are grateful for the financial support provided by
the Alexander von Humboldt Foundation (fellowship to N. L.)
and Helmholtz POF ‘‘STN’’. We also thank Ingrid Freuze for the
mass spectrometry measurements and Sven Stahl for the elemental
analysis measurements.
(
ESI†) in MeOH, DMF and DMSO with a qualitative rate of colour
change of the solution to dark blue (radical formation) increasing
in the order MeOH o DMF o DMSO, which is in line with a
better solubility of sodium dithionite in DMSO rather than in
MeOH (Fig. 4). However, as already known for Mbqn -H the
much better solubility of Rbqn -H in DMF or DMSO precludes
0
+5
Notes and references
0
+
0
+
precipitation of the Rbqn -H form which remains in solution
1 P. M. S. Monk, The Viologens: Physicochemical Properties,
ꢂ
+
+
0
in equilibrium with the corresponding radical Rbqn -H .
Thus, this explains the non-measurable yields reported using
DMF or DMSO solvents, the best yields being exclusively obtained
in MeOH.
Synthesis, and Application of the Salts of 4,4 -Bipyridine,
Wiley, 1998.
2 W. Kaim and W. Matheis, Chem. Ber., 1990, 123, 1323–1325.
3 W. Matheis and W. Kaim, J. Chem. Soc., Faraday Trans.,
1990, 86, 3337–3339.
Counter ion influence on yield
4 W. Matheis, J. Poppe, W. Kaim and S. Zalis, J. Chem. Soc.,
Perkin Trans. 2, 1994, 1923–1928.
The influence of the anion on the reaction yield has also been
investigated (ESI†). It is clear that the counter ion plays a signi-
ficant role with an increase in the yield of the [Rbqn ](BF
species (R = methyl, ethyl and benzyl) from 9% to 32% in the
5
N. Leblanc, S. Sproules, K. Fink, L. Sanguinet, O. Aleveque,
2
+
E. Levillain, P. Rosa and A. K. Powell, Chem. Sci., 2016, 7,
4 2
)
3820–3828.
ꢀ
ꢀ
ꢀ
6 N. Leblanc, D. Genovese, L. De Cola and A. K. Powell, Phys.
Chem. Chem. Phys., 2017, DOI: 10.1039/C6CP07538J.
7 O. N. Chupakhin, E. O. Sidorov, S. M. Shein and I. I. Bil’kis,
Zh. Org. Khim., 1976, 12, 2464–2468.
8 I. M. Piglosiewicz, R. Beckhaus, G. Wittstock, W. Saak and
D. Haase, Inorg. Chem., 2007, 46, 7610–7620.
order of starting anions I o Br o PF . This is in line with the
6
ꢀ
fact that I ions are more oxidisable and have a higher degree of
ion pairing compared to Br and also PF6 ions. This favours
quenching reactions involving iodide-mediated charge/electron
ꢀ
ꢀ
2
5–28
transfer or radical scavenging mechanisms.
However, the low yields observed in the case of R = propyl,
ꢀ
ꢀ
9 C. M. Fitchett and P. J. Steel, Polyhedron, 2008, 27, 1527–1537.
10 S.-r. Guo, P. S. Kumar and M. Yang, Adv. Synth. Catal., 2017,
either starting from I or PF
[
6
, suggest that the isolation of the
4 2
Rbqn ](BF ) species also depends on the intrinsic solubility
2
+
359, 2–25.
of the biquinoxen dications which in turn is governed by the
type of R substituent.
Further experiments will be performed in future investigating
these aspects in order to optimise the yields of the reactions.
1
1 G. W. H. Chesseman and R. F. Cookson, The Chemistry of
Heterocyclic Compounds, Condensed Pyrazines, John Wiley &
Sons, Inc., 2008, ch. 17, pp. 247–260.
12 O. N. Chupakhin and I. Y. Postovskii, Russ. Chem. Rev.,
1976, 45, 454.
1
1
3 S. K. Chung, J. Org. Chem., 1981, 46, 5457–5458.
4 D. R. Eaton, J. M. Watkins and R. J. Buist, J. Am. Chem. Soc.,
1985, 107, 5604–5609.
Conclusions
In conclusion we have reported the oxidative radical homo-
coupling of chemically reduced N-alkylquinoxalinium salts as 15 T. M. Bockman and J. K. Kochi, J. Org. Chem., 1990, 55,
0
an alternative method for the synthesis of N,N -diquaternised-
3
4127–4135.
0
,3 -biquinoxalinium ‘‘biquinoxens’’ dications. Although referr- 16 N. Kitamura, Y. Nambu and T. Endo, J. Polym. Sci., Part A:
ing back to Fig. 1 it is clear that left hand route can provide
Polym. Chem., 1990, 28, 3137–3143.
much better yields. This critically relies on the availability of 17 T. Yutaka, A. Kazuyuki, I. Toshiaki, K. Hitomi and
the corresponding oxonium derivatives. The right hand route, I. Kazuhiko, Chem. Lett., 1972, 847–848.
despite non regioselectivity, still provides yields of up to 32%. 18 L. Roullier and E. Laviron, Electrochim. Acta, 1980, 25, 795–804.
Since this method is very simple it allows for gram scale synthesis 19 W. Kaim, Res. Chem. Intermed., 1987, 8, 247–286.
at room temperature whilst using inexpensive reagents, no metal 20 N. Leblanc, S. Sproules, C. Pasquier, P. Auban-Senzier,
catalysts and no purification steps. Furthermore, the chemistry of
the quinoxaline quaternary salts used here is more accessible
H. Raffy and A. K. Powell, Chem. Commun., 2015, 51,
12740–12743.
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