anion in THF, whereas it is not possible to do so for sodium
benzophenone radical anion. Thus an important piece of
evidence which seems to emerge from the present work is the
effect that the alkyl groups may have on the degree of aggre-
gation of aromatic ketone radical anions in solution.
Lastly the present work might explain the success of the
method of preparing aromatic pinacols by reducing the corre-
sponding ketones with Mg–MgI2, the so-called Gomberg–
Bachmann pinacol synthesis.16
References
1 A. Behrendt, C. G. Screttas, D. Bethell, O. Schiemann and B. R.
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Conclusion
Interaction between alkali metal aromatic ketone radical
anions and the chlorides of lithium and magnesium leads to
either diminishing or complete disappearance of paramagnet-
ism. Magnetic titration studies revealed that the interaction in
most cases is stoichiometric and leads either to the establish-
ment of equilibria or to the formation of diamagnetic products
with rather well defined stoichiometry or composition. In the
latter case, stoichiometry appears to reflect the degree of asso-
ciation of the radical anion moiety in solution. Clustering of
the radical anion in solution and complex formation between
the radical anions and non-reducible salts are markedly
dependent on steric factors.
Experimental
The nuclear magnetic resonance spectra were recorded using a
Bruker AC 300 MHz instrument operating at a probe temper-
ature of ca. 300 K. Tetrahydrofuran was doubly distilled from
fluoren-9-ylpotassium shortly before use. LiCl was a commercial
product and was dried by heating under vacuum for several
hours. MgCl2 was prepared from magnesium and 1,2-dichloro-
ethane in THF, as described in a previous publication.17 An
almost saturated solution of MgCl2 in THF was approximately
0.66 M. Radical anion solutions were prepared and standard-
ised as described previously.14 Samples for NMR measurements
were prepared as described in a previous publication.14 Stand-
ard solutions were handled with microsyringes.
8 C. G. Screttas, G. I. Ioannou and D. G. Georgiou, Russ. Chem. Bull.,
1995, 44, 78.
9 S. Takeoka, K. Horiuchi and E. Tsushida, Solid State Ionics, 1992,
50, 175.
10 See for example, S. R. Zhu and T. Cohen, Tetrahedron, 1997, 53,
17607; G. H. Lee, S. J. Ha, I. K. Yoon and C. S. Pak, Tetrahedron
Lett., 1999, 40, 2581; C. S. Salteris, I. D. Kostas, M. Micha-Screttas,
B. R. Steele, G. A. Heropoulos, C. G. Screttas and A. Terzis,
J. Organomet. Chem., 1999, 590, 63.
Acid hydrolysis of the diamagnetic LiPh2CO–MgCl2 complex
To a solution of 20 mmol of lithium benzophenone radical
anion, prepared from 3.8 g of benzophenone, 0.140 g lithium
chips and 18 ml of anhydrous THF, were added 10 mmol of
MgCl2, as a standard solution. The bluish-purple solution
formed was acidified with 6 M hydrochloric acid. Solvent
was removed under vacuum and the product was taken up in
dichloromethane, the organic layer was washed with water,
dried over anhydrous sodium sulfate and evaporated to dryness
to yield a white solid (3.6 g) which exhibited a mp 168–70 ЊC.
After one recrystallisation from hexane–toluene the mp was
11 H. S. Sorensen and K. Daasbjerg, Acta Chem. Scand., Ser. A, 52,
51; C. Galli and P. Gentili, Acta Chem. Scand., Ser. A, 1998, 52, 67;
T. Holm, J. Am. Chem. Soc., 1999, 121, 515; S. Shaik, D. Danovich,
G. N. Sastry, P. Y. Ayala and H. B. Schlegel, J. Am. Chem. Soc.,
1997, 119, 9237.
12 M. Micha-Screttas, G. A. Heropoulos and B. R. Steele, J. Chem.
Soc., Perkin Trans. 2, 1999, 1443.
13 C. G. Screttas and M. Micha-Screttas, J. Am. Chem. Soc., 1987, 109,
7573.
14 C. G. Screttas and M. Micha-Screttas, J. Org. Chem., 1981, 46, 993;
J. Org. Chem., 1983, 48, 153.
1
182–185 ЊC (benzopinacol, lit.18 mp 182–183 ЊC). The H and
15 The observation that reaction between the diamagnetic complex
with methyl iodide led to formation of benzophenone is consistent
with this interpretation. Indeed, electron transfer from the
pinacolate dianion causes fragmentation via the formation of an
oxygen-centred radical and a β-scission process, see ref. 8.
16 J. March, Advanced Organic Chemistry, Reactions, Mechanisms and
Structures, Fourth Edition, John Wiley and Sons, Inc., New York,
1992, p. 1225.
13C NMR spectra were consistent with those of authentic
benzopinacol.
Reaction of the diamagnetic LiPh2CO–MgCl2 complex with
methyl iodide
To a solution of the diamagnetic title complex, prepared as
described in the previous paragraph, was added methyl iodide,
4 ml, and the resulting mixture was stirred at room temperature
for 72 hours. The mixture was hydrolysed with a dilute solution
of ammonium chloride, and the product was taken up in
dichloromethane. Evaporation of the extraction solvent left
a viscous liquid which by GC–MS was identified as benzo-
phenone. NMR analysis confirmed the GC–MS results and
indicated the absence of any Ph2C(Me)OH.
17 C. G. Screttas and M. Micha-Screttas, J. Organomet. Chem., 1985,
292, 325.
18 Handbook of Chemistry and Physics, 54th Edn., CRC Press,
Cleveland, Ohio, 1973–1974, Sect. C-2.
Paper 9/06337D
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J. Chem. Soc., Perkin Trans. 2, 1999, 2685–2690