Scheme 2 Proposed mechanism for the PIDA-mediated oxidation of 2,6-dmp.
"
A mixture of 2,6-dmp (1 g, 8.18 mmol) and DPQ (0.655 g, 2.73 mmol) in
herein reported thus represents a simple and novel method to
selectively produce at will, either DPQ by oxidation and coupling
of 2,6-dmp, or TMBD by means of comproportionation of the
quinone DPQ formed with additional 2,6-dmp added in the same
reaction mixture at 50 uC.
4
mL of chloroform was stirred 6 h at 50 uC. After cooling down to RT,
,2,4,5-tetrabromobenzene (0.322 g, 0.818 mmol, internal standard) was
1
added and the solvent evaporated under reduced pressure. The crude
1
residue was analyzed by H NMR and the yield of the reaction was
determined with respect to the internal standard. All DPQ had been
consumed and 1.17 g of TMBD was formed (yield = 89%).
In summary, the use of PIDA as an oxidant for the oxidative
coupling of 2,6-dmp allows the selective synthesis of TMBD.
Methodical investigations of this unprecedented PIDA-mediated
selective C–C coupling have allowed the proposition of a reaction
mechanism which significantly differs from the expected one for a
classical 2,6-dmp coupling reaction. Thus, the coupling of two
radical intermediates at their para-phenyl position directly leads to
the formation of DPQ which spontaneously comproportionates at
˚
I 4 A molecular sieves were activated at 180 uC under vacuum. 2,6-dmp
(1.86 g, 15.21 mmol) and activated 4 A˚ molecular sieves (1 g) were stirred
for 1 h at 50 uC under argon. The reaction was started upon addition of
PIDA (1 g, 3.04 mmol) in powder. After 6 h of stirring, 1,2,4,5-
tetrabromobenzene (0.120 g, 0.304 mmol) and 50 mL of hexane were added
and the reaction mixture was refluxed for 1 h. Cooling down to RT, and
filtration of the solution afforded 0.46 g of TMBD as a light yellow
precipitate (yield = 62%).
1
2
3
A. S. Hay, Prog. Polym. Sci., 1999, 24, 45.
A. S. Hay, J. Polym. Sci., Part A: Polym. Chem., 1998, 36, 505.
Mitsubishi Gas Chemical Co, US4211857, 1979.
4 General Electric, US4471068, 1984.
5 P. Gamez, J. A. P. P. v. Dijk, W. L. Driessen, G. Challa and J. Reedijk,
Adv. Synth. Catal., 2002, 344, 890.
50 uC in the presence of an excess of 2,6-dmp to produce TMBD.
This PIDA-induced oxidative C–C coupling is therefore a
convenient, simple and selective procedure for the preparation of
TMBD.
Support by NRSC Catalysis (a Research School Combination
of HRSMC and NIOK) and the Spanish Ministry of Science and
Technology (GA) is kindly acknowledged.
6
7
Konica Minolta Holdings Inc, JP2004085774, 2004.
General Electric, US2003198892, 2003.
8 Honshu Chemical Ind, EP856504, 1998.
Mitsubishi Petrochemical Co, US4564713, 1986.
0 M. J. S. Dewar and T. Nakaya, J. Am. Chem. Soc., 1968, 90,
134.
1 G. Pandey, C. Muralikrishna and U. T. Bhalerao, Tetrahedron Lett.,
990, 31, 3771.
2 H. Nishino, N. Itoh, M. Nagashima and K. Kurosawa, Bull. Chem. Soc.
Jpn., 1992, 65, 620.
9
1
Notes and references
7
1
{
0.82 mmol (DDQ or PIDA) or 1.64 mmol ([CuCl
added to a solution of 2,6-dmp (0.5 g, 4.09 mmol, for 1–3 and 19–39; 0.2 g,
.64 mmol, for 4 or 0.1 g, 0.82 mmol for 5) in CHCl (33 mL) under argon
2 2
(neo) ]) of oxidant was
1
1
1
3
at RT or 50 uC. After stirring overnight, 0.332 g (0.82 mmol) of 1,2,4,5-
tetrabromobenzene (internal standard) was added and the solvent was
1
3 T. Wirth, Angew. Chem., Int. Ed., 2005, 44, 3656.
14 L. K u¨ rti, P. Herczegh, J. Visy, M. Simonyi, S. Antus and A. Pelter,
J. Chem. Soc., Perkin Trans. 1, 1999, 379 and references therein.
1
evaporated under reduced pressure. The crude residue was analyzed by H
NMR and the yields were determined with respect to the internal standard.
1
5 A. Pelter and R. S. Ward, Tetrahedron, 2001, 57, 273.
{
DDQ, Cu(II) complexes and PIDA are known to mediate the oxidation
of phenol derivatives. DDQ is a quinone based stoichiometric 2-electron
16 T. Dohi, K. Morimoto, Y. Kiyono, A. Maruyama, H. Tohma and
Y. Kita, Chem. Commun., 2005, 2930.
17 General Electric, US3631208, 1971.
18 A. S. Hay, Tetrahedron Lett., 1965, 47, 4241.
19 H. D. Becker, J. Org. Chem., 1965, 30, 982.
20 C. Boldron, P. Gamez, D. M. Tooke, A. L. Spek and J. Reedijk, Angew.
Chem., Int. Ed., 2005, 44, 3585.
19
oxidant. Cu(II)/Neocuproine is a 1-electron stoichiometric oxidant due to
the extreme stability of the bis-neocuproine–Cu(I) complex formed during
the reaction preventing the reoxidation to Cu(II). PIDA is a stoichio-
20
14,15
metric 2-electron oxidant.
In the specific case of the quinone derivative DDQ, the comproportiona-
tion reaction was not observed. This phenomenon is under investigation.
§
5
810 | Chem. Commun., 2005, 5808–5810
This journal is ß The Royal Society of Chemistry 2005