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U. Kolle and H. Franzl: Oxidation of Alcohols by [Cp Rh(ppy)(OH)]
1326
Pentamethylcyclopentadienyl-(4S,40S)-4,40,5,50-tetrahydro-4,40-methyl-bisoxazole-chlororhodium
tetra¯uoroborate (2d)
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2d was prepared analogously to the polypyridine complexes [2] after dehalogenation of [Cp RhCl2]2
with one equivalent of AgBF4 in methanol or acetone. 1H NMR (300 MHz, ꢄ, acetone-d6): 1.63, 1.91
(d, J 6:3 Hz, Me), 1.95 (s, 15H, Cp ), 3.58 (m, 2H, H4), 4.8 (m, 2H, H40), 5.11 (m, 2H, H5) ppm;
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cyclic voltammogram (CH2Cl2:acetonitrile 1:1, Bu4NPF6, v 200 mV/s): Epc 0:94 (irrev),
Epa 0:43 (irrev).
Pentamethylcyclopentadienyl-(4S,40S)-4,40,5,50-tetrahydro-4,40-i-propyl-bisoxazole-chlororhodium
tetra¯uoroborate (2e)
1H NMR (300 MHz, ꢄ, CD2Cl2): 1.00, 1.07 (d, J 6:5 Hz, 2H each, Me), 1.5 (m, 3H, CH(Me)2 and
CH2), 1.87 (s, 15 H, Cp ), 4.3 (dd, J 7:3, 8.6 Hz, 2H, H4), 4.51 (d, J 8:6 Hz, 2H, H40), 4.77 (m,
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2H, H5) ppm.
Oxidation of alcohols was conducted in 5 cm3 septum sealed vessels charged with a 10
10 4 M
3
solution of the Rh(III) complex in H2O (ppy 2,20-bipyridine-4,40-dicarboxylate), THF, DMF, or the
alcohol as solvent, to which 0.1 M aqueous KOH and the alcohol were added. Experiments with
alcohols collected in Scheme 1 were performed with ꢃ10 3 M 3a, 10 2 M KOH, and 10 2 M
alcohol in THF:H2O 1:1 to ensure the solubility of 4a. After deaerating the vessels with Ar by
means of a syringe needle, the reaction starts by developing a blue to violet color. Repeated reduction
and reoxidation of the blue solution was effected by injecting air slightly less than required for
stoichiometric reoxidation of Rh(I). In one experiment, a 20 cm3 septum sealed vessel was charged in
the same way, and after the reduction had come to completion once, oxygen was continuously
injected by means of an automatic pipette (Metrohm 702 Titrino) at a rate ensuring the presence of
residual Rh(I) throughout. Polypyridine complexes generally allowed many reduction-reoxidation
cycles. Bisoxazole complexes which in cyclic voltammetry showed similar reduction-oxidation
behavior and developed dark red solutions in the presence of alcohol and base gave only one cycle.
Kinetic measurements were performed in a 5 mm optical glass cell ®tted with a rubber seal.
The absorption of the Rh(I) phenanthroline complex at 700 nm was followed with a J&M Tidas diode
array spectrophotometer at 1 min intervals. The reaction scheme was simulated with the CKS, Vers. 1,
kinetic simulator program [10] and compared with the experimental curves using Origin 6.01.
References
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[1] Kolle U, Gratzel M (1987) Angew Chem 99: 572; (1987) Angew Int Ed Engl 26: 567
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[2] Kolle U, Kang B-S, Infelta P, Comte P, Gratzel M (1989) Chem Ber 122: 1869
[3] Ladwig M, Kaim W (1991) J Organomet Chem 419: 233; Kaim W, Reinhardt R, Sieger M (1994)
Inorg Chem 33: 4453
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[4] Dadci L, Elias H, Frey U, Hornig A, Kolle U, Merbach AE, Paulus H, Schneider JS (1995) Inorg
Chem 34: 306
[5] a) Cosnier S, Deronzier A, Vlachopoulos N (1989) J Chem Soc Chem Commun 1259; b) Caix C,
Chardon-Noblat S, Deronzier A, Moutet J-C, Tingry S (1997) J Organomet Chem 540: 105
[6] Ruppert R, Herrmann S, Steckhan E (1987) Tetrahedron Lett 28: 6538; (1988) J Chem Soc Chem
Commun 1150
[7] a) Ziessel R (1988) J Chem Soc Chem Commun 16; b) Ziessel R (1991) Angew Chem 103: 863
[8] a) Zassinovich G, Mestroni G, Gladiali S (1992) Chem Rev 92: 1051; b) Gladiali S, Mestroni G
(1998) In: Beller M, Bolm C (eds) Transition Metals for Organic Synthesis, vol 2, Wiley-VCH, p 97 ff
[9] Chaudhuri P, Hess M, WeyhermuÈller T, Wieghardt K (1999) Angew Chem 111: 1165
[10] Chemical Kinetics Simulator (1995) IBM Almaden Research Center
Received July 27, 2000. Accepted September 18, 2000