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Helvetica Chimica Acta – Vol. 93 (2010)
(i.e., Mn-5) enhanced the catalytic performance and also the stability of the hindered
catalyst (TOF 960/min; TON 1002/24 h). The UV/VIS-spectral study of the Mn-5
catalyst during the oxidation of the less reactive octan-2-ol showed that 81% of the
catalyst remain intact. Probably, the prevention of the formation of catalytically
inactive species such as m-oxo dimer by hindered t-Bu groups near the coordination
sites may be a reason for increasing the activity and stability of the catalyst [16]. It is
noteworthy that, in the case of simple Mn-1 in the oxidation of octan-2-ol, the
percentage of the remaining catalyst was 64%, which is a promising result for the
stability of a simple Schiff-base complex against oxidative degradation in this oxidation
method.
Conclusions. – The catalytic efficiency and excellent selectivity of tetradentate
MnIII – Schiff base complexes in the oxidation of alcohols to the corresponding
aldehydes and ketones using Bu4NHSO5 under mild conditions were established. The
presence of an electron-donating group on the phenyl ring of the benzyl alcohol
increased the reactivity of the substrate, while electron-withdrawing and bulky groups
on the Schiff-base ligands enhanced the catalytic activity of the Mn catalyst. The
applicability of this simple catalytic system to a wide variety of primary and secondary
benzylic, allylic, and also saturated cyclic and alicyclic alcohols with excellent
selectivity, combined with high turnover rates and ready scalability of the method,
makes it attractive for practical purposes.
Support for this work by Research Council of University of Birjand is highly appreciated.
Experimental Part
Typical Procedure for Oxidation of Benzyl Alcohol. To a mixture of PhCH2OH (1 mmol, 0.108 g)
and [Mn(Schiff-base)Cl] [30 – 32] (0.01 mmol) in MeCN (5 ml) was added Bu4NHSO5 [33][34]
(1.7 mmol, 0.7 g). The mixture was stirred at 258 for an appropriate reaction time, and the reaction was
followed by TLC or GC. After completion of the reaction, PhCHO was isolated by silica-gel
chromatography with hexane/AcOEt 10 :1. Liquid. B.p.: 1788. IR (neat): 1701, 1600, 1460, 1312, 1204,
827, 749. 1H-NMR (250 MHz): 7.45 – 7.67 (m, 3 H); 7.87 – 7.90 (m, 2 H); 10.02 (s, 1 H). 13C-NMR
(63 MHz): 128.8; 129.7; 134.4; 136.5; 192.3. MS: 106 (Mþ).
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