Mn-complexes

The authors describe a clear enhancement of the catalyst activity by the addition of the ionic liquid even if the reaction medium consisted mainly of CH2CI2. In the presence of the ionic liquid, 86 % conversion of 2,2-dimethylchromene was observed after 2 h. Without the ionic liquid the same conversion was obtained only after 6 h. In both cases the enantiomeric excess was as high as 96 %. Moreover, the ionic catalyst solution could be reused several times after product extraction, although the conversion dropped from 83 % to 53 % after five recycles this was explained, according to the authors, by a slow degradation process of the Mn complex. 

Table 1 Results of the alkene epoxidation reactions with fluorinated (salen)Mn complexes under biphasic conditions ...

The first application of ionic hquids for salen complexes dealt with the epoxidation of alkenes [14]. Jacobsen s Mn complex was immobilized in [bmimjlPFe] and different alkenes were epoxidized with aqueous NaOCl solution at 0 °C. As the ionic solvent sohdified at this temperature, dichloromethane was used as a cosolvent. The recychng procedure consisted of washing with water, evaporation of dichloromethane, and product extraction with hexane. The results (Table 3) were excellent and only a slow decay in activity and enantioselectivity was detected after several cycles. 

Fig. 7 Methods for ship-in-bottle synthesis of (salen)Mn complexes inside the pores of zeolites...

Electrochemical studies, in combination with EPR measurements, of the analogous non-chiral occluded (salen)Mn complex in Y zeoUte showed that only a small proportion of the complex, i.e., that located on the outer part of the support, is accessible and takes part in the catalytic process [26]. Only this proportion (about 20%) is finally oxidized to Mn and hence the amount of catalyst is much lower than expected. This phenomenon explains the low catalytic activity of this system. We have considered other attempts at this approach using zeolites with larger pore sizes as examples of cationic exchange and these have been included in Sect. 3.2.3. 

A different strategy has very recently been applied to the immobilization of (salen)Mn complexes by electrostatic interactions. In these examples the charges were placed on substituents of the aromatic rings in salen Ugands (Fig. 14). 

A complementary approach has been reported very recently [43]. hi this case negative charges were introduced into the salen ligand Iq (Fig. 14) with the aim of exchanging it on cationic supports, such as a layered double (Zn, Al) hydroxide. The expansion in the basal spacing indicated intercalation, at least partially, of the Ig-Mn complex between the layers of [Zn2,i5Alo,86(OH)6,o2]- The complex was used in the epoxidation of (i )-limonene with molecular oxygen and pivalaldehyde. The use of N-... 

Encapsulation in Y zeohte was also the method chosen to immobihze Mn complexes of C2-symmetric tetradentate hgands (Fig. 24) [75]. These materials were used as catalysts for the enantioselective oxidation of sulfides to sulfoxides with NaOCl. The lack of activity when the larger io-dosylbenzene was used as an oxidant was interpreted as an indication that the reaction took place inside the zeolite microporous system. Both the chemo- and enantioselectivity were dependent on the structure of the sulfide. (2-Ethylbutyl)phenylsulfide led to better results than methylphenylsulfide, although in all cases the enantioselectivity was low (up to 21% ee). 

Fig. 4. Plot of enthalpies of complex formation in the gas phase of CpNi complexes (Cp = cyclopentadienyl) vs those for the corresponding Mn complexes. The ligands are segregated into correlations for soft (O) (N and S donors) and hard (oxygen donors) ( ). Energies are in kcal mol-1. Redrawn after Ref. (14). 

Further efficient ligands for the epoxidation of alkenes have been reported by Pozzi, but using PhIO as the oxidant and pyridine V-oxide as an additive in FBS.[7, 51-53] Chiral (salen)Mn complexes have been synthesised, which are soluble in fluorous solvents and active in the epoxidation of a variety of alkenes. The catalysts were of the form shown in Figure 6.14. 

There is interest in the possible use of other metal nitrosyl complexes as vasodilators, but from the series KJM(CN)6NO] where M = V, Cr, Mn, and Co (n = 3) or M = Mo (n = 4) neither the Cr nor the Mn complexes exhibit any hypotensive action (504). Iron-sulfur-nitrosyl clusters such as [Fe4S4(NO)4] are active, and their effects can be potentiated by visible light (505). 

The Mn + complexed ions appear to be the most selective and efficient initiator system for grafting to polysaccharides so far described. The mechanism of the initiation reaction has been studied in our laboratories by model experiments and ESR spectroscopy. There are two possible reactions indicated. Bond cleavage of a vicinal diol according to reaction (22) is one possibility. Another and faster reaction with Mn3+ giving radicals appears to be oxidation of aldehyde groups (24) to alkoxy radicals ... 

Al3+, Fe31, Mn31 [N(CH2CH2OH)3] (T riethanolamine) Al-complex colourless Fe-complex Yellow Mn-complex Green... 

Soluble polymer-bound catalysts for epoxidation reactions have also been explored, with a complete study into the nature of the polymeric backbone performed by Janda [70]. Chiral (salen)-Mn complexes were appended to MeO-PEG, NCPS, Jan-daJeF and Merrifield resin via a glutarate spacer. It was found that for the Jacobsen epoxidation of ds-/ -mefhylstyrene, the enantioselectivities for each polymer-supported catalyst were comparable (86-90%) to commercially available Jacobsen catalyst (88%). Both soluble polymer-supported catalysts could be used twice before a decline in yield and enantioselectivity was observed. However, neither soluble polymer support proved as suitable as the insoluble JandaJel-supported (salen)-Mn complex for the epoxidation because residual impurities during precipitation and leaching of Mn from the complex, resulted in lowered yields. 

It has been demonstrated that Mn is the preferred substrate for MnP (13-17). The enzyme oxidizes Mn to Mn and the Mn produced, complexed with a suitable carboxylic acid ligand (12-16), diffuses from the enzyme and in turn oxidizes the organic substrates (6,8,13-17). Thus the Mn ion participates in the reaction as a diffusible redox couple (Fig. 1) rather than as an enzyme-binding activator. In support of this concept, we have demonstrated that chemically prepared Mn complexed with a carboxylic acid ligand such as malonate or lactate mimics the reactivity of the enzyme (6,8,14,15). 

Figure 6. XANES energy (innection point) as a function of charge for 15 Mn complexes. (Reproduced from reference 37. Copyright 1981 American Chemical Society.)...

The question of the molecular basis for the S states has existed since the original proposal by Kok and coworkers. As first formulated, the S state designation referred to the oxidation state of the O2-evolving center which could, in principle, include all of photosystem II and its associated components. Indeed, there are a number of redox-active components on the electron-donor side of photosystem II in addition to the Mn complex, such as the tyrosine radical that gives rise to EPR signal, and cytochrome b jg. 

However, a change in the oxidation state of these species does not alter either the period-four oscillation of 0 yields in a series of flashes, provided that the flashes are sufficiently closely spaced (16), or the EPR spectral properties of the 2 state (18). Moreover, EPR (17-31) and X-ray absorption (32-38) studies have shown that Mn is oxidized in the to 2 transition. Hence, it appears that the S states should be interpreted in terms of distinct intermediate oxidation states of the Mn complex (see below). 

The alternate proposal is that both the multiline and g = 4.1 EPR signals arise from the same tetranuclear Mn complex (18,24-25). The conversion of the g = 4.1 EPR signal into the multiline EPR signal upon incubation at 200 K in the dark can then be explained by a temperature-dependent structural change in the Mn site upon... 

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