Mn-Catalyzed Reactions

Dihydroxyacetone (CH20HC 0) has a MW of 90. Another novel organic oxygen scavenger introduced 20 years ago with little or no commercial success, DHA is catalyzed by HQ and Mn. The reaction requires at least 2 ppm DHA per 1 ppm oxygen. 

Table 16-7 The surface or enzyme-catalyzed reaction rate constant, Mn/ for oxidation of Mn normalized for oxygen concentration [O2], pH and particulate concentration [X]. d[Mn ]/dt = Mn [Mn][02][0H] [X]...

All of the reactions discussed in the preceding section are stoichiometric in Mn. Over the past decade, however, Mn-catalyzed carbomagnesation reactions of potential synthetic utility have been developed by Oshima, as summarized in Scheme 63.271-275 Attempts to use PdCl2(MeCN)2, NiCl(PPh3)2, CrCI (, and PuOI ( were unsuccessful, and... 

The cis alkenes are more reactive and more selective than their trans counterparts. As with the Evans system, this reaction is not stereospecific. Acyclic cis alkenes provide mixtures of cis and trans aziridines. cis-p-Methylstyrene affords a 3 1 ratio of aziridines favoring the cis isomer, Eq. 67, although selectivity is higher in the trans isomer. A fascinating discussion of this phenomenon, observed in this system as well as the Mn-catalyzed asymmetric oxo-transfer reaction, has been advanced by Jacobsen and co-workers (83). Styrene provides the aziridine in moderate selectivity, Eq. 68, not altogether surprising since bond rotation in this case would lead to enantiomeric products. 

Stance, although Mn(TPP)Cl-catalyzed reaction of 19 with styrene affords aziridine derivative in 80% yield (Eq. 13), significantly lower yields are obtained with other olefins. Allylic insertion by the metal nitrenoid is frequently the major side reaction encountered during olefin aziridination (88TL1927) (Eq. 14). 

The base-catalyzed reaction of 4-hydroxycoumarin 184 with indolones 213 or with isatin and MN 27a on short heating leads to spiro pyrano-benzopyrans 220 (89JHC1097, 05RCB992, 08JCO741, 08RCB2373) (Scheme 82). 

This process does not require molecular hydrogen to generate an active catalyst species in contrast to the original Co-catalyzed process. Moreover, the pressure of CO to mn the reaction is much lower than that necessary for the Co-catalyzed process. 

This reaction is analogous to the Mn+3 oxalate reaction 84), except that the Tl+2-ethylenediamine complex is less stable, and the metal-catalyzed reaction is completely inhibited in acid solutions, and by the addition of competing ions like Zn+2. 

Some interesting work has been published dealing with the metal ion-promoted hydrolysis of epoxides. Hanzlik and Michaely499 first observed that in the presence of copper(II) the hydration of 2-pyridyloxirane (153) is accelerated by a factor of 1.8 x 104 and its reaction with Cl-, Br and MeO becomes 100% regiospecific for /3-attack. The magnitude of the catalytic effect decreases in the order Cun> Con>Zn11 Mn". The pH rate profile for the copper(II)-catalyzed reaction is a bell-shaped curve with a maximum at ca. pH 5. 

The [Mn(CO)3(P)2(CH2Cl2)][BArF] complex, 23, also catalyzes reaction of phenol with triethylsilane, presumably by a mechanism similar to that proposed for the Fe system (Scheme 9). The ratio of silane to the catalyst was about 24 1, and a slight deficiency of phenol was added at —78°C in an NMR tube reaction. XH NMR spectra recorded from... 

Immobilization often leads to much improved activity or lifetime of the catalyst. For example, the microenvironment of the catalytic center can be chosen to have the proper polarity, ionic strength, etc. for the catalytic activity (367). Prevention of bimolecular deactivation is the key to enhanced stability (115,128). Unprecedented activities have been discovered by immobilization of catalytic centers, as is the case for Mn-catalyzed cis dihy-droxylation (81). Another tantalizing reaction is the selective oxidation of primary carbon atoms, the industrial implementation of which is eagerly awaited (163). 

The Mn(II)/Mn(III) system has not been studied in as much detail as cobalt, but the principles discussed above are also probably applicable to the manganese-catalyzed reactions ... 

In 1998 evidence for the existence of a commonly invoked intermediate for a variety of oxygen transfer reactions involving the ]0 = Mn ] moiety was obtained by interception of an ]0 = Mn (salen)(OIPh)] complex (Fig. lA) and a binu-clear [pi-0(Mn (salen)(0IPh))2] complex (Fig. IB) [36]. Later, an ESI-MS study on the Mn-catalyzed oxidative kinetic resolution of secondary alcohols by PhI(OAc)2 reported the observation of a similar manganese salen intermediate [MnV(salen)(PhIO)(OCH(CH3)Ph]+ (Fig. 1C) [6]. This, along with the observation of [Mn "(salen)(PhI(OAc)2)] (Fig. ID), allowed the proposal of a possible catalytic cycle for the kinetic resolution of secondary alcohols by this system. 

Mn(0Ac)3 2H20 <03EJ01410> <03S1977>. Muller, on the other hand, investigated the enantioselective rhodium-catalyzed reactions between 2-diazocyclohexane-l,3-diones and alkenes, which led to products with similar structures <03HCA3164>. Palladium-mediated coupling of alkenes and a 1,3-dicarbonyl derivative was utilized in the total synthesis of ( )-brevione B <03TL5235>. 

Substrate Scope. Best results in the (salen)Mn -catalyzed epoxidation reaction have been obtained with cis-disubstituted, conjugated alkenes (Table 1). Epoxidation of 2,2-dimethylchromene derivatives occurs with especially high selectivity (>97% ee). frans-Disubstituted alkenes are epoxidized with low selectivity (20-50% ee), as are simple alkyl-substituted alkenes. 

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