Functionalized benzylic manganese

We developed an alternative synthetic route for the direct formation of non-functionalized and functionalized benzylic manganese halides from highly reactive manganese and benzylic halides [13]. 

Table 8.7 Cross-coupling reaction of benzylic and functionalized benzylic manganese sulfonates. 

Homocoupling Reactions of Functionalized Benzylic Manganese Reagents... 

Typical Preparation of Functionalized Benzylic Manganese Halides and Their Cross-Coupling Reactions with Aryl Iodides under a Palladium Catalyst... 

The resulting benzylic manganese reagents were found to undergo a variety of cross-coupling reactions. It was also found that homocoupled products of functionalized benzyl halides could be readily prepared depending on reaction conditions. 

Functionalized benzylic mesylates containing a halogen atom were also investigated. Reactions of the halogenated benzyl manganese mesylates with acid chlorides, aldehydes, and ketones yielded the corresponding ketone, secondary alcohol, and tertiary alcohol in good to excellent isolated yields as shown in Table 8.11. As mentioned earlier, it is of interest that the mesylates... 

At even higher pH values, metal catalysis is required for chlorination to proceed. Thus, at pH =11, nickel(salen) catalyzes the chlorination of adamantane, cyclohexane and toluene785 and manganese porphyrin promotes the chlorination of cyclohexane786. Selective side-chain chlorination by sodium hypochlorite under PTCs at pH = 8.5 has been used for benzylic chlorinations787,788 and for the functionalization of poly(4-methyl-styrene)789,790. Similarly, with calcium hypochlorite in acetic acid, ring chlorination is reported for toluene, xylenes, anisole and other activated aromatics791. 

To oxidize the benzylic alcohol function of compound 20, a mixture of this compound and a large excess of manganese dioxide is stirred in dichloro-methane at room temperature under argon for 1.5 h. After a short work-up, compound 21 may be obtained quantitatively (Scheme 9.16). 

The rather drastic conditions are required because in this particular case the COOH group deactivates the intermediate p-toluic acid towards further oxidation, and some p-carboxybenzaldehyde is found as a side-product, which is hydrogenated back to p-toluic acid. Other than that, a large number of functional groups are tolerated (see Table 4.7) [129]. The combination of cobalt, manganese and bromide ions is essential for optimum performance. The benzylic radicals are best generated with bromine atoms (see above) which in turn are more easily produced... 

With the help of metal catalysis amidation takes place at room temperature under apparently neutral conditions. Manganese dioxide in large quantities, or, even better, manganese dioxide on silica gel, is well suited to this conversion. Even compounds with functional groups labile to manganese dioxide, for example benzylic groups, are suited for the hydration. With pentacarlwnylmanganese bromide, nitrile hydration takes place under PTC conditions. ... 

Oxidation of oxygen functions. DDQ is useful for the selective oxidation of allylic " and benzylic alcohols. The reagent has the advantage of not showing the variability encountered with active manganese dioxide. It dehydrogenates 2-hydroxymethylene-3 ketosteroids in about 50% yield. The steroid is treated in dioxane solution at room temperature with 1.1 -1.5 equivalents of DDQ for 1 -5 min.. 

The formation of both Mn(CO)s and the CO-loss product, Mn2(CO)9, from UV photolysis of Mn2(CO)io has been investigated by ultrafast IR spectroscopy.Density functional calculations have been performed on the ground and excited states of MnCl(CO)5, as an aid to understanding the photochemistry of MX(C0)5 complexes (M = Mn, Re X = Cl, Br, I). The lowest energy excitations are to the Mn-Cl a orbital, and these states are dissociative for both axial and equatorial CO loss, but not for Mn-Cl bond homolysis. The photochemistry of manganese carbonyl complexes (58, R = Me, benzyl R = /-Pr, p-tolyl) depends critically on the R group.CO is released with R = Me, but with R = benzyl there is efficient homolysis of the Mn-R bond. ... 

It is sometimes required to oxidize the C-1 methyl group of a substituted isoquinoline to an aldehyde function. Such a transformation is usually carried out using selenium dioxide, in which case anhydrous conditions using dioxane as solvent often give superior yields. An alternate route, however, involves initial formation of the isoquinoline A-oxide followed by acetylation as indicated below. The final step is a manganese dioxide oxidation of a benzylic alcohol. ... 

Considering the high reactivity of the active manganese (Mn ) metal prepared by the Rieke method and the exceptional tolerance to a wide range of functionality in the organic moiety, we explored the possibility of the direct oxidative addition to a variety of carbon-oxygen bonds. Our first approach employed benzyl sulfonates. This was expanded to functionalized and... 

Best results were obtained when the active manganese was added to the neat benzyl bromide. The reaction was completed within 10 min at rt without a catalyst [39]. Several other benzyl halides produced good to excellent yields of homocoupling products, which showed a wide tolerance of functional groups such as nitrile, ester, nitro, chloro, bromo, methoxy, and methyl groups (Table 8.16). 

In Summary Benzyhc oxidations of alkyl groups take place in the presence of permanganate or chromate benzyhc alcohols are converted into the corresponding ketones by manganese dioxide. The benzyhc ether function can be cleaved by hydrogenolysis in a transformation that allows the phenylmethyl (benzyl) substituent to be used as a protecting group for the hydroxy function in alcohols. 

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