Organomanganese bromides

Direct Formation of Aryl-, Alkyl-, and Vinylmanganese Halides via Oxidative Addition 307 Table 8.1 Formation and coupling reactions of organomanganese bromides. 

The formation of bis-organomanganese bromides cannot be ruled out on the basis of gas chromatography monitoring. After acidic quenching of the reaction mixture followed by gas chromatography analysis, less than 5% of thiophene was detected. 

Alkylation of organomanganese reagents with alkyl bromides can also be improved by addition of CuCI (3 mol%). The reactions proceed at room temperature and are complete within a few hours [123, 130], The opening of epoxides is also improved under these conditions. The reaction also features good chemoselectivity, tolerating the presence of sensitive functions such as ketones (Scheme 2.59) [130]. 

Transmetallation of zinc organometallics with manganese(II) salts does not occur and cannot be used to produce functionalized organomanganese compounds. The reaction of unsaturated alkyl bromides furnishes, in the presence of a mixed metal-salt system composed of copper(I) chloride and manganese(Il) bromide [32], cyclization products in satisfactory yields (see Section 9.6.17 Scheme 9-55) [94]. 

A palladium-catalyzed cross-coupling reaction of organomanganese reagents with various aryl bromides including unreactive deactivated or hindered aryl bromides was performed successfully by the Riguet et al. (Scheme 5-120). They used in particular a system comprising of 1% Pd(dppp)Cl2 with four equivalents of dimethoxyethane. ... 

The reaction of manganese(II) salts with organozinc reagents does not provide the corresponding organomanganese reagents [319]. However, functionalized bromides can be metallated by EtzZn in the presence of catalytic amounts of MnBrj... 

Of course, this route seems very attractive for the preparation of functionalized organomanganese reagents since it is thus possible to circumvent the limitations due to the use of organomagnesium or lithium compounds as an intermediate. However, until now, only a few examples have been described since the activated manganese is often too reactive to be sufficiently chemoselective. Nevertheless, the first alkylmanganese bromide bearing an ester group has been prepared by this route (Sciieme 13.4). 

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