Manganese-catalyzed cross-coupling reaction between aryl Grignard reagents and alkenyl halides

Abstract

The replacement of palladium or nickel complexes by iron salts as catalysts is an emerging area in catalysis that is useful for sustainable development. This is manifested by scores of cross-coupling reactions that have been catalyzed by iron. Meanwhile, for both economical and environmental considerations, manganese salts are a useful substitute for iron salts. Nevertheless to date, Mn-catalyzed cross-coupling reactions have been limited to the use of only very reactive aryl halides or chloroenynes. In this report, Cahiez and his group demonstrated the first Mn-catalyzed cross-coupling reactions between aryl Grignard reagents and simple alkenyl halides (Table 1). Their early attempts involving various 2-bromostyrenes and 10% MnCl2 provided satisfactory yields, although the reaction was slow at room temperature (entries 1-7). By increasing the temperature to 50deg;C, the coupling occurred in a much shorter time (entries 5-7). Similar conditions were also applied to other alkenyl bromides (entries 8-10). These manganese-catalyzed coupling reactions (entries 1-7, 9, and 10) were stereospecific except for (Z)-3-(2-bromoethenyl)pyridine, where complete inversion of configuration was observed (entry 8). The notably slow rate of Mn-catalyzed arylation was attributed to the sluggish reductive elimination step, a reflection of the general stability of the Mn(II) species, such as MnAr2, at room temperature. The reaction was further explored using less reactive alkenyl halides (entries 11-24). Unlike the case of chlorides, which gave poor yields (entry 11), the coupling of alkenyl bromides and iodides was highly successful (entries 12 (Table Presented) and 13). It is noteworthy that the reaction of β-monosubstituted alkenyl iodides readily proceeded at room temperature (entries 13-20), but that of the less reactive β,β-disubstituted analogs had to be carried out at 50°C to effect coupling at a tolerable rate (entries 21-23). Furthermore, in some cases, alkyl Grignard reagents have also been used successfully to effect similar cross-coupling processes. © 2010 Data Trace Publishing Company.