A mechanistic insights into manganese-catalyzed oxidative homocoupling reactions of Grignard reagents: A computational DFT investigation

Tandem Mn–I Exchange and Homocoupling Processes Mediated by a Synergistically Operative Lithium Manganate

“…Germane to the formation of bis(aryls) 3 a – 3 o , work on Mn II ‐catalyzed homocoupling of ArM (M=Li, MgX) has shown that atmospheric oxygen is required in order to facilitate the Csp 2 −Csp 2 bond‐forming process [7, 9, 10] . Since our experiments were carried out under strict inert atmosphere conditions, it should then be the alkyl iodide Me 3 SiCH 2 I concomitantly generated during the Mn–I exchange process which acts as an oxidant of putative in situ generated lithium aryl manganates Li 2 MnAr 4 ( I ), facilitating the homocoupling process (Figure 2 i).…”

Manganese‐Catalyzed Aerobic Heterocoupling of Aryl Grignard Reagents

“…The mechanism was recently investigated by DFT calculations and a manganese(II) -(IV) cycle was proposed through the sequence R2Mn(II)  R2Mn(IV)(O2)  R-R + Mn(II)(O2)  R2Mn(II). [9] The formation of organomanganese species from organomagnesium halides and MnCl2 has been investigated earlier and complete conversion is observed upon mixing the reagents in THF solution. [10] We have previously studied the reactivity of Grignard reagents [11] and used MnCl2 as a catalyst for a radical-mediated C-C coupling reaction.…”

Tandem Mn–I Exchange and Homocoupling Processes Mediated by a Synergistically Operative Lithium Manganate

“…[15] Manganate-Mediated Aryl-Aryl Oxidative Homocouplings Germane to the formation of bis(aryls) 3a-3o,w ork on Mn II -catalyzed homocoupling of ArM (M = Li, MgX) has shown that atmospheric oxygen is required in order to facilitate the Csp 2 À Csp 2 bond-forming process. [7,9,10] Since our experiments were carried out under strict inert atmosphere conditions,i ts hould then be the alkyl iodide Me 3 SiCH 2 Ic oncomitantly generated during the Mn-I exchange process which acts as an oxidant of putative in situ generated lithium aryl manganates Li 2 MnAr 4 (I), facilitating the homocoupling process (Figure 2i). Previous work by Zhou has also shown that MnCl 2 can catalyze homocoupling of Grignard reagents using dichloroethane (DCE) as an external oxidant.…”

“…In previous MnCl 2 -catalyzed homocoupling studies of ArLi and ArMgX under air, it has been proposed that reactions take place via aM n IV bis(aryl)oxo complex where the O 2 is h 2 -bonded to Mn. [7,9,10] However,taking into account that the polar aryl reagent ArM (M = Li, MgX) is present in al arge excess in comparison to the MnCl 2 catalyst (10-20 mol %), we ponder whether amore accurate scenario may involve lithium (or magnesium) manganate intermediates.In an attempt to further understand the fate of Mn during the homocoupling process when atmospheric oxygen is used as an oxidant, an in situ prepared solution of 5 was exposed to dry air for 15 min at À78 8 8Cu sing ad rying tube charged with oven-dried CaCl In contrast, 7 incorporates LiCl in its constitution (present in the reaction media as co-product of the synthesis of the Li 2 MnAr 4 precursor;F igure 3, RHS). It comprises two antiferromagnetically coupled Mn III centers connected/ bridged by two oxo and two chloride anions.I na ddition, 4 anisolyl ligands coordinate through their ortho Cand the Oof the OMe group to Mn and Li, respectively.E ach Li atom present in the structure completes its coordination sphere by…”

“…[8] More recent work by Tailler has also proposed that in situ generated ArLi species can undergo homocoupling in the presence of catalytic amounts of MnCl 2 . [9] Supported by computational studies, [9,10] these reactions have been proposed to occur via formation of putative MnAr 2 intermediate species which in turn can undergo oxidative homocoupling in the presence of oxygen. However,l imited tangible experimental evidence is available on the actual formation of these compounds which are hypothesized to form by salt-metathesis in the presence of alarge excess of the polar organometallic aryl reagent.…”