Metathesis Reactions of a Manganese Borylene Complex with Polar Heteroatom–Carbon Double Bonds: A Pathway to Previously Inaccessible Carbene Complexes

A comprehensive
study has been carried out to investigate the metathesis
reactivity of the terminal alkylborylene complex [(η5-C5H5)(OC)2MnB(tBu)] (1). Its reactions with 3,3′,5,5′-tetrakis(trifluoromethyl)benzophenone,
4,4′-dimethylbenzophenone, 2-adamantanone, 4,4′-bis(diethylamino)benzophenone,
and 1,2-diphenylcyclopropen-3-one afforded the metathesis products
[(η5-C5H5)(OC)2MnCR2] (R = C6H3-3,5-(CF3)2
3a, C6H4-4-Me 3b, C6H4-4-NEt2
3d; CR2 = adamantylidene 3c, cyclo-C3Ph2
3e). The cycloaddition intermediates were detected
by NMR spectroscopy from reactions involving ketones with more electron-withdrawing
substituents. The reaction of 1 with dicyclohexylcarbodiimide
(DCC) only proceeds to form the cycloaddition product [(η5-C5H5)(OC)2Mn{κ2-C,B-C(NCy)N(Cy)B(tBu)}] (4), which upon warming, rearranges
to afford complex [(η5-C5H5)(OC)2Mn{CN(Cy)B(tBu)CN(Cy)}] (5). The reaction of 1 with triphenylphosphine
sulfide SPPh3 also yields the metathesis product [(η5-C5H5)(OC)2Mn(PPh3)] via an intermediate which is likely to be a η2-thioboryl complex [(η5-C5H5)(OC)2Mn{(η2-SB(tBu)}]
(6). Similar reactions have been studied using an iron
borylene complex [(Me3P)(OC)3FeB(Dur)]
(Dur = 2,3,5,6-tetramethylphenyl, 9). Extensive computational
studies have been also carried out to gain mechanistic insights in
these reactions, which provided reaction pathways that fit well with
the experimental data.