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Regioselective Hydroboration of Epoxides and Esters Enabled by Magnesium Neosilyl Complexes
Jyoti Sharma1, Susmita Mandal1, Himadri Karmakar1
1Department of Chemistry, Indian Institute of Technology Hyderabad, Sangareddy, Telangana, India.
Abstract:
The reaction of two α-diimine ligands, N,N-disubstituted 1,4-diaza-butadienes (DippDAD and t BuDAD) with dineosilylmagnesium [Mg(CH2SiMe3)2] directed the migration of mononeosilyl group(─CH2SiMe3) to one of the two C═N bonds of the diaza ligand, followed by intramolecular hydrogen migration, which resulted in the synthesis of amido-imino magnesium complexes, [κ2-{Dipp─N═C(CH2SiMe3)─CH2NDipp}-Mg(CH2SiMe3){THF}] (1) and [κ2-{tBu─N═C(CH2SiMe3)-CH2NtBu}Mg(CH2SiMe3)]2 (2), respectively in very good yields. Single-crystal X-ray diffraction analysis elucidated the THF-solvated monomeric structure of complex 1, whereas magnesium complex 2 crystallized as a dimeric form containing a Mg2N2 unit as a four-membered metallacycle. The magnesium neosilyl complexes 1 and 2 efficiently catalyzed the hydroboration of epoxides and esters in the presence of pinacolborane (HBpin), leading to the regioselective formation of alcohol derivatives and boronate esters, respectively. The hydroboration of several aryl and alkyl epoxides is converted to a selectively Markovnikov product, branched boronate esters instead of linear ones, which, upon hydrolysis, result in secondary alcohols in high yields. Furthermore, complexes 1 and 2 catalyzed the hydroboration of aryl, alkyl, and cyclic esters, resulting in the formation of benzylic and linear boronate esters.-. Both magnesium complexes and all boronate esters were characterized by multi-nuclear NMR [1H, 13C{1H}, 11B{1H}, and 1H-DOSY(for complex 2)] spectroscopy.
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