Chemoselective C(sp)-H Borylation of Terminal Alkynes Catalyzed by a Bis(N-heterocyclicsilylene) Manganese Complex

The manganese(II) complex [Mn(SiNSi)Cl2] (SiNSi = 2,6-[EtNSi(NtBu)2CPh]2C5H3N) was an efficient catalyst for the chemoselective C(sp)-H borylation of terminal alkynes. Aliphatic as well as aromatic alkynes containing electron-withdrawing and -donating substituents in different positions have been efficiently borylated. In all cases, the cata-lyst showed an excellent chemoselectivity towards C-H borylation and the reactions proceeded without additives or in-situ activators. Paramagnetic Mn complexes are involved in catalytic turnover which is proposed to occur by a re-dox-neutral Mn(II) cycle. Stoichiometric reactions support that the [Mn(SiNSi)Cl2] precatalyst enters the catalytic cycle by reaction with HBPin. KIE experiments point toward C-H activation of the alkyne as not being involved in the rate-determining step.