Open AccessC(sp2)–H Borylation of Heterocycles by Well-Defined Bis(silylene)pyridine Cobalt(III) Precatalysts: Pincer Modification, C(sp2)–H Activation, and Catalytically Relevant Intermediates
Author(s) -
Rebeca Arévalo,
Tyler P. Pabst,
Paul J. Chirik
Publication year - 2020
Publication title -
organometallics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.231
H-Index - 172
eISSN - 1520-6041
pISSN - 0276-7333
DOI - 10.1021/acs.organomet.0c00382
Subject(s) - borylation , chemistry , silylene , pincer movement , cobalt , pyridine , catalysis , benzofuran , medicinal chemistry , stereochemistry , organic chemistry , aryl , alkyl , silicon
Well-defined bis(silylene)pyridine cobalt(III) precatalysts for C(sp 2 )-H borylation have been synthesized and applied to the investigation of the mechanism of the catalytic borylation of furans and pyridines. Specifically, [( Ar SiNSi)CoH 3 ]·NaHBEt 3 ( Ar SiNSi = 2,6-[EtNSi(N t Bu) 2 CAr] 2 C 5 H 3 N, Ar = C 6 H 5 ( 1-H 3 ·NaHBEt 3 ), 4-MeC 6 H 4 ( 2-H 3 ·NaHBEt 3 )) and trans -[( Ar SiNSi)Co(H) 2 BPin] ( Ar = C 6 H 5 ( 1-(H) 2 BPin ), 4-MeC 6 H 4 ( 2-(H) 2 BPin ), Pin = pinacolato) were prepared and employed as single component precatalysts for the C(sp 2 )-H borylation of 2-methylfuran, benzofuran and 2,6-lutidine. The cobalt(III) precursors, 2-H 3 ·NaHBEt 3 and 2-(H) 2 BPin also promoted C(sp 2 )-H activation of benzofuran, yielding [( Ar SiNSi)CoH(Bf) 2 ] (Ar = 4-MeC 6 H 4 , 2-H(Bf) 2 , Bf = 2-benzofuranyl). Monitoring the catalytic borylation of 2-methylfuran and 2,6-lutidine by 1 H NMR spectroscopy established the trans -dihydride cobalt(III) boryl as the catalyst resting state at low substrate conversion. At higher conversion two distinct pincer modification pathways were identified, depending on the substrate and the boron source.