High-Activity Cobalt Catalysts for Alkene Hydroboration with Electronically Responsive Terpyridine and α-Diimine Ligands

Abstract

Cobalt alkyl complexes bearing readily available and redox-active 2,2′:6′,2″-terpyridine and α-diimine ligands have been synthesized, and their electronic structures have been elucidated. In each case, the supporting chelate is reduced to the monoanionic, radical form that is engaged in antiferromagnetic coupling with the cobalt­(II) center. Both classes of cobalt alkyls proved to be effective for the isomerization–hydroboration of sterically hindered alkenes. An α-diimine-substituted cobalt allyl complex proved exceptionally active for the reduction of hindered tri-, tetra-, and geminally substituted alkenes, representing one of the most active homogeneous catalysts known for hydroboration. With limonene, formation of an η3-allyl complex with a C–H agostic interaction was identified and accounts for the sluggish reactivity observed with diene substrates. For the terpyridine derivative, unique Markovnikov selectivity with styrene was also observed with HBPin.

Cobalt-Catalyzed 1,1-Diboration of Terminal Alkynes: Scope, Mechanism, and Synthetic Applications

“…Accordingly, its use with 1,1-diborylalkenes could proceed via intermediacy of a stabilized α -borylcobalt intermediate. 35 As shown in Scheme 8, sequential 1,1-diboration-hydroboration of terminal alkynes involving ( Cy APDI)CoCH 3 ( 1 ) and (Terpy)Co(CH 2 SiMe 3 ) ( 14 ) 34 furnished a collection of 1,1,1-triboronates 19 in 50–76% yield over two steps. Importantly, carbonyl-based functional groups were compatible with this hydroboration protocol, as evidenced by the synthesis of triboronates 19e , 19i , and 19g .…”

Recent Advances in Hydrometallation of Alkenes and Alkynes via the First Row Transition Metal Catalysis

“…[195] With a terpyridine-cobalt alkyl complex (74-CoCH 2 SiMe 3 ), Chirik and co-workers disclosed a highly regioselective Markovnikov hydroboration of styrene in 2015 (Scheme 69). [195] With a terpyridine-cobalt alkyl complex (74-CoCH 2 SiMe 3 ), Chirik and co-workers disclosed a highly regioselective Markovnikov hydroboration of styrene in 2015 (Scheme 69).…”

Cobalt–Pincer Complexes in Catalysis

“…Interestingly, the air‐stable Co–CCC‐NHC‐pincer complex 29 and the cobalt–alkyl‐NNN‐pincer complex 30 bearing the 2,2′:6′,2′′‐terpyridine ligand hydroborate styrene favouring the Markovnikov regiochemistry (Table , entries 4 and 5) instead of the usual anti‐Markovnikov product. When the bipyridyl–oxazoline cobalt complex 31 c was applied for the hydroboration of different substituted styrene derivatives the Markovnikov selectivity could be significantly improved .…”

“…[53] Most reactions proceeded to completion with 0.005-0.05mol %o f28 with or without solventu pon activation with NaBHEt 3 .I nalarger scale experiment the hydroboration of styrene (50 mmol) with HBPin was realised with a very low catalyst loading of 28 (0.005 mol %) providing ane xcellent turnover number (TON) of 19 800. Interestingly,t he air-stable Co-CCC-NHC-pincerc omplex 29 [54] and the cobalt-alkyl-NNN-pincer complex 30 [55] bearing the 2,2':6',2''-terpyridine ligand hydroborate styrene favouring the Markovnikov regiochemistry ( Table 9, entries4 and 5) instead of the usual anti-Markovnikov product. When the bipyridyl-oxazoline cobalt complex 31 c was applied for the hydroboration of different substituted styrene derivatives the Markovnikovs electivity could be significantly improved.…”