Cobalt-Catalyzed Reductive C–O Bond Cleavage of Lignin β-O-4 Ketone Models via In Situ Generation of the Cobalt–Boryl Species

Abstract

An efficient and mild method for reductive C−O bond cleavage of lignin β-O-4 ketone models was developed to afford the corresponding ketones and phenols with PDI-CoCl 2 as the precatalyst and diboron reagent as the reductant. The synthetic utility of the methodology was demonstrated by depolymerization of a polymeric model and gram-scale transformation. Mechanistic studies suggested that this transformation involves steps of carbonyl insertion, 1,2-Brook type rearrangement, β-oxygen elimination, and rate-limiting regeneration of the catalytic active Co−B species.

Cobalt-Catalyzed Z to E Geometrical Isomerization of 1,3-Dienes

“…Earth-abundant transition-metal catalysts have emerged as desired alternatives to noble metals fo r alkene isomerization. − On the basis of our previous investigations in cobalt-catalyzed new transformations, we postulated that an appropriate in situ-generated cobalt-hydride species may enable Z to E isomerization of 1,3-dienes to produce the thermodynamically more stable ( E ) isomers (Scheme d). Mechanistically, the reaction could begin with 3,4-migratory insertion of ( Z )-1,3-diene into the Co–H bond to generate a π-allyl cobalt intermediate ( I ), which then undergoes the σ-π-σ process to form interconverting isomer IV .…”

Na2S-Mediated One-Pot Selective Deoxygenation of α-Hydroxyl Carbonyl Compounds including Natural Products

“…The process requires the functionalization of phenylacetic acid to phenylacetyl chloride by stoichiometric PCl 3 or SOCl 2 prior to the C–C bond coupling [ 8 , 9 , 10 , 11 , 12 ]. Other elegant strategies, including hydration [ 13 ], olefin cleavage [ 14 ], benzylic oxidation [ 15 ] and C–O bond breaking protocols [ 16 , 17 , 18 ], have also been developed to access DOBs in recent years ( Scheme 1 ). However, these methods generally required the prefunctionalization of starting molecules or, alternatively, the use of expensive substrates [ 13 , 14 , 15 , 16 ].…”

“…Other elegant strategies, including hydration [ 13 ], olefin cleavage [ 14 ], benzylic oxidation [ 15 ] and C–O bond breaking protocols [ 16 , 17 , 18 ], have also been developed to access DOBs in recent years ( Scheme 1 ). However, these methods generally required the prefunctionalization of starting molecules or, alternatively, the use of expensive substrates [ 13 , 14 , 15 , 16 ]. Thus, it is highly desirable to develop practical processes for DOB production using cheap and easy-to-handle feedstocks.…”

Selective Hydrogenolysis of Lignin Model Compounds to Aromatics over a Cobalt Nanoparticle Catalyst

“…16,17 Under the guidance of this strategy, lots of non-noble metal catalysts have been designed to realize the efficient hydrogenolysis of β-O-4 ketone. For example, Ni/MgAlO−C catalyst achieved the cleavage by the modulation of the electronic structure via carbon modification; 6 a mild method with PDI-CoCl 2 as a reductant and a diboron reagent as a precatalyst was developed to afford the corresponding ketones and phenols, 18 and photocatalysis was also carried out. 19 Recently, nitrogen-doped carbon-supported metal catalysts (MNCs) have been drawing considerable attention because of their unique properties of a redox reaction in a hydrogen or oxygen atmosphere.…”

“…Oxidation of C α –OH to C α O can lower the C β –O bond energy, followed by the hydrogenolysis of the C β –O bond. , Under the guidance of this strategy, lots of non-noble metal catalysts have been designed to realize the efficient hydrogenolysis of β-O-4 ketone. For example, Ni/MgAlO–C catalyst achieved the cleavage by the modulation of the electronic structure via carbon modification; a mild method with PDI-CoCl 2 as a reductant and a diboron reagent as a precatalyst was developed to afford the corresponding ketones and phenols, and photocatalysis was also carried out …”