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

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.

本研究合成了一系列带有易得且具有氧化还原活性的2,2′:6′,2″-三联吡啶（2,2′:6′,2″-terpyridine）与α-二亚胺（α-diimine）配体的钴烷基配合物，并对其电子结构进行了系统阐明。在所有案例中，辅助螯合配体均被还原为单阴离子自由基形式，并与钴(II)中心发生反铁磁耦合。两类钴烷基配合物均可有效催化空间位阻较大烯烃的异构化-硼氢化反应。其中，经α-二亚胺修饰的钴烯丙基配合物对空间位阻较大的三取代、四取代及偕二取代烯烃展现出格外优异的还原催化活性，属于目前已知活性最高的硼氢化均相催化剂之一。以柠檬烯为反应底物时，体系中生成了带有C-H agostic相互作用的η³-烯丙基配合物，该中间体可合理解释二烯底物所表现出的迟缓反应活性。对于2,2′:6′,2″-三联吡啶衍生的钴配合物，在使用HBPin作为硼试剂时，其与苯乙烯反应还展现出独特的马氏区域选择性。