Selective hydroboration of unsaturated bonds by an easily accessible heterotopic cobalt catalyst

ARTICLE https://doi.org/10.1038/s41467-021-24117-5 OPEN Selective hydroboration of unsaturated bonds by an easily accessible heterotopic cobalt catalyst 1234567890():,; Chuhan Li1, Shuo Song1, Yuling Li1, Chang Xu2, Qiquan Luo 3, Yinlong Guo1 ✉ & Xiaoming Wang1,4 ✉ Homogeneous earth-abundant metal catalysis based on well-defined molecular complexes has achieved great advance in synthetic methodologies. However, sophisticated ligand, hazardous activator and multistep synthesis starting from base metal salts are generally required for the generation of active molecular catalysts, which may hinder their broad application in large scale organic synthesis. Therefore, the development of metal cluster catalysts formed in situ from simple earth-abundant metal salts is of importance for the practical utilization of base metal resource, yet it is still in its infancy. Herein, a mixture of catalytic amounts of cobalt (II) iodide and potassium tert-butoxide is discovered to be highly active for selective hydroboration of vinylarenes and dihydroboration of nitriles, affording a good yield of diversified hydroboration products that without isolation can readily undergo further one pot transformations. It should be highlighted that the alkoxide-pinacolborane combination acts as an efficient activation strategy to activate cobalt (II) iodide for the generation of metastable heterotopic cobalt catalysts in situ, which is proposed to be catalytically active species. 1 State Key Laboratory of Organometallic Chemistry, Center for Excellence in Molecular Synthesis, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai, China. 2 Department of Chemistry, Anhui University, Hefei, Anhui, China. 3 Institutes of Physical Science and Information Technology, Anhui University, Hefei, Anhui, China. 4 School of Chemistry and Materials Science, Hangzhou Institute for Advanced Study, University of Chinese Academy of Sciences, Hangzhou, China. ✉email: ; NATURE COMMUNICATIONS | (2021)12:3813 | https://doi.org/10.1038/s41467-021-24117-5 | www.nature.com/naturecommunications 1 ARTICLE H NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-021-24117-5 omogeneous earth-abundant metal catalysis is one of the keys to the sustainable future in organic synthesis benefited from the advantages of cheap, earth-abundant, and less toxic base metals1–5. Their well-defined metal complexes have achieved great advances as homogeneous catalysts in recent years (Fig. 1a). For example, cobalt is abundant, inexpensive and a variety of their salts are commercially available1–5. In the past decade, several well-defined alkene hydroboration catalysts with sophisticated ligands have been developed based on cobalt complexes6–10. Nevertheless, these reactions typically employed catalysts bearing sophisticated ligands, which can be expensive, air sensitive, or difficult to be synthesized. In addition, current methods for the activation of the pre-catalysts to lower oxidationstate catalytic species relied heavily on the use of various hazardous reducing reagents, such as main group organometallics or hydrides11,12. All of these hinder their broad application in largescale organic synthesis. Complementary to such molecularly defined systems, catalysis by metallic clusters has become a quite recent field of research which attracted the attention from the chemical community13–19. It has appeared that metal clusters which are formed in situ from simple metal salts may fill the gap between the single metal atom with sophisticated ligands and the metal nanoparticles (NPs) (Fig. 1a)20–29. They can also invoke distinct catalytic properties compared to conventional NPs. Taking cobalt again as an example, some ill-defined or nanoparticulate Co catalysts, prepared by in situ reduction of a cobalt salt with a reductant, have been reported to exhibit good hydrogenation activities30–38. Despite this enormous potential in catalysis, the development of metal cluster catalysts based on earth-abundant metals is still in its infancy20–29. From a practical perspective, the development of ligand-free heterotopic cobalt catalysts for synthetically useful alkene hydroboration reaction with HBPin, using readily available cobalt salts would be highly desirable39–46. Herein, we disclose a user-friendly catalytic protocol using the mixture of CoI2 and KOtBu for highly active and selective Markovnikov hydroboration of vinylarenes and double hydroboration of nitriles, without using any costly ligand/activator. It should be noted that Markovnikov hydroboration reactions are rarely approached with base-metal catalysts47–58. KOtBu is proposed to act as a masked reducing agent, by reacting with HBPin to form an ate-type complex that can then serve as a reductive pre-catalyst activator11. Preliminary mechanistic studies suggest that the Co(II) salt is most likely to be reduced in situ to some low-valent Co species, which undergo aggregation to form heterotopic Co catalysts responsible for the catalysis (Fig. 1b). Notably, using other strong reductants such as NaBHEt3 or Grignard reagents lead to poor results, demonstrating the formation of the heterotopic species is largely influenced by the reductants and the alkoxide-pinacolborane combination plays a key role in the success of the present catalysis. The as-synthesized hydroboration products can serve as valuable synthons in further synthetic manipulations in a one-pot transformation, demonstrating the practicality and utility of the present methodologies. Results Reaction development. The study was commenced by hydroboration of styrene 1a with HBPin, using commercially available CoI2 as the pre-catalyst in combination with substoichiometric amount of a reductant. The use of NaBHEt3, EtMgBr, or PhLi as the activator in the reaction only gave the desired hydroboration products in low yields with poor regioselectivities (Fig. 2, entries 1–3). Hydrogenated product 4a (ethylbenzene) was also detected in these reaction mixtures, which is consistent with the good hydrogenation activities of the generated nanoparticles30–38. In 2017, Thomas et al. reported that a boron ‘ate’ reductive species can be formed in situ by the reaction between alkoxide and HBPin and activate high-oxidation-state cobalt complexes immediately as hydride donors11,59. Inspired from this pioneering work, a catalytic amount of NaOtBu (10 mol%) was tested in the reaction, leading to a boosting in the yield of hydroboration product 2a to 82% with excellent Markovnikov regioselectivity (40/1), highlighting the importance of the activators in the catalytic activity (Fig. 2, entry 4 vs 1–3). Changing NaOtBu to KOtBu slightly improved the results (Fig. 2, entry 5), whereas use of LiOtBu resulted in a decrease in the regioselectivity (Fig. 2, entry 6). The addition of crown ethers to the reactions turned out to be deleterious for the catalysis (for details, see the Supplementary Information). These results suggested that the (...truncated)