Cobalt-nhc Complexes and Diazabutadienes in Activation of Mono/Diboron Compounds and Their Application in C-b Coupling Reactions
Boronic acid(esters) have been well recognized as an indispensable coupling partner in the Suzuki-Miyaura cross coupling reactions producing a vast spectrum of molecules, applicable in the diverse field ranging from medicinal to materials sciences. Transition metal catalyzed synthesis of boronic esters from diborons with the assistance of bases is a well-established methodology. In this thesis, the cobalt-N-Heterocyclic carbene complexes catalyzed borylation of organic compounds and interaction of diazabutadienes with diboron compounds will be discussed. (i) In the first section, Co(IMes)2Cl2 catalyzed borylation of aryl halides will be discussed. [3a] The robust protocol, operating under mild condition facilitate the borylation of a diverse range of aryl halides with great efficacy, which includes the challenging aryl chlorides. The preliminary mechanistic studies suggest that base-bis(pinacolato)diboron adduct reduces the Co(IMes)2Cl2 complex to generate Co(IMes)2Cl complex, which acts as an active catalytic species. (ii) The second section deals with catalytic synthesis of primary and secondary alkyl boronic esters using alkyl halides. [3b] The in situ generated Co-NHC complex, in assistance with base and diboron compound, produces the corresponding borylated product from alky halides. The reaction proceeds under very mild conditions and covers a wide range of alkyl halides, including chlorides having different functional groups. (iii) In the third section, development in selective hydroboration of vinyl arenes and aliphatic alkenes will be discussed. [3c] Catalyzed by Co(I)NHC complex, the alkene hydroboration by pinacol borane gives Markovnikov selective product with good selectivity, where the regio-selectivity is controlled by phenyl substituent. In absence of that, complete inversion in the selectivity has been observed. The preliminary mechanistic cycle suggests that the catalytic cycle proceeds via oxidative addition of pinacol borane to [Co] followed by alkene insertion and reduction elimination steps. (iv) The last section discusses the interaction of diazabutadiene molecules with diboron compounds. [3d] The diazabutadiene derivatives have been observed to completely cleave the B-B bond of Bis(catacolato)diboron and Bis(dithiocatacolato)diboron. The preliminary findings hint towards homolytic cleavage of the B-B bond by concerted interaction of the two nitrogen atoms of diazabutadiene with the two boron atoms of the diboron from the same face. References:  Boronic Acids-Preparation and Applications in Organic Synthesis, Medicine and Materials, 2nd ed.; Hall, D. G., Ed.; Wiley-VCH: Weinheim, 2011.  Neeve, E. C.; Geier, S. J.; Mkhalid, I. A. I.; Westcott, S. A.; Marder, T. B. Diboron(4) Compounds: From Structural Curiosity to Synthetic Workhorse. Chem. Rev. 2016, 116, 9091-9161.  (a) Verma, P. K.; Mandal, S.; Geetharani, K. ACS Catal. 2018, 8, 4049-4054. (b) Verma, P. K.; Prasad, K. S.; Varghese, D.; Geetharani, K. Org. Lett. 2020, 22, 4, 1431-1436. (c) Verma, P. K.; Setulekshmi, A. S.; Geetharani, K. Org. Lett. 2018, 20, 7840-7845. (d) Verma, P. K.; Meher, N. K.; Geetharani, K. Accepted for publication in Chem. Commun., Manuscript ID: CC-COM-06-2021-002881.R2.