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ABSTRACT: Enantiopure homoallylic boronate esters are versatile intermediates because the C–B bond in these com-pounds can be stereospecifically transformed into C–C, C–O and C–N bonds. Regio- and enantioselective synthesis of these precursors from 1,3-dienes has few precedents in the literature. We have identified reaction conditions and ligands for the synthesis of nearly enantiopure (er >97:3 to >99:1) homoallylic boronate esters via a rarely seen cobalt-catalyzed [4,3]-hydroboration of 1,3-dienes. Monosubstituted or 2,4-disubstituted linear dienes undergo highly efficient, regio- and enanti-oselective hydroboration with HBPin catalyzed by [(L*)Co]+[BARF]–, where L* is typically a chiral bis-phosphine ligand with a narrow bite angle. Several such ligands (examples: i-PrDuPhos, QuinoxP*, Duanphos and, BenzP*) that give high enantioselectivities for the [4,3]-hydroboration product have been identified. In addition, the equally challenging problem of regioselectivity is uniquely solved with a dibenzooxaphosphole ligand, (R,R)-MeO-BIBOP. A cationic cobalt(I) complex of this ligand is a very efficient (TON >960) catalyst, while providing excellent regioselectivities (rr >98:2) and enantioselectiv-ities (er >98:2) for a broad range of substrates. A detailed computational investigation of the reactions using Co-complexes from two widely different ligands (BenzP* and MeO-BIBOP) employing B3LYP-D3 density functional theory provides key insights into the mechanism and the origins of selectivities. The computational results are in full agreement with the exper-iments. For the complexes we have examined thus far, the relative stabilities of the diastereomeric diene-bound complexes [(L*)Co(4-diene)]+ leads to the initial diastereofacial selectivity, which in turn is retained in the subsequent steps, providing exceptional enantioselectivity for the reactions. 

Although cobalt( i ) bis-phosphine complexes have been implicated in many selective C–C bond-forming reactions, until recently relatively few of these compounds have been fully characterized or have been shown to be intermediates in catalytic reactions. In this paper we present a new practical method for the synthesis and isolation of several cobalt( i )-bis-phosphine complexes and their use in Co( i )-catalyzed reactions. We find that easily prepared ( in situ generated or isolated) bis-phosphine and (2,6- N -aryliminoethyl)pyridine (PDI) cobalt( ii ) halide complexes are readily reduced by 1,4-bis-trimethylsilyl-1,4-dihydropyrazine or commercially available lithium nitride (Li 3 N), leaving behind only innocuous volatile byproducts. Depending on the structures of the bis-phosphines, the cobalt( i ) complex crystallizes as a phosphine-bridged species [(P∼P)(X)Co I [μ-(P∼P)]Co I (X)(P∼P)] or a halide-bridged species [(P∼P)Co I [μ-(X)] 2 Co I (P∼P)]. Because the side-products are innocuous, these methods can be used for the in situ generation of catalytically competent Co( i ) complexes for a variety of low-valent cobalt-catalyzed reactions of even sensitive substrates. These complexes are also useful for the synthesis of rare cationic [(P∼P)Co I -η 4 -diene] + X − or [(P∼P)Co I -η 6 -arene] + X − complexes, which are shown to be excellent single-component catalysts for the following regioselective reactions of dienes: heterodimerizations with ethylene or methyl acrylate, hydroacylation and hydroboration. The reactivity of the single-component catalysts with the in situ generated species are also documented. 

Discovery of enantioselective catalytic reactions for the preparation of chiral compounds from readily available precursors, using scalable and environmentally benign chemistry, can greatly impact their design, synthesis, and eventually manufacture on the scale. Functionalized cyclobutanes and cyclobutenes are important structural motifs seen in many bioactive natural products and pharmaceutically relevant small molecules. The simplest approach to make cyclobutenes is through an enantioselective [2 + 2]-cycloaddition between an alkyne and an alkenyl derivative. Known reactions of this class that give acceptable enantioselectivities are of very narrow scope and are strictly limited to activated alkynes and highly reactive alkenes. Here, we disclose a broadly applicable enantioselective [2 + 2]-cycloaddition between a wide variety of alkynes and alkenyl derivatives, two of the most abundant classes of organic precursors. The key cycloaddition reaction employs catalysts derived from readily synthesized ligands and an earth-abundant metal, cobalt. Over 50 different functionalized cyclobutenes with enantioselectivities in the range of 86–97% ee are documented. In addition to this development, some of the novel observations made during these studies including a key role of a cationic Co (I)-intermediate, ligand and counter ion effects on the reactions, will be discussed. 

Discovery of base metal-catalyzed methods for the preparation of chiral intermediates has garnered great attention. Recently, through a systematic study of activators and ligands, we have discovered Co(I)-catalyzed enantioselective heterodimerization of linear 1,3-dienes with ethylene and acrylates. In these studies, cationic cobalt(I) has been invoked as an active catalyst to carry out the transformation. However, the synthesis and isolation of such active Co(I)-complexes which could give insight into of reaction’s mechanism, remains challenging. Herein, we disclosed a reliable procedure for the synthesis and isolation of Co(I)-complexes and characterized them by UV-Vis spectroscopy and X-ray crystallography. The bis-phosphine ligated Co(I) complexes in presence of activators, performed the regio- and enantioselective hydroboration of 2- substituted 1,3-diene with pinacolborane (HBPin) to obtain homoallylic boronates (enantiomeric excess, ee >90%). In the absence of activators, these complexes do not catalyze the reaction suggesting the key role of cationic Co(I)-species in the catalytic cycle. Currently, these Co(I) complexes are being further utilized in the hydroacylation of 1,3-dienes with simple aliphatic aldehyde to produce enantiopure ketones. The comprehensive protocols for the synthesis of Co(I) complexes and its application in hydrovinylation, heterodimerization with acrylates, hydroboration, and hydroacylation of 1,3-dienes will be discussed.