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.
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Ligand Control in Co-Catalyzed Regio- and Enantioselective Hydroboration: Homoallyl Secondary Boronates via Uncommon 4,3-Hydroboration of 1,3-Dienes
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.
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- Award ID(s):
- 1900141
- PAR ID:
- 10474202
- Publisher / Repository:
- American Chemical Society
- Date Published:
- Journal Name:
- Journal of the American Chemical Society
- Volume:
- 145
- Issue:
- 13
- ISSN:
- 0002-7863
- Page Range / eLocation ID:
- 7462 to 7481
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1021/jacs.3c00181
