skip to main content
US FlagAn official website of the United States government
dot gov icon
Official websites use .gov
A .gov website belongs to an official government organization in the United States.
https lock icon
Secure .gov websites use HTTPS
A lock ( lock ) or https:// means you've safely connected to the .gov website. Share sensitive information only on official, secure websites.


Title: Ligand-Driven Advances in Iridium-Catalyzed sp3 C–H Borylation: 2,2′-Dipyridylarylmethane
Abstract The field of catalytic C–H borylation has grown considerably since its founding, providing a means for the preparation of synthetically versatile organoborane products. Although sp2 C–H borylation methods have found widespread and practical use in organic synthesis, the analogous sp3 C–H borylation reaction remains challenging and has seen limited application. Existing catalysts are often hindered by incomplete consumption of the diboron reagent, poor functional-group tolerance, harsh reaction conditions, and the need for excess or neat substrate. These challenges acutely affect the C–H borylation chemistry of unactivated hydrocarbon substrates, which has lagged in comparison to methods for the C–H borylation of activated compounds. Herein, we discuss recent advances in the sp3 C–H borylation of undirected substrates in the context of two particular challenges: (1) utilization of the diboron reagent and (2) the need for excess or neat substrate. Our recent work on the application of dipyridylarylmethane ligands in sp3 C–H borylation has allowed us to make contributions in this space and has presented an additional ligand scaffold to supplement traditional phenanthroline ligands.  more » « less
Award ID(s):
1847813
PAR ID:
10256911
Author(s) / Creator(s):
;
Date Published:
Journal Name:
Synlett
Volume:
32
Issue:
09
ISSN:
0936-5214
Page Range / eLocation ID:
845 to 850
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Due to the scarcity of C–F bond-forming enzymatic reactions in nature and the contrasting prevalence of organofluorine moieties in bioactive compounds, developing biocatalytic fluorination reactions represents a pre-eminent challenge in enzymology, biocatalysis and synthetic biology. Additionally, catalytic enantioselective C(sp3)–H fluorination remains a challenging problem facing synthetic chemists. Although many non-haem iron halogenases have been discovered to promote C(sp3)–H halogenation reactions, efforts to convert these iron halogenases to fluorinases have remained unsuccessful. Here we report the development of an enantioselective C(sp3)–H fluorination reaction, catalysed by a plant-derived non-haem enzyme 1-aminocyclopropane-1-carboxylic acid oxidase (ACCO), which is repurposed for radical rebound fluorination. Directed evolution afforded a C(sp3)–H fluorinating enzyme ACCOCHF displaying 200-fold higher activity, substantially improved chemoselectivity and excellent enantioselectivity, converting a range of substrates into enantioenriched organofluorine products. Notably, almost all the beneficial mutations were found to be distal to the iron centre, underscoring the importance of substrate tunnel engineering in non-haem iron biocatalysis. Computational studies reveal that the radical rebound step with the Fe(III)–F intermediate has a low activation barrier of 3.4 kcal mol−1 and is kinetically facile. 
    more » « less
  2. null (Ed.)
    A nontrigonal phosphorus triamide ( 1 , P{N[ o -NMe-C 6 H 4 ] 2 }) is shown to catalyze C–H borylation of electron-rich heteroarenes with pinacolborane (HBpin) in the presence of a mild chloroalkane reagent. C–H borylation proceeds for a range of electron-rich heterocycles including pyrroles, indoles, and thiophenes of varied substitution. Mechanistic studies implicate an initial P–N cooperative activation of HBpin by 1 to give P -hydrido diazaphospholene 2 , which is diverted by Atherton–Todd oxidation with chloroalkane to generate P -chloro diazaphospholene 3 . DFT calculations suggest subsequent oxidation of pinacolborane by 3 generates chloropinacolborane (ClBpin) as a transient electrophilic borylating species, consistent with observed substituent effects and regiochemical outcomes. These results illustrate the targeted diversion of established reaction pathways in organophosphorus catalysis to enable a new mode of main group-catalyzed C–H borylation. 
    more » « less
  3. The methyl moiety is a key functional group that can result in major improvements in the potency and selectivity of pharmaceutical agents. We present a radical relay C–H methylation methodology that employs a β-diketiminate copper catalyst capable of methylating unactivated C(sp3)–H bonds. Taking advantage of the bench-stable DABAL-Me3, an amine-stabilized trimethylaluminum reagent, methylation of a range of substrates possessing both activated and unactivated C(sp3)–H bonds proceeds with a minimal amount of overmethylation. Mechanistic studies supported by both experiment and computation suggest the intermediacy of a copper(II) methyl intermediate reactive toward both the loss of the methyl radical as well capture of radicals R• to form R–Me bonds. 
    more » « less
  4. Amidyl radicals mediate a diverse array of intermolecular aliphatic C(sp3)–H and decarboxylative functionalizations. Interestingly, we have observed that decarboxylative processes proceed with excellent chemoselectivity even with substrates containing weak C(sp3)–H bonds. Herein, we report a mechanistic basis for understanding this high chemoselectivity of amidyl radicals through divergent reaction pathways. A computational assessment of the transition state SOMOs and intrinsic bonding orbitals for amidyl radical hydrogen atom transfer (HAT) and concerted proton-electron transfer (CPET) processes support a shift in mechanism between aliphatic C(sp3)–H or carboxylic acid O–H abstraction, which is supported by experimental studies. These findings provide a rationale for the chemoselectivity of decarboxylative reactions mediated by amidyl radicals. 
    more » « less
  5. C-H Bond dissociation energies for a unique selection of tertiary amines that are known substrates or inhibitors of monoamine oxidase have been calculated using density functional theory. These amines are unusual because they are the only tertiary amines that exhibit MAO substrate or inhibitor behavior. The unique structural feature common to these specific compounds is an sp3-hybridized CH2 moiety, which is -both to nitrogen and an C=C or C≡C. The stabilization afforded the resulting radicals by extended delocalization dramatically lowers both the C-H bond strength of the substrate (R-H → R• + H•) and pKa of the corresponding radical cation (RH•+ → R• + H+). This interplay of structure and thermodynamics may provide the driving force for an electron transfer mechanism for MAO catalysis and inhibition. 
    more » « less