40 research outputs found
Activation and discovery of earth-abundant metal catalysts using sodium tert-butoxide
First-row, earth-abundant metals offer an inexpensive and sustainable alternative to precious-metal catalysts. As such, iron and cobalt catalysts have garnered interest as replacements for alkene and alkyne hydrofunctionalization reactions. However, these have required the use of air- and moisture-sensitive catalysts and reagents, limiting both adoption by the non-expert as well as applicability, particularly in industrial settings. Here, we report a simple method for the use of earth-abundant metal catalysts by general activation with sodium tert-butoxide. Using only robust air- and moisture-stable reagents and pre-catalysts, both known and, significantly, novel catalytic activities have been successfully achieved, covering hydrosilylation, hydroboration, hydrovinylation, hydrogenation and [2Ï+2Ï] alkene cycloaddition. This activation method allows for the easy use of earth-abundant metals, including iron, cobalt, nickel and manganese, and represents a generic platform for the discovery and application of non-precious metal catalysis
Highly Selective Bis(imino)pyridine Iron-Catalyzed Alkene Hydroboration
Bis(imino)pyridine iron dinitrogen complexes have been shown to promote the anti-Markovnikov catalytic hydroboration of terminal, internal, and geminal alkenes with high activity and selectivity. The isolated iron dinitrogen compounds offer distinct advantages in substrate scope and overall performance over known precious metal catalysts and previously reported in situ generated iron species
Bis(imino)pyridine Cobalt-Catalyzed Alkene IsomerizationâHydroboration: A Strategy for Remote Hydrofunctionalization with Terminal Selectivity
BisÂ(imino)Âpyridine cobalt methyl
complexes are active for the catalytic
hydroboration of terminal, geminal, disubstituted internal, tri- and
tetrasubstituted alkenes using pinacolborane (HBPin). The most active
cobalt catalyst was obtained by introducing a pyrrolidinyl substituent
into the 4-position of the bisÂ(imino)Âpyridine chelate, enabling the
facile hydroboration of sterically hindered substrates such as 1-methylcyclohexene,
α-pinene, and 2,3-dimethyl-2-butene. Notably, these hydroboration
reactions proceed with high activity and anti-Markovnikov selectivity
in neat substrates at 23 °C. With internal olefins, the cobalt
catalyst places the boron substituent exclusively at the terminal
positions of an alkyl chain, providing a convenient method for hydrofunctionalization
of remote CâH bonds
Mechanistic Studies of Cobalt-Catalyzed C(sp<sup>2</sup>)âH Borylation of Five-Membered Heteroarenes with Pinacolborane
Studies
into the mechanism of cobalt-catalyzed CÂ(sp<sup>2</sup>)âH
borylation of five-membered heteroarenes with pinacolborane
(HBPin) as the boron source established the catalyst resting state
as the <i>trans</i>-cobaltÂ(III) dihydride boryl, <b>(</b><sup><b>iPr</b></sup><b>PNP)ÂCoÂ(H)</b><sub><b>2</b></sub><b>(BPin)</b> (<sup>iPr</sup>PNP = 2,6-(<sup>i</sup>Pr<sub>2</sub>PCH<sub>2</sub>)<sub>2</sub>(C<sub>5</sub>H<sub>3</sub>N)),
at both low and high substrate conversions. The overall first-order
rate law and observation of a normal deuterium kinetic isotope effect
on the borylation of benzofuran versus benzofuran-2-<i>d</i><sub>1</sub> support H<sub>2</sub> reductive elimination from the
cobaltÂ(III) dihydride boryl as the turnover-limiting step. These findings
stand in contrast to that established previously for the borylation
of 2,6-lutidine with the same cobalt precatalyst, where borylation
of the 4-position of the pincer occurred faster than the substrate
turnover and arene CâH activation by a cobaltÂ(I) boryl is turnover-limiting.
Evaluation of the catalytic activity of different cobalt precursors
in the CâH borylation of benzofuran with HBPin established
that the ligand design principles for CâH borylation depend
on the identities of both the arene and the boron reagent used: electron-donating
groups improve catalytic activity of the borylation of pyridines and
arenes with B<sub>2</sub>Pin<sub>2</sub>, whereas electron-withdrawing
groups improve catalytic activity of the borylation of five-membered
heteroarenes with HBPin. Catalyst deactivation by PâC bond
cleavage from a cobaltÂ(I) hydride was observed in the CâH borylation
of arene substrates with CâH bonds that are less acidic than
those of five-membered heteroarenes using HBPin and explains the requirement
of B<sub>2</sub>Pin<sub>2</sub> to achieve synthetically useful yields
with these arene substrates
Bis(imino)pyridine Cobalt-Catalyzed Alkene IsomerizationâHydroboration: A Strategy for Remote Hydrofunctionalization with Terminal Selectivity
BisÂ(imino)Âpyridine cobalt methyl
complexes are active for the catalytic
hydroboration of terminal, geminal, disubstituted internal, tri- and
tetrasubstituted alkenes using pinacolborane (HBPin). The most active
cobalt catalyst was obtained by introducing a pyrrolidinyl substituent
into the 4-position of the bisÂ(imino)Âpyridine chelate, enabling the
facile hydroboration of sterically hindered substrates such as 1-methylcyclohexene,
α-pinene, and 2,3-dimethyl-2-butene. Notably, these hydroboration
reactions proceed with high activity and anti-Markovnikov selectivity
in neat substrates at 23 °C. With internal olefins, the cobalt
catalyst places the boron substituent exclusively at the terminal
positions of an alkyl chain, providing a convenient method for hydrofunctionalization
of remote CâH bonds
C(sp<sup>2</sup>)âH Borylation of Fluorinated Arenes Using an Air-Stable Cobalt Precatalyst: Electronically Enhanced Site Selectivity Enables Synthetic Opportunities
Cobalt catalysts
with electronically enhanced site selectivity
have been developed, as evidenced by the high <i>ortho</i>-to-fluorine selectivity observed in the CÂ(sp<sup>2</sup>)âH
borylation of fluorinated arenes. Both the air-sensitive cobaltÂ(III)
dihydride boryl 4-Me-(<sup>iPr</sup>PNP)ÂCoÂ(H)<sub>2</sub>BPin (<b>1</b>) and the air-stable cobaltÂ(II) bisÂ(pivalate) 4-Me-(<sup>iPr</sup>PNP)ÂCoÂ(O<sub>2</sub>C<sup>t</sup>Bu)<sub>2</sub> (<b>2</b>) compounds were effective and exhibited broad functional
group tolerance across a wide range of fluoroarenes containing electronically
diverse functional groups, regardless of the substitution pattern
on the arene. The electronically enhanced <i>ortho</i>-to-fluorine
selectivity observed with the cobalt catalysts was maintained in the
presence of a benzylic dimethylamine and hydrosilanes, overriding
the established directing-group effects observed with precious-metal
catalysts. The synthetically useful selectivity observed with cobalt
was applied to an efficient synthesis of the anti-inflammatory drug
flurbiprofen
Cobalt-Catalyzed CâH Borylation
A family of pincer-ligated cobalt
complexes has been synthesized
and are active for the catalytic CâH borylation of heterocycles
and arenes. The cobalt catalysts operate with high activity and under
mild conditions and do not require excess borane reagents. Up to 5000
turnovers for methyl furan-2-carboxylate have been observed at ambient
temperature with 0.02 mol % catalyst loadings. A catalytic cycle that
relies on a cobaltÂ(I)â(III) redox couple is proposed
Cobalt-Catalyzed CâH Borylation
A family of pincer-ligated cobalt
complexes has been synthesized
and are active for the catalytic CâH borylation of heterocycles
and arenes. The cobalt catalysts operate with high activity and under
mild conditions and do not require excess borane reagents. Up to 5000
turnovers for methyl furan-2-carboxylate have been observed at ambient
temperature with 0.02 mol % catalyst loadings. A catalytic cycle that
relies on a cobaltÂ(I)â(III) redox couple is proposed
Cobalt-Catalyzed C(sp<sup>2</sup>)âH Borylation: Mechanistic Insights Inspire Catalyst Design
A comprehensive
study into the mechanism of bisÂ(phosphino)Âpyridine
(PNP) cobalt-catalyzed CâH borylation of 2,6-lutidine using
B<sub>2</sub>Pin<sub>2</sub> (Pin = pinacolate) has been conducted.
The experimentally observed rate law, deuterium kinetic isotope effects,
and identification of the catalyst resting state support turnover
limiting CâH activation from a fully characterized cobaltÂ(I)
boryl intermediate. Monitoring the catalytic reaction as a function
of time revealed that borylation of the 4-position of the pincer in
the cobalt catalyst was faster than arene borylation. Cyclic voltammetry
established the electron withdrawing influence of 4-BPin, which slows
the rate of CâH oxidative addition and hence overall catalytic
turnover. This mechanistic insight inspired the next generation of
4-substituted PNP cobalt catalysts with electron donating and sterically
blocking methyl and pyrrolidinyl substituents that exhibited increased
activity for the CâH borylation of unactivated arenes. The
rationally designed catalysts promote effective turnover with stoichiometric
quantities of arene substrate and B<sub>2</sub>Pin<sub>2</sub>. Kinetic
studies on the improved catalyst, <b>4-(H)</b><sub><b>2</b></sub><b>BPin</b>, established a change in turnover limiting
step from CâH oxidative addition to CâB reductive elimination.
The iridium congener of the optimized cobalt catalyst, <b>6-(H)</b><sub><b>2</b></sub><b>BPin</b>, was prepared and crystallographically
characterized and proved inactive for CâH borylation, a result
of the high kinetic barrier for reductive elimination from octahedral
IrÂ(III) complexes
Cobalt-Catalyzed Benzylic Borylation: Enabling Polyborylation and Functionalization of Remote, Unactivated C(sp<sup>3</sup>)âH Bonds
Cobalt
dialkyl and bisÂ(carboxylate) complexes bearing α-diimine
ligands have been synthesized and demonstrated as active for the CÂ(sp<sup>3</sup>)-H borylation of a range of substituted alkyl arenes using
B<sub>2</sub>Pin<sub>2</sub> (Pin = pinacolate) as the boron source.
At longer reaction times, rare examples of polyborylation were observed,
and in the case of toluene, all three benzylic C-H positions were
functionalized. Coupling benzylic C-H activation with alkyl isomerization
enabled a base-metal-catalyzed method for the borylation of remote,
unactivated CÂ(sp<sup>3</sup>)-H bonds