A bis(imino)pyridine cobalt-catalyzed hydroboration of terminal alkynes with HBPin (Pin = pinacolate) with high yield and (Z)-selectivity for synthetically valuable vinylboronate esters is described. Deuterium labeling studies, stoichiometric experiments, and isolation of catalytically relevant intermediates support a mechanism involving selective insertion of an alkynylboronate ester into a Co-H bond, a pathway distinct from known precious metal catalysts where metal vinylidene intermediates have been proposed to account for the observed (Z) selectivity. The identity of the imine substituents dictates the relative rates of activation of the cobalt precatalyst with HBPin or the terminal alkyne and, as a consequence, is responsible for the stereochemical outcome of the catalytic reaction.
A comprehensive study into the mechanism of bis(phosphino)pyridine (PNP) cobalt-catalyzed C-H borylation of 2,6-lutidine using B2Pin2 (Pin = pinacolate) has been conducted. The experimentally observed rate law, deuterium kinetic isotope effects, and identification of the catalyst resting state support turnover limiting C-H activation from a fully characterized cobalt(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 C-H 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 C-H borylation of unactivated arenes. The rationally designed catalysts promote effective turnover with stoichiometric quantities of arene substrate and B2Pin2. Kinetic studies on the improved catalyst, 4-(H)2BPin, established a change in turnover limiting step from C-H oxidative addition to C-B reductive elimination. The iridium congener of the optimized cobalt catalyst, 6-(H)2BPin, was prepared and crystallographically characterized and proved inactive for C-H borylation, a result of the high kinetic barrier for reductive elimination from octahedral Ir(III) complexes.
Combination of the readily available
α-diimine ligand, ((ArNC(Me))2 Ar = 2,6-iPr2–C6H3), (iPrDI) with
air-stable nickel(II) bis(carboxylates) generated a highly active
catalyst exhibiting anti-Markovnikov selectivity for the hydrosilylation
of alkenes with a variety of industrially relevant tertiary alkoxy-
and siloxy-substituted silanes. A combination of the method of continuous
variations with stoichiometric studies identified the formally Ni(I)
hydride dimer, [(iPrDI)NiH]2 as the nickel compound
formed following reduction of the carboxylate ligands. For the hydrosilylation
of 1-octene with (EtO)3SiH, a rate law of [Ni]1/2[1-octene][(EtO)3SiH] in combination with deuterium-labeling
studies establish dissociation of the nickel hydride dimer followed
by fast and reversible alkene insertion into (iPrDI)NiH,
consistent with turnover-limiting C–Si bond formation. The
hydrosilylation of 1-octene with triethoxysilane, a reaction performed
commercially in the silicones industry on a scale of >5 000 000
kg/year, was conducted on a 10 g scale with 96% yield and >98%
selectivity
for the desired product. Silicone cross-linking, another major industrial
application of homogeneous hydrosilylation, was also demonstrated
using the air-stable nickel and ligand precursors.
Cobalt
alkyl complexes bearing readily available and redox-active
2,2′:6′,2″-terpyridine and α-diimine ligands
have been synthesized, and their electronic structures have been elucidated.
In each case, the supporting chelate is reduced to the monoanionic,
radical form that is engaged in antiferromagnetic coupling with the
cobalt(II) center. Both classes of cobalt alkyls proved to be effective
for the isomerization–hydroboration of sterically hindered
alkenes. An α-diimine-substituted cobalt allyl complex proved
exceptionally active for the reduction of hindered tri-, tetra-, and
geminally substituted alkenes, representing one of the most active
homogeneous catalysts known for hydroboration. With limonene, formation
of an η3-allyl complex with a C–H agostic
interaction was identified and accounts for the sluggish reactivity
observed with diene substrates. For the terpyridine derivative, unique
Markovnikov selectivity with styrene was also observed with HBPin.
The addition of carbon dioxide to ((tBu)PNP)CoH [(tBu)PNP = 2,6-bis(di-tert-butylphosphinomethyl)pyridine] resulted in rapid insertion into the Co-H bond to form the corresponding κ(1)-formate complex, which has been structurally characterized. Treatment of ((tBu)PNP)CoH with PhSiH3 resulted in oxidative addition to form trans-((tBu)PNP)CoH2(SiH2Ph), which undergoes rapid exchange with excess free silane. With 0.5 mol % ((tBu)PNP)CoH, the catalytic hydrosilylation of CO2 with PhSiH3 to a mixture of oligomers containing silyl formate, bis(silyl)acetyl, and silyl ether subunits has been observed.
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