The aluminum-catalyzed hydroboration of alkenes with HBpin is reported using simple commercially available aluminum hydride precatalysts [LiAlH 4 or sodium bis(2-methoxyethoxy)aluminum hydride (Red-Al)]. Good substrate scope and functional group tolerance is demonstrated for alkene hydroboration, and the protocol was also applied to the hydroboration of ketone, ester and nitrile functional groups, showing the potential for wider application. The aluminum-catalyzed hydroboration is proposed to proceed by alkene hydroalumination which generates an alkyl aluminum species that undergoes s-bond metathesis with HBpin to drive turnover of the catalytic cycle.
An aluminum-catalyzed hydroboration of alkynes using either the commercially available aluminum hydride DIBAL-H or bench-stable Et Al⋅DABCO as the catalyst and H-Bpin as both the boron reagent and stoichiometric hydride source has been developed. Mechanistic studies revealed a unique mode of reactivity in which the reaction is proposed to proceed through hydroalumination and σ-bond metathesis between the resultant alkenyl aluminum species and HBpin, which acts to drive turnover of the catalytic cycle.
A mechanistic study on the origin of the difference in reactivity between Ir catalysts for C−H borylation reactions is reported. Catalytic reactions of B 2 pin 2 with a series of substrates that require high temperatures and long reaction times were conducted. These reactions catalyzed by the combination of [Ir(COD)(OMe)] 2 and 3,4,7,8-tetramethylphenanthroline (tmphen) occur in yields that are substantially higher than those of reactions catalyzed by [Ir(COD)(OMe)] 2 and 4,4′-di-tertbutylbipyridine (dtbpy). The electronic properties of Ir catalysts ligated by dtbpy or tmphen and their stoichiometric reactivity were investigated. It was found that a longer lifetime rather than higher reactivity of the catalyst leads to higher yields of reactions catalyzed by Ir-tmphen. The catalyst ligated by dtbpy decomposes principally by dissociation of the ligand and rapid borylation at the positions alpha to nitrogen. Thus, the greater stability of the catalyst containing tmphen results from its greater binding constant.
An ickel-catalyzed aryl thioether metathesis has been developed to access high-value thioethers.1 ,2-Bis(dicyclohexylphosphino)ethane (dcype) is essential to promote this highly functional-group-tolerant reaction. Furthermore,s ynthetically challenging macrocycles could be obtained in good yield in an unusual example of ring-closing metathesis that does not involve alkene bonds.In-depth organometallic studies support ar eversible Ni 0 /Ni II pathway to product formation. Overall, this work not only provides am ore sustainable alternative to previous catalytic systems based on Pd, but also presents new applications and mechanistic information that are highly relevant to the further development and application of unusual single-bond metathesis reactions.Aryl thioethers are often found in natural products, [1] Scheme 1. Context of the work.
dicyclohexylphosphino)ethane (dcype) complex for the catalytic Buchwald−Hartwig amination of aryl thioethers. The protocol shows broad applicability with a variety of different functional groups tolerated under the catalytic conditions. Extensive organometallic and kinetic studies support a nickel(0)−nickel(II) pathway for this transformation and revealed the oxidative addition complex as the resting state of the catalytic cycle. All the isolated intermediates have proven to be catalytically and kinetically competent catalysts for this transformation. The fleeting transmetalation intermediate has been successfully synthesized through an alternative synthetic organometallic pathway at lower temperature, allowing for in situ NMR study of the C−N bond reductive elimination step. This study addresses key factors governing the mechanism of the nickel-catalyzed Buchwald−Hartwig amination process, thus improving the understanding of this important class of reactions.
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