Activation and discovery of earth-abundant metal catalysts using sodium tert-butoxide

Abstract

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 [2π+2π] 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.

Manganese‐Catalyzed Hydrosilylation Reactions

“…Poly(siloxane)‐supported dimeric manganese complex 57 was used and the hydrosilylation product 58 was obtained in high yield with no olefin isomerization detected. In 2017, the group of Thomas described a series of earth‐abundant transition‐metal‐catalyzed hydrosilylation reactions with a focus on iron and cobalt catalysis . In the screening table of various transition metals, the authors showed that Mn II complex 59 together with NaO t Bu could enable the hydrosilylation of olefin 56 to give product 60 in moderate yield (Scheme c).…”

Cobalt Catalysts for Alkene Hydrosilylation under Aerobic Conditions without Dry Solvents or Additives

“…Moreover, another limitation of these catalysts is their high sensitivity toward air and moisture, which implies a careful preparation and usage. Thus, the active species can be generated in situ in order to face this disadvantage, by treating stable well‐defined iron(II) or cobalt(II) coordination compounds with an organometallic co‐catalyst like EtMgBr, BuLi, NaEt 3 BH, or NaO t Bu . However, this strategy implies three main disadvantages: 1) these activators are also moisture sensitive; 2) these activators are stronger nucleophiles, which could potentially provide the cleave the Si–O bonds in the alkoxysilanes and siloxanes; 3) the Li, Mg, or B metal from the activator could be incorporated in the final product.…”

Alkoxide‐Promoted Selective Hydroboration of N‐Heteroarenes: Pivotal Roles of in situ Generated BH3 in the Dearomatization Process

“…Seven 11 B signals appeared at δ =21.1, 8.6, 4.3, 0.2, −11.9, −12.9, and −16.1 ppm, in addition to signals due to HBpin ( δ =27.9 ppm), 2 a ( δ =23.6 ppm), and KBH 4 ( δ =−39.2 ppm; see the Supporting Information). Among them, three peaks in the lowest fields were assigned to pinBO t ‐Bu, KH 2 Bpin, and KO t ‐Bu⋅HBpin, respectively (vide supra) . A small signal at δ =0.2 ppm splits into a triplet ( 1 J BH =102 Hz), attributable to a 1,4‐dihydropyridylborane dimer ( A ) .…”

“…Based on the experimental observations, we propose a catalytic pathway of the KO t ‐Bu‐promoted 1,4‐hydroboration of pyridines (Scheme ). Initially, KO t ‐Bu reacts with HBpin (excess) to rapidly establish a pre‐equilibrium between several borohydride species including KH 2 Bpin, KO t ‐Bu ⋅ HBpin, KO t ‐Bu ⋅ BH 3 , and KBH 4 . The in situ generated pyridine‐BH 3 adduct undergoes a nucleophilic hydride attack by borohydride species to afford 1,4‐dihydropyridyl borohydride ( C ) that is a major resting species.…”