Regiodivergent Hydrosilylation, Hydrogenation, [2+2]-Cycloaddition and C–H Borylation using Counterion Activated Earth-abundant Metal Catalysis

The widespread adoption of earth-abundant metal catalysis lags behind that of the second- and third-row transition metals due to the often challenging practical requirements needed to generate the active low oxidation-state catalysts. Here we report the development of a single endogenous activation protocol across five reaction classes using both iron- and cobalt pre-catalysts. This simple catalytic manifold uses commercially available, bench-stable iron- or cobalt tetrafluoroborate salts to perform regiodivergent alkene and alkyne hydrosilylation, 1,3-diene hydrosilylation, hydrogenation, [2π + 2π]-cycloaddition and C-H borylation. The activation protocol proceeds by fluoride dissociation from the counterion, in situ formation of a hydridic activator and generation of a low oxidation-state catalyst

Direct Introduction of a Dimesitylboryl Group Using Base-Mediated Substitution of Aryl Halides with Silyldimesitylborane

The first dimesitylboryl substitution of aryl halides with a silylborane bearing a dimesitylboryl group in the presence of alkali-metal alkoxides is described. The reactions of aryl bromides or iodides with Ph2MeSi-BMes(2) and Na(OtBu) afforded the desired aryl dimesitylboranes in good to high yields and with high borylation/silylation ratios. Selective reaction of the sterically less-hindered C-Br bond of dibromoarenes provided monoborylated products. This reaction was used to rapidly construct a D-pi-A aryl dimesityl borane with a non-symmetrical biphenyl spacer

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 [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

Catalytic stereoselective addition to alkynes. Borylation or silylation promoted by magnesia-supported iron oxide and cis-diboronation or silaboration by supported platinum nanoparticles

Iron oxide nanoparticles supported on magnesia (FeO/MgO) have been prepared by NaBH4 reduction of Fe(SO4) on MgO and spontaneous reoxidation upon storage. XPS of FeO/MgO indicates the presence of Fe(0) (16%) and Fe(II) (84%) on this solid. TEM images show that catalytically active FeO/MgO is constituted by iron oxide nanoparticles of about 25 nm dispersed on fibrous MgO. FeO/MgO in the presence of catalytic amounts of triphenylphosphine promotes highly regio- and stereoselective monoborylation of aromatic, aliphatic, terminal and internal alkynes. Chemical analysis of the liquid after the reaction and control experiments using Fe(II) salts in the absence or presence of PPh3 supports that catalysis is heterogeneous. The possibility that trace amounts of copper impurities present in the iron precursor influence the catalytic activity of FeO/MgO was studied using a commercially available high-purity Fe(SO4) as precursor (99.999% Fe purity) showing again good (but lower) activity. In addition, a control experiment using as catalyst MgO containing 30 times higher amounts of Cu than that present in low purity Fe did not lead to complete alkyne conversion, although product formation was observed in a large extent. Alkynes react with complete chemoselectivity versus alkenes. In contrast to FeO/MgO, Pt supported on MgO or active carbon efficiently promotes the stereoselective diboronation and silaboration of alkynes in the absence of triphenylphosphine at lower temperature to render the cis configured diboronated and silaborated alkene.Financial support by the Spanish Ministry of the Economy and Competitiveness (Severo Ochoa and CTQ2012-32315) and the Generalidad Valenciana (Prometeo 2012-014) is gratefully acknowledged. This work was also funded by the Deanship of Scientific Research (DSR), King Abdulaziz University under grant No. 75-130-35-HiCi. The authors, therefore, acknowledge technical and financial support of KAU.Khan, A.; Asiri, AM.; Kosa, SA.; García Gómez, H.; Grirrane, A. (2015). Catalytic stereoselective addition to alkynes. Borylation or silylation promoted by magnesia-supported iron oxide and cis-diboronation or silaboration by supported platinum nanoparticles. Journal of Catalysis. 329:401-412. https://doi.org/10.1016/j.jcat.2015.05.006S40141232