Markovnikov‐Selective Hydroboration of Aryl Alkenes Enabled by a Simple Nickel Salt

Abstract

Comprehensive Summary The sustainable development of synthetic reactions catalyzed by earth‐abundant metals is one of the principal goals in homogeneous catalysis. However, so far most of the protocols are still plagued by sophisticated ligands, hazardous activators, high catalyst loading (1—10 mol%) and/or multistep synthesis of the metal complexes in the process development. Consequently, the development of earth‐abundant metal catalysts with high activity from commercially available metal salts is highly desirable for practical utilization of base metal catalyzed synthetic methodology. Herein, we report the catalyst generated in situ from a mixture of catalytic amounts of Ni(acac)2 (as low as 0.005 mol%) and CsF, which is found highly active for Markovnikov‐selective hydroboration of vinylarenes, including 1,1‐disubstituted vinylarenes and internal olefins, affording a wide range of secondary and tertiary alkyl boronates in excellent yields. Mechanistic experiments indicate that the key to the success of this catalysis is the use of CsF, which in combination with pinacolborane acts as an effective activator for Ni(acac)2, probably generating metastable Ni nanoparticles in situ that demonstrate high activity in the catalytic hydroboration (TON up to 18800).

Highly Selective Nickel‐Catalyzed Isomerization‐Hydroboration of Alkenes Affords Terminal Functionalization at Remote C−H Position

“…The reaction of styrene mediated by 2 afforded 78 % conversion with preferred formation of the branched (Markovnikov) borylation product (68 % vs. 10 % terminal; entry 11). Such Markovnikov selectivity has been observed in nearly all previously reported examples of Ni‐catalyzed vinyl arene hydroboration with HBPin, [6g,7a–f,h] and has been attributed to preferential 2,1‐insertion of styrene facilitated by the formation of a stabilized η 3 ‐benzyl intermediate [7h,25] …”

“…Faced with the need to develop increasingly sustainable chemical processes, interest in the reactivity of Earth‐abundant 3 d ‐metals has grown significantly, with the goal of discovering new, effective hydroboration catalysts to complement, and possibly supplant, platinum‐group metal based technology [6] . In this context, the utility of Ni‐based alkene hydroboration catalysts is relatively unexplored, [6e,g,7] with the handful of reported examples focusing almost exclusively on the reactivity of styrene derivatives [7a–f,h] …”

Thermally Robust and Highly Active PCP Cobalt(II) Catalyst for Linear-Selective Hydroboration of Terminal and Internal Olefins

“…Remarkably, this catalyst can tolerate temperatures as high as 150 °C, and at such a temperature, using merely 0.001 mol % catalyst loadings (17 ppm), the reaction produced 90% yield, accounting for a total turnover number (TON) of 90,000 (entry 17). To the best of our knowledge, this represents the highest TON in transition-metal-catalyzed hydroboration of unactivated olefins with HBpin, and the very low catalyst loadings make the method potentially practical for commodity-scale synthesis of alkylboronate esters. To be noted, no hydroboration products are formed in the absence of the Co catalyst (entry 18).…”

Synthesis, Structure, and Hydroboration Reactivity of Anionic Nickel(0) Complexes Supported by Bidentate NHC-Pyridone Ligands

“…Nickel-catalyzed hydroboration of alkenes is an attractive protocol to access alkylboron compounds using either borane derivatives such as HBpin or a combination of diboron reagent such as B 2 pin 2 and a proton source. ,− Nickel-NHC complexes have been shown to catalyze hydroboration of vinyl arenes using HBpin to access branched and/or linear alkylboronate esters. ,, Given this precedent, we selected hydroboration of styrene as a benchmark reaction to evaluate the catalytic activity of the (C,N)Ni complexes identified above.…”