Cobalt‐Catalyzed Suzuki Biaryl Coupling of Aryl Halides

Abstract

Readily accessed cobalt pre-catalysts with N-heterocyclic carbene ligands catalyze the Suzuki cross-coupling of aryl chlorides and bromides with alkyllithium-activated arylboronic pinacolate esters.P reliminary mechanistic studies indicate that the cobalt species is reduced to Co 0 during the reaction.The palladium-catalyzed cross-coupling of organoboronic acids or esters with organic halides or related substrates-the Suzuki reaction-is ap owerful and very widely exploited method for the formation of biaryl compounds (Scheme 1). [1] This reaction is used commercially for the production of arange of materials,including the sartan class of angiotensin inhibitors for the treatment of hypertension (e.g. Lorsartan) and Boscalid, abroad-spectrum agrochemical fungicide.[2]While palladium-based catalysts are ubiquitous in Suzuki cross-coupling reactions,there is agrowing impetus to replace them with more sustainable alternatives based on earthabundant metals (EAM). This is not only because of the fact that palladium, like all platinum-group metals (PGMs), is scarce and expensive,b ut also because of the relatively high toxicity of PGMs;asac onsequence of this toxicity there are strict regulations in place for the removal of PGMs to the low ppm level from active pharmaceutical intermediates.[3] Of the EAM alternatives to palladium for Suzuki biaryl coupling, first-row transition metals are particularly attractive,w ith nickel-based catalysts being the most well developed to date. [4,5] Iron-catalyzed Suzuki cross-coupling can be performed between avariety of organic halides and organoboron reagents, [6] but simple biaryl bond formation remains elusive and challenging. [6a, 7, 8] Recently,C hirik and co-workers reported early results in the coupling of aryl triflates with arylboron pinacol esters by using ac obalt PNP-pincer-based catalyst.[9] We now report the cross-coupling of aryl chlorides and bromides with activated arylboronic pinacol esters, [10,11] using simple cobalt catalysts prepared in situ from commercially available precursors.Ther esults shown in Table 1s ummarize selected [12] optimization studies using 4-chorotoluene and the activated phenylboronic ester 1a, [6b,c,e, 8] catalyzed by species formed in situ from cobalt(II) chloride with ar ange of different ligands and ligand precursors.No reaction was observed in the absence of added ligand (Table 1, entry 1) and little or no product 2a was obtained when mono,b i-, or tridentate phosphine ligands were tested Scheme 1. Palladium-catalyzed Suzuki biaryl coupling and selected products.

Co(Salox)‐Catalyzed Enantioselective Reduction of α,β‐Unsaturated Esters

“…Indeed, in the racemate synthesis campaign with 4,4′‐( t Bu) 2 ‐bipyridine as the ligand, we observed quantitative formation of products 3 a and 3 b starting respectively from 2 h and 2 i [43] . Thus under the reaction conditions hereby explored, debromination readily occurred in the presence of a neutral ligand possibly due to a more facile access to low valent Co‐species, in agreement with previously observed reactivity [44] . Substituents on the aromatic ring could be placed in para‐ or meta ‐positions ( 3 k – 3 l ) without resulting in significant changes in reactivity and enantioselectivity.…”

Visible-Light Photoredox-Catalyzed Amidation of Benzylic Alcohols

“…White solid (110 mg, 49% yield); mp 104–105 °C, 22 R f = 0.26 (hexane/ethyl acetate 2/3). 1 H NMR (400 MHz, CDCl 3 ) δ: 7.63–7.59 (m, 4H), 7.53–7.42 (m, 4H), 7.39–7.35 (m, 1H), 3.09 (brs, 6H).…”

Three-Coordinate Formal Cobalt(0), Iron(0), and Manganese(0) Complexes with Persistent Carbene and Alkene Ligation

“…44 A recent report by Bedford even showed that the three-coordinate iron(0) and cobalt(0) complexes [(IMes)Fe(η 2 :η 2 -dvtms)], [(sIPr)Co(η 2 :η 2 -dvtms)], and [(sIPr)Co(η 2 -norbornene) 2 ] can be formed upon the interaction of MCl 2 (M = Fe, Co) with NHCs, alkenes, and n-butyllithium-activated phenylboronic pinacolate ester, wherein the borane reagent seems to serve as a net reductant. 45,46 It is worth mentioning that in addition to these reductive methods, the formation of fourand three-coordinate cobalt(0) and iron(0) complexes via ligand replacement and reductive elimination reactions can also be found in the literature. Grubbs reported the preparation of the four-coordinate complex [(BAC) 2 Fe(η 2 -COT)] (COT = cyclooctatetraene) from the reaction of Fe(COT) 2 with BAC (Chart 1).…”