Cobalt‐Catalyzed Alkylation of Secondary Alcohols with Primary Alcohols via Borrowing Hydrogen/Hydrogen Autotransfer

Abstract

Alcohols are promising sustainable startingm aterials because they can be obtained from abundant and indigestible biomass. The substitution of expensive noble metals in catalysis by earth abundant 3d metals,s uch as Mn, Fe, or Co, (nonprecious or base metals) is ar elated key concept with respect to sustainability. Here, we report on the first cobalt-catalyzed alkylationo fs econdary alcohols with primary alcohols. Easy-to-synthesize and easy-toactivate PN 5 P-pincer-ligand-stabilized Co complexes developed in our laboratory mediate the reaction moste fficiently.T he catalysis is applicable to ab road substrate scope and proceeds under relatively mild conditions. We have even demonstrated the coupling of av ariety of purely aliphatic alcohols with ab ase or nonprecious metal catalyst. Mechanistic studies indicatet hat the reactionf ollows the borrowing hydrogen or hydrogen autotransfer concept.The borrowing hydrogen/hydrogen autotransferm ethodology (BH/HA) is ap rominente xample of an alcohol re-functionalization concept. [1,2] Thea lcohol is dehydrogenated to ac arbonyl compound, followed by ar eaction with an ucleophile and subsequent reduction with the "borrowed" hydrogen. Due to dwindling crude oil reserves and growingp ublica wareness for the consequences of climate change, the search for alternative carbon resources is getting increasingly urgent. [3,4] In this regard, alcohols are promising as startingm aterials due to the possibility of obtaining them from abundantlya vailable, indigestible,a nd barely used biomass (lignocellulose). [5][6][7] The direct hetero-coupling of two alcohols follows the BH/HA concept and exclusively uses alcohols as startingm aterials. This reaction has been reported to proceed employing am ultitude of noble metal catalysts.[8] However,n oble metals are rare and their conservation is ak ey issue with regard to as ustainable future.O ne possible approach is the replacemento fp recious metals in key technologies by more abundant metals. Beside Mn, [9] and Fe, [10] especiallyC oc omplexes have been disclosed as highly active and selectivecatalysts for av ariety of reactions following the BH/HA mechanism (see Scheme 1). [11][12][13][14][15][16][17] Iron- [18] and nickel-based [19] homogenousc atalysts have been described for thea lkylation of secondary alcohols by primary alcohols.[20] Unfortunately,t he substrate scopei sl imited, especially for the nickel-catalyzed version. Ab roadly applicable catalyst system based on an onprecious metal,w hich is capable of coupling aromatic, as well as purely aliphatic alcohols remains unknown.Herein, we report on the first cobalt-catalyzed version of the hetero-alkylation of alcohols by alcohols following the BH/HA concept. The catalyst is able to couple substituted 1-phenylethanol derivatives with primary aromatic or aliphatic alcohols as well as secondary aliphatic alcohols with primary aromatic or aliphatic alcohols. Triazine-based PN 5 Pp incer ligand complexesd eveloped in our laboratory are most efficient in these reactions. ...

Catalytic (de)hydrogenation promoted by non-precious metals – Co, Fe and Mn: recent advances in an emerging field

“…Very recently, a related Co catalyst M-Co-11 was shown to promote alkylation of secondary alcohols with primary ones at 2-5% mol loadings in the presence of 1.1 equivalents of KHMDS per substrate (Scheme 16). 148 A more challenging C-alkylation of amides and esters known to be promoted by noble-metal catalysts 149,150 was recently demonstrated by Deibl and Kempe. 151 Amides were converted in the presence of 2.5% mol M-Co-10 catalyst in THF at 100 1C while alkylation of esters was done with M-Co-11 in toluene at 5% mol loading.…”

“…Comparable loadings of base can be found in Co catalysis -M-Co-11 catalysing alcohol alkylation required 1.1 equivalents of KHMDS. 148 Similarly high loadings of KO t Bu were also required by M-Co-10,11 to promote amide and ester alkylation. 151 Non-anionic bases are also often applied as additives in base metal catalysis.…”

Recent advances in homogeneous borrowing hydrogen catalysis using earth-abundant first row transition metals

“…In 2017, Kempe and co-workers disclosed the cobalt-catalysed coupling of secondary alcohols with primary alcohols via the borrowing hydrogen approach (Scheme 21). 40 Using Co(II) Scheme 20 Co-Catalysed methylation of ketones, acetonitriles and indoles (Liu).…”

Manganese Complexes for (De)Hydrogenation Catalysis: A Comparison to Cobalt and Iron Catalysts

“…Minireviews 56 www.angewandte.org (Scheme 26). [115] It could be shown in parallel that the alkylation of secondary alcohols with primary alcohols is also catalyzed by the Co complex Co-15, [143] which was shortly before shown to efficiently catalyze the a-alkylation of esters. [144] Thef irst homogeneous Fe-catalyzed example of an alcohol alkylation stems from Sun and co-workers,w ho could couple an arrow array of alcohols using ferrocenecarboxaldehyde (Fe-36).…”