Co(II) PCP Pincer Complexes as Catalysts for the Alkylation of Aromatic Amines with Primary Alcohols

Self Cite

225

111

Herein, we describe an efficient coupling of alcohols and amines catalyzed by well-defined isoelectronic hydride Mn(I) and Fe(II) complexes, which are stabilized by a PNP ligand based on the 2,6-diaminopyridine scaffold. This reaction is an environmentally benign process implementing inexpensive, earth-abundant non-precious metal catalysts, and is based on the acceptorless alcohol dehydrogenation concept. A range of alcohols and amines including both aromatic and aliphatic substrates were efficiently converted in good to excellent isolated yields. Although in the case of Mn selectively imines were obtained, with Fe-exclusively monoalkylated amines were formed. These reactions proceed under base-free conditions and required the addition of molecular sieves.

Section: Methodsmentioning

confidence: 99%

Divergent Coupling of Alcohols and Amines Catalyzed by Isoelectronic Hydride Mn^I and Fe^II PNP Pincer Complexes

Mastalir

Glatz

Gorgas

et al. 2016

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225

111

“…Kirchner established two Co II complexes 51 a and 51 b , which are stabilised by an anionic PCP‐pincer ligand based on the 1,3‐diaminobenzene scaffold for this N‐alkylation reaction . While complex 51 a alkylated aromatic amines with primary alcohols in the presence of 1.3 equivalents of base at low temperature (80 °C), the Co species 51 b bearing the basic ligand CH 2 SiMe 3 worked under base‐free conditions requiring molecular sieves and 130 °C.…”

Section: Catalytic Dehydrogenation Reactionsmentioning

confidence: 99%

“…Twoa dditional examples for the cobalt-catalysed N-alkylation of amines with alcohols were reported parallela nd independently by the groups of Zhang [76] and Kirchner. [77] Based on the observation that during the imine formation up to 10 %o f imine are hydrogenated to amine, Zhang and co-workers optimised the reactionc onditions by simple addition of molecular sieves (4 ). [76] Thus, the same cobalt complex 10 a catalysed the selective formation of monoalkylated amines startingf rom amines and primary alcohols in high yields.…”

Section: Dehydrogenative Coupling Of Alcohols With Aminesmentioning

confidence: 99%

Cobalt–Pincer Complexes in Catalysis

Junge

Papa

Beller

2018

167

Non-noble metal catalysts based on pincer type compounds are of special interest for organometallic chemistry and organic synthesis. Next to iron and manganese, currently cobalt-pincer type complexes are successfully applied in various catalytic reactions. In this review the recent progress in (de)hydrogenation, transfer hydrogenation, hydroboration and hydrosilylation as well as dehydrogenative coupling reactions using cobalt-pincer complexes is summarised.

“…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.…”

mentioning

confidence: 99%

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

Freitag

Irrgang

Kempe

2017

118

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. ...