The use of earth-abundant transition metals as a noble metal replacement in catalysis is especially interesting if different catalytic reactivity is observed. We report, here, on the selective manganese-catalyzed base-switchable synthesis of N-alkylated amines or imines. In both reactions, borrowing hydrogen/hydrogen autotransfer (N-alkyl amine formation) or dehydrogenative condensation (imine formation), we start from the same amines and alcohols and use the same Mn precatalyst. The key is the presence of a potassium base to prefer N-alkylation and a sodium base to permit imine formation. Both bases react with the manganese hydride via deprotonation. The potassium manganate hydride reacts about 40 times faster with an imine to give the corresponding amine than the sodium manganate hydride. The selectivity seems unique for manganese complexes. We observe a broad scope with a complete product overlap, all amine alcohol combinations can be converted into an N-alkyl amine or an imine, and a good functional group tolerance.

We introduce a highly active and chemoselective manganese catalyst for the hydrogenation of imines. The catalyst has a large scope, can reduce aldimines and ketimines, and tolerates a variety of functional groups, among them hydrogenation sensitive examples such as an olefin, a ketone, nitriles, nitro groups, and an aryl iodo substituent or a benzyl ether. We could investigate the transfer step between imines and the hydride complex in detail. We found that double deprotonation of the ligand is essential and excess base does not lead to a higher rate in the transfer step. We identified the actual hydrogenation catalyst as a K–Mn-bimetallic species and could obtain a structure of the K–Mn complex formed after hydride transfer by X-ray analysis. NMR experiments indicate that the hydride transfer is a well-defined reaction, which is first order in imine, first order in the bimetallic (K–Mn) hydride, and independent in rate from the concentration of the potassium base. We propose an outer-sphere mechanism in which protons do not seem to be involved in the rate-determining step, leading to a transiently negatively charged nitrogen atom in the substrate which reacts rapidly with HO t Bu (2-methylpropan-2-ol) to produce the amine. This is based on several observations, such as no dependency of the reaction rate on the HO t Bu concentration, no observable manganese amide complex, and a high reaction constant in a conducted Hammett study. Furthermore, hydrogen transfer of the catalytic cycle was experimentally probed and monitored by NMR with subsequent quantitative regeneration of the catalyst by H2.

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. ...

The development of a new catalytic system for enantioselective Henry reactions, which permits superb 99 % ee with a broad variety of aldehydes, is presented. In‐depth structure–selectivity investigations with 33 5‐cis‐substituted prolinamines, prepared from methyl Boc‐l‐pyroglutamate, revealed that an aromatic or sterically demanding aliphatic substituent in 5‐cis position is crucial for high levels of stereocontrol, while bulkier substituents at the nitrogen atoms diminish both, enantioselectivities and reaction rates. The scope of the prime catalyst was expanded to gram‐scale and diastereomeric Henry reactions (up to 84:16 dr, 99 % ee). In the course of mechanistic studies, it was proven that the resulting β‐nitro alcohols are configurationally stable under the reaction conditions. In addition, competition experiments were used to determine the relative reaction rates of some of the prolinamine‐modified catalysts.

DedicatedtoProf. Dr.D r. h.c. mult. Gerhard Bringmann on the occasion of his 65th birthday.Scheme1.The enantioselective, copper-catalyzed Henry reaction and aselection of diamine(4-7) [5a,j,p, 8a] and proline-derived (7,8) [5j,m] ligandst hat give 99 % ee with at least one aldehyde substrate.Scheme2.The proline-derived diamines 9 and 10,their metal complexes [M·9/10], and enantioselective, copper-catalyzed Henry reactions in the presence of the chiral diamine 9a. [9] [a]