The borrowing hydrogen (BH) principle, also called hydrogen auto-transfer, is a powerful approach which combines transfer hydrogenation (avoiding the direct use of molecular hydrogen) with one or more intermediate reactions to synthesize more complex molecules without the need for tedious separation or isolation processes. The strategy which usually relies on three steps, (i) dehydrogenation, (ii) intermediate reaction, and (iii) hydrogenation, is an excellent and well-recognized process from the synthetic, economic, and environmental point of view. In this context, the objective of the present review is to give a global overview on the topic starting from those contributions published prior to the emergence of the BH concept to the most recent and current research under the term of BH catalysis. Two main subareas of the topic (homogeneous and heterogeneous catalysis) have been identified, from which three subheadings based on the source of the electrophile (alkanes, alcohols, and amines) have been consid...

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Advances in One-Pot Synthesis through Borrowing Hydrogen Catalysis

Transition-metal catalyzed reactions that are able to construct complex aliphatic amines from simple, readily available feedstocks have become a cornerstone of modern synthetic organic chemistry. I...

New Strategies for the Transition-Metal Catalyzed Synthesis of Aliphatic Amines.

Transition metal-catalyzed substitution of alcohols by N-nucleophiles (or N-alkylation of amines and related compounds with alcohols) avoids the use of alkylating agents by means of borrowing hydrogen (BH) activation of the alcohol substrates. Water is produced as the only by-product, which makes the “BH” processes atom-economic and environmentally benign. Diverse types of homogeneous organometallic and heterogeneous transition metal catalysts, and substrates such as N-nucleophiles including amines, amides, sulfonamides and ammonia, and various alcohols, can be used for this purpose, demonstrating the promising potential of “BH” processes to replace the procedures using traditional alkylating agents in pharmaceutical and chemical industries. Borrowing hydrogen activation of alcohols for C–N bond formation has recently been paid more and more attention, and a lot of new and novel procedures and examples have been documented. This critical review summarizes the recent advances in “BH” substitution of alcohols by N-nucleophiles since 2009. “Semi-BH” N-alkylation processes with or without an external hydrogen acceptor are also briefly presented. Suitable discussion of the “BH” strategy provides new principles for establishing green processes to replace the relevant traditional synthetic methods for C–N bond formation.

Substitution of alcohols by N-nucleophiles via transition metal-catalyzed dehydrogenation

Borrowing hydrogen (or hydrogen autotransfer) reactions represent straightforward and sustainable C-N bond-forming processes. In general, precious metal-based catalysts are employed for this effective transformation. In recent years, the use of earth abundant and cheap non-noble metal catalysts for this process attracted considerable attention in the scientific community. Here we show that the selective N-alkylation of amines with alcohols can be catalysed by defined PNP manganese pincer complexes. A variety of substituted anilines are monoalkylated with different (hetero)aromatic and aliphatic alcohols even in the presence of other sensitive reducible functional groups. As a special highlight, we report the chemoselective monomethylation of primary amines using methanol under mild conditions.

/pdf/efficient-and-selective-n-alkylation-of-amines-with-alcohols-4x8lkjds1b.pdf

Efficient and selective N-alkylation of amines with alcohols catalysed by manganese pincer complexes.

Borrowing hydrogen is a process that is used to diversify the synthetic utility of commodity alcohols. A catalyst first oxidizes an alcohol by removing hydrogen to form a reactive carbonyl compound. This intermediate can undergo a diverse range of subsequent transformations before the catalyst returns the “borrowed” hydrogen to liberate the product and regenerate the catalyst. In this way, alcohols may be used as alkylating agents whereby the sole byproduct of this one-pot reaction is water. In recent decades, significant advances have been made in this area, demonstrating many effective methods to access valuable products. This outlook highlights the diversity of metal and biocatalysts that are available for this approach, as well as the various transformations that can be performed, focusing on a selection of the most significant and recent advances. By succinctly describing and conveying the versatility of borrowing hydrogen chemistry, we anticipate its uptake will increase across a wider scientific audience, expanding opportunities for further development.

Borrowing Hydrogen for Organic Synthesis.

Using a simple amino amide ligand, ruthenium-catalyzed one-pot alkylation of primary and secondary amines with simple alcohols was carried out under a wide range of conditions. Using the alcohol as solvent, alkylation was achieved under mild conditions, even as low as room temperature. Reactions occurred with high conversion and selectivity in many cases. Reactions can also be carried out at high temperatures in organic solvent with high selectivity using stoichiometric amounts of the alcohol.

Ruthenium-catalyzed N-alkylation of amines with alcohols under mild conditions using the borrowing hydrogen methodology.

Reactions of CO2 and related heteroallenes with CF3-substituted aromatic silylamines of tin

Aqueous ring opening of N-tosylaziridine with aniline derivatives

Previous studies have shown that CO2 can insert into divalent Sn and Ge bis(silylamides) to give mixtures of sily- lated isocyanates and bis(silyl)carbodiimides, albeit rather sluggishly. In order to more effectively utilize the divalent metal bis(silylamides) in synthesis, more active and selective reactions to form only isocyanates or carbodiimides were needed. We have now shown that the more electropositive Group 2 complexes react with CO2 virtually instantaneously under the very mild conditions of room temperature and atmospheric pressure CO2. We have also demonstrated that CO2 can react under high pressure directly with the solid Mg bis(silylamides) to produce an unusual, high melting point solid. Different products are obtained when the cyclic, tied-back complex Mg(N(SiMe2CH2)2)2(Et2O)2 is used in place of Mg(N(SiMe3)2)2(THF)2. Various Zn bis(silylamides) can be designed to afford either the silyl isocyanate or the bis(alkyl)carbodiimide in near quantitative con- versions in ∼100% selectivities. Lastly, we have shown that the tied-back Zn complex can react with CO2 at room temperature and 4 atm CO2 pressure to cleave CO2 and generate a product containing a stable, (Zn4O) 6+ core with a bridging 4-O atom.

Insertion of CO 2 into divalent group 2 and 12 bis(silylamides)

The successful application of an aminoamide ligand for the ruthenium-catalyzed alkylation of amines under mild conditions is demonstrated.

Ruthenium‐Catalyzed N‐Alkylation of Amines with Alcohols under Mild Conditions Using the Borrowing Hydrogen Methodology.

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