Abstract
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 considered. Then the type of bond being formed (carbon-carbon and carbon heteroatom) has been taken into account to end-up with the intermediate reaction working in tandem with the metal-catalyzed hydrogenation/dehydrogenation step. The review has been completed with the more recent advances in asymmetric catalysis using the BH strategy.
Highlights
The borrowing hydrogen (BH) principle, called hydrogen autotransfer, is a powerful approach which combines transfer hydrogenation with one or more intermediate reactions to synthesize more complex molecules without the need for tedious separation or isolation processes
It is necessary to indicate that there are few examples that despite not strictly following the basic reaction sequence detailed in the Scheme 1, they fulfill the philosophy of the BH strategy in the sense that the dehydrogenation product is not a waste any more.[7,8]
Grigg and coworkers reported the synthesis of alkylated indoles using alcohols as hydrogen donors,[112] and more recently pyrrole and indole (52) molecules were alkylated with methanol using the same iridium complex as catalyst affording the corresponding methylated derivatives in high yields (53) (Scheme 24).[113]
Summary
Alkane activation has been a target for research in the last century and is even more important today due to the finding of valuable reserves of natural gas.[270] At the same time, the olefin metathesis, a reaction which has been receiving increasing attention since its discovery, has led to the implementation of relevant industrial procedures such as the Lummus ABB process, which converts the ethylene into propylene using heterogeneous catalysts.[271] Research into the synthesis of new chiral primary amines (from racemic alcohols and ammonia), enantiopure amines (through amination of racemic amines), or asymmetric functionalization of the β-position via enamines remain as the three main goals that should be undertaken
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