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

Two series of homoleptic phenolate complexes with fluorinated aryloxide ligands A2[M(OAr)4] with M=Co2+ or Cu2+, OAr-=(OC6F5)- (OArF) or [3,5-OC6H3(CF3)2]- (OAr'), A+=K (18-crown-6)+, Tl+, Ph4P+, Et3HN+, or Me4N+ have been synthesized. Two related complexes with nonfluorinated phenoxide ligands have been synthesized and studied in comparison to the fluorinated aryloxides demonstrating the dramatic structural changes effected by modification of OPh to OAr(F). The compounds [K(18-crown-6)]2[Cu(OArF)4], 1a; [K(18-crown-6)]2[Cu(OAr')4], 1b; [Tl2Cu(OArF)4], 2a; [Tl2Cu(OAr')4], 2b; (Ph4P)2[Cu(OArF)4], 3; (nBu4N)2[Cu(OArF)4], 4; (HEt3N)2[Cu(OArF)4], 5; [K(18-crown-6)]2[Cu2(mu2-OC6H5)2(OC6H5)4], 6; [K(18-crown-6)]2[Co(OArF)4], 7a; [(18-crown-6)]2[Co(OAr')4], 7b; [Tl2Co(OArF)4], 8a; [Tl2Co(OAr')4], 8b; (Me4N)2[Co(OArF)4], 9; [Cp2Co]2[Co(OAr')4], 10; and [(18-crown-6)])[Co2(mu2-OC6H5)2(OC6H5)4], 11, have been characterized with UV-vis and multinuclear NMR spectroscopy and solution magnetic moment studies. Cyclic voltammetry was used to study 1a, 1b, 7a, and 7b. X-ray crystallography was used to characterize 1b, 3, 4, 5, 6, 7a, 7b, 10, and 11. The related [MX4]2- compound (Ph4P)2[Co(OArF)2Cl2], 12, has also been synthesized and characterized spectroscopically, as well as with conductivity and single-crystal X-ray diffraction. Use of fluorinated aryloxides permits synthesis and isolation of the mononuclear, homoleptic phenolate anions in good yield without oligomerized side products. The reaction conditions that result in homoleptic 1a and 7a with OArF upon changing the ligand to OPh result in mu2-OPh bridging phenoxides and the dimeric complexes 6 and 11. The [M(OArF)4]2- and [M(OAr')4]2- anions in 1a, 1b, 3, 4, 5, 7a, 7b, 9, and 10 demonstrate that stable, isolable homoleptic phenolate anions do not need to be coordinatively or sterically saturated and can be achieved by increasing the electronegativity of the ligand.

Homoleptic cobalt and copper phenolate A2[M(OAr)4] compounds: the effect of phenoxide fluorination.

Metal chlorides are demonstrated to behave as bifunctional acid catalysts in organic media in the one-pot reductive etherification of 5-hydroxymethylfurfural (HMF) in 2-propanol toward production of biodiesel. Two competing reaction pathways, direct etherification to 5-(isopropoxymethyl)furfural and reductive etherification to 2,5-bis(alkoxymethyl)furan, are proposed with the selectivity depending on the metal ion. Furfural and furfuryl alcohol are used as model compounds to investigate each pathway individually. The roles of Lewis/Bronsted acidity of metal chlorides solution are elucidated by kinetic studies in conjunction with salt speciation using electrospray soft ionization mass spectrometer. Bronsted acidic species, generated from alcoholysis of the metal chlorides, are the predominant catalytically active species in etherification. On the other hand, partially hydrolyzed metal cations produced by alcoholysis/hydrolysis are responsible Lewis acid centers for furfural reduction to furfuryl alcohol. I...

Role of Lewis and Brønsted Acidity in Metal Chloride Catalysis in Organic Media: Reductive Etherification of Furanics

Formation of carboalkoxyiridium complexes by carbonylation of alkoxyiridium complexes and the crystal structure of trans-PhOIr(CO)(PPh 3 ) 2

This review focuses on the design, synthesis, and reactivity of low-coordinate base transition metal complexes supported by bulky alkoxide ligands. The unique electronic features of alkoxide ligands result in strongly electrophilic character at the metal centers, which enhances reactivity of metal-bound substrates and facilitates their transformation. However, insufficiently bulky alkoxide ligands tend to form oligomeric and polymeric aggregates, thus circumventing well-defined reactivity at isolated metal centers. This review examines first the requirements for the attainment of mononuclear complexes in alkoxide ligand environments. Coordination chemistry of the most prominent alkoxide ligands [OCR3] is discussed, with particular emphasis on the impact of the R group on the nuclearity of the resulting complex and the number of alkoxide ligands bound to it. Next, the behavior of such complexes in group transfer chemistry and ensuing reactivity is examined. While several previously synthesized alkoxide lig...

R.C. Mehrotra

Papers

3 – Heterometallic Alkoxides

4 – X-Ray Crystal Structures of Alkoxo Metal Compounds

5 – Metal Oxo-alkoxides

7 – Industrial Applications

2 – Homometallic Alkoxides