Olefin hydrogenation using borohydride reagents

Metal borohydride salts are simple and practical reagents that are widely used in modern synthesis, in both academic and industrial settings. As such, they are widely available from most commercial chemical vendors and are typically inexpensive. 

Despite their availability and practical utility, examples of iron-catalysed olefin reductions using borohydride reagents are quite limited. Both Ashby and Boger reported the use of either stoichiometric or superstoichiometric quantities of iron in combination with borohydride reagents for olefin hydrogenation (see Section 12.6.2).  

Iron has played an extremely important role in catalysis in the past, present and increasingly will in the future. The fundamental work carried out over a centuiy ago continues to he relevant and informative to modern catalysis. The discovery and development of heterogeneous iron-hased catalysts used in large-scale ammonia, methanol and hydrocarbon synthesis, amongst others, has undoubtedly sculpted modern science and society. Most crucial to the use of iron in modem catalysis is perhaps the excellent sustainability traits associated with iron. The high natural abundance, low cost and low toxicity of iron oxides and iron salts provides sustainable avenues for molecule diversification. In particular, the ability of simple iron oxides and iron salts to facilitate crosscoupling and olefin hydrofunctionalisation reactions, where noble metals are commonly required, demonstrates a significant advance towards more sustainable synthesis. 

Despite an ever-growing interest in iron catalysis, the mechanistic understanding of iron-catalysed processes is veiy limited, and key challenges for the future development of the field must certainly include greater mechanistic elucidation. However, it is almost certain that iron catalysis will, in the future, provide access to numerous new synthetic vectors, building upon established reactivity and expanding the portfolio of iron-catalysed processes. 

Max Appl Ammonia in Ullmann s Encyclopedia of Industrial Chemistry, 2006, Wiley-VCH, Weinheim. 

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Scheme 12.21 Hydrogenation of olefins using Fe(OTQ2 and a borohydride reagent.

The reduction of tosylhydrazones by complex metal hydrides has been used very effectively to prepare saturated steroid hydrocarbons in high yields.317 In certain cases this reduction (with lithium aluminum hydride) takes a different course, and olefins are formed.318 The effect is dependent on both the reagent concentration and the steric environment of the hydrazone.319 Dilute reagent and hindered hydrazone favor olefins borohydride gives the saturated hydrocarbon. The hydrogen picked up in olefin formation comes from solvent, and in full reduction one comes from hydride and the other from solvent. This was shown by deuteriation experiments with the hydrazone (150) 319... 

Precursor of Useful Chiral Ligands. OPEN is widely used for the preparation of chiral ligands. Organometallic compounds with these ligands act as useful reagents or catalysts in asymmetric induction reactions such as dihydroxylation of olefins, transfer hydrogenation of ketones and imines, Diels-Alder and aldol reactions, desymmetrization of meso-diols to produce chiral oxazolidinones, epoxidation of simple olefins, benzylic hydroxylation, and borohydride reduction of ketones, imines, and a,p-unsaturated carboxylates. ... 

Sodium borohydride is a useful reagent for replacing halogen by hydrogen in fluoro-olefins. It has now been found that in the presence of water, ethanol, or t-butanol in bis-(2-methoxyethyl) ether, it may cause reduction of the double bond, e.g ... 

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