Selective deoxygenations

The potential of combining a lower need for deoxygenation and a higher product value is illustrated in Fig. 2.15. It shows that the selective incorporation of oxygen into a hydrocarbon, as done in the petrochemical industry, is very expensive. In contrast, the bio-based alternative enjoys two advantages. Firstly, the feedstock is cheaper than crude oil, even on an energy and carbon base, as discussed above. Secondly, its selective deoxygenation has been proven to cheaper than the petrochemical route in a few cases, e.g., for ethanol and furfural. The same can be expected for other biomass derivates in the future. 

The transformation of L-arabinose (58) to lactone 57 was based on a route developed by Marquez and Sharma [51] Selective protection of the primary hydroxy group with TBDPSCl and oxidation of the lactol moiety with bromine afforded lactone 59. Subsequent selective deoxygenation a to the carbonyl group proceeded under Barton-McCombie conditions providing lactone 57 in 21% yield (Scheme 14). 

Schlaf, M., P. Ghosh, P.J. Fagan, E. Hauptman and R.M. Bullock, Metal-Catalyzed Selective Deoxygenation of Diols to Alcohols, Angewandte Chemie International Edition, 40, 3887-3890 (2001). 

Reaction of compound (194) with sodium hypochlorite or lead tetraacetate gave the 2-oxide (195) in 70 and 90% yields, respectively (Scheme 16) <87H(26)292i>. Treatment of the 2,4-oxide (196) with phosphorus trichloride in chloroform at reflux selectively deoxygenated compound (196) at the pyridine nitrogen and gave the 2-oxide (195) (63%). 

Deoxygenation of sulfoxides. Tris(phenylseleno)borane is an effective reagent for the reduction of sulfoxides under mild conditions. Although this reagent converts ketones to selenoketals, selective deoxygenation of keto sulfoxides is possible. Deoxygenation of vinyl sulfoxides also proceeds smoothly. 

The synthetic potential of reductions by formate has been extended considerably by the use of ammonium formate with transition metal catalysts like palladium and rhodium. This forms a safe alternative to use of hydrogen. In this fashion it is possible to reduce hydrazones to hydrazines, azides and nitro groups to amines, to dehalogenate chloro-substituted aromatics, and to carry out various reductive removals of functional groups. For example, phenol triflates are selectively deoxygenated to the aromatic derivatives using triethylammonium formate as reductant and a palladium catalyst. - These recent af li-cations have been reviewed. 

Upon irradiation with a Xe lamp (100 W), the typical five-line spectrum characteristic of the nitronyl nitroxide radical 114 (g= 2.0066) was progressively transformed into a seven-line spectrum characteristic of an imino nitroxide radical (Scheme 24). It was proposed that during the irradiation, selective deoxygenation of the radical 114 to the radical 115 had occurred <2003S2145>. 

Thermally labile telluriranes can also be generated from oxiranes by treatment with sodium 0,0-diethyl phosphorotelluroate, and this reaction was especially applied to the selective deoxygenation of terminal epoxides (see Section 1.07.6.2). 

Selective deoxygenation of isatins. involves brief heating with N2H4. 

Deoxygenation of ketosteroids. The deoxygenation of ketones with this combination has been extended to ketosteroids. -Cholestene can be obtained in about 70% yield from 5a-cholestane-3-one with trimethylchlorosilane and zinc dust in THE at reflux. The reaction fails with 6-, 7-, 12-, 17-, and 20-ketones. This result suggests that a very bulky intermediate is involved. Thus it is possible to selectively deoxygenate diketones. For example, A -Sa-cholestene-... 

Nakagawa, H., T. Higuchi, K. Kikuchi, Y. Urano, andT. Nagano (1998). Selective deoxygenation of heteroaromatic N-oxides with olefins catalyzed by ruthenium porphyrin. Chem. Pharm. Bull. 46, 1656-1657. 

Even as stoichiometric reagents, such nickel complexes as bipy-(COD)Ni have already demonstrated their value in desulfurizing and ring-contracting processes, in hydrodesulfurization, in selective deoxygenation of epoxides, and in the controlled cleavage of allyl ethers. Their ability to form metallocyclopentadienes from biphenylenes or from thiophenes can develop into a valuable alternative route to such reactive metal heterocycles. 

Internal epoxides react only slowly and, therefore selective deoxygenation of diepoxides is possible. The reaction can be carried out as a catalytic process by addition of (C2H50>2P0Na (1-2 equiv.) to a solution of the epoxide and Te (0.1 equiv.) in C2H5OH. The selenium salt corresponding to 1 is much less reactive in this deoxygenation and is less suitable for catalytic deoxygenations. ... 

Hardouin C, Doris E, Rousseau B, Mioskowski C (2002) Selective deoxygenation of leurosine concise access to anhydrovinblastine. J Org Chtan 67 6571-6574... 

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