Alcohols deoxygenation

The overall transformation of compound 13 into 15 employs a classical deoxygenation protocol. Suggest other ways of deoxygenating alcohols or ketones. 

Deoxygenation. Alcohols are converted to xanthates and then exposed to PhjP(0)H... 

Ar= 2,4,6-trichlorophenyl, ) Best method for deoxygenation 4-fluorophenyl J of primary alcohols... 

Reductive carbonylation of nitro compounds is catalyzed by various Pd catalysts. Phenyl isocyanate (93) is produced by the PdCl2-catalyzed reductive carbonylation (deoxygenation) of nitrobenzene with CO, probably via nitrene formation. Extensive studies have been carried out to develop the phosgene-free commercial process for phenyl isocyanate production from nitroben-zene[76]. Effects of various additives such as phenanthroline have been stu-died[77-79]. The co-catalysts of montmorillonite-bipyridylpalladium acetate and Ru3(CO) 2 are used for the reductive carbonylation oLnitroarenes[80,81]. Extensive studies on the reaction in alcohol to form the A -phenylurethane 94 have also been carried out[82-87]. Reaction of nitrobenzene with CO in the presence of aniline affords diphenylurea (95)[88]. 

Nitropyridazines are reduced catalytically either over platinum, Raney nickel or palladium-charcoal catalyst. When an N-oxide function is present, palladium-charcoal in neutral solution is used in order to obtain the corresponding amino N-oxide. On the other hand, when hydrogenation is carried out in aqueous or alcoholic hydrochloric acid and palladium-charcoal or Raney nickel are used for the reduction of the nitro group, deoxygenation of the N- oxide takes place simultaneously. Halonitropyridazines and their N- oxides are reduced, dehalogenated and deoxygenated to aminopyridazines or to aminopyridazine N- oxides under analogous conditions. 

Deoxygenative autoaromatization was reported to occur in the reaction of 3-amino-1,2,4-triazine 2-oxides 42 with alcohols in the presence of HCl or acetyl chloride. In this case the intermediate cr -adducts undergo elimination of water or acetic acid, resulting in 6-alkoxy-3-amino-l,2,4-triazines 75 (77JOC3498). 1,2,4-Triazine 1-oxides do not react with alcohols under these conditions (77JOC3498). 

Carbonyl reduction begins to compete with olefin saturation when the double bond is hindered or the carbonyl is aromatic or an aromatic vinylog. In conjugated systems, deoxygenation may occur via an intermediate allylic alcohol (51). 

Deoxygenation is sensitive lo solvent and structure. Alcohols lend lo favor loss of oxygen, as illustrated in selected data of Accrombessi et al. (/). Additionally, methanol and ethanol may give substantial amounts of solvolysis products, in this case methoxy- and ethoxycyclohexanols. 

Generation of phenylnitrcne by thermal decomposition of phenyl azide in the same solvent mixture, or by deoxygenation of nitrosobenzene with triethyl phosphite in the absence of the trifluoroethanol, fails to yield the 1//-azepine. The role of the alcohol in promoting l//-azepine formation is not understood. 

Attempts to prepare 2-butoxy-3//-azepines by heating nitro compounds with tris(di-ethylamino)phosphane in /m-butyl alcohol failed, as diethylamine, liberated by alcoholysis of the aminophosphane, reacts in preference with the alcohol (vide supra) to give the Ar,/V-di-ethyl-3//-azepin-2-amine in good yield.176 However, the deoxygenation of nitroarenes with tributylphosphane in the presence of primary and secondary alcohols furnishes 2-alkoxy-3//-azepines 98 in practicable yields.79... 

Alkoxy-3//-azepines 98 by Deoxygenation of Nitroarenes in Alcohol Solution Typical Procedures 79 Method A 2-Methoxy-3//-azepine (98, R = Me) ... 

Organic chemists have been aware of reversible addition-fragmentation involving xanthate esters in organic chemistry for some time. It is the basis of the Barton-McCombie process for deoxygenation of alcohols (Scheme 9.37).402 404... 

As for the synthesis of 5-e/j/-KDG, compound 6 seemed to be a suitable precursor of the methyl ester of 5-deoxy-KDG 20 since only the C-5 hydroxyl was unprotected. In this case the key step was not the epimerization but the removal of that hydroxyl. Our attempts of radicalar deoxygenation of 6 were unsuccessful because the intermediate radical was intramolecularly trappy by the C-2.C-3 double bound. Therefore we first reduced the double bond and then converted the resulting diastereoisomeric alcohols 14 into the corresponding triflates 15 which were submitted to the action of sodium iodide. Finally the iodides 16 Aus obtained were hydrogenolyzed in the presence of diisopropylethylamin to give 17. 

The alcohol derivatives that have been successfully deoxygenated include thiocar-bonates and xanthates.207 Peroxides can be used as initiators.208... 

Scheme 5.9 illustrates some of the conditions that have been developed for the reductive deoxygenation of alcohols. Entries 1 to 4 illustrate the most commonly used methods for generation of thiono esters and their reduction by tri-M-butylstannane. These include formation of thiono carbonates (Entry 1), xanthates (Entry 2), and thiono imidazolides (Entries 3 and 4). Entry 5 is an example of use of dimethyl phosphite as the hydrogen donor. Entry 6 uses r .s-(trimethylsilyl)silane as the hydrogen atom donor. 

Scheme 5.9. Deoxygenation of Alcohols via Thiono Esters and Related... 

There are also examples in which phosphate esters of saturated alcohols are reductively deoxygenated.229 Mechanistic studies of the cleavage of aryl dialkyl phosphates have indicated that the crucial C-O bond cleavage occurs after transfer of two electrons.230... 

An effective deoxygenation using enantiomerically pure epoxides from primary allylic alcohols ( Sharpless epoxides ) [44] to give enantiomerically pure secondary allylic alcohols was described by Yadav [45]. This approach circumvented a kinetic resolution of secondary allylic alcohols that implies a maximum yield of 50% ( Scheme 5). 

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