Phenols deoxygenation

On catalytic hydrogenation, the 5-aryloxytetrazoles 13 produce arenes Ar-H and a tetrazolone 14 this tetrazole-mediated sequence estabUshes a method to deoxygenate phenols Ar-OH. 

Photolysis of pyridazine IV-oxide and alkylated pyridazine IV-oxides results in deoxygenation. When this is carried out in the presence of aromatic or methylated aromatic solvents or cyclohexane, the corresponding phenols, hydroxymethyl derivatives or cyclohexanol are formed in addition to pyridazines. In the presence of cyclohexene, cyclohexene oxide and cyclohexanone are generated. 

The synthetic procedure described is based on that reported earlier for the synthesis on a smaller scale of anthracene, benz[a]anthracene, chrysene, dibenz[a,c]anthracene, and phenanthrene in excellent yields from the corresponding quinones. Although reduction of quinones with HI and phosphorus was described in the older literature, relatively drastic conditions were employed and mixtures of polyhydrogenated derivatives were the principal products. The relatively milder experimental procedure employed herein appears generally applicable to the reduction of both ortho- and para-quinones directly to the fully aromatic polycyclic arenes. The method is apparently inapplicable to quinones having an olefinic bond, such as o-naphthoquinone, since an analogous reaction of the latter provides a product of undetermined structure (unpublished result). As shown previously, phenols and hydro-quinones, implicated as intermediates in the reduction of quinones by HI, can also be smoothly deoxygenated to fully aromatic polycyclic arenes under conditions similar to those described herein. 

Hydrogenolysis of 1-phenyltetrazolyl ether has been applied to deoxygenation of several heavily substituted phenols, for example, ethyl orsellinate (4a). 

Transition-metal complexes such as [Rh(CO)2Cl]2,204 Rh(butadiene)2Cl,205 or Cr(CO)3(NH3)312 have also been used for the deoxygenation of oxepins to give 312,204 205 and benzoxepins to give 4.12,204 Occasionally, substantial amounts of phenolic compounds have been isolated due to the competing NIH shift of the arene oxide.204 1-Benzoxepin and 3-benzoxepin resist oxygen extrusion under these conditions probably due to their inability to form arene oxi-des.133,204... 

Deoxygenation of the nitroarene in the presence of phenol or an enol (acetyl acetone) fails to yield the 3//-azepine. On the basis of these studies it is concluded that there is a correlation between azepine formation and the p/(a of the hydroxy compound and only those hydroxy compounds of pATa >12 yield 3//-azepines. The detrimental effect of acid conditions on 3H-azepine formation has been referred to earlier.214... 

In a related approach from the same laboratory, the perfluorooctylsulfonyl tag was employed in a traceless strategy for the deoxygenation of phenols (Scheme 7.82) [94], These reactions were carried out in a toluene/acetone/water (4 4 1) solvent mixture, utilizing 5 equivalents of formic acid and potassium carbonate/[l,T-bis(diphe-nylphosphino)ferrocene]dichloropalladium(II) [Pd(dppf)Cl2] as the catalytic system. After 20 min of irradiation, the reaction mixture was subjected to fluorous solid-phase extraction (F-S PE) to afford the desired products in high yields. This new traceless fluorous tag has also been employed in the synthesis of pyrimidines and hydantoins. 

Scheme 7.82 Fluorous phase traceless deoxygenation of phenols.

Cleavage of aryl carbonates (general procedure)To a solution of NaHTe prepared in situ from tellurium powder (1.3 g) and NaBH4 (0.9 g) in EtOH (20 mL) bnffered with deoxygenated HO Ac (1.2 mL in 5 mL EtOH) is added the aryl ethyl carbonate (5 mmol) and the mixture refluxed under N2 for 30 min. The reaction mixture is filtered, the filtrate evaporated and the residue dissolved in H2O. The aqueous layer is slightly acidified with a few drops of HOAc. The respective phenols are obtained in quantitative yields. 

Although there are measured enthalpies of formation of phenyl and cyclohexyl benzoylperoxycarbonate, there are none for the corresponding deoxygenated benzoylcarbonates that replace the —OO- moiety by —0-. The difference between the gas phase enthalpies of formation of the peroxy compounds, ca 258 13 klmol", is the hydrogenation enthalpy of the phenyl compound. This value is far in excess of the hydrogenation enthalpy of any other phenylated species from the enthalpies of formation of gaseous phenol and cyclohexanol, the difference is but 189.98 2.3 kJmol. ... 

A useful variation for the deoxygenation reaction is to react the alcohol with thiophos-gene and then to treat the resultant thiocarbonyl group with the appropriate phenol [43],... 

The data in Table IV demonstrate the occurence of extensive phenol deoxygenation. The reactivity of -18(0) through -22(0) and -18(02) and -22(Op compounds appears to be independent of... 

Aromatic hydroxylation.1 The reaction of l-isopropylidene-2-indanone (1) with singlet oxygen (Rose bengal) in CH3CN at —35° results in an unstable product 2. which on deoxygenation with P(OC2Hs)3 affords the phenol 3 in 55% yield. The initial reaction is presumed to form an endoperoxide (a) by a [4+2]cycloaddition, which rearranges to 2. 

Fig (4) The transformation of the ketone (24) to the cyclic ether (9) applying the standard organic reactions is described It wa subjected to three sequencial reactions with reagents mentioned for the conversion to cyclic ether (30). Isopropylation and by aromatization, it produces the phenol (31), which is converted to pisiferol (4). This on subjection to oxidation, esterification and deoxygenation respectively, furnish O-methyl pisiferate (5) and this is easily converted to pisiferic acid (1). 

Fig (14) Olefin (107) has been converted to cyclic ether (114) by standard reactions. Its transformation to enone (115) is accomplished by annelation with methyl vinyl ketone and heating the resulting diketone with sodium hydride in dimethoxyethane. The ketoester (116) is subjected to Grignard reaction with methyllithium, aromatization and methylation to obtain the cyclic ether (117). Its transformation to phenolic ester (119) has been achieved by reduction, oxidation and esterification and deoxygenation. 

Phenols are transformed into aryl chlorides by PhPCl4779. Aromatic and aliphatic aldehydes give, in high yields, gem dibromides with combination of triphenylphosphite and bromine780. Brominative deoxygenation of ketones is achieved by 2,2,2-tribromo-l,2,3-benzodioxaphosphone781. Allylic alcohols are transformed into allylic iodides by reaction with P2I4782. 

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