Deoxygenation with triethyl phosphite

Diisobutyl-3miethoxypyrazine 1,4-dioxide (107) with phosphorus trichloride in ethyl acetate at 40° gave 2,5-diisobutyl-3-methoxypyrazine (980). 2-Amino-5-[(/7-chlorophenoxy)methyl]-3-cyanopyrazine 1-oxide was deoxygenated with triethyl phosphite in dimethylformamide at 120° (1041). 

Bis(methylamino)-2,6-bis(Af-methylcarbamoyl)pyrazine 1-oxide was deoxy-genated by refluxing with triethyl phosphite (462) and 2-amino-3-cyano-5-[A -(p-carboxyphenyl)-JV-methyl]aminomethylpyrazine 1 -oxide (56, R = OH) (and some derivatives) and similar compounds were deoxygenated with triethyl phosphite alone or with dimethylformaraide at 120 (762, 1038-1040). Catalytic reduction has been employed with 2-amino-5-methyIpyrazine 1-oxide (Pd/C) (535) and 23-diamino-5,6-dimethylpyrazine 1,4-dioxide (Raney Ni) in ethanol (907). 

Biphenyls carrying (a) an azido group or (b) a nitro group in the 2-position (1/5) can be transformed to carbazoles 3 (a) by thermolysis, photolysis, or catalysis of Rh(II) [199] and (b) by deoxygenating with triethyl phosphite. Both functionahties thus produce a nitrene intermediate 2, which cychzes to carbazole 3 by intramolecular C-H insertion ... 

Treatment of the substituted azobenzene (561) with triethyl phosphite gave the triazapen-talene (563). It is likely that the 2-substituted benzotriazole (562) was initially formed which then underwent deoxygenative cyclization to (563) (74CL951). 

Whereas the production of arylnitrenes by the deoxygenation of nitrosobenzenes or nitro-benzenes by trivalent phosphorus reagents and their subsequent intramolecular ring expansion to 3//-azepines are well-known processes, the corresponding intermolecular reactions to form 1//-azepines have been exploited only on rare occasions and appear to be of little preparative value. For example, the highly electrophilic pentafluorophenylnitrene, obtained by deoxygenation of pentafluoronitrosobenzene with triethyl phosphite in benzene solution, produced a low yield (2-10%) of l-(pentafluorophenyl)-l//-azepine, which was isolated as its [4 + 2] cycloadduct with ethenetetracarbonitrile.169 With anisole as the substrate l-(pentafluorophenyl)-l//-azepin-2(3//)-one (16% bp 128 —130 C/0.4 Torr) was obtained. 

More surprising, in view of its poor electrophilic character, is that phcnylnitrene, generated by deoxygenation of nitroso benzene with triethyl phosphite in a mixture of benzene and 2,2,2-trifluoroethanol, yields 1-phenyl-1//-azepine (26), a rare example of a stable 1-aryl-lH-azepine.170... 

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. 

Deoxygenations of 2-nitrosobiphenyls with triethyl phosphite in the presence of diethylamine produce mixtures of carbazoles and Ar,Ar-diethyl-3/f-azepin-2-amines.182 The yields, however, are substantially lower (10-51 %) than those obtained by the photolyses of 2-azidobiphenyls in diethylamine. 

Once again, the deoxygenation of nitro-compounds has found use in the synthesis of heterocyclic compounds.62-64 Several 2-nitrophenyl-substituted indoles, e.g. (50) and (51), are deoxygenated on reaction with triethyl phosphite.62 The reaction of... 

The cyclization of 1,5-dicarbonyl chains by means of the deoxygenating reagent triethyl phosphite provides an alternate strategy for building the fused hve-membered penem ring. The first example in fact actually comprises a 1,5-oxa-thio array. Reaction of the perhydrothiophene oxide (16-1) with the anion from carbon disulhde... 

Deoxygenation of the diaryl sulfide (56) with triethyl phosphite gives the phenothiazine (58) in good yield via formation and rearrangement of the spiro intermediate (57). 

The substituted azobenzene (160) with triethyl phosphite gives the triazapentalene (162) via the 2-substituted benzotriazole (161) which undergoes a second deoxygenative cyclization (74CL951). 

Alkylidene derivatives of phthalic thioanhydride are formed as shown in Scheme 160. Reaction of phthalic thioanhydride with hot triethyl phosphite produces trafts-S -bithioph-thalide (457), probably via the carbene and phosphorane (Scheme 161) (72AHC(14)331>. Support for this mechanism stems from the fact that brief treatment of phthalic thioanhydride with triethyl phosphite in the presence of phthalic anhydride gives (458) in the presence of benzaldehyde the same reaction leads to the benzylidene derivative (456). An alternative mechanism has also been suggested, in which the penultimate step is the formation of an epoxide, which is deoxygenated to yield the product (72AHC(14)331>. 

Tetracyclic benzimidazo[l,2- ]benzimidazoles 36 and 35 were prepared by deoxygenation of l-(2-nitrophenyl) benzimidazole with triethyl phosphite and thermal decomposition of l-(2-azidophenyl) benzimidazole, respectively (Scheme 24) <2000PJS168, CHEC-III(11.04.9.1.1)153>. 

When p-nitro-N,N-dimethylaniline (IVc) was ozonized at 0°C in ethyl acetate, methylene chloride, or methanol, a mixture of products resulted. In addition to the expected side-chain oxidation products— p-nitro-N-methylaniline (Vc), p-nitro-N-methyl formanilide (Vic)—a peroxide compound was formed. This peroxide, which is not formed until the solvents are removed, was shown by a series of experiments (described below) to be identical with di-[(N-methyl-p-nitrophenyl)-aminomethyl] peroxide (VII). Deoxygenation of VII with triethyl phosphite (7) yielded the ether VIII, which in turn decomposed at its melting point to the amine IX. 

The reaction of the nitrone (23) with alkyl phosphites has been studied. With trimethyl phosphite in refluxing MeOH (23) afforded a mixture of the epimers (32), whereas in AcOH (33) was obtained. It was shown that (33) was not formed from (32) by loss of MeOH. With triethyl phosphite analogous products were formed, but in this case a large amount of the deoxygenated compound (34) was also obtained. With trimethyl phosphite the formation of an immonium structure, reducible to conanine (35), was suggested by the presence in the n.m.r. spectrum of the reaction mixture of a N—Me group. The structures of these compounds were deduced from their spectral data. 

The allyl oxalate V-acylamide 1 reacts with triethyl phosphite at elevated temperature in a deoxygenation/intramolecular cyclopropanation sequence to give 2 and 3. It is assumed that an acylamino(allyloxycarbonyl)carbene is involved in this transformation (see Houben-Weyl, Vol.E19b, pl050). 

Thermolysis or photolysis of 2-azidobiphenyl produces a nitrene, as does deoxygenation of 2-nitrobi-phenyl with triethyl phosphite. The nitrene cyclizes immediately to give carbazole. Carbazoles can also be made by cyclodehydrogenation of diphenylamines. 

The deoxygenation of 1,2,3-triazine 1- and 2-oxides by catalytic hydrogenation to give 2,5-dihydro-l,2,3-triazines has been reviewed in Section 9.01.5.6 (Equation 46). 4,5,6-Triaryl-l,2,3-triazine 2-oxides are transformed into the corresponding 1,2,3-triazines by treatment with triethyl phosphite <1985LA1732>. In contrast to other heteroaromatic A -oxides, 4,6-dimethyl-l,2,3-triazine 2-oxide does not transfer its oxygen to alkenes under catalysis by [dioxo(tetramesitylporphyrinato)mthenium(vi)] <1991TL7435>. 

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