Pyridines deoxygenation

Dichlorocarbene, generated in a variety of ways, was shown to deoxygenate pyridine iV-oxide, being itself oxidized to phosgene. 

The terdentate cyclometalated complexes [Ir(L)(L )]2+ and [Ir(L )2]1, L = 2,6-bis(7 -methyl-4 -phenyl-2 -quinoyl)pyridine (233), L = monoanion of L (234), luminesce at 77 K in MeOH/EtOH (lmax = 592 nm, r =20 ps) and at room temperature in deoxygenated acetonitrile (imax 620 nm, r = 325 ns).405 Both compounds undergo four reversible, ligand-centered, one-electron reduction processes. 

The selective reduction of the D-ring olefin in 106 using a partially poisoned catalyst (Pd/C, 0.25 % pyridine) provided intermediate 107 (83 %), which was epimerized at -78 °C with sodium methoxide (HOAc quench at -78 °C, 89 %) (Scheme 10.9). Deoxygenation by means of tosyl hydrazone 108 and subsequent treatment with catechol borane and tetrabutylammonium acetate gave pentacyclic... 

Alternatively, the mother liquors remaining after the original preparation of the hydrates (in Sec. A) can be used. Since these solutions are very concentrated, it is necessary to dilute them with deoxygenated water before addition of pyridine. 

In a typical experiment, the Fe(n) derivative of (314) rapidly binds dioxygen in pyridine subsequent deoxygenation may be effected by freeze-thawing. Several such cycles can be performed with little deterioration of the system. Moreover, the Oz adduct in pyridine has a half-life of about 20 hours. Following this initial success, the 02-binding properties of a number of other related capped porphyrin derivatives have been investigated (Baldwin Perlmutter, 1984). 

Wittig olefination of 2-nitro-Z-cinnamaldehyde (1300) with the phosphonium bromide 1301 led to the diene 1302. The Diels-Alder cycloaddition of 1302 with maleimide (1303), followed by dehydrogenation with DDQ, afforded the phthali-mide 1304. Double deoxygenation of 1304 with triphenylphosphine (PPhs) in collidine gave O-methylarcyiiaflavin B (1305). Finally, heating of 1305 with molten pyridine hydrochloride led to arcyriaflavin B (346) (759) (Scheme 5.215). 

The A -oxide is prepared from pyridine by the action of a peradd (e.g. hydrogen peroxide in acetic acid, forming peracetic acid in situ, or m-chloroperbenzoic acid, MCPBA) pyridine is regenerated by deoxygenation by heating with triphenylphosphine (Ph3P -> PhjPO) (Scheme 2.8). 

Another dipolar cycloaddition is that between the sulfimide 46 and p-toluonitrile oxide, giving the triazolopyridine 3-oxide 47, which can be deoxygenated by phosphorus trichloride to give the 2-(p-tolyl)triazolo-pyridine.61... 

The only alkyl group reported as reductively removed is the di-phenylmethyl substituent from N3 in compound 165.153 Deoxygenation of N-oxides can be done thermally (heating in toluene with oxygen transfer to solvent) for compounds of type 166,44 or more commonly by use of phosphorus derivatives, as described for compounds 32, 33,45 166,44 and 167.61 Halogens can be reductively removed from [l,2,3]triazolo-[4,5-6]pyridines 160,146 168,220 and 169,142 and from pentachloro derivative 164, giving dichloro compound 170.208 Removal of thiol groups by reduction... 

Deoxygenation of pyridine N-oxides. This reaction can be carried out with P214 in refluxing CH2C12 in 10-30 minutes with yields of 80- 95%. 

Deoxygenation of epoxides. Hpoxidcs, particularly a,/)-disubstituted ones, are deoxygenated by reaction with P2I4 or PI3 in CS2 at room temperature in 70 90% yield. The reaction occurs with retention (>97%) of configuration. In the case of terminal, trisubstituted, and tetrasubstituted epoxides, the yields with P2I4 or PI3 are low ( 50%) unless pyridine or triethylamine is added. 

Deoxygenation of alcohols.2 Secondary alcohols react with 1 in the presence of pyridine or 4-dimethylaminopyridine to form thiono esters (2), which are cleaved... 

Deoxygenation of epoxides. Di-, tri-, and tetrasubstituted epoxides are converted into alkcncs by treatment with PI3 (or P2h). The reaction occurs most readily with a,/J-disubstiluted epoxides and with >98% retention of configuration. With more substituted epoxides addition of pyridine or triethylamine improves yields by preventing formation of alkyl iodides. 

Deoxygenation of N-oxides,4 TiCl4 -NaBH4 (1 2) in DME reduces the N-oxide of pyridine and mcthylpyridines (picolines) to the corresponding heterocycles in high yield. However the N-oxide of quinolines and isoquinolines is reduced further to dihydro derivatives of the hetcrocyclcs. Pyridine, quinoline, and isoquinoline themselves are not reduced by this low-valent titanium species. Reduction of heterocyclic N-oxides with TiCI, has been reported (6, 588). 

Pyridine and quinoline /V-oxides react with phosphorus oxychloride or sulfuryl chloride to form mixtures of the corresponding a- and y-chloropyridines. The reaction sequence involves first formation of a nucleophilic complex (e.g. 270), then attack of chloride ions on this, followed by rearomatization (see also Section 3.2.3.12.5) involving the loss of the /V-oxide oxygen. Treatment of pyridazine 1-oxides with phosphorus oxychloride also results in an a-chlorination with respect to the /V-oxide groups with simultaneous deoxygenation. If the a-position is blocked substitution occurs at the y-position. Thionyl chloride chlorinates the nucleus of certain pyridine carboxylic acids, e.g. picolinic acid — (271), probably by a similar mechanism. 

It is of interest that, although the Skraup reaction62 on 3-amino-pyridine 1-oxide affords 1,5-naphthyridine, the EMME synthesis on this compound affords the l,7-naphthyridine-7-oxide.28 It has been suggested that the 3-aminopyridine 1-oxide is deoxygenated prior to the Skraup reaction so that, in fact, it is 3-aminopyridine that is directly converted to the 1,5-naphthyridine. The EMME cyclization, on the other hand, follows the expected direction of cyclization. 

Deoxygenation of N-oxides.1 Pyridine N-oxides are deoxygenated by this combination generally in 80-90% yield. The coproducts are (CH3)3SiOSi(CII3)3, NaCl, and ZnCl2. 

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