Pyridine-chromium oxide

Chromium trioxide in pyridine selectively oxidizes the hydroxymethyl groups in thymidine, 2 -deoxyadenosine, 2 -deoxyguano-sine, and 2 -deoxycytidine to carboxyl groups,525 but the partial liberation of the free, heterocyclic bases in the reactions suggested that oxidation at C-3 also occurs to some extent. 

Extensive studies on diastereoselectivity in the reactions of 1,3-dipoles such as nitrile oxides and nitrones have been carried out over the last 10 years. In contrast, very little work was done on the reactions of nitrile imines with chiral alkenes until the end of the 1990s and very few enantiomerically pure nitrile imines were generated. The greatest degree of selectivity so far has been achieved in cycloadditions to the Fischer chromium carbene complexes (201) to give, initially, the pyrazohne complexes 202 and 203 (111,112). These products proved to be rather unstable and were oxidized in situ with pyridine N-oxide to give predominantly the (4R,5S) product 204 in moderate yield (35-73%). 

Cr(NH3) ]Cl3 Hexamminechro-mium(III) chloride, 2 196 [Cr(NH3)6](N03)3 Hexamminechro-mium(III) nitrate, 3 153 Cr02Cl2 Chromyl chloride, 2 205 Cr03-2CsH6N Pyridine-chromium-(VI) oxide, 4 94... 

Mitsui et al. studied the selectivity of transition metal catalysts in the hydrogenation of various pyridine V-oxides in ethanol at room temperature and atmospheric pressure with Raney Ni, Pd-C, Pt-C, Rh-C, and Ru-C, and at elevated temperatures and pressures with Raney Co, Raney Cu, and copper-chromium oxide.229 The IV-oxide groups of 4-benzyloxy- and 4-styrylpyridines were hydrogenated in preference to the other functional groups over Raney Cu, copper-chromium oxide, Raney Co, and Ru-C, similarly as over Raney Ni. Pt-C and Rh-C also showed similar selectivity in... 

Ni-kieselguhr. iV-Butylpiperidine was obtained in a 78.7% yield when pyridine was hydrogenated in butyl alcohol at 220°C and 8.8 MPa of initial hydrogen pressure (eq. 12.15).22 The reaction was also studied using Raney Ni and copper-chromium oxide as catalysts, resulting in somewhat lower yields of N-butylpiperidine.24... 

This order of elution for isomeric species with n = 2, 3, and 4 is observed also for chromium (III) species coordinated by water and pyridine N-oxide (12). 

Pyridine forms a number of complexes and salts which are useful synthetic reagents. These include pyridinium hydrobromide perbromidc (CjHjNH Brj ), which is a convenient crystalline, easily handled, bromi-nating agent, the pyridine sulfur trioxide complex (C H NrSOj) for sul-fonations, pyridine borane (C HjNrBH,) for hydroborations and the pyridine-chromium(VI) oxide complex (CjHjNiCrOj) for oxidations. 

Another pyridine-chromium trioxide complex, pyridinium chloro-chromate, CsHsNHCrOjCl (PCC), is prepared by adding pyridine to a solution of chromium trioxide in 6 M hydrochloric acid [605. This complex is superior to Collins reagent in that much a smaller excess is needed, with the ratio of the substrate to the oxidant being 1 1.5-2 (equation 211). 

A chromium(VI) oxidant that is applicable to oxidations of acid-sensitive substrates is the complex of chromium trioxide with two molecules of pyridine (Collins reagent). As described on pages 22 and 274, its preparation requires the portionwise addition of chromium trioxide to dry pyridine at 15-20 C (addition of pyridine to chromium oxide could cause ignition) [592, 595, 599]. Up to 6 mol of the complex is used to oxidize alcohols in dichloromethane solutions at 25 °C, and the reaction is finished in 5-15 min [595]. Alternatively, the oxidation can be carried out in pyridine cooled with an ice bath and is finished at room temperature within 15-22 h [592, 599]. 

Guatteria psilopus Mart. (Anonaceae) along with, the aporphine guatter-ine (3a). It was noted that atherospermidine (3) was formed from guat-terine using chromium trioxide in pyridine. Further oxidation of atherospermidine by chromium trioxide in sulfuric acid yielded 1-azaanthraquinone-4-carboxylic acid (la). 

In general, nickel in its various forms requires elevated temperature and pressure conditions for the catalytic reduction of pyridines. Hydrogenations with nickel on keiselguhr (4) or nickel chromite (5), for example, employ similar rigorous conditions. Copper chromite (6) (copper chromium oxide) has also been investigated. With this catalyst temperature conditions are usually higher than with nickel catalysts. There is a report of the use of a palladium catalyst in the reduction of some 2-(/3-hydroxyalkyl) pyridines at 130° and 200 atmospheres pressure (7). 

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