Nitrogen iron-catalyzed synthesis

Iron-Catalyzed Synthesis of Nitrogen-Containing Heterocycles... 

Efficient and regioselective iron-catalyzed aerobic oxidative reactions afforded 3,5-disubstituted isoxazoles 5 from homopropargylic alcohols 4, r-BuONO as the nitrogen source, and H2O under mild conditions (140L6298).A transition metal-free one-pot synthesis of 3,5-disubstituted isoxazoles used terminal alkynes by treatment with -BuLi, then aldehydes and iodine to afford intermediate a-alkynyl ketones 6 converted into isoxazoles 7 with hydroxylamine (14JOC2049). 

Molybdenum In its pure form, without additions, it is the most efficient catalyst of all the easily obtainable and reducible substances, and it is less easily poisoned than iron. It catalyzes in another way than iron, insofar as it forms analytically easily detectable amounts of metal nitrides (about 9% nitrogen content) during its catalytic action, whereas iron does not form, under synthesis conditions, analytically detectable quantities of a nitride. In this respect, molybdenum resembles tungsten, manganese and uranium which all form nitrides during their operation, as ammonia catalysts. Molybdenum is clearly promoted by nickel, cobalt and iron, but not by oxides such as alumina. Alkali metals can act favorably on molybdenum, but oxides of the alkali metals are harmful. Efficiency, as pure molybdenum, 1.5%, promoted up to 4% ammonia. 

Finally, as shown in Table V, activity of ferrochelatase is also decreased by -NO. This mitochondrial enzyme catalyzes the final step in heme synthesis, the insertion of ferrous iron into porphyrin, and has been shown to contain an Fc2S2 nonheme iron-sulfur cluster which is required for activity (Dailey et aL, 1994). This result suggests that loss of activity may occur by nitrogen oxide-mediated [i.e., -NO in the presence of dioxygen (Wink et aL, 1993a)] destruction of its iron-sulfur cluster. Consistent with this result, while inhibition of ALAS and increase in HO induced by SNAP requires a period of 4-8 hr (consistent with the established effects of heme on these enzymes, as described above), the inhibition of ferrochelatase is virtually complete within 1 hr (Kim et aL, 1995). 

It was found during studies of ammonia synthesis on iron that the incorporation of a condenser downstream of the sample valve in the external circulation loop of the HPLP apparatus (Fig. 7), enabled the system to be run as a flow rather than a batch reactor. This is true for any reaction system where the reactants are more volatile than the products, since the condenser temperature can be adjusted to trap the products almost exclusively, allowing a nearly pure stream of reactants to impinge on the catalyst. In the case of ammonia synthesis, (where, next to the product, nitrogen at a partial pressure of 5 atm was the most condensable species) a slurry of isopentane (— 159.9 °C) was found to be the ideal condenser medium. During a study of rhenium-catalyzed ammonia synthesis the isopentane condenser was switched in periodically to reduce the ammonia partial pressure to below that at which it appeared to poison the catalyst. In this way, the rhenium was able to produce ammonia in excess of the amount usually leading to poisoning. 

Reports on the synthesis of diene complexes using Fe2(CO)9 are more common. Reaction of 2-phenylsulfonyl-1,3-cyclohexadiene with 2equiv. of Fe2(GO)9 in refluxing ether formed the [l-(phenylsulfonyl)-l,3-cyclohexa-diene]iron(0)tricarbonyl isomer 26. The reaction is catalyzed by 1-aza-1,3-butadiene. In a separate study, 1-aza-1,3-butadienes were shown to effect a quantitative catalytic complexation of cyclohexadienes with Fe2(GO)9. Activities are greatly enhanced in the presence of aryl rings bonded to nitrogen. 

Driver et al. also studied the reactivity of aryl azides with oxime ethers 66 toward the synthesis of benzopyrazoles 67 through Fe(II)-catalyzed formation of an N—N bond (Scheme 16.30) [49]. In the reactions of oxime derivatives 66, the (Z)-isomer could not afford any desired product. It was suggested that N—N bond formation occurs through intermediate A produced from ( )-66 by coordination of the iron(II) catalyst to the terminal N atom of the azide. Subsequent nucleophilic attack of the oxime nitrogen on the activated azide forms an N—N bond, which is followed by extrusion of N2 and dissociation of the iron catalyst to provide the benzopyrazoles 67. 

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