Trapping by Carbon Monoxide

Heating aryl azides in benzene at 16Q-180°C in an autoclave in the presence of carbon monoxide at 136 atm yields the corresponding isocyanates. Use of a reactive solvent gives isocyanate derivatives  

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With wavelengths >300 nm dione 92 splits off carbon monoxide and oxopro-padienylidene C3O (90) is formed, whose preparation can also be achieved by pyrolysis of the educt molecules 89 and 91.125 Quite recently38 we found that C3O 90 is also formed when C2 is trapped by carbon monoxide in a matrix at 10 K. 

Carboxylic acids can also be formed by a reaction of small alkanes, carbon monoxide, and water on solid acid catalysts (93,94). By in situ C MAS NMR spectroscopy (93), the activation of propane and isobutane on acidic zeolite HZSM-5 was investigated in the presence of carbon monoxide and water. Propane was converted to isobutyric acid at 373 73 K, while isobutane was transformed into pivalic acid with a simultaneous production of hydrogen. On SZA, methyl isopropyl ketone was observed as evidence for the carbonylation of isobutane with carbon monoxide after the sample was held at 343 K for 1 h (94). When the reaction of isobutane and carbon monoxide was carried out in the presence of water, pivalic acid was identified as the main reaction product (94). These observations are rationalized by the existence of a small number of sites capable of generating carbenium ions, which can be further trapped by carbon monoxide (93). 

Alkyl radicals that formed from alkanes under the action of various radical-like species can react rapidly not only with molecular oxygen, but can also be trapped by carbon monoxide. In this case, carboxylic acids can be obtained. Thus adamantane has been recently converted into 1-adamantanecarboxylic acid as the main product (Scheme II.7) [50]. N-Hydroxyphthalimide was used for this transformation as an efficient radical catalyst. The reaction occurs under mild conditions (CO pressure up to 15 atm and temperature below 100 °C) and gives l-adamantanecarboxyUc acid with selectivity 56% and conversion 75%. 

Some studies have shown that the generation of the nitrene from pentafluorophenyl azide or from 2,6-dimethylphenyl azide from irradiation in a low-temperature matrix, permits its trapping by carbon monoxide and the formation of the corresponding isocyanate. No evidence for the ring-expansion process was seen in these experiments. Other low-temperature studies in the absence of CO trap and using N-labelled phenyl azide have shown that even at 12 K there is evidence for the ring expansion to the didehydroazepine. ... 

In the presence of superacids, alkynes generate vinyl cations which are trapped by carbon monoxide. Thus a mixture of but-2-yne and earbon monoxide, when bubbled into FSOsH-SbFg in an n.m.r. tube, gave (85), (86), and (87) (Scheme 5) (85) was explained by attack of adventitious water. Curiously, the stereoehemistry of (86) implies attack of carbon monoxide upon... 

Thirdly, trapping of highly unstable carbonium ions, such as primary alkyl and vinyl ions, by carbon monoxide has been shown possible, so that their existence as distinct species with a finite lifetime has gained in plausibility. 

These reactions may proceed through an M(C5H6)2 intermediate, but in the absence of isolated or trapped intermediates there is no compelling evidence that this is so. Chromium vapor-C5H6 condensates liberate H2 only on warming, and, in this case, it has been possible to isolate a complex by trapping with carbon monoxide ... 

Such a mechanism is also considered to operate in the photochemical reaction of Mo(jt-Cp)(CO)2(NO) with triphenylphosphine. Isolation of the isocyanate complex Mo(NCO)(jt-Cp)CO(PPh3)2 together with Mo(jt-Cp)(CO)NO(PPh3) results from trapping of the metal nitrene by carbon monoxide in an intramolecular process.122 Nitrene formation is also considered to participate in the process of conversion of Mo(NO)2(S2CNR2)2 to Mo(NO)(S2CNR2)3 using triphenylphosphine.119... 

Trapping of the cyclized a-alkylpalladium species by carbon monoxide (2 atm) is exemplified by Scheme 51. Again the use of acetic acid as a solvent was essential for the cyclization of the butadiene telomer (245) giving initially a-Pd species (246). Subsequent C— Pd/CO insertion (246) —> (247) + (248) was followed more readily an alkene/acylpalladium insertion-p-elimination sequence (248 -> 251) when the two adjacent moieties were cis disposed, whereas the trans intermediate (247) mainly provided the rran5-vinylcyclopentane acetic acid (249) (50% from 245). Ligand variations have so far failed to increase the diastereoselectivity of these cyclizations. 

Evacuation of the oxygen atmosphere down to 10- torr does not produce a desorption (24) and no thermal effect is registered (68). COa fads) is therefore a stable species at room temperature, in the presence of oxygen or under vacuum. Its spontaneous decomposition to yield carbon dioxide is not possible at room temperature but is observed at 200° (24). A subsequent adsorption of carbon monoxide is possible on the samples of NiO(200°) and NiO(250°) (Table V) and, in both cases, carbon dioxide is then found in the cold trap. This adsorption of carbon monoxide decreases the electrical conductivity of the samples which, however, remains higher than the conductivity of the initial nickel oxide [7 X 10-i< ohm-i cm-i for NiO(200°) 1.6 x lO- o ohm-i cm-i for NiO(250°)] (25, 41). It was concluded from these experiments that a fraction of C03 (ads) ions is decomposed at room temperature by carbon monoxide and that the interaction product is carbon dioxide, which is, at least partially, desorbed to the gas phase (0.5 cm /gm) (25). 

The mechanism proposed by Ddtz involves the insertion of a carbon monoxide into the vinyl carbene complex intermediate with the formation of the vinyl ketene complex (255). °° Electrocyclic ring closure of (255) leads to the cyclohexadienone complex (252), which is related to the final benzannulation product by a tautomerization when R is hydrogen. The mechanism proposed by Casey differs from that of Ddtz in that the order of the steps involving carbon monoxide insertion and cyclization to the aryl or alkenyl substituent is reversed.Specifically, the vinyl carbene complex intermediate (248) first undergoes cyclization to the metallacyclohexadiene (249), followed by carbon monoxide insertion to give the intermediate (251), and finally reductive elimination to give cyclohexadienone intermediate (252). At this time the circumstantial evidence favors the intermediacy of vinyl ketene intermediates since they can be trapped from these reactions and isolated where the metal is dispaced from the vinyl ketene functionality however, there is not any evidence which can rule out the alternative mechanism. 

Elemental selenium can be reduced by carbon monoxide and water in the presence of tertiary amine to produce an amine salt of hydrogen selenide ([HSe"j [R3NH ]) [79], which can be trapped by acid halides and alkyl halides to give selenol esters (Scheme 15) [80]. A variety of acyl or aroyl chlorides can be used as the substrates. However, the bulkiness of the alkyl halides influences the yields of the products. 

A number of carbonates and lactones have been shown to give rise to quinone methide intermediates on irradiation.90-92 For example, Padwa and coworkers92 demonstrated that 3-phenylisocoumaranone (146) will extrude a molecule of carbon monoxide when irradiated in methanol to generate ort/zo-quinone methide (24) with moderate efficiency ( = 0.058, Eq. 1.39). This intermediate is subsequently trapped by the methanol solvent to give 147. 

Lee [524] described a method for the determination of nanogram or sub-nan ogram amounts of nickel in seawater. Dissolved nickel is reduced by sodium borohydride to its elemental form, which combines with carbon monoxide to form nickel carbonyl. The nickel carbonyl is stripped from solution by a helium-carbon monoxide mixed gas stream, collected in a liquid nitrogen trap, and atomised in a quartz tube burner of an atomic absorption spectrophotometer. The sensitivity of the method is 0.05 ng of nickel. The precision for 3 ng nickel is about 4%. No interference by other elements is encountered in this technique. 

Hydroxyurea reacts with oxy, deoxy and metHb in vitro to form iron nitrosyl hemoglobin (HbNO) and transfers NO to 2-6% of the iron heme groups [115]. Trapping studies using cyanide and carbon monoxide indicate that hydroxyurea oxidizes both oxy and deoxyHb to metHb and reduces metHb to deoxyHb specifically identifying the reaction of hydroxyurea and metHb as the critical reaction in the formation of HbNO from hydroxyurea [115]. Scheme 7.16 depicts the proposed mechanisms of N O and HbNO formation during the reaction of deoxy and metHb with hydroxyurea. Oxidation of hydroxyurea by metHb produces deoxyHb and the nitroxide radical (25,... 

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