Isocyanates, reactions with metal carbonyls

Zirconium and hafnium dialkylamides are highly reactive compounds. They undergo (i) protolytic substitution reactions with reagents such as alcohols, cyclopentadiene and bisftrimethylsilyOamine 63 64 (ii) insertion reactions with C02, CS2, COS, nitriles, phenyl isocyanate, methyl isothiocyanate, carbodiimides and dimethyl acetylenedicarboxylate 69-72 and (iii) addition reactions with metal carbonyls.73 These reactions are summarized with reference to Zr(NMe2)4 in Scheme 1. 

Interaction of isocyanates with metal carbonyls [for instance reaction (2.11)] leads to decarbonylation of the ligand and thus, to the formation of a cluster structure 413 [713] ... 

In general high pressure and temperature are required for these reactions to occur. However there are significant examples of reactions catalysed at atmospheric pressure, in part icular for the synthesis of isocyanates (4.2.5.). In the majority of cases the most important steps of these reactions are supposed to be the deoxygenation of the nitro function by carbon monoxide iving a nitrene residue bound to the metal centre, followed by insertion of carbon monoxide into the metal-nitrene bond. This is a likely hyphotesis since nitrene complexes can be obtained by stoichiometric reactions of nitro compounds with metal carbonyls. Conversion of the imido metal complex to the observed... 

Across other double bonds Isocyanates also undergo [2+2] cycloaddition reactions with metal/carbon double bonds. For example, liganded tungsten carbonyl complexes 143 add... 

Other important tests are for acid and alkalinity number and for water content (266), because water content and alkalinity of the polyether glycol can influence the reaction with isocyanates. The standard ASTM test for acid and alkalinity number, ASTM D4662 (267), is not sensitive enough for the low acidity and alkalinity numbers of PTMEG, and special methods have been developed. A useful alkalinity number (AN) has been defined as milliequivalents KOH per 30 kg of PTMEG, as titrated in methanol solution with 0.005 N HC1 (268). Other useful nonstandard tests are for heavy metals, sulfated ash, and peroxide. The peroxides formed initially in oxidations are quickly transformed into carbonyl groups, which are detectable by infrared spectroscopy. On oxidation, a small C—O peak develops at 1726 cm-1 and can be detected in thick (0.5-mm) films. A relative ratio of this peak against an internal standard peak at 2075 C—O is sometimes defined as the carbonyl ratio. 

Metal-bound isocyanates give similar results to nitrosyl compounds.363,367 Starting with [Ru4(NCO)(CO)13] , one can obtain [Ru4(CO)i2N] in 78% yield upon thermolysis. Treatment of [Ru2(CO)r(NCO)] (x = 10,11) with Ru3(CO),2 leads to the hexanuclear nitride [Ru6(CO)i6N] in 82% yield. Organic nitroso and nitro can be similarly reduced by metal carbonyls or carbonylate anions. These reactions are discussed in the later section on reactions with organic substrates. 

The uranium-carbon multiple bond has an extensive insertion chemistry with polar unsaturated molecules including carbon monoxide, nitriles, isocyanides,and isocyanates. Metal carbonyls also insert into this bond to form metallaphosphoniumenolates, which undergo novel reactions... 

HIDROXILAMINA (Spanish) (7803-49-8) A powerful reducing agent. Aqueous solution is a base. Contact with water or steam causes decomposition to ammonium hydroxide, nitrogen, and hydrogen. Contaminants and/or elevated temperatures above (reported at 158°F/70°C and 265°F/129°C) can cause explosive decomposition. Moisture in air or carbon dioxide may cause decomposition. Violent reaction with oxidizers, strong acids, copper(II) sulfate, chromium trioxide, potassium dichromate, phosphorus chlorides, metals calcium, sodium, zinc. Incompatible with carbonyls, pyridine. Forms heat-sensitive explosive mixtures with calcium, zinc powder, and possibly other finely divided metals. Aqueous solution incompatible with organic anhydrides, acrylates, alcohols, aldehydes, alkylene oxides, substituted allyls, carbonyls, cellulose nitrate, cresols, caprolactam solution, epichlorohydrin, ethylene dichloride, glycols, isocyanates, ketones, nitrates, phenols, pyridine, vinyl acetate. Attacks aluminum, copper, tin, and zinc. 

In Reppe reactions organic substrates are reacted with carbon monoxide in the presence of a metal carbonyl catalyst (or their precursors). Industrial chemicals such as acetic acid, methyl formate, formamide, di-methylformamide, formic acid, methyl methacrylate, and the emerging nonphosgene intermediates for isocyanates and dimethyl carbonate for polycarbonates are manufactured via Reppe reactions. 

Aromatic amines have been shown to be intermediates in the metal catalyzed carbonylation of nitroaromatics to aryl carbamates. Previous research established that the novel bis(methoxycarbonyl) complex, Ru(dppe)(C0)2[C(0)0Me]2, was the most abundant species present during catalysis. In this study, the complete kinetic analysis of the reaction of p-toluidine with Ru(dppe)(C0)2[C(0)0Me]2 established that the C-N bond formed by nucleophilic attack on a metal carbonyl, and that the organic product was removed from the metal by an intramolecular elimination of aryl isocyanate. 

Unfortunately, the available information on the systems which directly yield the isocyanate is scant. Although the reported kinetic equations [26, 38, 44] are consistent with the reaction scheme proposed, they may arise from other pathways so that the real mechanism in these cases is still questionable. In numerous papers it has been proposed that the synthesis of the isocyanates proceeds though the carbonylation of an intermediate nitrene complex, with the metal cocatalyst accelerating this step. This possibility will be discussed in a special paragraph devoted to the role of nitrene intermediates (paragraph 6.7.). 

Some metal carbonyl or metal isonitrile complexes react with isocyanates via [2+2] cycloaddition sequences to cause exchange reactions. In the first step, the carbonyl complex reacts with the isocyanate to give an isonitrile complex 63 and carbon dioxide. The generated isonitrile complex reacts subsequently with a second molecule of the isocyanate to give a carbodiimide 64 with generation of the original complex". ... 

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