Catalytic cycle under anhydrous conditions

Scheme 8. Catalytic Cycle under Anhydrous Conditions...

This catalytic cycle, generating acetyl iodide from methyl iodide, has been demonstrated by carbonylation of anhydrous methyl iodide at 80°C and CO partial pressure of 3 atm using [(C6H5)4As][Rh(CO)2X2] as catalysts. After several hours reaction, acetyl iodide can be identified by NMR and infrared techniques. However, under anhydrous conditions some catalyst deactivation occurs, apparently by halogen abstraction from the acetyl iodide, giving rhodium species such as frans-[Rh(CO)2I4] and [Rh(CO)I4] . Such dehalogenation reactions are common with d8 and d10 species, particularly in reactions with species containing weak... 

In the case of [Os(bpy)2(CO)II] as an electrocatalyst of the reduction of carbon monoxide in AN and in the presence of tetra- -butylammonium hexafluorophosphate as a supporting electrolyte, the main product is carbon monoxide under anhydrous conditions and formate anion in the presence of water (88OM238, 92IC4864). In the catalytic cycle for tr6z 5-[Os(bpy)(PPh3)2(CO)H] reversible protonation to f/ /7,s-[Os(bpy)(PPh3)(CO)(II)2] occurs and the product was characterized as the dihydrido cation (87IC1247). 

If nitration under acidic conditions could only be used for the nitration of the weakest of amine bases its use for the synthesis of secondary nitramines would be severely limited. An important discovery by Wright and co-workers " found that the nitrations of the more basic amines are strongly catalyzed by chloride ion. This is explained by the fact that chloride ion, in the form of anhydrous zinc chloride, the hydrochloride salt of the amine, or dissolved gaseous hydrogen chloride, is a source of electropositive chlorine under the oxidizing conditions of nitration and this can react with the free amine to form an intermediate chloramine. The corresponding chloramines are readily nitrated with the loss of electropositive chlorine and the formation of the secondary nitramine in a catalytic cycle (Equations 5.2, 5.3 and 5.4). The mechanism of this reaction is proposed to involve chlorine acetate as the source of electropositive chlorine but other species may play a role. The success of the reaction appears to be due to the chloramines being weaker bases than the parent amines. 

However, several observations of note were made. First and foremost was that H2 was necessary for catalysis. This fact is not incorporated into the catalytic cycle usually cited (Scheme 7), but rather, H2 is considered to be involved only in activating the precursors. Reduction of Ru(III) (and higher valent species) to Ru(II) is known to be preceded by heterolytic cleavage of H2 (79,80), and under the anhydrous conditions of the system, it is possible that H2 is constantly necessary to maintain the ruthenium in a lower oxidation state (the catalytic cycle may generate an otherwise inactive high-valent species, either directly or by some side reaction). The presence of H + and an iodide promoter is also requisite for reactivity. It has also been concluded that for catalysis, the I/Ru ratio must equal or exceed 3 (73, 78). 

The formation of an iron(II) phase in an excess of oxygen is quite remaiitable. Thermodynamic calculations indicate that under reaction conditions anhydrous iron(IlI)sulfate Fe2(S04)3 i the only stable compound Kinetic experiments reveal that in the catalytic redox cycle the reoxidation of the active sites is the rate-determining step The order with respect to oxygen is almost unity, whereas the order with respect to H2S is close to zero [9]. Under steady slate conditions therefore the concentration of the iron(m) species and, hence, the concentration of reactive oxygen species will be low. The high steady state surface concentration of Fe(U) species brings about the formation of the... 

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