Osmium-based catalyst systems

The magnitudes of the rate constants for the iridium catalyst were close to those obtained for rhodium 3 and osmium 5 based catalyst systems at similar conditions. However, the unusual dependence on catalyst concentration affects its general utility in comparison to other homogeneous catalysts for the hydrogenation of NBR. 

Because of the importance of olefin metathesis in the industrial production of olefins and polymers, many different catalysts have been developed. Almost all of these are transition metal-derived, some rare exceptions being EtAlCl2 [758], Me4Sn/Al203 [759], and irradiated silica [760]. The majority of catalytic systems are based on tungsten, molybdenum, and rhenium, but titanium-, tantalum-, ruthenium-, osmium-, and iridium-based catalysts have also proven useful for many applications. 

The computational study of the osmium dihydroxylation of aliphatic al-kenes is much more complicated than the case of aromatic alkenes due to the large number of conformations that the former could adopt. To overcome this issue, we considered the system to be composed of two different parts the catalyst and the olefin. For the catalyst, the conformation considered is that from the X-ray structure. As already shown in the study of styrene [95], and in some experimental works [98], the catalyst is a fairly rigid molecule. For the aliphatic alkenes under study, there is a large number of possible conformations in addition, the stability of an olefin conformation is also affected by the interactions between the olefin substituent and the catalyst. Therefore, the catalyst must be included in the conformational search. The conformational analysis was done using a scheme based on the systematic search approach [99]. The strategy consisted of two parts first we developed a method to identify all of the possible conformations afterwards, we screened all of the possible conformations at MM level to select the most stable. Finally, we only carried out the relatively expensive QM/MM calculations on these selected conformations. 

Examples include acetal hydrolysis, base-catalyzed aldol condensation, olefin hydroformylation catalyzed by phosphine-substituted cobalt hydrocarbonyls, phosphate transfer in biological systems, enzymatic transamination, adiponitrile synthesis via hydrocyanation, olefin hydrogenation with Wilkinson s catalyst, and osmium tetroxide-catalyzed asymmetric dihydroxylation of olefins. 

We also have studied other metal carbonyl complexes in alkaline ethoxyethanol to survey the generality of the shift-reaction catalysis. Under conditions (0.9 atm CO, I00°C) comparable with those used for the ruthenium catalyst described above, iron, rhodium, osmium, and iridium carbonyls all proved active but rhenium carbonyl did not. For systems starting with the listed complexes, the normalized catalytic activities see Table I normalized activity is based on the number of... 

Carbonyl complexes of rhodium, ruthenium, osmium, iridium, and platinum, in the presence of H2O and a weak base (e.g., trimethylamine), act as catalysts for the conversion of propene to a mixture of butanol and methylpropanal with the exception of the platinum system, these catalysts are considerably more active than Fe(CO)s as reported by Reppe. Under the same conditions, but in the absence of olefin, the carbonyls act as catalysts for the conversion of CO and H2O to CO2 and H2. The metal carbonyls, together with Fe(CO)s, in the presence of H2O, CO, and a weak base such as McsN, serve as catalysts for the conversion of nitrobenzene, dinitrobenzene, and 2,4- and 2,6-di-nitrotoluene to the corresponding aminobenzene derivatives. 

The calculations were done using the MM2 force field, including new parameters developed for this type of complexes. A new set of parameters were developed for the metal catalyst, concretely for an osmium tetroxide with a tertiary amine coordinated. They developed these parameters based on X-ray structures and DFT calculations on related systems. As they pointed out in their paper, the scarcity of data makes quantitative predictions from these models unreliable. The purpose of this paper is to qualitatively identify the factors responsible for the observed face selectivities and rates in the AD reaction . 

The mechanism for this ostensibly homogeneous process, the Chalk-Harrod mechanism, [264] was based on classical organometallic synthetic and mechanistic research. Its foundation lies in the oxidative addition of the silane Si-H bond to the low oxidation state metal complex catalyst, a reaction which is well established in the organometallic literature. Lewis reported in 1986 that the catalyti-cally active solutions contained small (2.0 nm) platinum particles, and demonstrated that the most active catalyst in the system was in fact the colloidal metal. [60, 265] Subsequent studies established the relative order of catalytic activity for several precious metals to be platinum > rhodium > ruthenium = iridium > osmium. [266] In addition, a dependence of the rate on colloid particle morphology for a rhodium colloid was observed. [267]... 

This catalytic system allows three independent transformations to occur in sequence the Heck reaction, N-oxidation and asymmetric dihydroxylation (AD). The mechanism of the Heck reaction is discussed in the previous section. Here we take a closer look at the last two steps. They are coupled processes, based on the Sharpless asymmetric dihydroxylation reaction [22, 23]. Several recent reviews on Sharpless asymmetric dihydroxylation cover the general synthetic aspects [24-27], together with methods for immobilization of the osmium catalysts [28]. 

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