Osmium ligand substitution reactions

Figure 5.11 Scheme for the synthesis of a pyridinylimidazolyl ligand, its copolymerization with acrylic acid (AA) and butyl acrylate (BA), and subsequent ligand substitution reaction with an osmium complex to yield a redox polymer. From [147] with permission from Elsevier. 

Intermolecular addition of carbon nucleophiles to the ri2-pyrrolium complexes has shown limited success because of the decreased reactivity of the iminium moiety coupled with the acidity (pKa 18-20) of the ammine ligands on the osmium, the latter of which prohibits the use of robust nucleophiles. Addition of cyanide ion to the l-methyl-2//-pyr-rolium complex 32 occurs to give the 2-cyano-substituted 3-pyrroline complex 75 as one diastereomer (Figure 15). In contrast, the 1-methyl-3//-pyrrolium species 28, which possesses an acidic C-3-proton in an anti orientation, results in a significant (-30%) amount of deprotonation in addition to the 2-pyrroline complex 78 under the same reaction conditions. Uncharacteristically, 78 is isolated as a 3 2 ratio of isomers, presumably via epimerization at C-2.17 Other potential nucleophiles such as the conjugate base of malononitrile, potassium acetoacetate, and the silyl ketene acetal 2-methoxy-l-methyl-2-(trimethylsiloxy)-l-propene either do not react or result in deprotonation under ambient conditions. 

In transition metal-main group element clusters, there is the possibility of ligand substitution at either type of element center. Displacement of an exo-cluster ligand on one of the metal centers (equation 10) is to be expected (see Mechanisms of Reaction of Organometallic Complexes) However, displacement at a main group cluster site has also been observed (equation 11 ). Indeed, phosphine substitution takes place exclusively at the boron atom, and the osmium-substituted BCO complex can only be prepared by synthesizing it from the phosphine-substituted osmium carbonyl starting material. 

Table VI lists the known trifluoroacetato complexes of iron, ruthenium, and osmium. Photochemical substitution reactions of various tricovalent phosphorus-donor ligands with (7r-allyl)Fe(C0)2(02CCF3) and (iT-Cp)Fe(C0)3(02CCF3), analogous to those described in the preceding for Mn(C0)5(02CCF3), have afforded (ir-allyl)Fe(CO)(cis-Ph2PCH=CHPPh2)(02CCF3), (7r-Cp)Fe(C0)(PR3)(02CCF3) (where...

The mechanism of this activation of the C-H bond is unknown although the reaction may proceed by an oxidative addition. Generally, the pentaammine-osmium(II) system is known to activate phenols, anilines, and anisoles toward electrophilic addition and substitution reactions by binding the aromatic ligand in an T -fashion. Protonation, for example, results in the formation of a heterotriene system [30b] ... 

The most extensive studies of the chemistiy of cluster complexes have been associated with the trinuclear cluster unit, as may be anticipated. A wide range of substitution reactions has been demonstrated for both Ru3(CO)i2 and Os3(CO)i2, with the full range of ligands normally employed in the study of metal carbonyl chemistry. In genera 1, the trinuclear osmium cluster is more readily maintained, ruthenium often giving rise to cluster breakdown, yielding mononuclear and binu-clear adducts. This reflects the increased bond enei of the metal-metal bond on descending the triad (see Table X later in this section). 

The stoichiometric enantioselective reaction of alkenes and osmium tetroxide was reported in 1980 by Hentges and Sharpless [17], As pyridine was known to accelerate the reaction, initial efforts concentrated on the use of pyridine substituted with chiral groups, such as /-2-(2-menthyl)pyridine but e.e. s were below 18%. Besides, it was found that complexation was weak between pyridine and osmium. Griffith and coworkers reported that tertiary bridgehead amines, such as quinuclidine, formed much more stable complexes and this led Sharpless and coworkers to test this ligand type for the reaction of 0s04 and prochiral alkenes. 

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