Arenes Os

OsA A number of complexes built on the (Ty -arene)Os template 19990M1185... 

The syntheses of other diene derivatives have also been reported, ineluding (i) the reduetion of the bis(arene) [Os rf -G6H6)(77 -[22]-l,4-cyclophane)][BF4]2 with Red-Al (Red-Al = sodium bis(2-methoxyethoxy)dihydroaluminate) whieh produces [Os( 7 -G6H8)(77 -[22]-l,4-cyclophane)] in good yield," " and (ii) the formation of r/" -coordinated isoquinoline heterocycles by reaction of the G,A-cycloruthenated compounds 352 with 3-hexyne and 3-phenyl-2-propyne in methanol (Equation (36)). " Two regioisomers, 353a and 353b, are obtained in the case of the unsymmetrieal alkyne PhG=GMe. 

A review describes the asymmetric epoxidation of allylic alcohols,369 another the role of metal oporphyrins in oxidation reactions.370 jhe TiiOPrMi, catalysed self-epoxidation of allylic peroxides proceeds via an intermolecular mechanism.371 Racemic allyl alcohols can be resolved by asymmetric epoxidation (eq.35).372 a Pd(II)/Mn02/benzoquinone system catalyses the oxidative ring-closure of 1,5-hexadienes (eq.36).373 propenyl phenols are oxidatively degraded to aryl aldehydes and MeCHO in the presence of Co Schiff-base catalysts.374 An Oppenauer-type oxidation with Cp2ZrH2/cyclohexanone converts primary alcohols selectively into aldehydes.375 co macrocycles catalyse the oxidation of aryl liydrazones to diazo compounds in high yields.376 similar Co complexes under CO oxidise primary amines to azo compounds.377 Arene Os complexes in the presence of base convert aldehydes and water slowly into carboxylic acids and H2.378... 

We have recently extended our interest to the analogous halfsandwich osmium-arene complexes and are exploring the chemical and biological properties of [Os(r 6-arene)(XY)Z]ra 1 complexes (Fig. 25) (105). Both the aqueous chemistry and the biological activity of osmium complexes have been little studied. Third-row transition metals are usually considered to be more inert than those of the first and second rows. Similar to the five orders of magnitude decrease in substitution rates of Pt(II) complexes compared to Pd(II), the [Os(ri6-arene)(L)X]"+ complexes were expected to display rather different kinetics than their Ru(II)-arene analogs. A few other reports on the anticancer activity of osmium-arene complexes have also appeared recently (106-108). 

The use of an extended arene (tetrahydroanthracene) in [OsCl(en)(ri6-tha)]+ (29) gave rise to a similar potency (112). This is in contrast with the data for ruthenium-arenes, where the same substitution gave rise to a 10-fold increase in activity. Further work therefore needs to determine if the extended Os-arenes can intercalate into DNA in a manner similar to Ru-arenes. Replacement of the iV /V-chelating ligand en for other AyV-bidentates with pyridine, aliphatic amine, or azopyridine donor atoms leads to loss of activity, probably because of slower hydrolysis and higher acidity of the coordinated water (112). 

Fig. 26. Bar charts relate the influence of different chelates in [Os(r 6-arene)Cl(XY)]n+ (XY = NJV- N,0- or 0,0-) on cytotoxicity, stability with respect to hydroxido-dimer formation, hydrolysis rates, and pKa of the aqua adduct for osmium-arene complexes. Shading indicates the range in observed values. Adapted from Ref. (III).

Radical cations of n donors are derived typically from substrates containing one or more N, O, or S atoms they are substituted frequently with alkene or arene moieties. Among these systems, we mention only a few examples, including two radical ions derived from l,4-diazabicyclo[2.2.2]octane (2) and the tricyclic tetraaza compound (3). For both ions, ESR as well as OS/PES data were measured. The bicyclic system (an = 1-696 mT, 2N ah = 0-734 mT, 12H) ° shows... 

In the area of Os organometallic chemistry, a comprehensive review of the literature up until 1982 has been produced (4). More recent reviews include the annual surveys on the organometallic chemistry of Ru and Os (8-10), organometallic arene chemistry of Ru and Os (11), and terminal methylene complexes (12). 

Because of the sheer volume of work published over the last 5 years, only certain specialist topics will be discussed in any depth (Sections III-V). Whereas this survey covers many areas in a cursory fashion, it amply illustrates the rapid growth of knowledge in Os chemistry. It also serves to indicate where future advances are likely to occur, especially in the organometallic chemistry of Os(NH3) Lm]JC+ complexes, which often mimics or surpasses the extensive chemistry normally associated with phosphine ligands. Examples wherein novel organometallic chemistry has occurred are the Tj2-ketone and rj2-arene complexes of penta-ammineosmium. 

N-, 0-, and S-heterocyclic ligands also form [Os(NH3)5 t)2-(C,C)-L ]2+ complexes [L = 2,6-lutidine, 2,6-lutidinium, pyridinium, N-methylpyridinium, and lV-methyl-4-picolinium (85, 167), NJV -dimethylimidazolium (90), pyrrole (90, 179), IV-methylpyrrole (90, 179), thiophene (90,179), furan (90,179), and 1,3-dimethyluracil (72, 73)]. On oxidation to Os(III), arene ligands are rapidly lost from the coordination sphere, or in the case of the substituted arene ligands with good a donors, rapid linkage isomerization reactions occur (Section V,D). 

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