Arene osmium alkyl complexes

As noted in Chapters 2 and 11, a series of -q -arene complexes of osmium have been prepared, and the reactivity of these species has been studied extensively by Harman. The reactions of iq -arene complexes of Os(II) illustrate how strong backbonding can cause the uncoordinated portion of an aromatic system to be more susceptible to electrophilic attack than the corresponding free arene. ° Osmium(II) pentamine complexes of phenols, anilines, acetanilides, and anisoles react with electrophiles at the uncoordinated portion of the ring. For example, the simple phenol complex in Equation 12.78 reacts with Michael acceptors at the 4-position of the coordinated phenol in the presence of a mild tertiary amine base. This reactivity and selectivity for reaction at the 4-position is greater than the reactivity of free phenol. The reactions of electrophiles with aniline derivatives occur in a similar fashion and lead to products from alkylation of the aromatic ring predominantaly at the 4-position (Equation 12.79). Related reactions occur with complexes of electron-rich five-membered pyrrole and furan heterocycles. Examples of electrophilic attack on -q -pyrrole complexes of Os(II) are shown in Equation 12.80. ... 

C-M bond addition, for C-C bond formation, 10, 403-491 iridium additions, 10, 456 nickel additions, 10, 463 niobium additions, 10, 427 osmium additions, 10, 445 palladium additions, 10, 468 rhodium additions, 10, 455 ruthenium additions, 10, 444 Sc and Y additions, 10, 405 tantalum additions, 10, 429 titanium additions, 10, 421 vanadium additions, 10, 426 zirconium additions, 10, 424 Carbon-oxygen bond formation via alkyne hydration, 10, 678 for aryl and alkenyl ethers, 10, 650 via cobalt-mediated propargylic etherification, 10, 665 Cu-mediated, with borons, 9, 219 cycloetherification, 10, 673 etherification, 10, 669, 10, 685 via hydro- and alkylative alkoxylation, 10, 683 via inter- andd intramolecular hydroalkoxylation, 10, 672 via metal vinylidenes, 10, 676 via SnI and S Z processes, 10, 684 via transition metal rc-arene complexes, 10, 685 via transition metal-mediated etherification, overview,... 

Osmium forms a wide variety of alkyl and aryl complexes including homoleptic alkyl and aryl complexes and many complexes with ancillary carbonyl (see Carbonyl Complexes of the Transition Metals), cyclopentadienyl (see Cyclopenta-dienyl), arene (see Arene Complexes), and alkene ligands (see Alkene Complexes). It forms stronger bonds to carbon and other ligands than do the lighter elements of the triad. Because of this, most reactions of alkyl and aryl osmium complexes are slower than the reactions of the corresponding ruthenium complexes. However, because osmium is more stable in higher oxidation states, the oxidative addition (see Oxidative Addition) of C-H bonds is favored for osmium complexes. The rate of oxidative addition reactions decreases in the order Os > Ru Fe. 

Addition has also been shown to occur when the pyridinlum iodide (139) is irradiated. The product Is assigned the structure (140) and the relevance of this reaction to the photoreactivity of Kosower solvent polarity probes has been discussed. The use of osmium tetroxide to hydroxylate alkenes is a well known procedure which is often carried out in aromatic solvents. These arene solvents form charge transfer complexes with the osmium tetroxide and the photochemistry of these has now been examined. It is shown that with benzene and alkyl benzenes isolable adducts are formed that from benzene Is assigned structure (141). 

Friedel-Crafts reactions. The reaction of osmium-complexed anisoles with electrophiles such as enones is catalyzed by TfOH. Benzylation of arenes by a reductive alkylation with arenecarbaldehyde acetals involves an intramolecular redox process (hydride shift) after protonation of the benzylic ether intermediates. 

The subsequent chemistry of these complexes is not directly the chemistry of the osmium complex, but the chemistry of the remaining portion of the arene, which behaves as if dearomatized. Treatment of the phenol complex 10.236 with pyridine and a Michael acceptor yields the 4-alkylated dienone complex 10.238 (Scheme 10.62). The ring may be rearomatized on treatment with an amine base (stronger than pyridine) and decomplexed by heating. 

---