Osmium-Based Initiators

Reaction of dineopentylosmium dioxide with 2 equiv. of Ta(CHR)(CH2R)3 was reported to result in the formation of osmium dineopentylbisneopentylidene [269, 270]. Interconversion of the two isomeric forms [syn-anti and anti-anti) of this compound was postulated to proceed via an osmium tris(neopentyl)neo-pentylidene. So far, no reports on the ROMP activity of these complexes have been given. Similar to OsCls [271] and (p-cyrnene)OsCl2P(c-H,.) j [272], (/z-H)20s3(CO)io was reported to be active in the ROMP of NBE to yield all cis-poly-NBE [273]. 

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Carretero and co-workers found that dqjrotection and cyclization of the vinyl-sulfone 294 produced a 4 1 mixture of 2,3-cu-disubstituted pyrrolidine 295 and its tram isomer (Scheme 40). A/-Alkylation of the nuxture. with 3-chloro-2-chloro-methylprop-l-ene followed by chromatogttqjhy led to isolation of the pure 2,3-c/s product 296, silylation and base-initiated cyclization of which gave indolizidine 297. Ozonolysis and elimination of the sulfone group yieldied another pivotal intermediate, the bicyclic enone 298. Reduction with L-Selectride afforded an inseparable nuxture of two diastereomeric alcohols 299 (9 1). Separation was accomplished only after dihydroxylation with osmium tetroxide and peracetylation of the resdting tetrols. The synthesis of ( )-241 was completed by hyc lysis of the m or tetraacetate 300. 

The Sharpless asymmetric hydroxylation can take one of two forms, the initially developed asymmetric dihydroxylation (AD)1 or the more recent variation, asymmetric aminohydroxylation (AA).2 In the case of AD, the product is a 1,2-diol, whereas in the AA reaction, a 1,2-amino alcohol is the desired product. These reactions involve the asymmetric transformation of an alkene to a vicinally functionalized alcohol mediated by osmium tetraoxide in the presence of chiral ligands (e.g., (DHQD)2-PHAL or (DHQ)2-PHAL). A mixture of these reagents (ligand, osmium, base, and oxidant) is commercially available and is sold under the name of AD-mix p or AD-mix a (vide infra). 

For a synthesis of leurosidine (56), 15,20-dihydrocatharanthine iV-oxide (57) was subjected to coupling with vindoline (3) under the modified Polo-novski conditions. The initial adduct, imonium salt 58, was converted to the enamine 59 in base. Oxidation of this product with osmium tetrox-ide proceeded chemo- and stereoselectively, without reaction of the... 

A recent development31 is the preparation of metal polymer complexes directly on the electrode via the electrochemically induced polymerization of the metal complex. Ruthenium(II) and osmium(II) complexes with ligands containing aromatic amines, e.g. 3- or 4-aminopyridine or 5-amino-1,10-phenanthroline, are electrochemically polymerized to yield a film of the metal polymer on the electrode surface. The polymerization involves free radicals, which are formed via the initial oxidation of the metal complex to a radical cation and subsequent reaction of the radical cation with a base to yield the free radical. 

If the nitrogen donor is a secondary 1,2-diamine, the initially formed hexacoordinated osmi-um(VI) ester of type 4 loses a molecule of water upon heating forming a five-coordinated square-based pyramidal osmium(VI) complex of type 647... 

Although C4 addition occurs with phenol complexes even for cases where C4 is substituted, in many cases, ortho addition is thermodynamically favored. In this scenario, the regiochemistry can be effectively controlled by adjusting reaction variables such as temperature, time, and catalyst [44]. Under basic conditions, the active form of the phenol complex is the phenoxide species, which can undergo reversible Michael reactions at C4 and C2, provided that the resulting enolate is not protonated. For instance, the addition of MVK to the osmium complexes of para-cresol (31) or estradiol (27, Fig. 8) occurs at C4 to give the 4H-phenol product (28,32) at -40 °C with an amine base. However, if the reaction is carried out at 20 °C or is run in the presence of a Zn " co-catalyst, the initially formed enolate may undergo retroaddition, and ultimately, the reaction yields the orthoalkylated product (30,33 see Fig. 8). Electrophilic addition at the ortho... 

The initial work on the reaction of osmium tetroxide with olefins was carried out by Hofmann early in the last century [3,4]. In the 1930s, Criegee [5] showed that the addition of a base, such as pyridine, accelerates the reaction rate. Later on, catalytic variations of the reaction were developed employing relatively inexpensive reagents for the reoxidation of the osmate species [6]. More recently, several advances were achieved to immobilize the catalyst and to improve the catalytic oxidation chain. In particular, recent work couples the oxidation of the osmium(VI) compound to a green terminal oxidant such as air or hydrogen peroxide [7-9]. 

Enantioselectivity in this process arises from reaction of the alkene and osmium tetroxide within an asymmetric environment created by the ligand. However, the precise shape of this binding pocket and the nature of the interactions between the substrate and ligand has been the subject of some debate. Corey has advanced a model based on a U-shaped ligand conformation initially derived from an inspection of molecular models while Sharpless has proposed an L-shaped active site based on the results of molecular mechanics studies. 

A new photoconversion method for TEM immunolabeling promises, at least in certain cases, to reduce these problems of penetration and local accessibility. This method is based on the generation of free radicals by excitation of the fluorochrome the oxidation of diaminobenzidine (DAB) by these free radicals generates precipitates that are converted to an electron-dense stain by osmium tetroxide (Deerinck et al., 1994). The ultimate resolution of this method should be determined by the diffusion of the free radicals prior to reaction with DAB and the size of the precipitate initial results indicate resolution comparable to immunogold methods (Deerinck et ai, 1994 Huang et al., 1994), although a detailed comparison of the sensitivity of the photoconversion method has not yet been done. 

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