Osmium structural transformations

Osmium (continued) carbide, 24 233 dianion, lA. Xil, 317-319 with phosphines and diphosphines, 30 191 protonation/deprotonation, 30 169 raft hexaosmium clusters, 30 180-182 reactions of condensation, 30 145 with hexafluoroacetone, 30 288 redox, 30 184-185 structural transformations, 30 203 sulfur-containing, synthesis of, 30 147 sulfur derivatives, 24 269, 300-310 synthesis... 

A catalytic enantio- and diastereoselective dihydroxylation procedure without the assistance of a directing functional group (like the allylic alcohol group in the Sharpless epox-idation) has also been developed by K.B. Sharpless (E.N. Jacobsen, 1988 H.-L. Kwong, 1990 B.M. Kim, 1990 H. Waldmann, 1992). It uses osmium tetroxide as a catalytic oxidant (as little as 20 ppm to date) and two readily available cinchona alkaloid diastereomeis, namely the 4-chlorobenzoate esters or bulky aryl ethers of dihydroquinine and dihydroquinidine (cf. p. 290% as stereosteering reagents (structures of the Os complexes see R.M. Pearlstein, 1990). The transformation lacks the high asymmetric inductions of the Sharpless epoxidation, but it is broadly applicable and insensitive to air and water. Further improvements are to be expected. 

Figure 8.40 The k ySk) extended X-ray absorption fine structure (EXAFS) signal, Fourier transformed and then retransformed after application of the filter window indicated, in (a) osmium metal and (b) a 1% osmium catalyst supported on silica. (Reproduced, with permission, Ifom Winnick, FI. and Doniach, S. (Eds), Synchrotron Radiation Research, p. 413, Plenum, New York, 1980)...

The a-osmiumdiazo compound 91 decomposes in a thermal reaction to yield the metallacyclic complex 93 (130). This resembles the electrophilic carbene insertion reaction forming OsCl(CO)2(PPh2C6H4CHCl) (PPh3) (77) (see Section V,D,2), and we suggest that a similar insertion reaction of an electrophilic, cationic osmium carbyne 92 is the key step in this transformation. An X-ray structure determination has confirmed the formulation of 93. 

The /Tamino alcohol structural unit is a key motif in many biologically important molecules. It is difficult to imagine a more efficient means of creating this functionality than by the direct addition of the two heteroatom substituents to an olefin, especially if this transformation could also be in regioselective and/ or enantioselective fashion. Although the osmium-mediated75 or palladium-mediated76 aminohydroxylation of alkenes has been studied for 20 years, several problems still remain to be overcome in order to develop this reaction into a catalytic asymmetric process. 

Some other natural compounds have been transformed for their use in the synthesis of polymers via olefin metathesis processes. As mentioned in the introduction, furans, which are obtained from carbohydrates, are perfect precursors of monomers for ROMP via simple Diels-Alder cycloadditions (n) (Scheme 25) [26]. In this regard, the first example of the ROMP of 7-oxabicyclo[2.2.1]hept-5-ene derivatives was reported by Novak and Grubbs in 1988 using ruthenium- and osmium-based catalysts [186]. The number of examples of ROMP with monomers with this generic structure is vast, and it is out of the scope of this chapter to cover all of them. However, it is worth mentioning here the great potential of a renewable platform chemical like furan (and derived compounds), which gives access to such a variety of monomers. 

Plots of the function K xUO vs. K at 100°K for the extended fine structure beyond the osmium edge for pure metallic osmium, and for the osmium-copper clusters in the catalyst containing 2 wt% Os and 0.66 wt% Cu, are shown in the left-hand sections of Figure 4.13. The associated Fourier transforms of the functions are shown in the right-hand sections of the figure. As previously noted, the Fourier transform yields the function n(R), the peaks of which are displaced from the true interatomic distances because of the phase shifts. Similar plots for the extended fine structure beyond the copper K edge for pure metallic copper and for the osmium-copper catalyst are given in our 1981 paper (32). 

Osmylation and Epoxidation With osmium tetroxide, carbon nanotubes react as expected for a compound containing double bonds. The osmylation adduct with the respective double bond being replaced by two C-O-bonds is formed as shown in Figure 3.78. However, the process is normally conducted in a photochemical way here. The intermediates thus obtained can be transformed into hydroxylated nanotubes by hydrolysis. In doing so, it is advisable to effect a reoxidation of the resultant osmium(Vl) by hydrogen peroxide in order to minimize the consumption of osmium. The osmylation of carbon nanotubes is reversible so the process may also be employed for purification or separation steps. Contrary to an ozonoly-sis with subsequent reductive work-up, the osmylation does not give rise to holes in the side wall. Hence the electronic structure is less affected. 

A transformation showing enhancement of the reactivity of phenol through transition metal complexation occurs in the reaction of [Os(NH3)s(fi -phenol]-(OTO2 with maleic anhydride in acetonitrile over 20 hours at ambient temperature followed by recovery of the product, dimethyl (4-hydroxyphenyl)succinate in 85% yield by simple ethereal precipitation and removal of the osmium by refluxing in acidic methanol (ref.39). These last two examples illustrate the versatility of the appropriately modified phenolic structure to function either in a nucleophilic or in an electrophilic manner. 

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