Osmium complexes, reaction with pyridines

Phosphonate Arene Osmium(II) Complexes. Benzene osmium(II) phosphonates have been made via the initial reaction of phosphite-containing osmium cations of type 147 with Nal. Demethylation occurs, and the anionic complex 182 is obtained. Protonation with trifluoroacetic afforded adduct 183 which generated complex 184 on treatment with pyridine. The latter was shown to be a precursor of the thallium complex 185 containing the chelating osmium phosphonate 115). 

In the absence of tertiary amines, osmium tetroxide reacts with alkenes via 1,3-dipolar addition to generate a monomeric Os(VI) ester such as 252,352 where L is a ligand that can be a solvent molecule or an added substrate such as pyridine. Sharpless et al. proposed that hydroxylation proceeds by an allowed [2-1-2]- cycloaddition reaction, producing an Os(VII) intermediate, followed by reductive insertion of the Os—C bond into an Os=0 bond.353 This complex can be decomposed in aqueous or alcoholic solution, but the hydrolysis is... 

Photochemical reactions of Ru3(CO)i2 and Os3(CO)i2 with nitrogen hetero-cycles have been studied, with substitution products being formed in most cases. For example, with pyridine the ruthenium compound forms the ortho-metallated complex Ru3H(CO)i i(CsH4N), while the osmium compound gives the simple substitution product Os3(CO)n(py). The novel dihydrido triruthenium cluster H2Ru3(CO)io can be synthesized by photolysis of Ru3(CO)i2 under an atmosphere of dihydrogen. 

Preparation of dihydroxytiagabine (VII) was accomplished by the method shown in Scheme 29.19. Synthesis of the 9-0-(4 -Methyl-2 -quinolyl) ether of dihydroquinidinol proved difficult in that the central double bond is hindered and proved to be refractory to attack by reagents such as m-chloroperbenzoic acid and hydrogen peroxide. The putative epoxide was not detected under a variety of reaction conditions a complex mixture of products was always obtained. Reaction with osmium tetroxide/pyridine/V-methylmorpholine-V-oxide was slow and yielded the requisite diol in low yield, but extraction of the product from water proved to be a problem. 

Another approach is based on the application of redox polders, e.g. osmium complex-modified poly(vinyl pyridine) (9-11) or ferrocene-modified poly(siloxanes) (12,13X crosslinked together with an enzyme on the top of the electrode. The electron transfer fi-om the active site of the polymer-entrapped enzyme to the electrode surfece occurs to a first polymer-bound mediator which has suflSdently approached the prosthetic group to attain a fast rate constant for the electron-tranrfer reaction. From this first mediator the redox equivalents are transported along the polymer chains by means of electron hopping between adjacent polymer-linked mediator molecules (Fig. 2). Extremely fast amperometric enzyme electrodes have been obtained with si ificantly decreased dependence fi-om the oxygen partial pressure. However, die redox polymer/enzyme/crosslinker mbcture has to applied either manually or by dipcoating procedures onto the electrode surface. 

In summary, the reaction of osmium tetroxide with alkenes is a reliable and selective transformation. Chiral diamines and cinchona alkakoid are most frequently used as chiral auxiliaries. Complexes derived from osmium tetroxide with diamines do not undergo catalytic turnover, whereas dihydroquinidine and dihydroquinine derivatives have been found to be very effective catalysts for the oxidation of a variety of alkenes. OsC>4 can be used catalytically in the presence of a secondary oxygen donor (e.g., H202, TBHP, A -methylmorpholine-/V-oxide, sodium periodate, 02, sodium hypochlorite, potassium ferricyanide). Furthermore, a remarkable rate enhancement occurs with the addition of a nucleophilic ligand such as pyridine or a tertiary amine. Table 4-11 lists the preferred chiral ligands for the dihydroxylation of a variety of olefins.61 Table 4-12 lists the recommended ligands for each class of olefins. 

This new process has one unexpected benefit the rates and turnover numbers are increased substantially with the result that the amount of the toxic and expensive 0s04 is considerably reduced (usually 0.002 mole %). The rate acceleration is attributed to formation of an Os04-alkaloid complex, which is more reactive than free osmium tetroxide. Increasing the concentration of 1 or 2 beyond that of 0s04 produces only negligible increase in the enantiomeric excess of the diol. In contrast quinuclidine itself substantially retards the catalytic reaction, probably because it binds too strongly to osmium tetroxide and inhibits the initial osmylation. Other chelating tertiary amines as well as pyridine also inhibit the catalytic process. 

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. 

---