Imido-osmium compound

The direct aminolysis of metal oxides with primary amines is a useful synthetic route for imido compounds of rhenium and osmium (equation 41). 

Muniz K. Imido-osmium(VIII) compounds in organic synthesis I. Aminohydroxylation and diamination reactions. Chem. Soc. Rev. 2004 33 166-174. 

Cycloadditions are among the more common reactions of imido and 0x0 groups in catalytic transformations. Both [2+2] cycloadditions and [3+2] cycloadditions are well established. The [2+2] cycloaddition chemistry occurs during hydroaminations of alkynes, allenes, and strained olefins catalyzed by imido compounds. - The [3+2] cycloaddition reactions appear to occur during dihydroxylation and aminohydroxylation of olefins catalyzed by osmium complexes. ... 

In 1975 Sharpless and coworkers discovered the stoichiometric aminohydrox-ylation of alkenes by alkylimido osmium compounds leading to protected vicinal aminoalcohols [1,2]. Shortly after, an improved procedure was reported employing catalytic amounts of osmium tetroxide and a nitrogen source (N-chlo-ro-N-metallosulfonamides or carbamates) to generate the active imido osmium species in situ [3-8]. Stoichiometric enantioselective aminohydroxylations were first reported in 1994 [9]. Finally, in 1996 the first report on a catalytic asymmetric aminohydroxylation (AA) was published [10]. During recent years, several reviews have covered the AA reaction [11-16]. 

While inorganic complexes of osmium in oxidation states +4 through +8 have been known for many years, the study of high-valent alkyl and aryl complexes of osmium is much more recent. The organometallic complexes include homoleptic see Homoleptic Compound) alkyls and aryls, oxo alkyls and aryls, nitrido/imido alkyls and aryls, and cyclopentadienyl see Cyclopentadienyl) alkyls and aryls. The majority of these are complexes of osmium(VI). 

The electrochemical properties of a number of isostructural rhe-nium(V) oxo, rhenium(V) imido, osmium(VI) nitrido, of formula [M(E)(X)(Y)(Tp )] (Tp = Tp or Tp, E = O, N-tolyl, N X, Y = hydrocarbyl, halide, triflate), have been described. The reactivity of these complexes as inner-sphere oxidants does not correlate with their peak reduction potentials, whereas the ease of the oxidation of these compounds well parallels their reactivity as oxidants.206... 

The synthetic procedure is very critical. In our case, we believe that the imido-lithium compound (Li2NR) is present in the solution of butyl lithium and para-toluidine, in diethyl ether, as reported for the dilithiated a-naphtylamine. This is a noticeable difference in comparison to typical preparations of ruthenium, osmium, and iridium imido complexes, " in which a dichlorometal complex and the monolithium salt (LiNHR) in a molar ratio 1 2, appropriate for a ftA-amido precursor, are used. In these cases a subsequent removal of amine, or a dehydro-halogenation step with LiNHR, is required to afford the products and free amine. Equation (1) summarizes our synthetic procedure ... 

Common solvents used in experiment are water/alcohol mixtures. Three major groups of substituents R on the imido nitrogen atom are known [20] These are sulfonyl groups (e.g., mesyl or tosyl substituents), esters (e.g., tert-Bu-carbamate substituent), and carbonyl groups (e.g., acetamido substituents) (Scheme 8). Early publications referred to alkyl imido osmium compounds [18,90,104]. The reaction is, depending on the nature of R, usually accelerated by nitrogen ligands [20]. 

Dihydroxylated by-products might indeed result from dihydroxylation mediated by the imido osmium compound so2 Toig. l s a very low kinetic selectivity of only 0.9 kcal/mol (14.3-13.4 kcal/mol) has been obtained. In case of the methyl-substituted imido osmium, another pathway becomes more probable, which is the dihydroxylation by in situ formed OSO4 NH3. It was predicted to have a reaction barrier of only 17.0 kcal/mol and might be responsible for the experimental observation of a significant amount of dihydroxylated by-product in case of R = tert-Bu [18, 90]. 

Both substituents R and R play an important role and exhibit an influence on the calculated activation energy of the reaction. Lower activation energies were predicted for imido osmium compounds bearing electron-withdrawing substituents R in reaction with electron-rich olefins (e.g., L + butadiene, 11.2 kcal/mol). 

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