Osmium allenes

Reaction (105) could also be induced in benzene at room temperature by addition of silver perchlorate. Of related interest is the decarboxylation of diketene either by an osmium cluster to give an i73-allyIosmium complex [Eq. (106)] (114) or by (T7-C5H5)Mn(CO)2(thf) to give (77-C5H5)-Mn(CO)2(allene) (115). 

We have previously seen examples of the carbon-like formulas of mononuclear and dinuclear osmium compounds, namely the methane-like tetrahydride (4.50c), ethylene-like H20s=CH2 (4.51c) and H2Os = OsH2 (Table 4.15), acetylenelike HOs = CH (4.54c) and HOs = OsH (Table 4.15), allene-like H2C = Os = CH2 (4.55a), and so forth. While the coordination numbers and Lewis-like formulas are formally analogous, the actual structures of Os and C species may be quite similar (e.g., the Td structures of OsfL and CH4) or dissimilar (e.g the strongly bent Cs structure of H20s = CH2 [Fig. 4.13(c)] versus the planar D2h structure of H2C = CH2). 

Unlike palladium(II), osmium tetraoxide and ruthenium tetraoxide catalyze the dihydroxylation of one or both double bonds of an allene. The osmium tetraoxide-catalyzed dihydroxylation of unsymmetrically substituted allenes 45 can lead to two different a-ketols, 46 and 47, depending on which of the double bonds is oxidized. David et al. studied this reaction using NMO as a stoichiometric oxidant and found good product selectivity in a few cases, but the yields were only moderate (Scheme 17.15) [16]. They showed that the most substituted double bond was oxidized preferably when the bulkiness of the allene substituents did not interfere. 

Asymmetric, osmium-catalysed dihydroxylation of 1,1-disubstituted and 1,3-disub stituted allenes has been employed to synthesize chiral a-hydroxy ketones, a,a - (g) Dihydroxy ketones were obtained from 1,3-disubstituted allenes with high enantio-... 

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. ... 

The reactions of allenes and dienes with iron and ruthenium compounds form a large number of various allyl complexes (see allene complexes). Osmium forms allyl compounds of the formulas [OsCl(C3H5)(PPh3)3] and [Os(C8Hg)(CO)3]. 

Castro-Rodrigo R, Esteruelas MA, Lopez AM, Mozo S, Onate E. C-H bond activation reactions in p-allene-osmium-ttiisopropylphosphine complexes with cyclopentadienyl or hydridotris(pyrazolyl)borate ligands formation of isopropenyldiisopropylphosphine versus hydride-alkenylcarbyne derivatives. 

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