Osmium cycloadduct

Osmium(VIII) tetraoxide (0s04) is an effective reagent for the cis hydroxylation of olefins under stoichiometric conditions as well as in a variety of catalytic variants.213 Under both catalytic and stoichiometric conditions, the critical step is the formation of an osmium(VI) cycloadduct, the formation of which is dramatically accelerated in the presence of amine bases such as pyridine,214 i.e.,... 

In Bettolo and co-workers approach to (+)-methyl trachyloban-18-oate (16), enone 13 was subjected to a photocycloaddition with 1,2-propadiene (1) to afford the [2 + 2]-cycloadduct 14 as a single product in 67% yield (Scheme 19.3) [5]. The addition proceeded exclusively from the /3-face. The resulting exocyclic olefin was eventually converted to a ketone using osmium tetroxide and NaI04 and taken on to 15, constituting a formal total synthesis of 16. 

The regio- and stereochemistry of the cycloadduct was determined by H-NMR spectroscopy68. Further transformations, involving cis hydroxylation with osmium tetroxide and reductive cleavage of the N-O bond, performed with Raney nickel at 40 °C, lead after acetylation, to an ( )-aminoallose derivative67,127. 

Osmium(VIII) tetroxide is an efficient reagent for cw-hydroxylations of alkenes [157, 158] via the osmium(VI) cycloadduct [159], which is stabilized by bases such as pyridine (pyr) [157, 160], (Eq. 28) ... 

Molecular complexes of OSO4 with various substituted benzenes, naphthalenes, and anthracenes have been identified by their charge-transfer absorption, which follows the Mulliken correlation in Eq. 8 [114, 161], The arene-0s04 complexes are quite stable when kept in the dark and only very slowly form osmium(VI) cycloadducts by thermal osmylation (Eq. 30). 

On the other hand, charge-transfer irradiation of the same molecular complexes in -hexane solution rapidly yields dark brown solids which—after dissolving in pyridine—have been identified by X-ray crystallography (see Figure 9), IR and NMR spectroscopy as the pyridine-ligated analogs of the osmium(Vl) cycloadduct in Eq. 30 [114, 161], (Eq. 31) ... 

Note that the spectral absorption of the reduced acceptor OSO4" is obscured by the arene absorption.) This ion-radical pair is formed unambiguously as the first reactive intermediate upon charge-transfer excitation, and its collapse ultimately leads to the formation of the osmium(VI) cycloadduct observed upon steady-state CT irradiation (vide supra), as depicted for the simple benzene donor in Eq. 34 ... 

In the course of the synthesis, the mixture of benzylated cycloadduct 257, obtained from diene 255 and glyoxylic acid ester 256, was hydroxylated with osmium tetraoxide to afford mixture of unseparable diols 258 (hydroxyalation onto si face of the si cycloadducts) and 259 (hydroxyalation onto re face of the re cycloadducts). This mixture... 

Liganded osmium complexes react with isothiocyanates across their C=S bond to give [2+1] cycloadducts 10. ... 

A similar [2+2] cycloaddition reaction occurs with iminophosphane complexes The [2+2] cycloaddition reaction of SO2 to an osmium carbon double bond in 6 affords the cycloadduct 7. ... 

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