| abstract |
Electron-deficient olefins are oxidised with the aid of osmium cpds. the olefins pref. of formula (I) (Where R1-R4 = electron-withdrawing gps. whose atom- or gp.-electronegativity is less than that of hydrogen). (I) is such that, pref. R1-R4 = H, F, Cl, Br, I, CN, NO2, SO3H, ReO3, COOH, COOR5, OR5 (R5 = 1-12C alkyl or aryl), or 1-32C alkyl or alkenyl(each opt. substd. by any of the substits. already given for R1-R4 with the exception of OR5) or R1+R3 or R2+R4 can form a ring, with the provisos that (i) at least one of R1-R4 is not H and at most two are OR5 and (ii) OR5 can only exist when F, Cl, Br, I, CN, NO2, SO3H, Re03, COOH or COOR5 is present. The Os cpd. is pref. OsxO (where x = 2, 3 or 4 or is an anionic Os cpd., esp. an alkali salt of (OsO2(OH)4)-. The oxidiser which can be catalysed by the Os cpd. is a hexacyanoferrate (III) salt, H202, t.Bu hydropeoxide or N-methylmorpholine-N-oxide. USE/ADVANTAGE - To oxidise e.g. partially or fully fluorinated olefins. Used for prodn. of 1,2-diols or 1,2-diketones or, in the case of cyclic olefins, for stereospecific cis hydroxylation. The Os cpd. is reacted stoichiometrically with the olefin and then the prod. is hydrolytically cleaved to the 1,2-diol or the Os cpd. |