Osmium reactions with halogens

Neutral formyl complexes which contain ligating CO often decompose by decarbonylation however, several exceptions exist. For instance, the osmium formyl hydride Os(H)(CO)2(PPh3)2(CHO) evolves H2(54). Although the data are preliminary, the cationic iridium formyl hydride 49 [Eq. (14)] may also decompose by H2 evolution (67). These reactions have some precedent in earlier studies by Norton (87), who obtained evidence for rapid alkane elimination from osmium acyl hydride intermediates Os(H)(CO)3(L)(COR) [L = PPh3, P(C2H5)3], Additional neutral formyls which do not give detectable metal hydride decomposition products have been noted (57, 65) however, in certain cases this can be attributed to the instability of the anticipated hydride under the reaction conditions (H2 loss or reaction with halogenated solvents). 

Electrophilic attack at carbyne complexes may ultimately place the electrophile on either the metal or the (former) carbyne carbon, the two possibilities being related in principle by a-elimination/migratory insertion processes (Figure 5.39). The reactions of the osmium carbyne complex are suggestive of an analogy with alkynes. Each of these reactions (hydro-halogenation, chlorination, chalcogen addition, metal complexation see below) have parallels in the chemistry of alkynes. 

Substituted carbonyl clusters react with halogens in much the same way as their parent carbonyls. The final products of the reaction of halogens with Ru3(CO)9(PPh3)3 are the dimers [Ph3PRu(CO)2X2]2, as detected by Piacenti and co-workers (325, 326), but the initial products are the mononuclear Ph3PRu(CO)3X2 which dimerize with loss of CO just as the Ru(CO)4X2 compounds did (212). No trinuclear products were detected. The corresponding trisubstituted osmium carbonyl cluster... 

As with chlorine-containing oxidants, JV-bromo species have been used to oxidize sulphoxides to sulphones (with no bromine incorporation) through the initial formation of a bromosulphonium ion, by nucleophilic attack of the sulphoxide sulphur atom on the electrophilic halogen atom. Such reactions involve JV-bromosuccinimide ° bromamine-T, iV-bromoacetamide ° and iV-bromobenzenesulphonamide. All reported studies were of a kinetic nature and yields were not quoted. In acid solution all oxidations occurred at or around room temperature with the nucleophilic attack on the electrophilic bromine atom being the rate-limiting step. In alkaline solution a catalyst such as osmium tetroxide is required for the reaction to proceed . ... 

Whereas osmium dithiocarboxylato complexes are very rare, some dithiocar-boxylato iron and ruthenium complexes have been synthesized by substituting into the metal-halogen complex with lithium dithiocarboxylate [77] and by inserting CS2 into the carbon-metal bond [78,79]. The reaction of a vinylidene complex 32 with CS2 and NaOMe has also been reported to undergo a facile loss of HCl, followed by insertion of CS2 to give the dithiocarboxylato complex 33 (Scheme 7) [80]. 

In several of the reactions in previous sections, one heteroatom (halogen, oxygen) was added to one side of a 7i-bond and a hydrogen or another heteroatom to the other side. There is another type of reaction in which heteroatoms are incorporated on both carbons of a 71-bond. A specific example incorporates two hydroxyl (OH) units. In one experiment, cyclohexene is treated with OSO4 (osmium tetroxide) in anhydrous er -butyl alcohol (2-methyl-2-propa-nol) at 0°C. After standing overnight, a 45% yield of cis-l,2-cyclohexanediol (124) was obtained.Analysis of this reaction shows that two OH units added and that both oxygen atoms were derived from osmium tetroxide. This reaction is termed a dihydroxylation. Furthermore, the two OH units have a cis relationship. 

---