Osmium-substituted carbene complexes

Trimethylsilyldiazomethane, 327 Silyl substituted arenes Bis(trimethylsilyl)acetylene, 97 Chromium carbene complexes, 82 Titanium(IV) chloride-Diethylalu-minum chloride, 309 Other organosilanes Osmium tetroxide-Trimethylamine N-oxide-Pyridine, 223 Tributyltin chloride, 315 Di- x-carbonylhexacarbonyldicobalt, 99 Trimethylsilyl trifluoromethanesul-fonate, 329... 

Instead of adding 4-substituted pyridine ligands trans to the carbene ligand, the complexes Os(CHR XTTP) (R = SiMe3, C02Et) add these donors to the carbene carbon atom with formation of an ylide species. This reaction is not observed for Os(CR2XTTP) with R = Ph [313c]. Nevertheless, the diphenylcar-bene or trimethylsilylmethylene osmium porphyrins crystallize as Os(CRR )-(TTP)THF with a THF molecule in trans position. 

Dimetallacycles are commonly synthesized by substitution of a geminal dihalide (Equation (27)) <86JOM(3l4)C43, 89JOM(359)205>, (Equation (28)) <88JA7868>. The osmium compound can also be prepared by elimination of methane from an acyclic complex (Equation (29)) <82JA7325>. Many dimetallics have been synthesized by additions to metal carbenes <82AOC(20)159>. Transmetallation reactions are also known (Equation (30)) <90ICAll>. 

The reluctance of the carbyne carbon to react with nucleophiles is revealed by the reaction with LiEt3BH (see Scheme 6). Here the most electrophilic site is not the carbyne carbon but the ipara position of the aryl ring in the carbyne substituent Both ruthenium and osmium five coordinate, cationic, carbyne complexes undergo this reaction. The structure of a representative example, the osmium compound derived from the p-tolyl carbyne complex, has been determined by X-ray crystallography [16]. The unusual vinylidene complex reacts with HCl to produce a substituted benzyl derivative. The reaction may proceed through the intermediate a-vinyl complex depicted in Scheme 6 although there is also the possibility that the vinylidene compound is in equilibrium with the carbene tautomer as shown below. 

---