Ruthenium hydride dimerization

Ruthenium hydride complexes, e.g., the dimer 34, have been used by Hofmann et al. for the preparation of ruthenium carbene complexes [19]. Reaction of 34 with two equivalents of propargyl chloride 35 gives carbene complex 36 with a chelating diphosphane ligand (Eq. 3). Complex 36 is a remarkable example because its phosphine ligands are, in contrast to the other ruthenium carbene complexes described so far, arranged in a fixed cis stereochemistry. Although 36 was found to be less active than conventional metathesis catalysts, it catalyzes the ROMP of norbornene or cyclopentene. 

DFT calculations have shown that the experimentally observed decomposition pathway likely occurs through insertion of the benzylidene into the chelating Ru-C bond. The computed free-energy profile for the decomposition of complex 22 is shown in Figure 7.21. Insertion of the alkylidene into the chelating ruthenium-carbon(adamantyl) bond required 29.7 kcalmoC and formed alkyl ruthenium complex 28. Complex 28 then underwent facile fi-hydride elimination to form ruthenium hydride 30, which then converted to the q -bound olefin complex 32 and eventually the dimer complex 26. The a-hydride ehmination pathway from intermediate 28 via 33-ts required much higher activation energy than the fi-hydride elimination. 

On the other hand, treatment of norbornadiene with a catalytic amount of Ru(cod)(cot) (cod = cyclo-l,5-octadiene, cot = cyclooctatetraene) and an electron-deficient olefin caused a unique dimerization reaction to afford cage compound 18 (Scheme 7.6) [8]. Although the precise reaction mechanism is unclear, it is proposed that the reaction proceeds through the alkylruthenium intermediate 13. It adds intramolecularly to the alkene moiety to form 14, which further adds to the remaining alkene moiety. Subsequent oxidative addition of a C-C bond located in proximity to the ruthenium center affords 15. Reductive elimination ensues to give the alkylruthenium intermediate 16. Subsequent P-carbon elimination breaks the strained norbornane skeleton to furnish alkylruthenium 17. P-Hydride elimination produces the cage molecule 18 along with a ruthenium hydride species. 

Heterometal alkoxide precursors, for ceramics, 12, 60-61 Heterometal chalcogenides, synthesis, 12, 62 Heterometal cubanes, as metal-organic precursor, 12, 39 Heterometallic alkenes, with platinum, 8, 639 Heterometallic alkynes, with platinum, models, 8, 650 Heterometallic clusters as heterogeneous catalyst precursors, 12, 767 in homogeneous catalysis, 12, 761 with Ni—M and Ni-C cr-bonded complexes, 8, 115 Heterometallic complexes with arene chromium carbonyls, 5, 259 bridged chromium isonitriles, 5, 274 with cyclopentadienyl hydride niobium moieties, 5, 72 with ruthenium—osmium, overview, 6, 1045—1116 with tungsten carbonyls, 5, 702 Heterometallic dimers, palladium complexes, 8, 210 Heterometallic iron-containing compounds cluster compounds, 6, 331 dinuclear compounds, 6, 319 overview, 6, 319-352... 

The ruthenium allenylidene complexes W are excellent precursors for the catalytic dimerization of tributyltin hydride under mild conditions [ 109] (Eq. 15). In the presence of Bu3SnH, the hydride addition at Cy provides a catalytically active alkynyl ruthenium-tin species (Scheme 22). 

Enyne derived from ditosyl o-phenylenediamine 257 formed in the presence of benzylidene ruthenium carbene complex a nine-membered ring 258 in 5% yield (Equation 30) <20000L543, 2001S654>. Dimerization was a major by-process (22% yield) along with formation of a small amount of 259 (5% yield), which was explained by /3-hydride elimination from the intermediary ruthenacyclobutane. 

Rhodium-based catalysis suffers from the high cost of the metal and quite often from a lack of stereoselectivity. This justifies the search for alternative catalysts. In this context, ruthenium-based catalysts look rather attractive nowadays, although still poorly documented. Recently, diruthenium(II,II) tetracarboxylates [42], polymeric and dimeric diruthenium(I,I) dicarboxylates [43], ruthenacarbor-ane clusters [44], and hydride and silyl ruthenium complexes [45 a] and Ru porphyrins [45 b] have been introduced as efficient cyclopropanation catalysts, superior to the Ru(II,III) complex Ru2(OAc)4Cl investigated earlier [7]. In terms of efficiency, electrophilicity, regio- and (partly) stereoselectivity, the most efficient ruthenium-based catalysts compare rather well with the rhodium(II) carboxylates. The ruthenium systems tested so far seem to display a slightly lower level of activity but are somewhat more discriminating in competitive reactions, which apparently could be due to the formation of less electrophilic carbenoid species. This point is probably related to the observation that some ruthenium complexes competitively catalyze both olefin cyclopropanation and olefin metathesis [46], which is at variance with what is observed with the rhodium catalysts. 

Reaction of Ru(III)Cl3 with a precursor 1,4-cyclic diene gives rise to a Ru(II) arene dimer such as 14 (Fig. 2.4). This is often a convenient synthetic route to Ru(II) arenes, and the structures of several such dimers have been determined [34—36]. Recent interest in the design of ruthenium arene complexes as catalyst precursors has led to the exploration of a wide range of synthetic routes to complexes containing various arenes, substituted arenes, together with other ligands such as hydride, phosphines, aUcyl and aryl groups [37]. 

Ura and Kondo have recently reported the ruthenium-catalyzed co-dimerization of N-vinylamides with alkynes. The process leads to the formaticHi of 1-amido-1,3-dienes 207 with a preferential (l ,3 )-selectivity. The authors proposed a mechanism involving the insertion of the alkyne into an Ru-H bond (generated in situ), leading to complex 205, followed by a chelaticui assisted insertion of A -vinylamide into the Ru-C bond and subsequent p-hydride elimination (Scheme 82) [176]. 

---