Osmium complexes with olefins

Che and coworkers [152] were able to isolate and characterize a pure bis-carbene (TPFPP)Os(CPh2)2 (Fig. 3). The bis-carbene species represents the first structurally characterized fra s-bis-carbene metal complex whose carbene groups are not stabilized by heteroatoms. The related pentacoor-dinated mono-carbene complex was also prepared and characterized by an X-ray structure. A comparison of the reactivity of these complexes with olefins suggests that the bis-carbene species acts as an intermediate in cy-clopropanation. Thus, the inertness of the mono-carbene complex towards stoichiometric styrene cyclopropanation and the observation of an efficient cyclopropanation of styrene in the presence of the bis-carbene complex as a catalyst support this suggestion [152]. A recent X-ray structure determination for (TPFPP)Os(CPh2)(MeIm) revealed an Os = C distance of 1.902(3) A (Table 3) [141]. Recently, Che and coworkers [153] and Miyamoto and coworkers [154] reported oxo-bridged carbene complexes of osmium porphyrins (see Table 3). These compounds are rare examples of oxo-binuclear carbene complexes. 

Diols are applied on a multimilhon ton scale as antifreezing agents and polyester monomers (ethylene and propylene glycol) [58]. In addition, they are starting materials for various fine chemicals. Intimately coimected with the epoxidation-hydrolysis process, dihydroxylation of C=C double bonds constitutes a shorter and more atom-efficient route to 1,2-diols. Although considerable advancements in the field of biomimetic nonheme complexes have been achieved in recent years, still osmium complexes remain the most efficient and reliable catalysts for dihydroxylation of olefins (reviews [59]). 

In summary, the reaction of osmium tetroxide with alkenes is a reliable and selective transformation. Chiral diamines and cinchona alkakoid are most frequently used as chiral auxiliaries. Complexes derived from osmium tetroxide with diamines do not undergo catalytic turnover, whereas dihydroquinidine and dihydroquinine derivatives have been found to be very effective catalysts for the oxidation of a variety of alkenes. OsC>4 can be used catalytically in the presence of a secondary oxygen donor (e.g., H202, TBHP, A -methylmorpholine-/V-oxide, sodium periodate, 02, sodium hypochlorite, potassium ferricyanide). Furthermore, a remarkable rate enhancement occurs with the addition of a nucleophilic ligand such as pyridine or a tertiary amine. Table 4-11 lists the preferred chiral ligands for the dihydroxylation of a variety of olefins.61 Table 4-12 lists the recommended ligands for each class of olefins. 

Model of a chirally modified dinuclear osmium complex reacting with olefins at 0+. 

Scheme 6D.1. Complexation and reaction of osmium tetroxide with tetiary amines and olefins.

Only one example of an NHC-containing olefin metathesis catalyst containing a transition metal other than ruthenium has been reported in the literature. The NHC-osmium complexes 53a and 53b (Scheme 2) are synthesized from the dichloro(i]6-p-cymene)osmium dimer by addition of the NHC prepared in situ and abstraction of the chloride, followed by introduction of the ben-zylidene moiety with phenyl diazomethane. 

In a second paper Criegee reported that pyridine markedly catalyzes the reaction of osmium tetroxide with an olefin and that an osmate ester combines with 2 molecules of pyridine to form a complex which can be crystallized from methylene... 

In the case of lead ruthenate, the oxygen non-stoichiometry concept can be developed further by combining it with the known reactions of the variable valence ruthenium. It has been shown in this work that these same catalysts can cleave carbon-carbon double bonds (3) in a manner analogous to that of osmium and ruthenium tetroxiHe (1,1). It is known (12) that OSO4 (and presumably RUO4) cleave olefins via complexes with the structure ... 

Tomioka et al. [61,62] demonstrated a highly enantioselective dihydroxyla-tion of olefins using stoichiometric amoimts of an osmium tetroxide complex with chiral diamine 45 (Fig. 6). 

He also pioneered the osmium-catalyzed vicinal aminohydroxylation reaction (AH), where imido osmium complexes react with olefins to form p-aminoalcohols [18]. Closely related to the DH and AH reactions is the amination of olefins leading to vicinal diamines, which was explored by Sharpless [19, 20] and Muniz [21, 22]. 

As diol by-products are frequently observed, it is highly probable that also osma-2,5-dioxolanes like II L or 12 L (Scheme 9) are formed. They could be formed either by reaction with OSO4 L (2 L), which could be formed hydrolytically, or by reaction of the olefin with 0=0s=0 rather than N=Os=0 of the trioxo imido osmium complexes 8 L (Scheme 9) [103, 105]. 

Therefore, a better regioselectivity should be obtained if electron-poor imido osmium complexes react with electron-poor olefins. This corresponds to the substrate-based methodology developed by Janda [96]. 

Oxidative rearrangements, via oxythallation, have been improved in yield and selectivity by the use of thallium(iii) nitrate supported on clay rather than in methanolic solution. Thus, cyclohexene gave an 85% yield of dimethoxymethyl-cyclopentane while 1-tetralone, which normally gives a complex mixture of products, gave a 1 1 mixture of methyl indane-l-carboxylate and 2-methoxytetralone. An efficient, large-scale procedure for the direct cis-dihydroxylation of olefins has been reported. The oxidant is t-butyl hydroperoxide and the catalyst osmium tetroxide, with the reaction conducted under alkaline conditions (E%N OH ), so facilitating a rapid turnover of catalyst via enhanced hydrolysis of the osmate esters. The method appears to be more advantageous for the more substituted olefins than the Hofmann and Miles procedure. 

The mechanism of the reactions of complexes containing more than one 0x0 group with olefins has been studied for many years in the context of the catalytic dihydroxylation and aminohydroxylation of olefins. Both the combination of a [2+2] cycloaddition followed by rearrangement to the final [3+2] addition product and direct [3+2] reactions of the olefin with the osmium species have been proposed (Scheme 13.21). Computational... 

Evidence in favor of the [2+2] mechanism is circumstantial, but it does include several types of studies. This evidence includes nonlinear free energy relationships between the substituent parameters on vinylarenes and the rates of the dihydroxylation, and it includes temperature effects on selectivity. It also includes the results of studies on the cleavage of Cp Re(0)(diolate) complexes to Cp ReOj and free olefin. The electronic effects, enthalpy of activation versus the strain of the olefin, and secondary deuterium isotope effects obtained from these studies on the rhenium complex support a stepwise cleavage process that could occur by initial formation of an oxametallacyclobutane intermediate. In the end, however, the combination of computational data and isotope-effect measurements seem to have led the community to accept that osmium tetroxide reacts by the direct [3+2] pathway. The mechanism of the reaction of OjOsNR with olefins during aminohydroxylation presumably follows the same type of [3+2] pathway. 

Ruthenium(III), d, is ruthenium s most stable oxidation state and resembles rhodium(III) and iridium(III) more than osmium(III). The salts inelude the halides, hydroxides, and oxides RuCls SHaO is most important because it is a good starting material for other compounds and reacts readily with olefins and phosphines. Complexes of this oxidation state are known with water, eyanide, oxygenated organies, sueh as diketones and earboxylates, pyridines, earbonyls, ey-elopentadienyls, phosphine, and arsine ligands. A notable differenee between ruthenium(II) and ruthenium(III) is the absenee of ruthenium(III) nitrosyl complexes. 

In subsequent chapters, only hydroformylations with Co, Rh, Ru, Pd, Pt, Ir, and Fe will be discussed in detail. Occasionally also molybdenum complexes (e.g., wer-Mo(CO)3(/ -C5H N-CN)3) [18] or osmium complexes (e.g., HOs(r -02CR)(PPhg)2) have been investigated [19]. Only recently, HOs(CO)(PPh3)3Br was evaluated for the hydroformylation of several olefins [20]. A main concern was the high isomerization tendency (up to 39%) noted. 

A comparison of Rh and Ru catalysts in the hydroformylation of linear butenes [4] or the strong electron-deficient substrate 3,3,3-trifluoropropene led to the conclusion that the latter are less active [5]. Moreover, in the hydroformylation of propene in comparison with Co and Rh catalysts, an inferior selectivity was noted [6]. In a competition experiment with the iridium-catalyzed hydroformylation of several a-olefins at 13 bar syngas pressure and 100 C, a related PPhj-modified Ru complex revealed no activity [7]. On the other hand, unmodified ruthenium based catalysts were shown to be more active than osmium complexes [8], thus the following rough order of reactivity results ... 

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