Dioxoruthenium

ENANTIOSELECTIVE EPOXIDATION OF ( >(i-METHYLSTYRENE BY D2-SYMMETRIC CHIRAL TRANS-DIOXORUTHENIUM(VI) PORPHYRINS... 

PREPARATION OF THE rRAAA-DIOXORUTHENIUM(VI) COMPLEXES WITH D2 SYMMETRIC PORPHYRINS (H2L1 3)[7]... 

A Z>2-symmetric chiral /ra/7,v-dioxoruthenium( VI) porphyrin, [RuVI(L1)02], bifa-cially encumbered by four threitol units can effect enantioselective epoxidation of (fs)- 3-methylstyrene in up to 70% ee. For the asymmetric styrene oxidation, a lower enantioselectivity of 40 % ee was obtained (c.f. 62 % ee, see Table 6.3) when... 

These tru 5-dioxoruthenium(VI) complexes have characteristic UV-vis absorption spectra. The fj-saturated nature of the macrocyclic tertiary amine ligands enables the high-energy metal-localized transition to be observed. " The weak vibronic structured band at 370-400nm has been assigned to (0 ) —> Ru charge transfer transition that is vibronically coupled to the... 

In general, tertiary amines are better cr-donors than pyridines. Hence dioxoruthenium(VI) complexes of macrocyclic tertiary amines are more stable and weaker oxidants than those of polypyridyls. 

The electrochemistry of dioxoruthenium(VI) and dioxoosmium(VI) complexes with polypyridyl and macrocyclic tertiary amine ligands has been extensively studied by cyclic voltammetric techniques. In general, cA-dioxo species have higher reduction potentials than the corresponding trans-Aiaxo species. " " For the trans-Aioxo species, the d, orbital ordering... 

L = (bpy)2 > N2O2 > CRMe3 > pytn > TMC. Replacing an amine nitrogen with a pyridine nitrogen in L will cause an increase in redox potentials and rates of reaction. cw-Dioxoruthenium(VI)... 

The tra x-[Ru (0)2(por)] complexes are active stoichiometric oxidants of alkenes and alkylaro-matics under ambient conditions. Unlike cationic macrocyclic dioxoruthenium I) complexes that give substantial C=C bond cleavage products, the oxidation of alkenes by [Ru (0)2(por)] affords epoxides in good yields.Stereoretentive epoxidation of trans- and cw-stilbenes by [Ru (0)2(L)1 (L = TPP and sterically bulky porphyrins) has been observed, whereas the reaction between [Ru (0)2(OEP)] and cix-stilbene gives a mixture of cis- and trani-stilbene oxides. Adamantane and methylcyclohexane are hydroxylated at the tertiary C—H positions. Using [Ru (0)2(i)4-por)], enantioselective epoxidation of alkenes can be achieved with ee up to 77%. In the oxidation of aromatic hydrocarbons such as ethylbenzenes, 2-ethylnaphthalene, indane, and tetrahydronaphthalene by [Ru (0)2(Z>4-por )], enantioselective hydroxylation of benzylic C—H bonds occurs resulting in enantioenriched alcohols with ee up to 76%. ... 

Table 12 UV-visible spectral data of selected dioxoruthenium(VI) porphyrins in CH2CI2...

Table 13 E° values (V vs. Cp2Fe ) for selected dioxoruthenium(VI) porphyrins. ...

Dioxoruthenium(V) and dioxoosmium(V) species containing polypyridyl ligands, cis- and trans-... 

Although the chiral ketoiminatomanganese(lll) complexes were reported to catalyze the asymmetric aerobic alkene epoxidations, an aldehyde such as pivalaldehyde is required as a sacrihcial reducing agent. Groves reported that the dioxo(porphyrinato)ruthenium complexes 31, prepared with m-chloroperoxyben-zoic acid, catalyzed the aerobic epoxidation without any reductant. " On the basis of these reports, Che synthesized the optically active D4-porphyrin 35 and applied it to the truly aerobic enantioselective epoxidation of alkenes catalyzed by the chiral frani-dioxo (D4-porphyrinato)ruthenium(Vl) complex. The dioxoruthenium complex catalyzed the enantioselective aerobic epoxidation of alkenes with moderate to good enantiomeric excess without any reductant. In the toluene solvent, the turnovers for the epoxidation of T-(3-methylstyrene reached 20 and the ee of the epoxide was increased to 73% ee. 

There continues to be an increasing level of activity centered about the use of porphyrin catalysts for the epoxidation of alkenes of various configurations. For example, the sterically encumbered fra/w-dioxoruthenium(VI) porphyrin (26) was found to catalyze the epoxidation of a variety of alkenes in yields from fair to excellent e.g., 27 -> 28). Kinetic studies on a series of para-substituted styrenes point to a mechanism which proceeds via a rate-limiting benzylic radical formation. The high degree of stereoretention in cir-alkenes was attributed to steric crowding which prevents C-C bond rotation of the intermediate radical. This same steric bulk prevents the familiar side-on approach of the alkene substrate, so that a head-on approach is postulated <99JOC7365>. 

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