Thiophenes osmium

The synthesis and chemistry of metal complexes of thiophenes have been reported including the electrophilic additions to osmium-thiophene complexes <9902988> and nucleophilic additions to ruthenium-thiophene complexes <99JOMC242>. The selectivity for the insertion of ruthenium into 3-substituted thiophenes was studied <99CC1793>. For example, treatment of 3-acetylthiophene (84) with Ru(cod)(cot) led to a regioselective 1,2-insertion of ruthenium giving thiaruthenacycle 85. 

SC, Thiocarbonyls, osmium, 26 185-187 SC4Hb, Thiophene, tetrahydro-gold complexes, 26 85-87 SCjHj, 2,4-Pentadienthial osmium complex, 26 188 SC H6, Benzenethiol osmium complex, 26 304 SClF,0,PtCiiHio, Platinum(II), chloro-bis-(triethylphosphine)(trifluoromethane-sulfonato)-cis-, 26 126... 

The first successful osmium-catalyzed asymmetric aminohydroxylation and dihydroxylation of thiophene acrylates have been reported. The aminohydroxylation of 2-thienyl-, 5-bromo-2-thienyl-, and 3-thienylacrylates proceeds with high regio- and enantioselectivity (Equation 75) <1999SL1907. Yields were about 67-68%, with the regioselectivity of 151 over 152 being >15 1. The ee was 99%. 

The 7] -osmium complexes of a-unsubstituted thiophenes undergo Lewis acid-promoted addition with acetals at C-2 to give the thiophenium complexes in good yields <19990M2988>. These can be deprotonated to give the 2-substituted thiophene complexes. The electrophile attacks the substrate on the rivn-face (Scheme 83). 

Alkylation of the thiophene 7] -osmium complex with hard electrophiles such as MeOTf or Et30 takes place at the sulfur atom <20010M1259>. 

The high cost associated with the use of stoichiometric quantities of osmium has limited the general use of this strategy. It is interesting that a new molybdenum 7] -thiophene complex has now been developed <2003JA2024>. Its usefulness for further reactions has not yet been demonstrated. 

SC, Thiocarbonyls, osmium, 26 185-187 SC4H, Thiophene, tetrahydro-, gold complexes, 26 85-87... 

Although C-S bond cleavage reactions between thiophenes and osmium clusters have not been observed, selenophene and tellurophene undergo ring opening reactions with [Os3(CO)io(NCMe)2] to give complexes 28a and 28b (Scheme T). " It is likely that the sulfur-extrusion reactions of iron and ruthenium carbonyl clusters with thiophenes proceed via ring-opened intermediates of this type. 

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