Rh-W complexes

Scheme 77. Synthesis of Rh-W complexes from Tp-alkylidyne complexes (R = C6H4Me-4, Me, SMe Ind = indenyl). Reagents (i) Rh(fy-C2H4)2( y -C9H7) and (ii) Rh CO)2(r] -C9Hj).

Fig. 4 Coupling of the redox participants to the DNA w-stack is requisite to DNA-mediated charge transport. Rapid (>109 s 1) photoinduced electron transfer occurs between the metallointercalators, [Ru(phen)2dppz]2+ and [Rh(phi) 2phen]3+, when they are tethered to opposite ends of a DNA duplex over 40 A apart. Conversely, electron transfer does not occur between non-intercalated Ru(II) and Rh(III) complexes tethered to DNA...

Besides the electrochemical application, the (Cp )Rh(bpy)-complex 9 can also be used to reduce cofactors with hydrogen. In a recent study it was compared with ruthenium complex 13 [RuC12(TPPTS)2]2 (TPPTS tris(w-sulfonatophenyl)-phosphine Scheme 43.5). Both complexes were used to regenerate the cofactors in the reduction of 2-heptanone to (S)-2-heptanol, catalyzed by an ADH from Thermoanaerobium brockii (TfrADH) [46, 47]. The TON for both catalysts was 18. 

On the basis of these initial results, various rare earth metal triflates, including Sc(OTf)3, Hf(OTf)4 and Yb(OTf)3 were applied as catalysts [27-29]. Recently Beller and coworkers developed efficient Friedel-Crafts alkylations with catalytic amounts of Rh, W, Pd, Pt and Ir complexes [30] or FeCl3 [31-34] as Lewis acid catalysts. However, in the latter cases high catalyst loadings had to be applied. To overcome these major drawbacks, we decided to develop a Bi(III)-catalyzed Friedel-Crafts alkylation of arenes with benzyl alcohols. Although bismuth-catalyzed Friedel-Crafts acylations were well known at this time, Friedel-Crafts alkylations using benzyl alcohols had not been reported. 

Acidity usually decreases down a periodic column, e.g., the acidity of h CjHj-M(CO)jH decreases Cr > Mo > W HjOs(CO) is a weaker acid than H2Fe(CO) and HRe(CO)j cannot be deprotonated under the conditions employed for the titration and pK determination of HMn(CO)j. However, there is little difference in pK between the Co and Rh dimethylglyoxime complexes (see Table 1), and IR observations... 

IW. Oppolzer and R. J. DeVita, J. Org. Chem., 1991,56, 6256. Oppolzer has also used Rh(I) complexes to catalyze the ene reaction. Yields are similar, but the stereochemical results are opposite those obtained with Pd catalysts. This difference may be a function of the octahedral geometry of Rh(III) intermediates, presumably generated during the catalytic cycle. See W. Oppolzer and A. Fiirstner, Helv. Chim. Acta., 1993, 76, 2339. 

It was considered that if these anchored species were ion pairs, flien one should be able to remove them by treatment with an excess of another anion. To test this, a sample of Rh(dppb) anchored onto PTA/A1203 was stirred for 30 hours with a large excess of LiBF4 in ethanol under an inert atmosphere. Analysis of the reaction liquid showed that, at most, 2-3% of the Rh present on the catalyst was lost. However, tungsten was also detected with a Rh W ratio of 1 12. Since there are twelve tungsten atoms in a PTA molecule, these data indicate that this procedure apparently did not remove only flie Rh complex from the PTA but, instead, somehow removed the Rh/PTA from the alumina. 

We are grateful to W. H. Myers and T. E. Nappier, Jr. for exploratory studies on the free-radical catalyzed syntheses of ligands and the Rh-ttp complexes, respectively, and to Matthey Bishop Inc. for a loan of rhodium trichloride. 

Wisden, W. and Seeburg, RH., A complex mosaic of high-affinity kainate receptors in rat brain, J. NeuroscL, 13, 3582, 1993. 

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