Heterobidentate

In 2007, Fernandez et al. demonstrated that transition-metal complexes with heterobidentate S/C ligands based on imidazopyridin-3-ylidene and thioether functionalities could be readily prepared from the corresponding azolium salts by reaction with Ag20 and transmetalation of the resulting silver carbenes with appropriate metal sources. The cationic Pd(allyl)(carbene-S) complexes have proven to be active catalysts in the test reaction, reaching enantioselectivities of... 

Additional combinatorial variation sites allow the heterocyclic self-assembly units. Thus, it has been shown that heterocycles 11 and 14-17 can serve as A-analogous donor-acceptor ligands self-assembling with the T-analogous acceptor-donor ligands isoquinolone 12 and 7-azaindole 18 (Scheme 30) [92]. All combinations form the heterobidentate ligands exclusively upon simple mixing in the presence of a transition metal salt (proven by X-ray, NMR). 

However, by far the most common approach toward catalytic asymmetric hydroboration is to use a chiral catalyst and an achiral borane source. Both homobidentate F Pand heterobidentate P,N ligand classes have been employed in this transformation with varying degrees of success. 

Reek, van Leeuwen et al. have also reported the template-induced generation of bidentate ligands onto a rigid bis-zinc(II) salphen platform. Homobidentate [41] or heterobidentate [42] P,P-ligands have been prepared and tested in hydroformylation and hydrogenation reactions. 

Scheme 8.7 Selective formation of heterobidentate ligands on a template with identical binding sites.

The assembly formation of heterobidentate ligated complexes was studied in detail using bidentate ligand 13"b as a typical example (Figure 8.8). UV/Vis titrations and NMR spectroscopy experiments showed that the pyridyl moiety of b selectively coordinates to thezinc(ii)porphyrinl3withabindingconstantofK(ib) = 3.8x lO m. An increase in the Kfor 13 b to 64.5 X lO was observed in the presence of metal... 

Template-induced formation of heterobidentate ligands and their application in the asymmetric hydroformylation of styrene. Chem. Commun., 4679 681. (b) Kuil, M., Goudriaan, P.E.. Kleij, A.W., Tooke, D.M., Spek, A.L.. van Leeuwen, P.W.N.M. and Reek, J.N.H. (2007) Rigid bis-zinc([[) salphen building blocks for the formation of template-assisted bidentate ligands and their application in catalysis. Dalton Trans., 2311-2320. 

Ionic Interactions as a Means to Form Heterobidentate Assembly Ligands 269... 

It occurred to us that ionic interactions might be a highly suitable binding motif to enforce the formation of heterobidentate ligand combinations [48[. The assembly ligand 14 /IS has been formed from the well-known TPPMS (14, monosulfonated triphenylphosphine sodium salt) and 3-(diphcnylphosphinyl)aniline hydrochloride (IS) by a simple ion-exchange reaction (Scheme 10.6). The coordination behavior ofthe ion-pair 14 /I S has been tested with various transition metal complexes. Other... 

Ionic Interactions as a Means to Form Heterobidentate Assembly Ligands 271 Table 10.5 Reaction of triphenylphosphine derivatives and Pt(l,5-cod)Cl2. ... 

In conclusion, the ionic bond as driving force for the formation of a heterobidentate bisphosphine seems a useful tool. Sulfonation of one ligand, and nitration followed by reduction to amine, may afford a relatively easy entry into this chemistry. Several... 

P.W.N.M. (2007) Ionic interaction as a powerful driving force for the formation of heterobidentate assembly ligands. Chem.-Eur. J., 13, 3424-3430. 

Although amino acids and related compounds frequently react in a monodentate fashion with bisperoxovanadate, they can react in a bidentate manner, but the products are often not bisperoxo complexes rather, one peroxide is eliminated in the condensation reaction. This is not necessarily true for all conditions, and bisperoxo-heterobidentate-ligand complexes are known, although in the solid state where a number of x-ray structures have been reported. In this case, one coordination site is an apical position, and a pentagonal bipyramidal product is formed, in the fashion of the oxalato [15] and picolinato (pyridine-2-carboxylato) [16] complexes, as represented diagrammatically in Scheme 6.3. There seems to be no detailed study of these and similar complexes in aqueous solution. Preliminary studies [17,18] of the... 

Another heterobidentate chiral ligand that has been used with success in catalysis is the phosphine QUINAP , 90. It is prepared using a palladium-catalysed coupling procedure, followed by a resolution of the racemic product using a chiral palladium salt (Scheme 32).58... 

COMPLEXES OF LIGANDS WITH MIXED DONOR SETS 3.7.8.1 Complexes of Heterobidentate Ligands... 

In this chapter, we focus on a more recent direction in asymmetric hydrogenation, the development of Ir catalysts based on heterobidentate ligands, which have considerably enhanced the substrate scope for this transformation. In particular, the major ligand classes and the underlying design principles as well as the special features and selected applications of these catalysts are discussed. We also dwell on recent mechanistic... 

Heterobidentate k -P,N ligands chemoselectivity and room temperature reactions... 

With Rh(I) salts heterobidentate ligands such as 3 form catalysts of greater activities than homobidentate ligands 4, for enantioselective hydrogenation of acrylate and cinnamate esters. The findings are rationalized in terms of conformational and allosteric effects of the substrates. ... 

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