Binding ring rhodium complexes

However, because the rhodium complexes of tetraphosphine ligand 4 behave differently than the rhodium complex of Josiphos ligand 3 for both substrates, we postulate that either a) different rhodium complexes are being formed (different binding modes as discussed above, see Fig. 2) by 3 and 4, or b) the substituents on the second cyclopentadiene (cp) ring influence the catalytically active site in the rhodium complexes of 4. 

Catalysis and Binding by 5-Ring Chelate Biphosphine Rhodium Complexes... 

Binding of Enamides and Catalysis by 6-Ring Chelate Rhodium Complexes... 

The proposed catalytic cycle is outlined in Scheme 9.9 [14]. Dimeric complex 23 is cleaved to give the monomeric complex 24 by solvation, substrate binding, or reaction with the nucleophile. Reversible exo-coordination of the substrate is followed by oxidative insertion with retention into a bridgehead C-O bond to give the 7r-aUyl or u-rho-dium aUcoxide complexes 26 or 27. It is likely that the formation of these rhodium] 111) aUcoxide complexes is irreversible due to the release of the ring strain present in the oxabicyclic aUcene substrate. The oxidative cleavage of the C-O bond is proposed to be the enantiodiscriminating step in the catalytic cycle. 

Halpern has made some significant observations in the chemistry of the cationic rhodium catalysts. Because much of this relates to asymmetric induction, it is treated in Chapter 4. It is appropriate to mention here that the catalyst formed from [Rh cod) dpe)]A dpe = 1,2-diphenylphosphino-ethane) has been shown to dimerize reversibly in the absence of substrate. An aromatic ring from one of the dpe ligands of the first metal binds in an T7 -fashion to the second metal and vice versa The system is labile, in contrast to the nonlability of simple arene complexes of Ir(I). 

Tryptophan offers an indole side chain that can be used for ligation chemistry. A water-compatible rhodium carbene can be added to the indole ring (19) [105,139]. The reactive species is generated in situ by a conjugated diazo compound by a rhodium catalyst like rhodium(II) acetate [63,139,149]. The reaction takes place in the two- and three-position of indole. Thus, a mixture of N-alkylated and C-alkylated product is obtained. It is necessary to add hydroxylamine hydrochloride as an additive to bind to the distal rhodium carbenoid complex. The usage of this salt lowers the pH value below 3.5 and therefore limits the scope of this methodology. As a side reaction, the carbene inserts into the O-H bond of water (Table 6). 

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