Trans-chelating diphosphines

Excellent ee-values (up to 94%) have been obtained for the hydrogenation of various 2-substituted N-acetyl indoles with an in-situ prepared rhodium catalyst modified with the trans-chelating diphosphine (S,S)-(R,R)-2,2"-bis[l-(diphenyl-phosphino)ethyl]-1,1"-biferrocene (Scheme 16.23) [88]. A strong base was required as co-reagent to observe both high conversion (TOFs of 50-100) and en-... 

Scheme 8 Rhodium complexes of trans-chelating diphosphine ligands...

In this fashion the metal complex should resemble the trans chelating diphosphine 1. (31)... 

The complete inhibition of CH3I oxidative addition observed in the case of trans chelating diphosphines complexes has been attributed to the fact that one of the axial sites of the rhodium center is blocked by the ligand backbone, avoiding the CH3I addition to proceed (Figure 1.3). 

The proposed catalytic cycle is analogous to that for the Monsanto process, but it involves neutral species with a trans-chelating diphosphine. Presumably, the isolated dinuclear compounds 62 a and 62 b act as resting states for the ace-... 

At an early stage, only for diphosphine ligands capable of trans-coordination were detected hydride solvate intermediates [71], but a catalytic pathway based on diphosphine trans-chelation has not been developed to date (see [80b]). 

Jimenez-Rodriguez, C., Roca, F.X., Bo, C, Benet-Buchholz, )., Escudero-Adan, E.C., Ereixa, Z. and van Leeuwen, P.W.N.M. (2006) SPANphos trans-sparming diphosphines as cis chelating ligands . Dalton Trans., 268-278. 

The complexes ci -[PtCl(SnCl3)P2] may be obtained starting from either a cis-bis-phosphine complex of a tertiary aryl phosphine or a chelating diphosphine, e.g. PPh3 or DIOP complexes (20), (although careful examination of the P-31 spectra of the monodentate systems suggests that the trans isomer is also present). 

Aliphatic phosphines add to CrX2 (X = Cl, Br), yielding adducts of uncertain composition, for example [Cr(PR3) X2] (n = 1-3). With chelating diphosphines, for example dmpe, well-characterized six-coordinate complexes have been prepared. The latter have low-spin electronic confignrations. A spin-crossover situation has been found in the complex [Cr(depe)2l2] (depe = l,2-bis(diethylphospino)ethane). At ca. 170 K, this trans octahedral complex exhibits a discontin-nous spin transition from S = I (2.8 /u-b) to S = 2 (4.8 /xb). 

Oxidative addition of H-H, C-H, and other a bonds to a transition-metal complex is one of the most widely studied reactions in organotransi-tion-metal chemistry both experimentally and theoretically [1-9]. In the oxidative addition of H2 to Pt(PR3)2, almost all hydrides assume a trans position except for the cases where the ligands are a chelating diphosphine... 

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