Trans-chelator

In the ligand l,2-bis-(2-pyridylethynyl)benzene the pyridyl N atoms easily attain the appropriate separation for trans-chelation of metal cations. The 1 1 complex of the ligand with palladium(II) chloride has been structurally characterized.171,182... 

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]). 

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-... 

The trap (trans-chelating phosphines) ligands developed by Ito and co-workers [33] form nine-membered metallocycles where trans-chelation is possible. However, it is not clear whether the cis isomer which has been shown to be present in small amounts or the major trans isomer is responsible for the catalytic activ-... 

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

The cationic aqua complexes of the C2-symmetric trans-chelating tridentate ligand 447 proved also highly effective chiral catalysts. The complexes involving the metal(II) perchlorates of iron, cobalt, nickel, copper and zinc produced the main endo adduct of cyclopentadiene and N - aery loy 1-1,3 -oxazo I i din -2 -one with very high ee values281. 

Kuwano, R., Uemura, T., Saitoh, M. and Ito, Y., A trans-chelating bisphosphine possessing only planar chirality and its application to catalytic asymmetric reactions. Tetrahedron Asymm.flWSA,... 

The first such ligand system (14) was developed in Burgess s group and was used in the synthesis of the palladium(II) complex 15, in which the bis-carbene acts as a trans-chelating ligand (Scheme 13) [38]. However, there are no reports of the use of 15 in Pd-catalyzed reactions. 

The trans geometry is confirmed in solution for 65b-67b by the appearance of a singlet for the methylene protons in the NMR spectra due to C2h symmetry of the trans chelates. 

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

With long chain bis(phosphines) (n = 4 - 16), trans chelate rings become possible, although with the longest chains bridged dinuclear species would be expected. 

Low-valent Ru(II) [150] and Rh(I) complexes catalyze aldol and Michael reactions of 2-nitrilo esters. The sequence is thought to be initiated by nitrile complexation to the transition metal. This Lewis acid-activation is followed by an oxidative addition to give a metal hydride and a nitrile complexed enolate as shown in Sch. 36. Examples including diastereoselective Ru(II) catalyzed reactions [151] and enantioselective Rh(I)-catalyzed reactions [152-154] with the large trans-chelating chiral ligand PhTRAP are shown in Tables 8 and 9. 

Biomimetic oxidation catalysis has largely focused on complexes with planar tetradentate ligands such as manganese or iron porphyrins and related macrocyclic trans-chelates[5]. There is considerable interest in the synthesis of multinuclear metal complexes since these molecules might be useful as building block for magnetic molecular materials[6] and model compounds for understanding the properties of metalloproteins[7]. 

A chiral bisphosphine containing two ferrocene molecules has been prepared using Ugi s lithiation as a key step (Scheme 2-9) [20], Thus, the diphenylphosphinyl group is introduced at the ferrocenylmethyl position of the iodide obtained from lithiated ferrocene 2. Oxidative coupling of the iodoferrocene followed by reduction of the phosphine oxide gives the bisferrocene-bisphosphine 15. The bisphosphine is unique in that a trans-chelate is formed on its coordination to a metal. 

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