Ligands halide

This synthesis is possible with other halide ligands... 

A model consistent with the stereochemical outcome of the reaction and with the role of the halide has been proposed (Fig. 10). This model suggests side-on olefin binding and reorganization of the halide ligands. In such geometry, a steric interaction between the unbound olefin and apical halide may justify the dramatic increase in enantioselectivity observed upon changing the halide from cr to r. 

L = neutral monodentate ligand L-L = neutral bidentate ligand X = halide ligand Ar = aromatic ligand Cp = cyclopentadiene. 

Subsequently, these catalysts were evaluated in the enantioselective desymmetri-sation of achiral trienes, and three distinct trends in catalyst selectivity were found. Firstly, catalysts 56a-b with two phenyl moieties on the backbone of the A -heterocycle exhibited higher enantioselectivity than those with a fused cyclohexyl group as the backbone 55a-b. Secondly, mono-ort/io-substituted aryl side chains induced greater enantioselectivity than symmetrical mesityl wing tips. Thirdly, changing the halide ligands from Cl to I" increased the enantioselectivity. As a result, catalyst 56b turned out to be the most effective. For example, 56b in the presence of Nal was able to promote the desymmetrisation of 57 to give chiral dihydrofuran 58 in up to 82% conversion and 90% ee (Scheme 3.3). 

Both Ni and Pd reactions are proposed to proceed via the general catalytic pathway shown in Scheme 8.1. Following the oxidative addition of a carbon-halogen bond to a coordinatively unsaturated zero valent metal centre (invariably formed in situ), displacement of the halide ligand by alkoxide and subsequent P-hydride elimination affords a Ni(II)/Pd(ll) aryl-hydride complex, which reductively eliminates the dehalogenated product and regenerates M(0)(NHC). ... 

Iridium(IV) complexes are less common when compared to Ir111 or Ir1. There are examples of N, P, As, Sb, O, S, Se, Te, and halide ligands, often prepared by oxidation from an Ir111 salt. Recent developments (in 1991) in the chemistry of the platinum metals in high oxidation states were addressed by Levason, including complexes of IrIV and Irv.25... 

For halide ligands, a coordination number of four (/u4-X) is rare. Self-assembly of ds metal centers and halides around a pyramidal halide gives tetrapalladium complexes. In this unprecedented case the metallamacrocycle owes its formation to the halide acting as a template.347 The complexes (PPN)[Pd4(Fmes)4] (Fmes = 2,4,6-tris(trisfluoromethyl)phenyl) display fluxional behavior in solution in noncoordinating solvents.347... 

Another general procedure to prepare gold(II) complexes consists of substitution reactions on gold(II) derivatives. Halide ligands can be substituted by neutral donor ligands2025-2032 such as... 

Ligand Substitutions in Alkyl Halides Ligand substitutions in alkyl halides,... 

Katsuki et al. have reported that the CoIII(salen) ((98) X = I, Y = t-Bu) bearing an apical halide ligand shows high trara-selectivity in the cyclopropanation of styrene and its derivatives, albeit with moderate enantioselectivity (Scheme 71).267 The enantioselectivity is influenced, however, by the natures of the apical ligand and the 5,5 -substituents, and high enantio- and traMs-selectivity has been realized by their appropriate tuning ((98) X = Br, Y = OMe).268 It is noteworthy that the CoIII(salen) complex bearing substituents at C3 and C3 shows no catalytic activity. 

The chloride or, generally, halide ligands X in compounds shown in Scheme 39 can readily be replaced by other anionic or neutral ligands. Treatment with AgC104 in the presence of donors L like R3P, tht, or even H20 leads to... 

In general the stability of hexahalo complexes of the transition metals tends to decrease appreciably with increasing atomic number and with increasing size of the halide ligand. (See Ref. (7) for discussion of the 3d situation.) Thus for example in the 4 d series MF - species are known for Mo, Ru, Rh, and Ag, but MCI - ions have been reported only for the first three of these metals. Similarly in the 5 d series MFelements from Ta to Au, but MCI 6 species have been reported only as far as WCle, whilst MFg compounds extend from W to Pt, but MCle systems only up to ReClg. 

R. J. Puddephatt No. Nobody has prepared such complexes and the synthesis is not trivial. Substitution of halide ligands in octahedral platinum(IV) derivatives is typically very slow, and a better route (suggested by J. K. Kochi) might involve oxidation of platinum(II) metallacyclobutanes with peroxides. It would certainly be worthwhile to attempt this synthesis in view of the promise of enhanced reactivity. 

These studies demonstrate that two alkoxide ligands can take the place of an oxo ligand and that aluminum halides, by coordinating to a halide ligand, can generate an efficient and long-lived catalyst. It is possible that [W(CHR)(0R)2(Br)]+AlBr4- is responsible for the catalytic activity, but at low concentrations, and in the... 

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