Halide effect

Halide effects have been reported for the Cr reduction of Co(NH3)5L" complexes (where L = NH3, acetato or fumarato, FuH ) . Chloride exerts a stronger catalytic effect than bromide in all cases the second-order rate coefficient takes the form where k is pH-independent for L = NH3... 

Recently, Stair and coworkers [10, 11] developed a method to produce gas-phase methyl radicals, and used this to study reactions of methyl groups on Pt surfaces [12] and on molybdenum oxide thin films [13]. In this approach, methyl radicals are produced by pyrolysis of azomethane in a tubular reactor locat inside an ulttahigh vacuum chamber. This method avoids the complications of co-adsorbcd halide atoms, it allows higher covraages to be reached, and it allows tiie study of reactions on oxide and other surfaces that do not dissociate methyl halides effectively. 

Nitryl halides effect the nitration of aromatic substrates in the presence of Lewis acids, although competing halogenation is a side-reaction in some cases."" ... 

Although the initial report included amine nucleophiles, the scope was limited to activated amines such as indole (which actually undergoes C-alkylation at the 3-position), phthalimide, and 7/-methylaniline. Furthermore, enantioselectivities were inferior to those observed with alcohols as nucleophiles. Lautens and Fagnou subsequently discovered a profound halide effect in these reactions. The exchange of the chloride for an iodide on the rhodium catalyst resulted in an increased enantioselectivity that is now comparable to levels achieved with alcoholic nucleophiles ... 

Examination of the stereospecificity of the etherification indicated that the reaction was subject to a dramatic halide effect (Tab. 10.9). Treatment of enantiomerically enriched allylic carbonate (R)-53 (94% ee) under optimized conditions furnished the allyl ether (R)-54 in 84% yield (2° 1° >99 1), although with poor enantiospecificity (41% cee ... 

For a recent review on halide effects in transition-metal catalysis, see Fagnou, K. Lautens, M. Angew. Chem., Int. Ed. 2002,... 

For recent examples of halide effects on en-antioselectivity in transition metal-catalyzed allylic substitutions, see (a) Bartels, B. Helmchen, G. Chem. Commun. 1999, 741. (b) Burckhardt, U. Baumann, M. ... 

Electronic Defect States in Alkali Halides Effects of Interaction widi Molecular Ions By V. Dierolf 2003. 80 figs., XII, 196 pages... 

The presence of catalytic amounts of halide salts (e.g. BU4NI) was found to enhance dramatically the efficiency of hydroesterification of alkenes (e.g. cyclohexenone) and alkynes catalysed by [(CO)i2Ru3] in the presence of the chelating 2-pyridylmethyl formate (2-C5H4NCH2OCHO). On the basis of IR and NMR studies, the halide effect... 

K. Fagnou, M. Lautens, Halide Effects in Transition Metal Catalysis, Angew. Chem. Int. Ed. Engl. 2002, 41, 26-47. 

Zinc halides effectively accelerated cycloadditions with cyclopentadiene, but they had little influence on the stereoselectivity. Thus, (Z)-72 yielded only one adduct, endo(t)-74, whereas (E)-72 gave a mixture of the four possible diaste-reoisomers (Scheme 40). 

Other ligand effects may be interpreted by use of Figure 1 on the basis of ring-strain effects, cisjtrans geometry effects, halide effects, and so on, but will not be further discussed here. 

ECT model [50a-c] has also been proposed [50d-e] and theoretical studies have shown that covalent 7r-elfects and tr-effects, and electrostatic effects and the ionicity of the M X bond, might have a significant effect on trends in vco and other observed parameters. The physical manifestations resemble those expected on the basis of n donation, but the explanation may be much more complex it might more appropriate to discuss the halide effects in terms of an apparent n donor strength. 

Comparative Solvation of OH" by Water and the Hydrogen Halides. Effects of Acidity, J.Phys. Chem. 74,1483 (1970). 

Fagnou K, Lautens M (2002) Halide effects in transition metal catalysis. Angew Chem Int Ed 41 26-47... 

Lautens M, Fagnou K, Yang D (2003) Rhodium-catalyzed asymmetric ring opening reactions of oxabicyclic alkenes applications of halide effects in the development of a general process. J Am Chem Soc 125 14884-14892... 

Micellar and microemulsion effects on reactivity in aquation and base hydrolysis reactions of iron(II)-diimine complexes have been much studied/ The latest contribution deals with the effects of added potassium chloride or bromide to micelles of the respective cetyltrimethylammonium halides. Effects on base hydrolysis of [Fe(phen)3] and its 4,7-diphenyl and 3,4,7,8-tetramethyl derivatives can be interpreted in terms of competitive binding to the micelles in a pseudophase-ion exchange model. In connection with these secondary effects of added halides it should be mentioned that further studies of kinetics of aquation of [Fe(bipy)3] and of [Fe(phen)3] in strong aqueous solutions of chlorides have been interpreted in terms of water and of chloride attack, with the postulation of transient diimine-chloride-iron(II) intermediates. ... 

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