Bromide trans effect

Surfactants greatly improve the performance of trans-cinnamaldehyde as a corrosion inhibitor for steel in HCl [741,1590,1591]. They act by enhancing the adsorption at the surface. Increased solubility or dispersibility of the inhibitor is an incidental effect. N-dodecylpyridinium bromide is effective in this aspect far below its critical micelle concentration, probably as a result of electrostatic adsorption of the monomeric form of N-dodecylpyridinium bromide. This leads to the formation of a hydrophobic monolayer, which attracts the inhibitor. On the other hand, an ethoxylated nonylphenol, a nonionic surfactant, acts by incorporating the inhibitor into micelles, which themselves adsorb on the steel surface and facilitate the adsorption of trans-cinnamaldehyde. 

The syntheses of iron isonitrile complexes and the reactions of these complexes are reviewed. Nucleophilic reagents polymerize iron isonitrile complexes, displace the isonitrile ligand from the complex, or are alkylated by the complexes. Nitration, sulfonation, alkylation, and bromina-tion of the aromatic rings in a benzyl isonitrile complex are very rapid and the substituent is introduced mainly in the para position. The cyano group in cyanopentakis(benzyl isonitrile)-iron(ll) bromide exhibits a weak "trans" effect-With formaldehyde in sulfuric acid, benzyl isonitrile complexes yield polymeric compositions. One such composition contains an ethane linkage, suggesting dimerization of the transitory benzyl radicals. Measurements of the conductivities of benzyl isonitrile iron complexes indicate a wide range of A f (1.26 e.v.) and o-o (1023 ohm-1 cm.—1) but no definite relationship between the reactivities of these complexes and their conductivities. 

If the counterion (X) in the oxidative addition complex is iodide or bromide (and no thallium or silver salts are present) the dissociation of one of the phosphorus atoms in the bidentate ligand from the metal is probably attributed to the relatively high trans effect exerted by the halidesJ This reversible displacement facilitates formation of a neutral rr-complex, in which the rr-system of the electron-rich alkene is only weakly polarized. Therefore, after insertion and hydridopalladium halide elimination, a larger fraction of /3-arylated product is formed, since steric factors always favor terminal ary-lation. 

The role of protonic acids in the stepwise oxidative addition of five-coordinate complexes is well illustrated by reactions of acids with the osmium(O) complex (XXXV). Hexafluorophosphonic and tetrafluoroboric acids form stable adducts (XXXVla) and (XXXVIb) which have been characterized as 1 1 electrolytes (39, 103). Similar cationic adducts (XXXVIc) have been formed by reaction of Os(CO)3(PPh3)2 with dry HCl and HBr gas [103). The osmium(II) hydrido cation (XXXVI) possesses trans CO groups (marked with an asterisk) which are mutually labilized through the trans effect. In the presence of a nucleophilic conjugate anion such as bromide one of these CO groups is replaced to yield a neutral adduct... 

The selective C-3 arylation of unprotected p3uidines with aryl halides took place when a Pd(OAc)2/phenanthroline system was employed (eq 144). It was necessary to use excess (50 equiv) pyridine in the reaction to obtain synthetically useful yields. Aryl bromides and iodides underwent the coupling, but aryl chlorides were found unreactive. The selectivity for the C-3 position was explained through a trans effect . The use of aryl tosylates for functionalization of the C-3 has also been disclosed under similar reaction conditions. ... 

The hydrosulfide ligand in [M(CO)5SH] (M = Cr or W) is a good cis labilizer, better than bromide and slightly weaker than acetate. The trans effect for the dissociative ligand substitution shown in equation (8) spans... 

The chemoselectivity of the other alkenes of Table 1 is more variable. It appears that bulky substituents favour bromide over methanol attack of the bromonium ion, since dibromlde increases from 39 to 70 % on going from methyl to tert-butyl in the monosubstituted series. The same trend is observed in the disubstituted series with a contraction of the chemoselectivity span (37 to 43 % on going from methyl to teH-butyl) for the trans isomers. Since the solvated bromide ion can be viewed as a nucleophile larger than methanol, the influence of steric effects, important in determining the regioselectivity, does not seem very significant as regards the chemoselectivity. This result has been interpreted in terms of a different balance between polar and steric effects of the substituents on these two selectivities. 

O-as- S p i ro p h os p h or a n e s 69, which display anti-apicophilicty, exhibit enhanced reactivity toward nucleophiles compared to 0-/ra r-spirophosphoranes 70 for example 69 produces hexacoordinate 157 on treatment with MeLi at 0°C while 70 is unreactive under the same conditions (Scheme 16). Compound 69 (R = Bn) is more reactive toward bases than 70 (R = Bn) with KHMDS deprotonating 69 (R = Bn) in THF at 0 °C but not 70 (R= Bn). -BuLi was sufficiently basic to deprotonate both 69 and 70 <2002JA13154>. Further differences are observed in the stability and stereochemistry of the brominated products formed between the reaction of the anion with BrCF2CF2Br. The effect of the bromide substituent in the O-m-derivative is such that it accelerates pseudorotation to the O-trans-isomer (Scheme 17). 

H. Chaimovich, V. R. Correia, P. S. Araujo, M. V. Aleixo, I. M. Cuccovia, A Quantitative Analysis of the Effect of Hexadecyl-Trimethylammonium Bromide Micelles on the Rate of Alkaline Hydrolysis of Benzylpenicillin , J. Chem. Soc., Perkin Trans. 2 1985, 925-928. 

---