Counterion alkyl-substituted

With a-alkyl-substituted chiral carbonyl compounds bearing an alkoxy group in the -position, the diastereoselectivity of nucleophilic addition reactions is influenced not only by steric factors, which can be described by the models of Cram and Felkin (see Section 1.3.1.1.), but also by a possible coordination of the nucleophile counterion with the /J-oxygen atom. Thus, coordination of the metal cation with the carbonyl oxygen and the /J-alkoxy substituent leads to a chelated transition state 1 which implies attack of the nucleophile from the least hindered side, opposite to the pseudoequatorial substituent R1. Therefore, the anb-diastereomer 2 should be formed in excess. With respect to the stereogenic center in the a-position, the predominant formation of the anft-diastereomer means that anti-Cram selectivity has occurred. 

An unusual solvent effect was observed in cycloadditions of aromatic nitrile N-oxides with alkyl-substituted p-benzoquinones in ethanol-water (60 40) the reaction rates were 14-fold greater than those in chloroform (148). The use of ion pairs to control nitrile oxide cycloadditions was demonstrated. A chiral auxiliary bearing an ionic group and an associated counterion provides enhanced selectivity in the cycloaddition the intramolecular salt effect controls the orientation of the... 

Alkyl-substituted pyrroles are not easy to synthesize and relatively few reports on poly (3-alky Ipyrrolejs have been published [213-216]. These polymers are much more conductive than the corresponding poly(A -substituted-pyrrole)s and, under some selected experimental conditions (propylene carbonate as solvent and PF,- as counterions), even more conductive (270 S cm ) than PP itself [215]. 

With the first reports on EDOT s special polymerization behavior yielding conductive polymers, several simple derivatives also were published simultaneously. Short-chain alkyl substitution at the dioxane ring does not change the EDOT and PEDOT properties very much. As is possible with EDOT-CH2OH, EDOT-CH3 can be polymerized oxidatively with PSS as the counterion in water. Conductivity is decreased, compared to... 

The effect of the nature of ion pairs as nucleophiles in a metal-catalysed substitution reaction has been investigated by determining product ratios for the Pd-catalysed allylic alkylations of substrates (9)-(ll) under various conditions, particularly with respect to catalyst ligands, nucleophiles, and counterions. Each dienyl acetate ionizes to form initially the vinyl (7r-allyl)-Pd intermediate corresponding most closely to the leaving group, i.e. (12) from (9), (13) from (11), but (12) and (13) from (10). The initial intermediate can then either be trapped by the nucleophile or it can begin to equilibrate to some mixture of vinyl 7r-allyl intermediates. If nucleophilic addition occurs before full equilibration, the product ratio is different for each substrate if... 

Alkylation, acylation, etc. at the heteroatom lead to substitution of an NH or to onium salts. Three-membered ring onium salts are difficult to isolate, and very weakly nucleophilic counterions must be... 

Reed s group reported the X-ray structures of trialkyl-silicenium ions R3ST6 complexed to low-nucleophilic hexahalo carborane anions90,91. The authors varied systematically both the alkyl substituents R (R = Me, Et, z -Pr and t-Buj and the halogen substitution of the XgCBnlL (X = Cl, Br, I) counterions. The degree of silicenium ion character in the... 

Electrophilic substitutions.1 The anion 1 reacts with water and R3SiCl exclusively at the y-position. Alkylation is less selective both positions are attacked, but the y-position is usually preferred. The selectivity is not altered by addition of TMEDA or HMPT or by change of the counterion to K+ or Zn2 +. The y-silylated products provide a route to vinyl ketones (equation I). 

Metal-14 anions react with alkyl halides (RX) mostly by nucleophilic substitution (Sn2), the stereochemistry of which is dependent on the structure of R and X, the solvent and the nature of the counterion. Other reactions were also observed nucleophilic substitution at halogen [also called halogen/metal exchange (HME)] and single electron processes. In some cases steric hindrance around the reactant results in elimination. 

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