Oxygen nucleophiles substitution reactions

Ab initio calculations suggested that the oxygen-substituted compounds 40 and 41 should be much better candidates for nucleophilic substitution reactions that follow pathway b than the halo salts belonging to the 7a-7i family (Scheme 9). Tire properties of the CCls-substituted cation of 43 should reveal more similarities with those of 7a than with those of 40 and 41. 

For carbon-carbon bond-formation purposes, S 2 nucleophilic substitutions are frequently used. Simple S 2 nucleophilic substitution reactions are generally slower in aqueous conditions than in aprotic organic solvents. This has been attributed to the solvation of nucleophiles in water. As previously mentioned in Section 5.2, Breslow and co-workers have found that cosolvents such as ethanol increase the solubility of hydrophobic molecules in water and provide interesting results for nucleophilic substitutions (Scheme 6.1). In alkylations of phenoxide ions by benzylic chlorides, S/y2 substitutions can occur both at the phenoxide oxygen and at the ortho and para positions of the ring. In fact, carbon alkylation occurs in water but not in nonpolar organic solvents and it is observed only when the phenoxide has at least one methyl substituent ortho, meta, or para). The effects of phenol substituents and of cosolvents on the rates of the competing alkylation processes... 

Biirgi studied also a series of five coordinated cadmium complexes, 38, that contain three equatorial sulfur ligands, but in which the fourth and fifth, axial ligands, X and Y, are sometimes iodine, sometimes sulfur, and sometimes oxygen (84). The structural correlations have a clear interpretation in terms of the ligand exchange reaction and are reminiscent of the kind of process that is believed to occur in S 2-type nucleophilic substitution reactions ... 

We have examined the competing isomerization and solvolysis reactions of 1-4-(methylphenyl)ethyl pentafluorobenzoate with two goals in mind (1) We wanted to use the increased sensitivity of modern analytical methods to extend oxygen-18 scrambling studies to mostly aqueous solutions, where we have obtained extensive data for nucleophilic substitution reactions of 1-phenylethyl derivatives. (2) We were interested in comparing the first-order rate constant for internal return of a carbocation-carboxylate anion pair with the corresponding second-order rate constant for the bimolecular combination of the same carbocation with a carboxylate anion, in order to examine the effect of aqueous solvation of free carboxylate anions on their reactivity toward addition to carbocations. 

The first step in the addition of alkoxides to Cgg is, consistently, the formation of the alkoxy Cgg anion. The subsequent process is strongly dependent on the presence of oxygen. In the presence of oxygen, 1,3-dioxolane derivatives of Cgg are formed [110]. In the absence of oxygen the oligo alkoxy fullerenide anions can be formed [111, 112]. Reaction of alkoxides with Cgg usually results in complex mixtures. This may be why only a few reactions of Cgg with alkoxides have been described [113]. Nevertheless, defined alkoxy fullerenes can be obtained by nucleophilic substitution reactions of alkoxides with halogenofullerenes (Chapter 9) [113]. 

The hydroxyl (—OH) group in alcohol is polarized due to the electronegativity difference between atoms. The oxygen of the —OH group can react as either a base or a nucleophile in the nucleophilic substitution reactions. 

The regioselectivity in the methoxydefluorination reactions of difluoroquinolines has been examined.48 Studies of the substitution patterns in 2-cyano-3-nitroimidazole[l,2-a lpyridine have shown that nitrogen and oxygen nucleophiles substitute the 2-cyano group whereas sulfur nucleophiles replace the 3-nitro group.49... 

According to kinetic measurements (21), this reaction proceeds as a nucleophilic substitution reaction with sulfur acting as EPD and apparently involves an electron shift from the more negative to the more positive part of the hypochlorite ion. Formation of a coordinate bond between sulfur and oxygen has been demonstrated using relabeled 0C1-. 

Alkyl halides undergo nucleophilic substitution reactions by unimolecular (S l) and bimolecular (S 2) pathways. The substitution reactions of alkyl halides, and of derivatives of alcohols, utilize oxygen, nitrogen, sulfur and carbon nucleophiles. 

Sodio-organophosphine reagents have also found considerable use in the past years. Aminyl radicals, R2N, are involved in the photo-assisted radical-nucleophilic substitution reactions between sodium diphenylphosphide and N-cyclopropyl-A-ethyl-/ -toluenesulfonamide in liquid ammonia, which after oxygenation, leads to the aminoalkyldiphosphine dioxide (80) as the principal product. The reactions of sodio-organophosphide reagents with chloroalkyl... 

Nucleophilic substitution reactions are limited to exchange of substituents. Bromo- and chloro-l,2,3-triazines are substituted by oxygen, sulfur, nitrogen, and carbon nucleophiles (84M15). These reactions occur preferentially at the 4-position. There are no recent examples. 

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