Nucleophilic attack reactions

The Cr(CO)3 unit in these compounds is strongly electron withdrawing and activates the ring to nucleophilic attack. Reactions with certain carbanions results in arylation.321... 

Compounds 260a-c have been studied with respect to their vulnerability to nucleophilic attack. Reactions were carried out between these compounds and pyridine, pyridine/water, DBU, and DBU/water (Scheme 25). The products formed were found to depend on the nucleophilicity and/or the basicity of the reagents used < 1998PS( 132)183>. 

Zerovalent transition metal carbonyl moieties may act as electron acceptors, and thus activate coordinated polyene ligands toward nucleophilic attack. Reaction of (C.411<5 )-Fe(CO)3 with KBHEt3 (—80 °C) proceeds via attack at a coordinated carbon monoxide to generate the anionic iron-formyl species 185 (Scheme 47)184. Upon warming to... 

These findings are compatible with a mechanism of intramolecular catalysis for both acyl migration and hydrolysis, as proposed in Fig. 8.5. Also, the possibility that both reactions share a common intermediate is emphasized. Reactions a and b in Fig. 8.5 involve a first step of deprotonation, in agreement with the observed specific base catalysis. Intramolecular nucleophilic attack (Reactions c and d) generates a tetrahedral intermediate that can result in acyl migration or hydrolysis (Reaction e). 

Fig. 8.5. Mechanism postulated for competitive, specific base catalyzed hydrolysis and acyl migration of catechol monoesters, as seen with 4-pivaloyl-L-dopa (8.81) [114a]. Deprotonation (Reactions a and b) accelerates intramolecular nucleophilic attack (Reactions c and d) to form a tetrahedral transition state. The latter is postulated to be the intermediate common to hydrolysis (Reaction e) and acyl migration. 

The coordinated ligands in Cp complexes (as well as other 71-complexes) are subject to reactions of various types, in particular nucleophilic attack reactions. 

A strong dependence on the leaving group is also noted in preparation of Cp(CO)3-MRe(CO)s (M = Mo, W) . These differing product mixtures result from three different mechanisms direct nucleophilic attack [reaction (f)], electron transfer [reaction (g)], and a two-electron transfer [reaction (h)] . Reaction of Fe(CO)4" with Re(CO)sBr gives the heterobimetallic complex ... 

This example shows that azinium cations generated from N-oxides are not always attacked by a nucleophile at C-a or C-y positions. In the above-mentioned reaction, it is the C-(3 position that is the preferential site for nucleophilic attack. Reactions of this type are usually accompanied by loss of the N-oxide function. The presence of a substituent able to stabilize the positively charged species (e.g., 4) seems to be an important feature in determining the site selectivity for these deoxidative nucleophilic substitutions (Scheme 6) (78JQC2514). Similar substitution reactions at C-3 (C-0) of the pyridine ring have also been observed in pyridine N-oxides (86H161). 

Pyrylium salts are prone to nucleophilic attack. Reaction of pyiylium salts 339 with ammonia or anunonium salts undergo ring-opening/ring-closing reaction sequences to afford the corresponding pyridine derivatives 340. This reaction was also called the Baeyer pyridine synthesis. ... 

All of the reactions so far studied which show these maxima have been classified as involving rate determining formation of the solvent separated ion pair followed by nucleophilic attack. Reactions going via free carbonium ions are much rarer, and although they undoubtedly exist, we have not measured a-d ef.-fects for any of them. 

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