Transition metals, heteroatom nucleophilic reactions

The transition metal catalysed nucleophilic attack of heteroatoms onto triple bonds has also been exploited in the preparation of larger rings. The intramolecular reaction of the phenol derivative shown in 5.12. and the pendant triple bond could result in the formation of a seven or an eight membered ring. Although the former system would be favoured by geometric considerations, the only product formed in the process is the benzoxazocin derivative, whose formation is probably driven by electronic factors.12... 

Haloiminium salts can react with metallates or similarly nucleophilic transition metal complexes to yield heteroatom-substituted carbene complexes (Figure 2.7) [120]. This reaction is closely related to the acylation of metallates with derivatives of carboxylic acids (Section 2.1.1.2). Examples are given in Table 2.5. 

The introduction of nucleophiles onto five membered heterocycles through non-catalyzed aromatic nucleophilic substitution is of little synthetic value, since the comparatively high electron density of the aromatic ring makes the nucleophilic attack unfavourable. The introduction of transition metal catalyzed carbon-heteroatom bond forming reactions overcame this difficulty and led to a rapid increase in the number of such transformations. 

The same transition metal systems which activate alkenes, alkadienes and alkynes to undergo nucleophilic attack by heteroatom nucleophiles also promote the reaction of carbon nucleophiles with these unsaturated compounds, and most of the chemistry in Scheme 1 in Section 3.1.2 of this volume is also applicable in these systems. However two additional problems which seriously limit the synthetic utility of these reactions are encountered with carbon nucleophiles. Most carbanions arc strong reducing agents, while many electrophilic metals such as palladium(II) are readily reduced. Thus, oxidative coupling of the carbanion, with concomitant reduction of the metal, is often encountered when carbon nucleophiles arc studied. In addition, catalytic cycles invariably require reoxidation of the metal used to activate the alkene [usually palladium(II)]. Since carbanions are more readily oxidized than are the metals used, catalysis of alkene, diene and alkyne alkylation has rarely been achieved. Thus, virtually all of the reactions discussed below require stoichiometric quantities of the transition metal, and are practical only when the ease of the transformation or the value of the product overcomes the inherent cost of using large amounts of often expensive transition metals. 

Carbenes, generated by several methods, are reactive intermediates and used for further reactions without isolation. Carbenes can also be stabilized by coordination to some transition metals and can be isolated as carbene complexes which have formal metal-to-carbon double bonds. They are classified, based on the reactivity of the carbene, as electrophilic heteroatom-stabilized carbenes (Fischer type), and nucleophilic methylene or alkylidene carbenes (Schrock type). 

Telomerization is defined as an oligomerization of dienes accompanied by addition of a heteroatom or carbon nucleophilic reagent10. It is catalyzed by various organometallic compounds of transition metals, especially palladium compounds. The nucleophiles, such as water, alcohols, amines or carboxylic acids, as well as enamines, nitroalkanes and stabilized carban-ions, are mainly introduced in the terminal position of the dimeric molecule in excellent yield10. It is also possible to direct the reaction towards an internal product functionalization. Telo-merizations with heteronucleophiles are regarded as heterocarborative addition reactions and are described in Section 1.5.8.4. 

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