Building molecular complexity through transition-metal-catalyzed oxidative annulations/cyclizations: Harnessing the utility of phenols, naphthols and 1,3-dicarbonyl compounds (original) (raw)

Template catalysis via non-transition metal complexes. New highly selective syntheses on phenol systems

Pure and Applied Chemistry, 1983

A methodological approach to conventional phenol chemistry based on the concepts and achievements of modern coordination chemistry allowed to find specific conditions for improving classical reactions and discovering new selective processes. Substrates, reagents, and ligands could be organized around non transition metal cations in suitable complexes, which were able to control the reactions and to determine highly selective ortho-attack on phenol systems. Template reactions of phenol substrates with carbonyl compounds in the presence of suitable ligands led to new general processes of ortho-formylation, ortho-acylation, ortho-alkylation and allylation. A rational route to all-ortho regular novolac resins was also disclosed. By using the basic strategy of Ziegler-Natta catalysis, the first synthesis of isotactic and syndiotactic all-ortho ethylidene-linked polyphenols was performed. Other synthetically useful Friedel-Crafts processes were performed according to an intramolecular template mechanism leading to new syntheses of important classes of oxygen heterocycles such as flavenes, chromanes, benzofuran derivatives, benzodioxins, and benzopyrylium salts. STRATEGY Activation of phenols, enols and indoles, which are typical ambident nucleophilic systems, is traditionally achieved by using strong bases which convert the substrates into the corresponding anions (phenolate, enolate, etc.), along with the use of dipolar aprotic solvents of high donicity (1) or phase transfer catalysis (2). In these conditions, the negative influence of the cation on reactivity is strongly reduced and, as a consequence, high activation of the substrate is usually observed. Moreover, predominant or exclusive functionalization at the more negative center (0, N) of the ambident system occurs (3). Reversing this general synthetic strategy, some years ago we focused our attention on the possible positive role of the cation and looked at these reactions in terms of coordinated processes in self-organized systems rather than of anion activation only (4).