Group 6 Dihapto-Coordinate Dearomatization Agents for Organic Synthesis

Abstract

This review covers publications ranging from 2005 to 2017 concerning the organic reactions of aromatic ligands η-coordinated to tungsten or molybdenum and the use of these reactions in the synthesis of novel organic substances. An emphasis is placed on C-C bond-forming reactions using conventional building blocks of organic synthesis such as acetals, enolates, Michael acceptors, acylating reagents, and activated aromatics. Substrates activated by the metal include arenes, pyridines, pyrroles, pyrimidines, furans, and thiophenes. General reactivity patterns are elucidated, as well as stereochemical preferences. These trends are compared to those of osmium and rhenium forebears as well as to the reactivity patterns of other methods of stoichiometric transition-metal-based dearomatization (i.e., η-arene complexes).

Palladium-Catalyzed Asymmetric Domino Heck/Carbocyclization/Suzuki Reaction: A Dearomatization of Nonactivated Naphthalenes

“…Dearomatization is an important reaction for the transformations of readily available planar aromatic compounds into complex 3-dimentional structures such as spirocycles, polycycles and bridged-cycles. [1][2][3][4][5] Recently, transition-metal-catalyzed dearomatization reaction of furans, pyrroles, indoles, pyridines, phenols, anilines, and naphthols, have been developed as powerful strategies to construct complex structures, [6][7][8][9][10][11][12][13][14][15] and catalytic asymmetric dearomatization has becoming an emerging concept in organic synthesis. 16 However, due to the inherent stability and high resonance energy of nonactivated arenes, the dearomative transformation of the nonactivated arenes remains an important challenge.…”

“…16 However, due to the inherent stability and high resonance energy of nonactivated arenes, the dearomative transformation of the nonactivated arenes remains an important challenge. 1,17 Over the past years, several approaches, including high pressures hydrogenations of arenes, [18][19][20] photocycloadditions of arenes and alkenes, 3 cyclopropanations of arenes and diazo compounds, 21,22 stoichiometric reactions of transition-metalarene complexes, 2,23 and visible-light-mediated 1,4-and 1,2-cycloadditions of arenes with arenophiles 1,24,25 have been developed for the dearomatization of nonactivated arenes. Nevertheless, new protocols, especially for enantioselective protocols for the dearomatization of nonactivated benzene or naphthalene remain highly desirable.…”

Regiochemistry Control by Bipyridine Substituents in the Deprotonation of ReI and MoIIN‐Alkylimidazole Complexes

“…Formally related are alkyl transfers from f‐block metals to pyridines. In the overwhelmingly large majority of their transition metal complexes, pyridine donors are κ 1 ‐ N coordinated . Outer‐sphere nucleophilic attack on coordinated pyridines in these species has been proposed long ago by Gillard and Lyons, but the few uncontroversial examples required hydrothermal conditions…”

Palladium‐Catalyzed Dearomative syn‐1,4‐Oxyamination

“…[5] Such processes are particularly challenging as the loss of aromatic stabilization during the functionalization event yields unsaturated products that are more reactive than their parent arenes (Figure 1A). [7] While powerful, this methodology often requires metal complexes that are toxic or expensive. An elegant strategy to control the reactivity of intermediates during the dearomative functionalization is the stoichiometric use of transition metals.…”