Publications

The dearomatization of naphthalene and anthracene is explored by their η² coordination to {TpMo(NO)(MeIm)} and {TpMo(NO)(DMAP)} (where Tp = hydridotris(pyrazolyl)borate, MeIm = 1-methylimidazole, and DMAP = 4-(dimethylamino)pyridine). The DMAP and MeIm complexes have nearly identical redox properties and abilities to bind these polycyclic aromatic hydrocarbons (PAHs), but unlike MeIm, the DMAP ligand can be protonated at N while remaining bound to the metal. This action enhances the π-acidic properties of DMAP, resulting in greater stability of the molybdenum toward oxidation by acid. Utilizing this feature of the DMAP ligand, several new 1,2-dihydronaphthalenes and 1,2-dihydroanthracenes were prepared. Furthermore, it was found that acetals and Michael acceptors could function as electrophiles for the PAHs using the DMAP system, resulting in several new mono- and 1,4-dialkylated products.

This review covers publications ranging from 2005 to 2017 concerning the organic reactions of aromatic ligands η²-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., η⁶-arene complexes).

The transiently stable indoline complex TpW(NO)(PMe₃)(η²-N-ethyl-6,7-indoline) is shown to be a synthon for a broad range of novel hexahydroindoles. The tungsten dearomatizes the indoline, allowing for its sequential protonation, electrophilic addition (e.g., protonation, fluorination, hydroxylation, acylation), nucleophilic addition (e.g., amination, arylation, etheration, alkylation), and reduction, all with well-defined and predictable stereochemistry. The tungsten stereocenter directly governs the stereoselective formation of four adjacent asymmetric carbons originating from the aromatic ring of the indoline.

Polycyclic aromatic hydrocarbons (PAHs; e.g., naphthalene and anthracene) form stable η²-bound complexes with the dearomatizing fragment {TpMo(NO)(MeIm)} (where Tp = hydridotris(pyrazolyl)borate; MeIm = 1-methylimidazole). These complexes undergo protonation at the α carbon followed by regioselective nucleophilic addition at the adjacent β carbon. The nucleophile (a pyrrole or an enolate) adds stereoselectively, anti to the face of metal coordination. The resulting 1,2-dihydroarene ligand may be isolated via metal oxidation by iodine to provide the free 1,2-dihydroarene in moderate yield (∼60%), as well as TpMo(NO)(MeIm)(I), the precursor of the original PAH complex (∼80–90%). Thus, a formal catalytic cycle for the dearomatization of naphthalene and anthracene has been demonstrated.

A method is described for the resolution of the versatile dearomatization reagent TpW(NO)(PMe₃)(η²-benzene), in which the 1,3-dimethoxybenzene (DMB) analogue of this complex is synthesized. In turn, the coordinated arene of TpW(NO)(PMe₃)(DMB) is protonated with either d or l dibenzoyl tartaric acid (DBTH₂) in a butanone/water or 2-pentanone/water solution. Sustained stirring of this mixture results in the selective precipitation of a single form of the diastereomeric salt [TpW(NO)(PMe₃)(DMBH)](DBTH). After isolation, the salt can be redissolved, and the DMB ligand can be deprotonated and exchanged for benzene to produce the desired product TpW(NO)(PMe₃)(η²-benzene) in either its R or S form. The absolute configuration of the tungsten stereocenter in TpW(NO)(PMe₃)(η²-benzene) can be determined in either case by substituting the naturally occurring terpene (S)-β-pinene for benzene and evaluating the 2D NMR spectrum of the corresponding β-pinene complex.

Indoline and tetrahydroquinoline derivatives form η²-bound complexes with the dearomatization agent {TpW(NO)(PMe₃)} that can be isolated as triflic acid salts. Protonation occurs selectively in the aromatic ring, either ortho or para to the nitrogen. With the tetrahydroquinoline complexes, para protonation dominates. For the indoline ligands, alkylation of the nitrogen dramatically affects the ortho/para isomer ratio present after protonation. In the case of the N-ethylindoline and N-isopropylindoline ligands, the bridgehead-protonated isomer (ortho) is formed with >10/1 selectivity. This indolinium isomer is found to undergo acid-catalyzed hydroarylation or hydroamination with various heterocycles. Oxidative decomplexation is demonstrated for the pyrazole and 2-methylfuran derivatives using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). The resulting tetrahydroindolines feature three new stereocenters determined by the configuration of the tungsten complex.

Naphthalene, when bound to the complex {TpW(NO)(PMe₃)}, is shown to be activated toward Diels–Alder cycloaddition in the bound ring with N-methylmaleimide (NMM). In its most stable form, the tungsten in this complex binds C1 and C2. However, the cyclization reaction is thought to proceed through an intermediate in which the metal binds C2 and C3, rendering the uncoordinated portion of the hydrocarbon similar to a quinodimethane. The reaction rates and regio- and stereochemistry are investigated for a series of substituted naphthalene ligands, and these observations support the proposed mechanism. An X-ray structure of the complex TpW(NO)(PMe₃)(η²-L), where L is the cycloadduct of NMM and 1,4-dimethylnaphthalene, confirms the endo stereochemistry of the cycloaddition.

The complexes TpW(NO)(PMe₃)(L), where L = phenol, N,N-dimethylanilinium, or naphthalene, undergo protonation followed by addition of an aromatic nucleophile. The addition of aromatic molecules occurs at the para carbon of the phenol or aniline ring or the beta carbon of the naphthalene. The addition occurs anti to the metal fragment, as determined by X-ray crystallography. In the case where L = phenol or N,N-dimethylanilinium, treatment of the bound arene with an electrophilic heteroatom followed by an aromatic nucleophile sets two stereocenters, with both additions occurring anti to the metal. The resultant ligands have been removed from the metal by oxidative decomplexation using ceric ammonium nitrate.

The attachment of a tungsten complex to various aromatic molecules across two carbons localizes the remaining p system in the molecules and renders them highly activated toward electrophilic reagents. As a result, new bonds are formed between the electrophile and the aromatic scaffold, giving rise to stereogenic centers with predictable configurations. Treatment of the resulting adducts with various oxidants frees the organic product from the tungsten fragment. This approach permits the synthesis of a number of small molecules that are difficult to prepare by conventional methods.