The effects of an electron-withdrawing group on the organic chemistry of an η-bound benzene ring are explored using the complex TpW(NO)(PMe)(η-PhCF). This trifluorotoluene complex was found to undergo a highly regio- and stereoselective 1,2-addition reaction involving protonation of an ortho carbon followed by addition of a carbon nucleophile. The resulting 1,3-diene complexes can undergo a second protonation and nucleophilic addition with a range of nucleophiles including hydrides, amines, cyanide, and protected enolates. Interestingly, the addition of the second proton and nucleophile occurs in a 1,4-fashion, again with a high degree of regio- and stereocontrol. Oxidation of the metal allows for the isolation of highly substituted trifluoromethylcyclohexenes with as many as four stereocenters set by the metal. The ability to synthesize enantio-enriched organics was also demonstrated for a diene and a trisubstituted cyclohexene. Substitution from an enantio-enriched η-dimethoxybenzene complex in neat trifluorotoluene yielded enantio-enriched trifluorotoluene complex, which was elaborated into cyclohexadienes and cyclohexenes with ee's ranging from 92 to 99%.

The preparation of the complexes TpMo(NO)(DMAP)(η-PhCF) (5) and TpMo(NO)(DMAP)(η-benzene) (3) is described. The CF group is found to stabilize the metal-arene bond strength in 5 by roughly 3 kcal/mol compared to that in 3, allowing the large-scale synthesis and isolation of the trifluorotoluene analogue (5, 37 g, 70%). When a benzene solution of 5 is allowed to stand, clean conversion to the benzene analogue 3 occurs, and this complex may be precipitated from solution upon the addition of pentane and isolated. The trifluorotoluene complex is shown to be a synthetic precursor to functionalized cyclohexadienes: In solution, it selectively protonates at the ortho position, and the resulting η-arenium species undergoes reactions with nucleophiles at the adjacent meta carbon. Thus, reactions of 5, triflic acid, and either N-methylpyrrole or 1-methoxy-2-methyl-1-(trimethylsilyloxy)-1-propene result in 5-substituted-1,3-cyclohexadienes after removal of the metal.

The dearomatization of naphthalene and anthracene is explored by their η 2 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 πacidic 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.

The preparation and properties of the complex (R Mo ,R)-MoTp(NO)(DMAP)(η 2 -α-pinene) are described (∼10 g scale; DMAP = 4-(dimethylamino)pyridine; Tp = hydridotris(pyrazolyl)borate). This complex undergoes exchange of the pinene with a wide range of other π ligands including acetone, ethyl acetate, N,N-dimethylformamide, acetonitrile, and naphthalene. Treatment of the α-pinene complex with iodine results in the complex (S)-MoTp(NO)(DMAP)(I), which is recovered in enantioenriched form (er = 99:1; yield >90%; scale 4.6 g). Reduction of this molybdenum(I) precursor results in enantioenriched molybdenum(0) complexes, including (R)-MoTp-(NO)(DMAP)(η 2 -trifluorotoluene). Sequential treatment of this arene complex with acid, a masked enolate, and iodine regenerates MoTp(NO)(DMAP)(I) along with an alkylated 1-(trifluoromethyl)cyclohexa-1,3-diene with an er value as high as 99:1. This process demonstrates the efficient transfer of asymmetry from α-pinene to the diene product. Accompanying studies with (1R)-myrtenal reveal a redox-catalyzed pinene/myrtenal ligand exchange occurring through Mo(I) intermediates.

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.