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.

The dihapto-coordination of benzene to the πbasic fragment {TpW(NO)(PMe 3 )} (Tp = hydridotris-(pyrazolyl)-borate) enhances the basicity of the arene ligand to the point that it can be protonated with a mild Brønsted acid (diphenylammonium triflate; pK a ∼ 1). The resulting η 2benzenium complex reacts with a wide range of nucleophiles including protected enolates, cyanide, amines, methoxide, and aromatic nucleophiles to form 5-substituted 3,4-η 2 -1,3-cyclohexadiene complexes in good yield (42−70%). These coordinated dienes were successfully taken through a second protonation and nucleophilic addition with a similar scope of nucleophiles (54− 80%). The resulting cis-3,4-and cis-3,6-disubstituted η 2 -cyclohexene complexes were prepared with high regio-and stereocontrol, as governed by the asymmetric nature of π-allyl intermediates. In some cases, a diene linkage isomerization from 3,4-η 2 to 1,2-η 2 could be effected with a redox catalyst, and reactions of the latter species led to cis-3,5-disubstituted cyclohexene products exclusively. Oxidative decomplexation afforded the free cyclohexene products in moderate yield (37−68%). Additionally, when a single enantiomer of the chiral dearomatization agent was used, the elaborated cyclohexenes were able to be synthesized in enantioenriched forms (86−90% enantiomeric excess). Full characterization of 40 new compounds is provided that includes two-dimensional NMR, IR, electrochemical and in some cases crystallographic data.

An oxidant-initiated, substitution process for dihapto-coordinated ligands is described for the {MoTp­(NO)­(DMAP)} system. Complexes of the form MoTp­(NO)­(DMAP)­(η2-alkene), MoTp­(NO)­(DMAP)­(η2-ketone), and MoTp­(NO)­(DMAP)­(η2-arene) (where Tp = hydridotris­(pyrazolyl)­borate and DMAP = 4-(dimethylamino)­pyridine) undergo an alkene-to-ketone exchange that is catalyzed by the addition of <0.1 equiv of a metallocene oxidant (ferrocenium, permethylferrocenium, or cobaltocenium). A similar acceleration was observed in the presence of the H-bond donor hexafluoroisopropanol (HFIP). From experimental observations, a radical chain propagation mechanism is proposed that is dependent on the equilibrium between dihapto-coordinated (C, O-η2) and monocoordinated (κ-O) isomers and the differing redox characteristics of these two isomeric forms. This concept was then applied to the search of sodium-free reduction conditions for the conversion of MoTp­(NO)­(DMAP)­(I) to various molybdenum(0) complexes of unsaturated ligands, including MoTp­(NO)­(DMAP)­(η2-naphthalene) and MoTp­(NO)­(DMAP)­(α-pinene).

A method for the resolution of η 2 -alkene-complex isomers of the type MoTp(NO)(DMAP)(η 2 -alkene) and WTp-(NO)(PMe 3 )(η 2 -alkene) (where Tp = hydridotris(pyrazolyl)borate and DMAP = 4-(dimethylamino)pyridine) has been explored. Alkene and polyene compounds form as a mixture of kinetically trapped isomers. For both types of complexes, it was found that addition of either a fluorinated alcohol or one-electron oxidant reduces the number of isomers in solution. Accelerated ligand exchange was also observed, although these reactions were accompanied by significant decomposition.

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