Publications

Octahedral tetraammineosmium(II) species are generated from their Os^III precursors containing an amine ligand cis to a labile alcohol or triflate. These compounds undergo reversible β-hydride eliminations resulting in the formation of cis-η²-iminium hydride complexes. Judging from NMR data, the η²-iminium group in these products lies parallel to the osmium-hydride bond with the iminium carbon eclipsing the hydride. Attempts to form η²-arene complexes of an Os^II ammine system bearing a stereogenic carbon are also described.

A series of anisoles are complexed by pentaammineosmium(II), and the resulting 5,6-η²-anisole complexes are treated with activated olefins or acetals in the presence of triflic acid to form 4H-anisolium complexes. These intermediates are capable of undergoing inter- or intramolecular nucleophilic addition reactions at C3, and 2-methoxy-1,3-cyclohexadiene complexes are formed. These complexes are readily converted into functionalized cyclohexenones, cyclohexadienes, and cyclohexenes. When BF₃·OEt₂ is used, it is possible to form a 4H-anisolium complex with a pendent boron enolate, which can ultimately undergo intramolecular addition to C1 to form the corresponding [4 + 2] cycloadduct. For cases in which an activated alkyne is added to a C4-alkylated anisole, a migration of the vinyl group occurs, leading to 4-methyl-3-vinylanisoles.

The complex [Os(NH₃)₅(η²-anisole)]²⁺ reacts with acetals or Michael acceptors to form 4-methoxystyrene complexes that are exclusively coordinated at the arene (η²). These styrene complexes are active dienes and readily participate in Diels−Alder reactions with electron-deficient olefins to form tetrahydronaphthalene complexes. The cycloadducts are formed typically as single diastereomers and are valuable as precursors to functionalized tetralins and decalins.

3-Alkylated anisole complexes of pentaammineosmium(II) are treated with methyl vinyl ketone and triflic acid to form 4H-anisolium Michael adducts. These compounds deprotonate regioselectively at the benzylic position adjacent to C3 and then undergo an aldol cyclization with the pendant carbonyl to form the decalin core. Reduction of the dienonium product can be directed in a 1,4 fashion to generate a trans decalin or in a 1,2 fashion to provide a methoxydiene complex that serves as a precursor to other functionalized decalins.

The complex TpRe(CO)₂(THF) (3) (Tp = hydridotris(pyrazolyl)borate) reacts with a variety of aromatic molecules to form stable binuclear complexes of the form {TpRe(CO)₂}₂(μ-η²-η^2‘-L), where L = furan (6), N-methylpyrrole (7), or naphthalene (8). With thiophene, the TpRe(CO)₂ fragment yields the sulfur-bound mononuclear complex TpRe(CO)₂(η¹-thiophene) (5). In the absence of a suitable ligand, complex 3 reacts under a nitrogen atmosphere to form Tp(CO)₂Re(μ-N₂)Re(CO)₂Tp (9). An X-ray diffraction study of a single crystal of 9 provides geometric details.

A series of η²-arene complexes (2a−i) of the form [Os(NH₃)₅(ArOCHR¹CH₃)]²⁺ was prepared in which the alkoxy substituent R¹bears a potential hydrogen bond acceptor. Using an ester or amide group as the hydrogen bond acceptor, a two-point interaction was achieved between the pentaammineosmium system and the organic ligand. For these complexes, a single coordination diastereomer was observed. For compound 2a, evidence for hydrogen bonding between the carbonyl oxygen and the pentaammineosmium fragment was obtained through ¹H NMR studies and by a comparison of the rate of arene displacement for 2a compared to that of the parent complex [Os(NH₃)₅(anisole)]²⁺. Complex 2a undergoes stereospecific protonation to form a 4H-anisolium product 3a. Complex 3a undergoes nucleophilic addition of a silylketene acetal at C3 to generate an alkoxydiene complex (4) as a single diastereomer. Hydrolysis of 4 removes the chiral auxiliary, and subsequent oxidative decomplexation yields a cyclohexenone with a new stereogenic center.

A series of η²-arene complexes (2a−i) of the form [Os(NH₃)₅(ArOCHR¹CH₃)]²⁺ was prepared in which the alkoxy substituent R¹bears a potential hydrogen bond acceptor. Using an ester or amide group as the hydrogen bond acceptor, a two-point interaction was achieved between the pentaammineosmium system and the organic ligand. For these complexes, a single coordination diastereomer was observed. For compound 2a, evidence for hydrogen bonding between the carbonyl oxygen and the pentaammineosmium fragment was obtained through ¹H NMR studies and by a comparison of the rate of arene displacement for 2a compared to that of the parent complex [Os(NH₃)₅(anisole)]²⁺. Complex 2a undergoes stereospecific protonation to form a 4H-anisolium product 3a. Complex 3a undergoes nucleophilic addition of a silylketene acetal at C3 to generate an alkoxydiene complex (4) as a single diastereomer. Hydrolysis of 4 removes the chiral auxiliary, and subsequent oxidative decomplexation yields a cyclohexenone with a new stereogenic center.

A series of complexes has been prepared of the form [Re(dien)(PPh₃)(PF₃)(L)]⁺ (dien = diethylenetriamine), where L is an unsaturated organic molecule. The range of ligands which form η²-coordinate complexes with this metal center includes aldehydes, olefins, and dienes. In addition, thiophene, benzo[b]thiophene, and acetonitrile bind through their heteroatoms. Although {Re(dien)(PPh₃)(PF₃)}⁺ (dien = diethylenetriamine) displays chemical and spectroscopic characteristics of a potent π-base, it fails to form stable η²-coordinated complexes with aromatic molecules. However, its carbonyl analogue {Re(dien)(PPh₃)(CO)}⁺, when combined with furan, forms the complex [Re(η²-furan)(dien)(PPh₃)(CO)][OTf], a rare example of a thermally stable η²-heterocycle.

Complexes of the form [Os(NH₃)₅(L)](OTf)₂ (where L = an unactivated arene or polyaromatic hydrocarbon) are readily protonated by triflic acid (HOTf) to generate stable arenium, naphthalenium, and anthracenium cations. A series of substituted anisole complexes were also investigated. The metal stabilizes the hydrocarbon arenium system, in most cases, by coordinating the organic ligand in an η³ fashion. Where L = m-xylene, however, NMR data strongly suggest that the arenium ion is essentially dihapto-coordinated, where an allyl cation fragment remains uncoordinated. For the corresponding anisolium systems, NMR data indicate η²-coordination. It is likely that η² and η³ geometries represent limiting cases for a continuum of distorted allyl (pseudo-allyl) complexes. The pK_a values determined for these complexes are dramatically higher than those of the free arenium cations.

A series of substituted-naphthalene complexes have been synthesized of the form [Os(NH₃)₅(η²-L)](OTf)₂ (where L = 1-methylnaphthalene, 2-methylnaphthalene, 2-methoxynaphthalene, 2-hydroxynaphthalene, 1-methoxynaphthalene, 1-hydroxynaphthalene, 1-acetonaphthone, 2-acetonaphthone, and phenanthrene), with two or more linkage isomers identified for several of these complexes. The ratio of these isomers changes over time in solution (acetone-d₆) and eventually reaches dynamic equilibrium. The final equilibrium ratios and the associated specific rates are reported. In several cases, protonation of an isomer mixture generates a single naphthalenium species, which upon deprotonation provides the original η²-naphthalene complex as a single set of ring isomers.