Publications

Several S-alkylthiophenium complexes of the type [Os(NH₃)₅(4,5-η²-L)](OTf)₃ (where L = S-alkylthiophenium, S-methylbenzo[b]thiophenium) are prepared by alkylation of the corresponding thiophene complexes. The S-alkylthiophenium species are proposed to undergo rapid and reversible cleavage of the C5−S bond, forming highly electrophilic metallacyclopropene intermediates. Although not directly observable, these vinyl cation intermediates may be trapped with both anionic and neutral nucleophiles affording η²-4-(alkylthio)-1,3-butadiene complexes. Treatment of selected 4-(alkylthio)-1,3-butadiene complexes with an oxidant (e.g., DDQ) affords the organic ligand in good yield.

A structurally and electronically diverse set of anisoles are dihapto-coordinated to the π-base pentaammineosmium(II) and treated with a variety of carbon electrophiles (e.g. Michael acceptors, acetals). After deprotonation of a 4H-anisolium intermediate with a tertiary amine base, C(4)-substituted anisole complexes are isolated. The functionalized arenes are removed from the metal center either by mild heating or treatment with an oxidant (e.g. AgOTf, DDQ, CAN). The resulting substituted anisoles are isolated with yields ranging from 55−95%.

Conclusion: The true significance of this work lies not so much in the demonstration of a transition metal localizing aromatic π systems, but rather in the ability of an η²-coordinated metal to activate π systems toward electrophilic addition often at a site away from metal coordination even if this causes disruption of an aromatic system. Thus, arenium, allyl, and even vinyl cations are stabilized by the metal to the point that nucleophilic addition becomes competitive with electrophilic substitution. In this regard, the chemistry exhibited by the osmium system provides a new method for activating unsaturated molecules that is complementary to the more established chemistry of nucleophilic addition to metal-coordinated π systems. The true potential of this approach, however, will not be realized until alternative transition metal systems become available. The successful system, in addition to having the appropriate steric requirements, must contain ancillary ligands that tolerate moderate electrophiles and a metal center that resists oxidative addition, either by direct attack of electrophile or by insertion of a C−H bond. The author hopes that a convincing case has been made to search for new transition metal systems that can function as this type of π base. ⁹²

A series of rhenium(I) dinitrogen complexes were prepared containing predominantly amine ligands. On the basis of infrared and electrochemical data, a system was selected that was anticipated to be a suitable precursor for an elongated dihydrogen complex. Upon oxidation by AgOTf, the dinitrogen ligand of fac-[Re(PPh₃)(PF₃)(dien)(N₂)]⁺ (3) is replaced with triflate to generate 13, fac-[Re(PPh₃)(PF₃)(dien)(OTf)]OTf, a convenient precursor to rhenium(II) and rhenium(I) amine complexes. Reduction of the rhenium(II) triflate 13 under 1 atm of hydrogen gas generates the complex fac-[Re(dien)(PPh₃)(PF₃)(dien)(H₂)]⁺ (fac-14). T₁ measurements indicate a dihydrogen species with a H−H distance of 1.38 ± 0.03 Å. The HD analog displays a J_HD of 6.7 Hz, corresponding to a H−H distance of 1.31 ± 0.03 Å, a value in good agreement with that determined from T₁ data and among the largest ever measured for an elongated dihydrogen system.

A series of d⁶ transition-metal complexes of the type ML₅(η²-CpH), where ML₅ = [Os^II(NH₃)₅]²⁺, [Ru^II(NH₃)₅]²⁺, and [Re^I(PPh₃)(PF₃)(dien)]⁺, were synthesized as their triflate salts and combined with electrophiles (HOTf, CH₂(OMe)₂) to form η³-allyl complexes. Treatment of these π-allyl complexes with the mild carbon nucleophile 1-methoxy-2-methyl-1-(trimethylsiloxy)propene (MMTP) followed by decomplexation affords substituted η²-cyclopentene derivatives with excellent regio- and stereocontrol. Deuteration and NOE studies for the π-allyl complexes along with stereochemical analysis of the organic products confirm that both electrophilic and nucleophilic addition occurs exclusively from the exo face of the ring (opposite to metal coordination) for all three systems.

η²-Aniline complexes of the π-base pentaammineosmium(II) are synthesized with a variety of different aniline ligands in good yield. Once synthesized, these complexes display ligand-based reactivity with a variety of electrophiles. Reaction with trifluoromethanesulfonic (triflic) acid produces 2H- and 4H-anilinium complexes, species resulting from protonation on the ring. The regioselectivity of protonation depends on a variety of factors, including the aniline substitution pattern, temperature, and solvent. In addition, the η²-aniline complexes undergo regiospecific, chemospecific, and stereospecific reactions with carbon electrophiles such as Michael acceptors, acetals, and ketals. The resulting anilinium complexes are stabilized by a strong metal-to-ligand back-bonding interaction. As a result, these 4H-anilinium complexes may be functionalized further by their reaction with nucleophiles and complete dearomatization of the aniline ligand is thus achieved. The resulting allylamine and imine complexes are oxidatively removed from the metal center to provide organic allyl amine and cyclohexenone products in moderate yield. This methodology is complementary to that based on η⁶-complexation of an electron-deficient metal center to the aniline.

A series of 4,5-η²-Os(II)pentaammine-3-vinylpyrrole complexes are synthesized from the corresponding 1-methylpyrrole complex by an electrophilic addition at the β-carbon, C(3), of the pyrrole ring. Four independent synthetic routes to β-vinylpyrrole complexes are described, each introducing different functionality on the pendant double bond. The uncoordinated portion of these complexes chemically and structurally resembles an aminodiene and as such readily undergoes Diels−Alder reactions with suitably activated dienophiles to generate the 5,6,7,7a-tetrahydroindole nucleus. These tetrahydroindole complexes can be decomplexed and oxidized with DDQ to generate a series of highly functionalized indoles.

A series of 4,5-η²-Os(II)pentaammine-3-vinylpyrrole complexes are synthesized from the corresponding 1-methylpyrrole complex by an electrophilic addition at the β-carbon, C(3), of the pyrrole ring. Four independent synthetic routes to β-vinylpyrrole complexes are described, each introducing different functionality on the pendant double bond. The uncoordinated portion of these complexes chemically and structurally resembles an aminodiene and as such readily undergoes Diels−Alder reactions with suitably activated dienophiles to generate the 5,6,7,7a-tetrahydroindole nucleus. These tetrahydroindole complexes can be decomplexed and oxidized with DDQ to generate a series of highly functionalized indoles.