Publications

In conclusion, the dihapto coordination of benzene by the π-basic metal fragment {[TpRe(CO)(MeIm)} effectively dearomatizes benzene. The electron-donating metal activates the arene to such an extent that the unbound portion of the ligand acts as an electron-rich diene undergoing a Diels−Alder cycloaddition with NMM with a specific rate that is 60% greater than that observed for cyclohexadiene. Furthermore, the metal controls the stereochemistry of the cycloaddition and protects the resulting cycloadduct from undergoing a retrocycloaddition by blocking one of the π bonds. Oxidation of the rhenium metal center promotes decomplexation, providing access to the bicyclo[2.2.2]octadiene. Optionally, decomplexation accompanied by oxygen insertion yields bicyclo[2.2.2]octenone products. The strategy of using a transition metal to “switch off” resonance stabilization in arenes and aromatic heterocycles could prove to be a powerful new tool for synthetic chemists.

Dihapto-coordination of aromatic ligands by electron-rich transition metals can effectively dearomatize the bound aromatic molecule. The pentaammineosmium(II) system forms thermally stable η²-complexes with a variety of arenes and aromatic heterocycles, and has been used for a variety of organic transformations on the bound aromatic fragments. The systematic variation of isoelectronic rhenium(I) systems has provided the necessary electronic and steric characteristics needed for a less expensive, chiral alternative to the {Os(NH₃)₅}²⁺ system. The {TpRe(CO)(PMe₃)} system has been shown to form stable dihapto complexes with furan, thiophene and naphthalene. Accordingly, the {TpRe(CO)(PMe₃)} fragment and analogous {TpRe(CO)(L)} fragments represent the first class of asymmetric surrogates to the pentaammineosmium(II) system.

A series of Michael acceptors has been combined with the Os(II) η²-naphthalene complex (1) to form stable 1H-naphthalenium species. Under acidic conditions, these complexes undergo ring closure at C2 to form the phenanthrenone core. In contrast, the corresponding 1-methylnaphthalene complex (15) upon addition of MVK at C8 undergoes ring closure at C5 to form a bridged tricyclic complex (18). Michael addition of MVK to the naphthalene complex (1) followed by deprotonation, an inter-ring linkage isomerization, and ring closure forms a 9-methylphenalene complex (21). In all cases, the organic cyclization products may be decomplexed by heating with silver triflate and isolated in moderate yield.

Synthetic access to {TpRe(CO)(PMe₃)} and the ability of this fragment to bind unsaturated compounds are reported. A variety of complexes of the type TpRe(CO)(PMe₃)(η²-L) (L = cyclohexene, cyclopentene, naphthalene, phenanthrene, thiophene, 2-methylthiophene, furan, or acetone) have been isolated and characterized, and stereochemical and stability issues of aromatic molecules bound to {TpRe(CO)(PMe₃)} are discussed in detail. In particular, a solid-state structural study of TpRe(CO)(PMe₃)(η²-cyclohexene) has provided a foundation for a discussion of the stereoelectronic features of the Re(I) fragment, and substitution reactions of an aromatic ligand by acetone provide insight into the stability of these aromatic complexes. In addition, a solid-state X-ray diffraction study of the Re(II) complex TpRe(CO)(PMe₃)(OTf) (OTf = trifluoromethanesulfonate) is presented.

A nonracemic η²-arene complex (2) was prepared from (R)-(+)-methyl-2-phenoxypropionate and pentaammineosmium(II) with high coordination diastereoselectivity (>9:1). Complex 2 was then treated with HOTf, an acetal, or a Michael acceptor to generate 4H-oxonium complexes at low temperature. These products were then combined with a nucleophile (silylketene acetal or hydride) to form stable alkoxydiene complexes. Further reaction of these complexes with triflic acid yielded isolable enonium complexes, and subsequent hydrolysis formed substituted cyclohexenone complexes. Upon oxidative decomplexation, these materials provided substituted cyclohexenones isolated with high ee's (80−85%). The reaction of cycloenonium complexes with hydride led to the formation of cyclohexenyl ether complexes, and their oxidative decomplexation yielded substituted cyclohexenyl ethers with de and ee values >90%. Treatment of a cyclohexenyl ether complex with triflic acid gave an isolable π-allyl complex. The reaction of this material with a nucleophile followed by decomplexation afforded cis-1,4,5-trisubstituted cyclohex-2-enes with ee's ranging from 80 to 90%. The increase in the steric bulk of the alkyl group of the alkoxy chiral auxiliary (methyl vs isopropyl) led to a more complete transfer of chirality.

A series of complexes has been prepared of the form [Os(NH₃)₅(4,5-η²-L)]²⁺ where L=furan and various 2-alkylated furans. Electrophilic addition to C(3) results in an unstable reaction intermediate, a 4,5-η²-3H-furanium species, that leads to several novel cyclization reactions with tethered nucleophiles to form new heterocycles.