Publications

A series of complexes of the form [Os(NH₃)₅(η²-L)](OTf)₂ (where L = thiophene and OTf = trifluoromethanesulfonate, i.e., triflate) are synthesized and characterized and their reactivities with electrophilic reagents are examined. Depending on the electrophile and the substitution pattern of the coordinated thiophene, direct electrophilic addition occurs at either the sulfur, C2, or C3, affording thiophenium complexes. Some elementary transformations of selected thiophene complexes are exploited to prepare novel 1- and 2-thiafulvenium salts.

The complex trans-[Re(terpy)(Br)(PPh₃)₂][OTf] (2) (terpy = 2,2‘:6‘,2‘ ‘-terpyridine) is a convenient precursor to the electron-rich π-basic fragment {(terpy)(L)₂Re}⁺ (L = ^tBuNC or PMe₃). Reduction of 2 with activated magnesium in the presence of unsaturated organic molecules and an excess of either ^tBuNC or PMe₃ yields complexes of the type trans-[(terpy)(L)₂Re(η²-π)][OTf] (L = ^tBuNC or PMe₃; π = olefin, aldehyde, or ketone). The dihapto-coordinated organic moieties show a preference for binding in the plane of the terpy ligand. Reaction of trans-[Re(terpy)(^tBuNC)₂(η²-acetone)][OTf] (9) with MeOTf yields an observable η²-ketonium complex. The electronic environment of these complexes has been probed by cyclic voltammetry, and the details of ligand exchange for the η²-ketone complexes are presented. Geometric features determined from X-ray crystal structure analyses of trans-[(terpy)(^tBuNC)₂Re(η²-cyclopentene)][OTf] (4) and trans-[(terpy)(^tBuNC)₂Re(η²-acetophenone)][OTf] (11) are reported.

In summary, a synthetic route to the π-basic {TpRe(CO)(PMe₃)} fragment has been developed. This metal system forms stable η²-aromatic complexes with naphthalene, thiophene, and furan that approach the stability of the pentaammineosmium(II) system. While the chiral metal fragment demonstrates only modest ability to discriminate enantiofaces with the heterocycles thiophene and furan, excellent stereo-differentiation is achieved upon coordination of naphthalene. Thus, it is possible that an entirely new class of dearomatization agents may be obtained of the form {TpRe(CO)(L)} that may ultimately prove to be powerful tools for the synthetic chemist.

In contrast to eta-1 and eta-5-pyrrole complexes, which generally react either at the nitrogen or at the s-positions, rl2-coordination of pyrrole by osmium(II)pentaammine activates the pyrrole ring toward regioselective protonation and electrophilic addition at the ~-position. Depending on the reaction conditions, the resulting products are either 1H-pyrrole or 3H-pyrrolium complexes, the latter of which are several orders of magnitude less acidic than their uncomplexed counterparts and resist rearomatization, rl2-Pyrrole complexes also undergo a dipolarcycloaddition reaction with certain electrophiles to generate a wide variety of 7-azanorbornene complexes. In most cases, the metal can be removed from the products by oxidation or heating to generate functionalized pyrroles, azanorbornanes, or pyrrolizinines. A series of I]-vinylpyrrole complexes can be synthesized via initial electrophilic addition to the pyrrole ring. These vinylpyrrole complexes readily undergo a Diels-Alder cycloaddition reaction with electron-deficient olefins to give functionalized 5,6,7,7a-tetrahydroindole complexes. These complexes, which are stable to isomerization, can be decomplexed and oxidized with DDQ to generate highly substituted indoles in moderate to good yields.

The naphthalene complex of pentaammineosmium(II) (1) reacts with four different classes of electrophiles to form 1-naphthalenium species 2−5. These η³-allyl complexes react stereospecifically with a variety of nucleophiles to form cis-1,4-dihydronaphthalene complexes. The entire reaction sequence may be performed outside a glovebox in two steps using conventional techniques.

A series of complexes of the form [Os(NH₃)₅(η²-L)](OTf)₂ (where L = thiophene and OTf = trifluoromethanesulfonate, i.e., triflate) are synthesized and characterized and their reactivities with electrophilic reagents are examined. Depending on the electrophile and the substitution pattern of the coordinated thiophene, direct electrophilic addition occurs at either the sulfur, C2, or C3, affording thiophenium complexes. Some elementary transformations of selected thiophene complexes are exploited to prepare novel 1- and 2-thiafulvenium salts.