A Ru carbene (8, Scheme ) that contains an internal metal−oxygen chelate is an active metathesis catalyst and is readily obtained by the sequential treatment of Cl2Ru(PPh3)3 with (2-isopropoxyphenyl)diazomethane and PCy3. This Ru-carbene complex offers excellent stability to air and moisture and can be recycled in high yield by silica gel column chromatography. The structures of this and related complexes have been unambiguously established by NMR and single-crystal X-ray diffraction studies.

Small peptide catalysts containing modified histidine residues are reported that effect enantioselective acylation reactions. The catalysts described include octapeptide β-hairpins (e.g., 11) that exhibit high selectivities (up to k rel = 51), tetrapeptide β-turns (e.g., 7) that afford moderate selectivities (up to k rel = 28), and several simple derivatives of the modified histidine amino acid that do not exhibit appreciable enantioselectivity. Supporting structural studies (1H NMR and X-ray) are presented which lead to the proposal of a model in which catalyst rigidity and structural complexity contribute to higher degrees of enantioselection. A covalently rigidified octapeptide (20) is prepared through solid-phase Ru-catalyzed ring-closing metathesis; kinetic evaluation of this peptide reveals that substituents along the peptide backbone may be more important than covalent stabilization of a structural motif. Detailed kinetics studies on the most selective peptide catalysts are presented that suggest the reactions are first order in catalyst and substrate. Additional kinetic studies indicate unambiguously that enantioselectivities are due to specific acceleration of reaction for one substrate enantiomer, rather than the deceleration of the reaction for the other. The results are presented in the context of a possible enantiomer-specific hydrogen-bonding interaction in the stereochemistry-determining step for these processes.

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The zirconium dimethyl complexes [N2NX]ZrMe2 (N2NX = [(MesNCH2CH2)2NX]; Mes = mesityl; X = H (1a), Me (1b)), have “mer” structures in the solid state in which the amido nitrogens occupy “axial” positions in a trigonal bipyramid. The reaction of 1b with 1 equiv of [Ph3C][B(C6F5)4] followed by addition of diethyl ether yielded the ether adduct {[N2NMe]ZrMe(Et2O)}+ (with [B(C6F5)4]- as the anion), an X-ray study of which revealed it to be a fac trigonal-bipyramidal species in which the diethyl ether is coordinated in an apical position. The reaction of 1b with 1 equiv of [PhNMe2H][B(C6F5)4] led to {[N2NMe]ZrMe(NMe2Ph)}[B(C6F5)4], solution NMR studies of which suggest a structure analogous to that of {[N2NMe]ZrMe(Et2O)}+. Heating solutions of {[N2NMe]ZrMe(NMe2Ph)}[B(C6F5)4] led to C−H activation in one mesityl o-methyl group and formation of methane. The reaction of 1b with 0.5 equiv of [Ph3C][B(C6F5)4] yielded [{[N2NMe]ZrMe}2(μ-Me)][B(C6F5)4] (5b), an X-ray diffraction study of which revealed an almost linear (167.4°) methyl bridge linking two distorted TBP moieties through the apical positions (average Zr−C(bridge) = 2.48 Å, average Zr−C(terminal) = 2.24 Å). The equatorial methyl groups in 5b exchange readily between Zr centers, while the bridging methyl group and the equatorial methyl groups exchange relatively slowly on the NMR time scale, but still rapidly on the chemical time scale. Exchange of free [N2NMe]ZrMe2 with the [N2NMe]ZrMe2 fragment in 5b is also facile on the chemical time scale. The reaction of 1b with 1.0 equiv or more of [Ph3C][B(C6F5)4] led to formation of a cationic species (6b), two forms of which could be observed at low temperature. Activation of 1a with [Ph3C][B(C6F5)4] yielded only one cationic form of 6a at low temperatures. Exchange of methyl groups between 6a and 6b is slow on the chemical time scale. All evidence is consistent with the observation of different ion pairs of 6b at low temperatures.

{[(MesNCH 2 CH 2 ) 2 NMe]ZrMe}[B(C 6 F 5 ) 4 ] and intermediates in the polymerization reaction of 1-hexene that are formed from it decompose as a consequence of CH activation in an ortho methyl group in the mesityl substituent. The intermediates in the polymerization reaction decompose significantly more readily than does {[(MesNCH 2 CH 2 ) 2 NMe]ZrMe}[B(C 6 F 5 ) 4 ]. On the other hand, analogous cationic complexes that contain the [(2,6-Cl 2 C 6 H 3 NCH 2 CH 2 ) 2 NMe] 2ligand are relatively stable and will consume 1-hexene in a strictly first-order and apparently living manner at 0 °C in chlorobenzene.