Multiple spectroscopic and computational methods were used to characterize the ground-state electronic structure of the novel {CoNO}(9) species Tp*Co(NO) (Tp* = hydro-tris(3,5-Me(2)-pyrazolyl)borate). The metric parameters about the metal center and the pre-edge region of the Co K-edge X-ray absorption spectrum were reproduced by density functional theory (DFT), providing a qualitative description of the Co-NO bonding interaction as a Co(II) (S(Co) = 3/2) metal center, antiferromagnetically coupled to a triplet NO(-) anion (S(NO) = 1), an interpretation of the electronic structure that was validated by ab initio multireference methods (CASSCF/MRCI). Electron paramagnetic resonance (EPR) spectroscopy revealed significant g-anisotropy in the S = ½ ground state, but the linear-response DFT performed poorly at calculating the g-values. Instead, CASSCF/MRCI computational studies in conjunction with quasi-degenerate perturbation theory with respect to spin-orbit coupling were required for obtaining accurate modeling of the molecular g-tensor. The computational portion of this work was extended to the diamagnetic Ni analogue of the Co complex, Tp*Ni(NO), which was found to consist of a Ni(II) (S(Ni) = 1) metal center antiferromagnetically coupled to an S(NO) = 1 NO(-). The similarity between the Co and Ni complexes contrasts with the previously studied Cu analogues, for which a Cu(I) bound to NO(0) formulation has been described. This discrepancy will be discussed along with a comparison of the DFT and ab initio computational methods for their ability to predict various spectroscopic and molecular features.

A novel chelated ruthenium-based metathesis catalyst bearing an N-2,6-diisopropylphenyl group is reported and displays near-perfect selectivity for the Z-olefin (>95%), as well as unparalleled TONs of up to 7400, in a variety of homodimerization and industrially relevant metathesis reactions. This derivative and other new catalytically-active species were synthesized using an improved method employing sodium carboxylates to induce the salt metathesis and C-H activation of these chelated complexes. All of these new ruthenium-based catalysts are highly Z-selective in the homodimerization of terminal olefins.

The first example of ruthenium-mediated ring-opening metathesis polymerization (ROMP) generating highly cis, highly tactic polymers is reported. While the cis content varied from 62 to >95% depending on the monomer structure, many of the polymers synthesized displayed high tacticity (>95%). Polymerization of an enantiomerically pure 2,3-dicarboalkoxynorbornadiene revealed a syndiotactic microstructure.

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In recent years, enantiopure C 2 -and C 1 -symmetric dienes have been established as authoritative ligands for asymmetric reactions mediated by late transition metals. 1 Despite their utility, current synthetic routes to these ligands remain unsatisfying and often rely upon preparative chiral HPLC, chiral auxiliaries, chiral pool starting materials, or chemoenzymatic reactions to access enantiopure material. [2][3][4][5][6] Hayashi and co-workers reported an effective enantioselective synthesis of C 2 -symmetric dienes based upon the desymmetrization of norbornadiene.2a-b In the first step in the synthesis of the chiral ligand, a double asymmetric hydrosilylation of norbornadiene with HSiCl 3 catalyzed by [PdCl(η 3 -C 3 H 5 )] 2 and a chiral phosphine ligand was reported to yield an enantiopure di(silane). However, a multi-step reaction sequence was required to re-establish the diene moiety and build steric bulk into the chiral scaffold.2a-b , 3If the synthesis of C 2 -dienes could be achieved via a direct desymmetrization of the parent diene by an asymmetric C-H functionalization of the sp 2 -carbons, it would represent a concise approach to this important class of chiral ligands (Scheme 1). In addition, the use of a prochiral electrophile in the reaction would allow for the preparation of enantiopure ligands that bear additional stereocenters, provided these reactions proceed with high stereoselectivities.Recently, we reported a stepwise C-H functionalization of the sp 2 carbon of simple alkenes mediated by the reactive intermediate [CpCo(NO) 2 ] (Scheme 2). 7-9 Herein we report that N-benzylated ammonium chloride salts (Chart 1, 1-4) combined with sodium hexamethyldisilazide (NaHMDS) can promote this reaction, while serving as a source of chirality to desymmetrize norbornene and norbornadiene. This methodology has been applied to a modular asymmetric synthesis of C 1 -and C 2 -symmetric diene ligands.Initial experiments demonstrated that addition of a mixture of NaHMDS and the quininium salt 1 to a mixture of 2-cyclohexen-1-one and cobalt complex 5 gave adduct 6 in 87% d.e. and high yield. Subsequent cycloreversion of 6 in the presence of norbornene yielded 5 and the diastereomerically impure alkene 7 which was obtained in a low but significant 18% e.e. (Table 1, entry 1). The order of reagent addition proved to be critical to the enantioinduction; in fact, adding NaHMDS to a mixture containing the remaining reagents gave the product with only 2% e.e. (entry 2). As NaHMDS alone can promote a racemic background reaction, we suspect a chiral base formed upon mixing 1 with NaHMDS is responsible for the observed enantioinduction.Several alternative solvents gave comparable or inferior results compared to those observed with THF/hexamethylphosphoramide (HMPA). Performing reactions in THF, however, fdtoste@berkeley.edu, rbergman@berkeley.edu. Supporting Information Available: Experimental procedures and spectroscopic data for all new compounds; conditions for chiral GC and HPLC methods and CIF file for 12b. This ma...