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The synthesis, structural characterization, and abstraction chemistry of ansa-zirconocene ester enolate complexes relevant to the isospecific polymerization of methacrylates are reported. Reactions of rac-(EBI)ZrMe(OTf) and rac-(EBI)Zr(OTf)(2) [EBI = C(2)H(4)(Ind)(2)] with 1 and 2 equiv of lithium isopropylisobutyrate in toluene produce the first examples of ansa-zirconocene mono- and diester enolate complexes: rac-(EBI)ZrMe[OC(O(i)Pr)=CMe(2)] (1) and rac-(EBI)Zr[OC(O(i)Pr)=CMe(2)](2) (2) in 89% and 50% isolated yields, respectively. The reaction of 1 with B(C(6)F(5))(3) was investigated in six different organic solvents; in THF at ambient temperature, this reaction cleanly produces the isolable cationic ansa-zirconocene ester enolate complex rac-(EBI)Zr(+)(THF)[OC(O(i)Pr)=CMe(2)][MeB(C(6)F(5))(3)](-) (3) in quantitative yield. The analogous reaction of 1 with Al(C(6)F(5))(3) in toluene, however, proceeds through a proposed novel, intramolecular proton transfer process in which propylene is eliminated from the isopropoxy group, subsequently producing a carboxylate-bridged tight ion pair rac-(EBI)Zr(+)(Me)OC((i)Pr)OAl(C(6)F(5))(3)(-) (4). In addition to standard spectroscopic and analytical characterizations for the isolated complexes 1-4, complexes 2 and 4 have also been structurally characterized by X-ray diffraction studies. Polymerization of methyl methacrylate (MMA) and n-butyl methacrylate (BMA) has been investigated using complexes 1, 3, and 4. Both the isolated cationic 3 and neutral 1 (the latter combined with B(C(6)F(5))(3) in situ) are highly active (10 min for quantitative MMA conversion) and highly isospecific ([mm] > 95% for PMMA; [mm] > 99% for PBMA) via enantiomorphic-site control, producing polymethacrylates with extremely narrow molecular weight distributions (M(w)/M(n) = 1.03). The aluminate complex 4, however, produces syndiotactic PMMA predominantly via chain-end control.

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Stereoblock polymerization with chiral ansa-metallocene/strong Lewis acid hybrid catalysts capable of switching stereospecificity of the methyl methacrylate polymerization produces the highly stereoregular isotactic-b-syndiotactic stereoblock poly(methyl methacrylate).

C-H bond activation has been extensively studied with (Cp*)M(L) n (M = Ir, Rh), but cobalt, the third member of this triad, has not previously been shown to activate sp 3 C-H bonds. Further, practical functionalization of the metal alkyl products of oxidative addition has not been fully explored. Towards these ends, we have developed catalytic dehydrogenation of alkyl amines with a Co(I) catalyst. Amine substrates are protected with vinyl silanes, followed by catalytic transfer hydrogenation to yield a broad range of stable protected enamines and 1,2-diheteroatom substituted alkenes, including several unprecedented heterocycles. (Cp*)Co(VTMS) 2 catalyzes transfer hydrogenation under surprisingly mild conditions with high chemo-, regio-, and diastereoselectivity, while tolerating additional functionality.Twenty-five years after the seminal reports by Bergman 1 and Jones 2 on oxidative addition of C-H bonds to (Cp*)M(L) complexes of iridium and rhodium respectively, extensive progress has been made in mechanistic understanding, 3 and functionalization for applications in synthesis 4 . Cobalt, the analogous metal from the first transition series, while known to activate sp 2 aromatic and aldehydic C-H bonds 5 , has been notably absent from reports of sp 3 C-H activation. In fact, experimental 6 and computational 7 studies have indicated that C-H bonds should not oxidatively add to the 16-electron Co(I) center as they do to Ir and Rh. This communication describes for the first time the facile and highly selective activation and functionalization of sp 3 C-H bonds by [(Cp*)Co(VTMS) 2 ] (1) (VTMS = vinyltrimethyl silane), allowing synthesis of unique heterocycles. Direct functionalization of C-H bonds α to nitrogen is a particularly attractive transformation, but catalytic examples are still somewhat rare 8 . One strategy for functionalization of C-H oxidative addition products is the net dehydrogenation of organic substrates, 9 transforming C-H bonds into carbon-carbon bonds. In this area, our group has developed methodology for the synthesis of silyl enol ethers 10 and 1,2-diheteroatom alkenes 11 via [(Cp*)Rh(VTMS) 2 ] (2) -Email: mbrookhart@unc.edu. Goldman 13 has reported synthesis of enamines from 3° alkyl amines via Ir-catalyzed intermolecular hydrogen transfer. Our intra-molecular approach to hydrogen transfer is complementary and applicable to (protected) 2° amines. The silicon protecting group serves additionally as hydrogen acceptor and directing group. This strategy affords protected endocyclic enamines which are difficult to access via conventional methodology. NIH Public AccessSubstrate and catalyst screening was efficiently conducted in screw-cap NMR tubes with C 6 D 12 for convenient monitoring of reaction progress. Preparative scale reactions were then performed in Kontes flasks in pentane solvent (5 mmol substrate, 2% Co catalyst loading, 6h, 80°C) with isolated yields of metal-free products up to 90% (See Supporting Information for details).Initially we explored conversion of piperidine 3 int...