Laurel L. Schafer

Broadening the scope of group 4 hydroamination catalysis using a tethered ureate ligand.

A broadly applicable group-4-based precatalyst for the hydroamination of primary and secondary amines was developed. Screening experiments involving a series of amide and urea proligands led to the discovery of a tethered bis(ureate) zirconium complex with unprecedented reactivity in the intermolecular hydroamination of alkynes and the intramolecular hydroamination of alkenes. This catalyst system is effective with primary and secondary amines, 1,2-disubstituted alkenes, and heteroatom-containing functional groups, including ethers, silanes, amines, and heteroaromatics. The gem-disubstituent effect is not required for cyclization. The catalyst is generally regioselective for the anti-Markovnikov product of intermolecular alkyne hydroamination, and chemoselective for hydroamination over alpha-alkylation when forming 6- and 7-membered rings from aminoalkenes.

Selective C−H Activation α to Primary Amines. Bridging Metallaaziridines for Catalytic, Intramolecular α-Alkylation

Selective alpha-C-H activation results in the synthesis of the first bridging metallaaziridine complex for the catalytic alpha-alkylation of primary amines. Reaction development led to the preparation of new Zr 2-pyridonate complexes for this useful transformation. No nitrogen protecting groups are required for this reaction, which is capable of assembling quaternary chiral centers alpha to nitrogen. Preliminary mechanistic investigations suggest bridging metallaaziridine species are the catalytically active intermediates for this alpha-functionalization reaction, while monomeric imido complexes furnish azepane hydroamination products.

Amidate complexes of titanium and zirconium: a new class of tunable precatalysts for the hydroamination of alkynesElectronic supplementary information (ESI) available: experimental details. See http://www.rsc.org/suppdata/cc/b3/b304176j/

A series of bis(amidate)group 4-bis(amido) complexes have been prepared, characterized and have been shown to be highly tunable precatalysts for both the intra- and intermolecular hydroamination of alkynes.

Mechanistic Elucidation of Intramolecular Aminoalkene Hydroamination Catalyzed by a Tethered Bis(ureate) Complex: Evidence for Proton-Assisted C–N Bond Formation at Zirconium

A broad mechanistic investigation regarding hydroamination reactions catalyzed by a tethered bis(ureate) zirconium species, [ureate(2-)]Zr(NMe(2))(2)(HNMe(2)), is described. The cyclization of both primary and secondary aminoalkene substrates gives similar kinetic profiles, with zero-order dependence on substrate concentration up to ∼60-75% conversion, followed by first-order dependence for the remainder of the reaction. The addition of 2-methylpiperidine changes the observed substrate dependence to first order throughout the reaction, but does not act as a competitive inhibitor. The reactions are first order in precatalyst up to loadings of ∼0.15 M, indicating that a well-defined, mononuclear catalytic species is operative. Several model complexes have been structurally characterized, including dimeric imido and amido species, and evaluated for catalytic performance. These results indicate that imido species need not be invoked as catalytically relevant intermediates, and that the mono(amido) complex [ureate(2-)]Zr(NMe(2))(Cl)(HNMe(2)) is much less active than its bis(amido) counterpart. Structural evidence suggests that this is due to differences in coordination geometry between the mono- and bis(amido) complexes, and that an equatorial amido ligand is required for efficient catalytic turnover. On the basis of the determination of kinetic isotope effects and stoichiometric reactivity, the catalytic turnover-limiting step is proposed to be a concerted C-H, C-N bond-forming process with a highly ordered, unimolecular transition state (ΔS(‡) = -21 ± 1 eu). In addition to this key bond-forming step, the catalytic cycle involves an on-cycle pre-equilibrium between six- and seven-coordinate intermediates, leading to the observed switch from zero- to first-order kinetics.

Catalytic synthesis of amines and N-containing heterocycles: Amidate complexes for selective C―N and C―C bond-forming reactions

The direct, 100 % atom-economic, and selective synthesis of amines is a challenging task that can be achieved, making use of early transition-metal catalysts. Here we report the synthesis and application of group 4 and 5 high-oxidation-state metal amidate complexes in catalytic C–N (hydroamination) and C–C (hydroaminoalkylation) bond-forming reactions to access substituted amines.

Bis(amidate)bis(amido) titanium complex: a regioselective intermolecular alkyne hydroamination catalyst.

An efficient and selective bis(amidate)bis(amido) titanium precatalyst for the anti-Markovnikov hydroamination of alkynes is reported. Hydroamination of terminal and internal alkynes with primary alkylamines, arylamines, and hydrazines is promoted by 5-10 mol % of Ti catalyst. Various functional groups are tolerated including esters, protected alcohols, and imines. The in situ generated complex shows comparable catalytic activity, demonstrating its synthetic versatility for benchtop application. Applications of this catalyst for the synthesis of amino alcohols and a one-pot procedure for indole synthesis are described. A mechanistic proposal that invokes turnover-limiting protonolysis is presented to rationalize the observed regioselectivities.

N,O-Chelating Four-Membered Metallacyclic Titanium(IV) Complexes for Atom-Economic Catalytic Reactions

Titanium, as the second most abundant transition metal in the earths crust, lends itself as a sustainable and inexpensive resource in catalysis. Its nontoxicity and biocompatibility are also attractive features for handling and disposal. Titanium has excelled as a catalyst for a broad range of transformations, including ethylene and α-olefin polymerizations. However, many reactions relevant to fine chemical synthesis have preferrentially employed late transition metals, and reactive, inexpensive early transition metals have been largely overlooked. In addition to promising reactivity, titanium complexes feature more robust character compared with some other highly Lewis-acidic metals such as those found in the lanthanide series. Since the advent of modulating ligand scaffolds, titanium has found use in a growing variety of reactions as a versatile homogeneous catalyst. These catalytic transformations include hydrofunctionalization reactions (adding an element-hydrogen (E-H) bond across a C-C multiple bond), as well as the ring-opening polymerization of cyclic esters, all of which are atom-economic transformations. Our investigations have focused on tight bite angle monoanionic N,O-chelating ligands, forming four-membered metallacycles. These ligand sets, including amidates, ureates, pyridonates, and sulfonamidates, have flexible binding modes offering a range of stable and reactive intermediates necessary for catalytic activity. Additionally, the simple form of these ligands leads to easily prepared proligands, along with facile tuning of steric and electronic factors. A sterically bulky titanium amidate complex has proven to be a leading catalyst for the selective formation of anti-Markovnikov addition products via intermolecular hydroamination of terminal alkynes, while sterically less demanding titanium pyridonates have opened the path to the selective formation of amine substituted cycloalkanes via the intramolecular hydroaminoalkylation of aminoalkenes over the competing hydroamination pathway. Sulfonamidates have boosted reactivity for hydrofunctionalization and polymerization reactions compared with amide ligands not bearing a sulfonyl group. N,O-Chelated titanium complexes have been used to synthesize ultrahigh molecular weight polyethylene and have been utilized in the challenging task of realizing equal incorporation of two different cyclic esters in a random ring-opening copolymerization. These discrete complexes have allowed for careful study of fundamental coordination chemistry and stoichiometric organometallic investigations. With inexpensive starting materials and modular ligands, titanium N,O-chelated complexes are well-suited to address the challenges of achieving greener chemical processes while accessing useful reaction manifolds for sustainable synthesis.

Catalytic asymmetric synthesis of substituted morpholines and piperazines.

Hydroamination is a powerful C N bond forming reaction that has been used for over two decades in the preparation of heterocyclic compounds. Indeed, various pyrrolidine and piperidine alkaloids, and derivatives thereof, have been prepared using a broad range of hydroamination reaction conditions. However, the extension of such strategies to biologically active heterocycles with more than one heteroatom, such as morpholines and piperazines, has only recently been achieved using late transition metal intramolecular hydroamination or carboamination catalysts. Early transition and lanthanide metals, to the best of our knowledge, have never been reported for intramolecular transformations demanding such functional group tolerance. Herein, we disclose a one-pot enantioselective synthesis of 3-substituted morpholines and a modular diastereoselective synthesis of 2,5-substituted piperazines (Scheme 1) using recently devel-

Zirconium catalyzed alkyne dimerization for selective Z-enyne synthesis

The regioselective head-to-head dimerization of alkynes is catalyzed by a dibenzyl tethered bis(ureate) zirconium precatalyst with aniline as an additive. This system also gives outstanding stereoselectivity to furnish Z-enynes in high yields. A dinuclear reactive intermediate has been characterized, which provides a potential mechanistic rationale for the unexpected regio- and stereoselectivity in this catalytic system.

2-Pyridonate Titanium Complexes for Chemoselectivity. Accessing Intramolecular Hydroaminoalkylation over Hydroamination

Chemoselectivity of intramolecular hydroaminoalkylation over hydroamination has been achieved with a bis(3-phenyl-2-pyridonate) titanium complex. Primary aminoalkenes are selectively α-alkylated by C-H functionalization adjacent to nitrogen to access five- and six-membered cycloalkylamines with a good substrate-dependent diastereoselectivity of up to 19:1.