Laurel Schafer

Professor

Relevant Degree Programs

 

Great Supervisor Week Mentions

Each year graduate students are encouraged to give kudos to their supervisors through social media and our website as part of #GreatSupervisorWeek. Below are students who mentioned this supervisor since the initiative was started in 2017.

 

So glad to work with @LaurelSchafer, a #GreatSupervisor who constantly challenges me to improve and encourages diversity with an inaugural #UBC #WomeninChem event!

 

I owe so much to my research supervisors @JenniferLoveUBC and @LaurelSchafer for all they have taught me! During #GreatSupervisor week at UBC I'm reminded of how much work our PIs put in for us behind the scenes; all the grants, paper edits, and networking to further our success.

 

Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - May 2019)
Metal element multiply bonded group 6 complexes with 1,3-N,O-donor ligands (2018)

The full abstract for this thesis is available in the body of the thesis, and will be available when the embargo expires.

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Sequential intermolecular hydroamination of alkynes and amines towards the synthesis of nitrogen-containing compounds (2018)

No abstract available.

1,3-N,O-Chelated complexes of rhodium and iridium : harnessing metal-ligand cooperativity for bond activation processes (2017)

This thesis explores the use of 1,3-N,O chelating ligands, amidates and phosphoramidates as ligands for rhodium and iridium in the +1 and +3 oxidation states. Toward this end, a series of novel group 9 complexes were prepared, characterized, and employed for cooperative small molecule activation and element-hydrogen (E-H) bond cleavage reactions. In Chapter 1, amides and phosphoramides are introduced as ligands for late transition metals. In particular, it is highlighted that such ligands have traditionally been used to form early transition metal and lanthanide N,O-chelated complexes, with the late transition metal chemistry being highly underdeveloped. In Chapter 2, the fundamental reactivity of rhodium(I) complexes having amidate ancillary ligands is presented including catalytic oxygen atom transfer using O₂ – the products of such reactions: η²-O₂ complexes were characterized using NMR spectroscopy and density functional theory (DFT). Chapter 3 details the use of 1,3-N,O chelated complexes of monovalent Rh(I) and Ir(I) for the controlled capture of HBCy₂, providing six-membered genuine metallaheterocycles bearing a δ-B-H agostic interaction, which can be employed for chemoselective boron transfer reactions. In this chapter, the inclination of this ligand class to change the chemoselectivity of HBCy₂ hydroboration toward carbonyl-containing substrates (in the presence of an alkene) is provided. Chapter 4 examines the preparation of the first unsaturated Cp*Ir(III) (Cp* = C₅Me₅) phosphoramidate complex for use in element-hydrogen (E-H) bond activation (E = H, C, Si, B). The syntheses of coordinatively unsaturated (E)-vinyloxy Cp*Ir(III) complexes, which are prepared from regioselective 1-alkyne C-H bond activation and O-phosphoramidation is discussed. This regioselectivity is completely inverted from that of free phosphoramidates, which undergo preferential N-alkylation. Finally, we illustrate how aminoborane (H₂B=NR₂) B-N bond rotation can be accessed using joint metal-ligand stabilization between Ir and a phosphoramidate coligand. All complexes were rigorously characterized using NMR spectroscopy, X-ray diffraction, as well as by DFT. Chapter 5 surmises a new protocol for rhodium-mediated ethylene amination (nitrogen-carbon bond formation) using diazenes (RN=NR) as the N-atom source. This work provides a “proof-of-principle” for the functionalization of simple C₂-synthons using easily handled nitrogen-sources, providing rhoda(III)heterocycles, which undergo [Rh]-N bond protonolysis to provide ethyl-substituted hydrazine complexes.

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Catalytic synthesis of amines : from small molecules to nitrogen-containing polymers (2017)

The research presented in this thesis highlights the utility of N,O-chelated complexes of early transition metals for the catalytic synthesis of amines. Two atom economic transformations to make carbon-carbon and carbon-nitrogen bonds, hydroaminoalkylation and hydroamination, were investigated. A significant expansion of the substrate scope of a pre-established catalytic system is presented, and a variety of novel titanium precatalysts were developed and screened for catalytic activity. The products of hydroaminoalkylation were employed as monomers for ring-opening metathesis polymerization using known ruthenium-based catalysts. These amine-containing polymers show interesting rheological behavior attributed to the presence of hydrogen bonding interactions. The substrate scope of a previously reported tantalum phosphoramidate complex capable of room temperature reactivity was expanded to include a solvent-free protocol. This methodology was shown to afford amine products in comparable or greater yields than the related diluted reactions. Titanium metal was investigated as an alternative to tantalum-based precatalysts. A family of mono-, di-, and tri-substituted phosphoramidate complexes was presented, with mono-phosphoramidates outperforming the others in the catalytic synthesis of amines, despite being susceptible to unwanted ligand redistribution reactions. The selective mono-alkylation of cyclic diene substrates was shown to generate strained alkene products suitable for use in ring-opening metathesis polymerization. Despite having unsaturated, Lewis-basic moieties, these monomers were polymerized to generate a variety of viscoelastic homopolymers bearing substituted aryl-amines with tunable hydrogen bonding potential. Thermal and rheological analysis revealed behaviours characteristic of hydrogen bonding, such as increased glass transition temperatures, and viscosity dependent on molecular weight and amine substitution. An expansion of the monomers amenable to polymerization was presented, and a variety of copolymeric structures were synthesized. Efforts focused on pairing amine-containing monomers with those of commodity plastics, such as norbornene. Excellent control over monomer incorporation was achieved in most cases. Preliminary investigations into useful applications of these novel polymers were presented and include anti-microbial materials, agents for sequestering metal ions, and compatibilizers for polymer blends. These preliminary experimental results show promise and future efforts will focus on realizing the full potential of these unique materials.

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Catalytic synthesis of N-heterocycles and alpha-alkylated amines by hydroamination and hydroaminoalkylation (2017)

The research presented in this thesis emphasizes the versatility and utility of N,O-chelated early transition metals for the catalytic synthesis of -alkylated amines. Two major transformations were studied extensively in this work, hydroamination and hydroaminoalkylation. For both reactions, the synthetic utility and substrate scope has been expanded by the work presented herein. In the field of hydroamination, N-heterocycles with more than one heteroatom can now be synthesized using early transition metal catalysts from prochiral substrates. Hydroamination with a bis(amidate)bis(amido) complex of titanium of ether-containing aminoalkyne substrates yield cyclic imines, which are subsequently reduced via asymmetric transfer hydrogenation using the Noyori-Ikariya catalyst, RuCl [(S,S)-Ts-DPEN] (η⁶-p-cymene). 3-Substituted morpholines are synthesized using a one-pot sequential catalysis protocol, in good yields and high enantiomeric excesses. Substrate scope investigations reveal that high enantioselectivities in the asymmetric transfer hydrogenation reaction arise from key hydrogen bonding interactions between the oxygen heteroatom of the ether-containing cyclic imine and the [(S,S)-Ts-DPEN] ligand of Noyori-Ikariya catalyst. This mechanistic insight informed the proposal that this synthetic strategy can be extended to other substrates containing functional groups with hydrogen bond acceptors. As such, 3-substituted piperazines are also prepared with high enantioselectivities using this one-pot protocol.Advances to the hydroaminoalkylation transformation have also been made with the first reported example of room temperature reactivity observed using a phosphoramidate-tantalum complex. The preparation and characterization of a series of N,O-chelated phosphoramidate-tantalum complexes is described. These complexes were easily synthesized from either Ta(NMe₂)₅ by protonolysis or a simple organometallic precursor, TaMe₃Cl₂, by salt metathesis. Reactivity towards catalytic hydroaminoalkylation was explored and the results highlight that the choice of tantalum starting material dramatically affects the reaction temperatures required for catalytic turnover. N,O-chelated phosphoramidate dimethylamido tantalum complexes showed reactivity occurred only at elevated temperatures (≥ 90 °C), whereas phosphoramidate-tantalum complexes derived from TaMe₃Cl₂ exhibited unprecedented catalytic activity at room temperature. Preliminary efforts indicate that there is potential for an asymmetric version of hydroaminoalkylation at room temperature. Chiral phosphoramidate-tantalum complexes were prepared and studied as the first examples of asymmetric hydroaminoalkylation reactions at room temperature.

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Early transition metal complexes of pyridine derivatives : applications in the catalytic synthesis of amines and N-heterocycles (2015)

The work described in this thesis focuses on the development of pyridine-derived group 4 and 5 complexes for application in the catalytic synthesis of selectively substituted amines. Two different catalytic alkene hydrofunctionalization reactions were targeted: hydroamination and hydroaminoalkylation. Respectively, these transformations provide atom-economical strategies for the formation of new C–N and C–C bonds on amines, using simple alkenes as the alkylating agents. A series of bulky mono(2-aminopyridinate)tris(dimethylamido)titanium complexes with varying steric parameters were synthesized and explored for intramolecular hydroamination reactivity using aminoalkene substrates. A titanium catalyst capable of room-temperature hydroamination reactivity was identified for the synthesis of gem-disubstituted 5- and 6-membered-ring products. This catalyst has good breadth of reactivity, including the challenging 7-membered-azepane ring formation and hydroamination with internal alkenes. A catalytically active 2-aminopyridnate-supported imido titanium complex was prepared, and reactivity investigations of this complex suggest that this reaction proceeds via an intermediate imido [2+2] cycloaddition pathway. Various 3-substituted-2-pyridonate ligands were synthesized and examined as ancillary ligands for targeting chemoselectivity for intramolecular hydroaminoalkylation over hydroamination. Systematic ligand screening studies showed that bis(3-phenyl-2-pyridonate)bis(dimethylamido)titanium complex is selective for hydroaminoalkylation over hydroamination. This is the first catalyst that can selectively α-alkylate primary aminoalkenes to access both 5- and 6-membered-cycloalkylamines with good substrate-dependent diastereoselectivity. Mechanistic and stoichiometric experiments using this complex support the involvement of a bimetallic imido species in the reaction. Notably, a titanium(III) species was isolated during these investigations. Reliable synthesis of this titanium(III) complex and examination of its reactivity showed that it is not active for hydroaminoalkylation. Varying combinations of mixed 2-pyridonate/alkyl/amido/chloro tantalum complexes were targeted to expand the substrate scope for intermolecular hydroaminoalkylation. The synthesis of mono(2-pyridonate)/alkyl/chloro tantalum complex was unsuccessful. Instead, bis(2-pyridonate)tantalum alkyl complexes were formed. While these complexes showed hydroaminoalkylation reactivity for terminal alkenes, their thermal and light sensitivity presents difficulty for synthetic application. The design and synthesis of a mixed 2-pyridonate-Ta(NMe₂)₃Cl provided a sterically accessible metal center for the hydroaminoalkylation of sterically demanding disubstituted alkenes. This complex is the first effective precatalyst for the alpha-alkylation of unprotected secondary amines using unactivated (E)- and (Z)-internal alkenes without C=C bond isomerization.

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Synthetic applications of zirconium and titanium amidate complexes (2015)

The use of titanium or zirconium amidate complexes as either reagents or catalysts for targeted applications is described herein. The investigation is focused on a novel class of zirconocene amidate hydride complexes primarily used for the hydrozirconation reaction and a previously disclosed bis(amidate) bis(amido) titanium complex for the regioselective alkyne hydroamination reaction. A novel class of zirconocene amidate hydride complexes is synthesized and characterized. The amidate binding mode is significantly influenced by sterics. A rare example of an equilibrium between the structural isomers where the amidate ligand adopts either the κ¹ O-bound or κ² is shown. Reaction of styrene with these complexes resulted in the formation of the branched insertion products, which is in contrast to the observed regioselectivity when the well-known Schwartz’s reagent is used. Asymmetric insertion was attempted with a complex bearing an amidate ligand with a stereocenter. Reactivity with other alkenes and phenylacetylene were explored. Under harsh reaction conditions, the zirconocene amidate hydride complexes undergo a halide exchange reaction with phenyl halides. A competition experiment suggests two separate mechanistic pathways for styrene insertion and halide exchange. A primary kinetic isotope effect suggests Zr─H cleavage is involved in the rate-determining step. Experimental evidence is consistent with a coordination-insertion mechanistic proposal. The synthetic utility of a previously reported bis(amidate) bis(amido) titanium complex for the regioselective alkyne hydroamination reaction is further explored. The reactivity and regioselectivity of hydroamination with benchmark substrates using this bis(amidate) titanium complex is directly compared to other titanium based hydroamination catalysts. The substrate scope of this bis(amidate) titanium hydroamination catalyst is extended to include more difficult substrates, such as protected propargyl alcohols. Modifications to the reaction protocol allow for facile bench-top use. The bis(amidate) titanium complex was applied to tandem sequential reactions featuring hydroamination to afford secondary amines, a primary amine and a substituted primary allylamine. This hydroamination catalyst is also used for the oligomerization of alkynylanilines. By tuning the alkynylaniline monomer, a soluble N-containing oligomer was synthesized, which shows a degree of conjugation. This bis(amidate) titanium hydroamination catalyst was employed to assemble a small library of aminoether compounds targeted as T-type calcium channel blockers.

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Bis(amidate) and bis(ureate) complexes of zirconium and tantalum : synthesis and catalytic application in C-N and C-C bond formation (2014)

This is a study of early metal organometallic complexes for catalytic C-N and C-C bond formation using amines. The use of zirconium complexes as hydroamination catalysts is explored first. New axially-chiral bis(amide) and bis(urea) proligands are designed. Synthetic methods used to generate these compounds are described and X-ray crystallographic analysis of a bis(sulfonamide) establishes the absolute configuration of the chiral proligands. Installation of the new bis(amidate) and bis(ureate) ligands onto zirconium is undertaken and the structure of these complexes is examined in solution and, where possible, in the solid state. Where well-defined zirconium complexes can be obtained, those complexes are tested for their efficacy in enantioselective catalytic hydroamination. In the absence of well-defined zirconium complexes, an in situ catalyst generation protocol is employed. Catalysts featuring a bis(amidate) ligand derived from 2,2ʹ-diamino-6,6ʹ-dimethylbiphenyl achieve modest hydroamination activity with a primary aminoalkene at 110 °C, with enantiomeric excesses (ee’s) of up to 25%. Catalysts featuring a bis(amidate) ligand derived from 3,3ʹ,5,5ʹ-tetrabromo-2,2ʹ-diamino-6,6ʹ-dimethylbiphenyl display impressive reactivity with a primary aminoalkene, including room temperature hydroamination, with ee’s ranging from 52-55%. Catalysts featuring a bis(ureate) ligand derived from 2,2ʹ-diamino-6,6ʹ-dimethylbiphenyl are shown to be capable of cyclizing secondary aminoalkenes with ee’s up to 63%. Capillary electrophoresis is developed as a method to determine the ee of tertiary amine products. A kinetic study of a catalyst featuring a bis(amidate) ligand derived from 2,2ʹ-diamino-6,6ʹ-dimethylbiphenyl supports established mechanistic proposals for neutral group 4 hydroamination catalysts. Solid state molecular structures, combined with existing knowledge of bonding and catalytic reaction pathways, are used to propose models for how enantioselectivity is achieved through the use of different ligand frameworks.The use of known ligands in the formation of tantalum complexes for hydroaminoalkylation catalysis is then explored. Installation of such axially-chiral bis(amidate) ligands onto tantalum centres is undertaken and the structure of these complexes is examined in solution and, where possible, in the solid state. Catalytic testing reveals general competence of these catalysts for the hydroaminoalkylation reaction.

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Development of a Molecular Synthesis of Asymmetrically Substituted Heterocycles with Functional Group Tolerant N, O-Chelated Group 4 Hydroamination Precatalysts (2014)

No abstract available.

Synthesis, structure, and reactivity of early transition metal precatalysts bearing (N,O)-chelating ligands (2014)

The synthesis, structure, and reactivity of early transition metal complexes containing(N,O)-chelating ancillary ligands are described. The ligands investigated include ureates,pyridonates, amidates, and sulfonamidates. These related ligands generate four-memberedmetallacycles when bound to the metal center in a κ²-(N,O) fashion. The zirconium and tantalumcomplexes have been examined in terms of their activity and selectivity as precatalyst systemsfor hydroamination or hydroaminoalkylation.A chiral cyclic ureate ligand has been synthesized from enantiopure L-valine forapplication in zirconium-catalyzed asymmetric hydroamination of aminoalkenes. Chiralzirconium complexes, prepared in situ from two equivalents of the urea proligand andtetrakis(dimethylamido) zirconium, promote the formation of pyrrolidines and piperidines in upto 12% ee. Isolation of an asymmetric bimetallic zirconium complex containing three bridgingureate ligands confirms that ligand redistribution occurs in solution and is most likelyresponsible for the low enantioselectivities.Mechanistic investigations focusing on the hydroaminoalkylation reactivity promoted bya bis(pyridonate) bis(dimethylamido) zirconium precatalyst expose a complex catalytic system insolution. Stoichiometric investigations reveal the formation of polymetallic complexes uponaddition of primary amines. The kinetic and stoichiometric investigations are most consistentwith a bimetallic catalytically active species.A series of mono(amidate) tantalum amido complexes with varying steric and electronicproperties have been synthesized via protonolysis. Solid-state and solution-phasecharacterization indicate that the amidate substituents influence the observed binding mode of the ligand. Salt metathesis and protonolysis routes to the synthesis of mixed tantalum chloro amidate complexes are investigated. Sulfonamide proligands react with pentakis(dimethylamido) tantalum to generate well-defined monomeric complexes containing a κ²-(N,O) bound sulfonamidate. The hemilabile (N,O)-chelating amidate ligands, which generate four-membered metallacycles, are the most active of the precatalysts examined for the intermolecular hydroaminoalkylation of terminal olefins with secondary amines.The substrate scope of a mono(amidate) tetrakis(dimethylamido) tantalum complex has been examined for the α-alkylation of unprotected piperidine, piperazine, and azepane N-heterocyclic amines. The lack of reactivity with pyrrolidine substrates is examined by quantum chemical calculations and isotopic labeling studies. Two (N,O)-chelating ureate ligands are also successful ancillary ligands for this transformation and, with a C₁-symmetric chiral ureate complex, enantioselective α-alkylation of piperidine is observed.

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Development of group 4 and 5 complexes with N,O chelating supporting ligands as catalysts for the alpha-alkylation of amines (2013)

The use of stoichiometric, catalytic and theoretical methods in the development of an early transition metal catalyst for the α-alkylation of amines is described herein. The investigation is primarily focused on a series of mono(amidate) complexes of tantalum with varying steric and electronic properties. The amidate binding mode and catalytic activity of these complexes is significantly influenced by sterics. Corresponding bis(amidate) complexes are less active as catalysts for the α-alkylation of amines but offers a platform to study the hemi-lability of amidate ligands as well as tantalaziridine formation in these systems. A model 5-membered metallacycle is synthesized and characterized.Isotopic labeling studies with the most active mono(amidate) precatalyst reveal off-cycle reactions and suggest that tantalaziridine formation is rapid and reversible. Preliminary kinetic investigations implicate alkene insertion as the turnover limiting step, consistent with stoichiometric investigations. In addition, the use of radical probes in ligand backbones and an alkene substrate contradicts a one electron mechanism.Quantum chemical calculations are used to develop a theoretical model of the proposed catalytic cycle. The hemi-lability of amidate ligands is highlighted with the optimization of both κ¹(O) and κ²(N,O) minima and transition states. Here, protonolysis is calculated to be the turnover limiting step with small changes in geometry having a significant effect on the potential energy surface. The unlikelihood of a radical mechanism is supported by the computations of triplet species. A survey of established steric parameters has been completed for asymmetric amidate ligands to be used as a predictive tool for catalyst design. The calculated values can be related to the catalytic activity of mono(amidate) and axially chiral tantalum precatalysts. Diamide and diurea proligands featuring a neutral chalcogen atom tether are installed on zirconium and tantalum. The zirconium species form well-defined κ⁴(N,N,O,O) complexes with fluxional behaviour observed for the tantalum species in solution. No evidence of bonding is observed between the chalcogen donor and any metal centre. Fundamental differences in the redox potentials for ligands and complexes are investigated using cyclic voltammetry. The tantalum complexes are found to catalyze the α-alkylation of amines with the zirconium species being competent precatalysts for hydroamination.

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Rheology and processing of biodegradable poly (E-Caprolactone) polyesters and their blends with polylactides (2013)

No abstract available.

Yttrium amidate complexes for the catalytic synthesis of biodegradable polymers and amides (2013)

Yttrium complexes are highly attractive systems for use in catalysis due to their high activity, low cost, and low toxicity. The highly modular amidate ligand set allows for easy variation of steric and electronic properties and, therefore, the reactive metal complex. This thesis explores the synthesis of yttrium amidate complexes and their use as catalysts in a variety of catalytic transformations.Mono-, bis-, and tris(amidate) complexes of yttrium are highly active initiators for the ring-opening polymerization of rac-lactide, yielding PLA with high molecular weight and narrow polydispersity. Termination of this polymerization is proposed to occur through formation of cyclic PLA with large ring sizes, once the monomer is consumed. Reaction of additional monomer is, therefore, not possible, and the polymerization is not living.The mechanical properties of the poly(ε-caprolactone) synthesized using tris(amidate) complexes of yttrium were determined to be consistent with those of commercially available polymer samples. Rheological testing of prepared poly(ε-caprolactone) suggested the possible formation of polymers with long-chain branching. However, the formation of long-chain branching in the poly(ε-caprolactone), synthesized with the tris(amidate) complex containing the naphthyl substituent, could not be confirmed or wholly refuted based on traditional chemical analyses. One yttrium tris(amidate) complex was also found to initiate the ring-opening polymerization of ε-caprolactone and rac-lactide to form diblock PCL/PLA copolymers.The mono-, bis-, and tris(amidate) complexes of yttrium are also highly active catalysts in the mild amidation of aldehydes with amines. The tris(amidate) complexes were most active, catalyzing the reaction in as little as 5 minutes. The best-performing catalyst can mediate the amidation of alkyl or aryl aldehydes, as well as aryl primary amines and alkyl or aryl secondary amines. This catalyst is also the first example of a rare-earth catalyst capable of tolerating pivalaldehyde in this transformation.The aforementioned class of complexes has been proven to be effective in a number of catalytic transformations. The modular synthesis of the amidate ligand, as well as the facile synthesis of yttrium amidate complexes, provide an easy means for altering the steric and electronic properties of the resulting complexes and, therefore, are ideal for further catalyst development.

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Bis(amidate) titanium, zirconium, and tantalum complexes : applications in catalytic synthesis of N-containing compounds (2012)

A two-part study involving the synthesis and catalytic investigations of amidate complexes of early transition metals is described. In the first part, the substrate scope of an achiral bis(amidate) titanium hydroamination precatalyst has been broaden to include challenging substrates. This complex efficiently catalyzes the hydroamination of heteroatom-containing allenes with arylamines. Control experiments rule out allene-alkyne isomerization as a reaction pathway during this catalysis. The hydrohydrazination of a variety of alkynes with 1,1-disubstituted hydrazines also proceed efficiently in the presence of this precatalyst giving the anti-Markovnikov hydrazone products predominantly. These hydrazones have been transformed into substituted indoles by a one-pot tandem sequential hydroamination/ZnCl₂-mediated cyclization. Importantly, the bis(amidate) titanium precatalyst can be generated in situ for these reactions with no impact on the reactivity or selectivity of the complex.The second part of this thesis focuses on the synthesis, characterization, stability and catalytic investigations of chiral zirconium and tantalum complexes ligated with amidate ancillary ligands. Seven new axially chiral proligands have been synthesized and used for in situ generation of zirconium hydroamination precatalysts. These chiral complexes efficiently produce N-heterocycles in up to 94% isolated yield with ee reaching 74%. Preparative scale synthesis and characterization of the zirconium complexes revealed coordination geometry that is greatly influenced by the steric properties of the ligand. Bulky proligands produce monomeric complexes, wherein the biphenyl ligand displays a κ²-O,O-bonding motif which accounts for the modest enantioselectivities realized with this system. The less sterically-congested proligands initially form similar monomeric complexes; however, these complexes dimerize diastereoselectively to κ⁴-N,O,O-N-bonding amidate complexes within a few hours in solution. The binding motif of the amidate ligand of the chiral biphenyl tantalum complexes is also dictated by the size of the N-substituent of the ligand. While a bulky proligand results in a discrete tantalum κ²-O,O-bonding amidate complex, less sterically-encumbered proligands produce a mixture of κ²-O,O-bonding and κ³-N,O,O-bonding amidate complexes. Using these tantalum complexes as precatalysts, alkenes undergo hydroaminoalkylation reactions with secondary amines to give branched alkylated secondary amines in isolated yields of up to 92% and enantiomeric excesses reaching 66%, for the first examples of an enantioselective hydroaminoalkylation reaction.

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Group 4 ureate complexes : synthesis, reactivity, and catalytic carbon-element bond formation (2010)

The synthesis, characterization, and reactivity patterns of three new classes of ureate- supported group 4 compounds are described, constituting the first comprehensive examination of the use of these electron-rich ligands. Four ureates with varying steric and electronic properties are included here: two mono(ureate)s and two tethered bis(ureate)s.Synthesis of dichlorobis(ureato) titanium and zirconium derivatives can be accomplished using several methods, with direct protonolysis between the urea proligand and M(NMe₂)₂Cl₂ precursors giving the highest yield. Solid-state and solution-phase characterization indicates the distal dialkylamino substituent on the ureate donates electron density into the chelate. While use of non-tethered ligands results in zirconium complexes that are fluxional in solution, tethered bis(ureato) ligands support well-defined species. These complexes retain neutral ligands, which are not easily removed. Ureate- supported zirconium dialkyl complexes can also be prepared by protonolysis between a urea and tetraalkyl zirconium compounds. The electron-rich nature of the ureate ligands allows the isolation of coordinatively unsaturated dialkyl complexes.Ureate-supported bis(amido) compounds of titanium and zirconium have been developed as precatalysts for hydroamination. A comprehensive structural comparison between related amidate and ureate complexes reveals that the ureate ligands bind tighter to their metal centers than amidates. As a consequence of this, and the electron-rich nature of the ureate ligands, amidate precatalysts are generally more effective for intramolecular hydroamination of alkenes than ureate precatalysts. In contrast, precatalysts with tethered ureate ligands are more effective than analogous amidates. The most active system identified through catalytic screening exhibits broad substrate scope and functional group tolerance. Most importantly, this is the first group 4 system that is highly effective with both primary and secondary amines.Mechanistic investigations have revealed that catalysis with the tethered bis(ureate) precatalyst does not proceed through an imido-mediated [2+2] cycloaddition-type mechanism. Instead, the key bond forming step is proposed to occur through concerted insertion of the alkene into a Zr–N bond and protonation of the terminal alkene carbon by a coordinated amine ligand. This proposal is supported by stoichiometric and kinetic investigations, which indicate that a proton-source accelerates alkene insertion, and a primary kinetic isotope effect when using an N-deuterated aminoalkene.

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Hydroamination and C-H activation reactivity of tetrakis(amido), bis(amidate) and bis(2-pyridonate) complexes of titanium and zirconium (2009)

The work reported herein focuses on expanding the reaction scope of known groupfour bis(amidate) and tetrakis(amido) complexes in hydroamination catalysis. Thedevelopment of new titanium and zirconium complexes exhibiting improved reactivity inhydroamination catalysis and unexpected C-C bond formation are disclosed. Theexceptional hydroamination activity of a bis(amidate) titanium bis(amido) precatalysttowards alkynes in the presence of aryl amine co-substrates is elucidated, and the scopeof this reactivity was found to include examples of room temperature intermolecularhydroamination. The application of commercially available tetrakis(dialkylamido)titanium(IV) as a precatalyst for the cyclohydroamination of aminoalkenes to form Nheterocyclic products is a particularly attractive contribution due to the ready availabilityand ease of use associated with this catalyst system.The second section involves efforts to develop more reactive and selectivebis(amidate) bis(amido) hydroamination precatalysts by the rational design andimplementation of new amidate ligands modified for enhanced reactivity and selectivityincluding attempts at enantioselective catalysis. The synthesis and characterization of abis(amidate) titanium bis(amido) complex incorporating electron withdrawingperfluorophenyl groups for enhanced reactivity, along with the assessment of this systemin terms of hydroamination is presented. The synthesis, characterization and evaluation ofchiral amidate ligands for the asymmetric cyclohydroamination of aminoalkenes is alsodescribed.In order to generate more reactive group four hydroamination precatalysts, 2-pyridone and its derivatives were investigated as a new class of amidate N,O chelating proligand. The synthesis and characterization of the first group four bis(2-pyridonate)bis(amido) complexes is presented along with their reactivity towards aminoalkenes.These novel complexes were found to be reactive for both cyclohydroamination andcatalytic intramolecular a-functionalization. The initial findings along with a substratescope analysis, and preliminary mechanistic investigations for this unique and exciting100% atom economic, catalytic C-C bond forming reaction is included.The work described in this dissertation contributes to understanding of group fourmetal catalyzed reactions by illuminating some previously unknown reactivity associatedwith titanium and zirconium as well as by providing further insight into how ligandstructure influences complex reactivity.

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Yttrium amidate complexes : fundamental reactivity and applications in catalysis and polymerization (2009)

Rare-earth complexes are attractive catalyst systems due to their low cost, low toxicityand high reactivity. Modular ligand sets are ideal for complex formation since the steric andelectronic properties of the resultant metal complexes can be easily varied. This thesisexplores the structure and reactivity of new yttrium amidate complexes, which combine thehighly reactive metal with the modular amidate ligand set. A library of tris, bis andmono(amidate) yttrium complexes have been directly synthesized from yttriumtris(trimethylsilyl)amidate and simple amide proligands.The tris(amidate) yttrium complexes are highly active initiators of ring-openingpolymerization of Ɛ-caprolactone, yielding some of the largest molecular weight values forpoly(Ɛ-caprolactone) reported. The initiation of this polymerization is proposed to be ligandinitiated; however, a side-reaction is postulated where formation of a Ɛ-caprolactone-enolateyttrium complex results in broad polydispersity values of the resultant polymers.The bis(amidate) yttrium complexes are also excellent precatalysts for thehydroamination of aminoalkenes. Simple modification to the amidate backbone to includeelectron-withdrawing groups was found to significantly enhance reaction efficiency. Thesecatalysts can mediate cyclohydroamination with both primary and secondary aminecontaining substrates.The mono(amidate) yttrium complexes were also investigated as novel precursors for thesynthesis of the elusive terminal yttrium imido complex. Mixed anilido/amidate yttriumcomplexes were synthesized in high yield and a-H abstraction and deprotonation reactionswere attempted in the hopes of isolating a crystalline compound. The addition of monodentate and neutral donors was required for isolation and characterization of the keyreactive intermediates.This new family of yttrium complexes has proven to be very successful in preliminarycatalytic studies. The ease with which the complexes can be synthesized and their steric andelectronic properties make these complexes ideal for further catalytic investigations.

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