Thesis advisor: Marc L. Snapper / Chapter 1. A Practical Synthesis of 3-Acyl Cyclobutanones by [2 + 2] Annulation. Mechanism and Utility of the Zn(II)-Catalyzed Condensation of alpha-Chloroenamines with Electron-Deficient Alkenes. We have developed a catalytic method for the synthesis of highly functionalized 3-acyl cyclobutanones which are useful building blocks for a variety of natural products. Methods for cyclobutanone synthesis have traditionally been limited to Lewis-acid promoted rearrangement of oxaspiropentanes or cyclizations of ketene and syntheses involving keteneiminium salts have required stoichiometric quantities of a Lewis acid. Furthermore, the mechanism for these types of cyclizations remains unknown. This portion of my research focused on an efficient, catalytic method for the synthesis of 3- acyl cyclobutanones and providing insight into the mechanism for cycloaddition. Chapter 2. Enantioselective Synthesis of Boron-Substituted Quaternary Carbons by NHC-Cu-Catalyzed Boronate Conjugate Additions to Unsaturated Carboxylic Esters, Ketones, or Thioesters. We have developed an enantioselective NHC-Cu-catalyzed conjugate addition of boronate esters to acyclic, trisubstituted alpha, beta-unsaturated carbonyl compounds. Enantioselective conjugate addition of a boronic acid pinacol ester through the use of bis(pinacolato)diboron [B2(pin)2,] to alpha, beta-unsaturated aliphatic and aryl esters promoted by 5 mol % of an NHC-Cu catalyst afforded tertiary beta-boryl carbonyls in high efficiency and enantioselectivity. Trisubstituted alpha, beta-unsaturated esters and thioesters were found to be reactive substrates in the presence of a stoichiometric quantity of methanol. Chapter 3. Metal-Free Catalytic C-Si Bond Formation in an Aqueous Medium and C-B Bond Formation in a Protic Medium. Enantioselective NHC-Catalyzed Silyl and Boryl Conjugate Additions to Cyclic and Acyclic alpha, beta-Unsaturated carbonyls. We have developed a method for enantioselective metal-free silyl conjugate additions through the use of dimethylphenylsilyl) boronic acid pinacol ester [PhMe2SiB(pin)] catalyzed by chiral N-heterocyclic carbenes (NHCs) in basic aqueous thf. Optimization of metal-free conditions demonstrated that the presence of water was required for high efficiency. alpha, beta-Unsaturated cyclic ketones and lactones were examined as substrates, and acyclic ketones, esters and aldehydes were also competent substrates for silyl conjugate addition. beta-Silyl carbonyls were isolated in up to >98% yield and >98:2 er. Chapter 4. Elucidation of Mechanism for Enantioselective NHC-Catalyzed Silyl Conjugate Addition. We propose a catalytic cycle for NHC-catalyzed enantioselective silyl conjugate addition. Mechanistic studies of NHC-catalyzed enantioselective silyl conjugate additions are presented. Optimization of conditions for an inefficient alpha, beta-unsaturated electron-deficient ketone provided insight into the roles for dbu and water. Kinetic data indicate that conjugate addition is first order in PhMe2SiB(pin) and carbene, and DFT calculations support the formation of an NHC-silyl anion as a reactive intermediate in the catalytic cycle. / Thesis (PhD) — Boston College, 2012. / Discipline: Chemistry.

Thesis advisor: Amir H. Hoveyda / Chapter 1 Catalytic Enantioselective Addition of Organoaluminum Reagents Catalytic methods involving the enantioselective addition of both commercially available as well as in situ generated organoaluminum reagents are reviewed. An overview of additions to aldehydes, ketones, and imines is provided as well as the difficulties and limitations of such transformations. Furthermore, additions to unsaturation adjacent to a leaving group to form a new stereogenic center are examined. Finally, conjugate addition reactions wherein an organoaluminum reagent is added to an olefin adjacent to a carbonyl or nitro group are discussed. Chapter 2 Synthesis of Quaternary Carbon Stereogenic Centers through Enantioselective Cu-Catalyzed Allylic Substitution with Alkenylaluminum Reagents A method for the formation of 1,4-diene containing quaternary stereogenic centers through catalytic enantioselective allylic substitution is disclosed. The addition of alkyl- and aryl-substituted alkenylaluminum reagents to trisubstituted allylic phosphates is promoted by 0.5–2.5 mol % of a sulfonate-containing bidentate N-heterocyclic carbene–copper complex. Products containing a quaternary stereogenic center as well as a newly formed terminal olefin are obtained in up to 97% yield and 99:1 er with high site selectivity (>98:2 SN2’:SN2). The requisite nucleophiles are generated in situ through hydroalumination of terminal alkynes. The utility of the method is demonstrated through a concise synthesis of natural product bakuchiol. Chapter 3 A Multicomponent Ni-, Zr-, Cu-Catalyzed Strategy for Enantioselective Synthesis of Alkenyl-Substituted Quaternary Carbons Despite the widespread use of conjugate addition in organic synthesis, few reports pertain to the addition of nucleophiles to acyclic systems and none in which the nucleophile is an alkene. Herein, we report the first examples of enantioselective conjugate addition of alkenylmetal reagents to trisubstituted enones to form all-carbon quaternary stereogenic centers. Alkenylaluminum nucleophiles are prepared through a site-selective Ni-catalyzed hydroalumination of terminal alkynes and the requisite E-trisubsituted enones are the products of a regioselective Zr-catalyzed carboalumination/acylation of a terminal alkyne. Products are obtained in up to 97% yield and 99:1 er. A model for enantioselectivity, supported by DFT calculations, is proposed. Chapter 4 Formation of Tertiary Centers through Catalytic Enantioselective Conjugate Addition of Alkenylaluminum Reagents to Acyclic Enones We have developed an enantioselective NHC–Cu catalyzed synthesis of tertiary centers in acyclic systems using in situ generated alkenylaluminum reagents, as current methods typically rely on Rh-catalysis at high temperatures with alkenyl boronic acids in protic solvents. Moreover, most examples include chalcone-derived substrates, which, while more reactive, often preclude further functionalization. With the current method, we are able to couple a variety of alkenyl nucleophiles with α,β-unsaturated ketones. E- or Z-silylalkenylaluminum reagents, derived from hydroalumination of silyl-protected alkynes, lead to products in good yields and high enantioselectivities. Additionally, both the α- and β-alkenylaluminum reagents participate in the reaction. Chapter 5 Development of N-Heterocyclic Carbene–Cu Catalyzed Allylic Substitution of Diboryl Methane to Morita-Baylis-Hillman Derived Allylic Phosphates We have developed a method for the coupling of a geminyl diboron reagent with Morita-Baylis-Hillman derived trisubstituted ester-containing allylic phosphates. With 10 mol % of an in situ generated NHC–Cu complex and 1.5 equivalents of the boron reagent, we are able to form the desired product in high regio- and enantioselectivity with a 2,5-ditert-butyl containing carbene. Simple aryl substituents as well as those containing a halogen or an electron-withdrawing group furnish the desired products in up to 85% yield and 98:2 er. Alkyl-containing substrates are also competent reaction partners, although longer chain aliphatics results in slightly diminished enantioselectivity. We are pursuing the application of this method to the synthesis of α-methylene lactones which can be further functionalized to natural products like tubulin polymerization inhibitor (–)-steganone and glaucoma medication (+)-pilocarpine. / Thesis (PhD) — Boston College, 2016. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

This thesis describes the synthesis, characterisation and reactivity of new manganese and rhenium(I) NHC complexes, which have been investigated both thermally and photochemically and the results contrasted with existing phosphine analogues in the literature. Cp’Mn(CO)2(NHC) (NHC = IEt2Me2 1, IMes 2, IiPr2Me2 3 and IPr 4) were synthesised and investigated by TRIR spectroscopy. Loss of CO was observed after 355 nm irradiation to form agostically stabilised intermediates, which reformed the parent species by recombination with CO on the nanosecond timescale. Loss of NHC was not observed, in contrast to Cp’Mn(CO)2(PPh3) which lost both CO and PPh3 upon photolysis. [Re(NHC)(Bpy)(CO)3]BAr4F (NHC = IEt2Me2 5, IMes 6) were synthesised and investigated by TRIR spectroscopy and UV/Vis absorption and emission spectrometry. Inclusion of an NHC altered the excited state manifold of the complexes, favouring population of the 3MLCT over the 3IL excited state. The lowest energy excited state for both 5 and 6 proved to be a 3MLCT excited state at 298 and 77 K. In contrast, [Re(PPh3)(Bpy)(CO)3]BAr4F exhibited 3MLCT at 298 K, but 3IL at 77 K. A series of complexes, M(NHC)(CO)4X and M(NHC)2(CO)3X (M = Re, X = Cl; M = Mn, X = Br) formed upon reaction of the corresponding M(CO)5X species and free NHC. The substitution pattern was dictated by the steric bulk of the NHC. Generation of the corresponding cations by halide abstraction was investigated. M(NHC)2(CO)3X was found to form agostic stabilised species upon halide abstraction by NaBAr4F in CH2Cl2. Under the same conditions, Re(IPr)(CO)4Cl was found to form the dichloromethane complex, [Re(IPr)(CO)4(η1-CH2Cl2)]BAr4F. In C6H5F solution under an atmosphere of dihydrogen, the CH2Cl2 ligand could be displaced by H2 to form the dihydrogen species, [Re(IPr)(CO)4(H2)]BAr4F.

The syntheses of a new class of polycyclic TriAmino DiCarbenes (TADCs), based on 3,9-diazajulolidine, and their precursors and adducts are described. Starting with 2,6-dimethyl-nitrobenzene, 2,6-bis ((alkylamino)methyl)anilines (alkyl = isopropyl, mesityl, and tert-butyl) were synthesized in 40% yield over five steps. These triamines were then di-cyclized stepwise to diformamidinium dications or formamidinium/2-methoxyformaminals using oxonium salts and trialkyl orthoformates. A diformamidinium dication was characterized by single-crystal X-ray diffractometry. Treatment with various bases, particularly lithium hexamethyldisilylazide, led to the novel TADCs and monocarbenes, two of which were isolated and characterized by 1H and 13C NMR spectroscopies. In both cases, treatment with elemental sulfur trapped the TADCs as dithiobiurets. No TADC-transition metal complexes were successfully isolated from reactions of the diformamidinium dications or LiHMDS TADC complex with a number of transition metal complexes. With the exception of these two cases, all other TADCs were not isolated because they rapidly reacted to form dimers, trimers, and tetramers. One of these dimers was isolated and its structure determined using 1D and 2D NMR spectroscopies, along with high-resolution electrospray ionization mass spectrometry. This revealed that the TADC had dimerized to form an ene-triamine, likely via 1,3-shift of a benzylic proton. Novel N-heterocyclic Carbene (NHC) complexes of molybdenum were also synthesized and characterized. Reaction of Cp2Mo2(CO)4 (Cp = C5H5) with dimesityl-imidazol-2-ylidenes (IMes) or dimesityl-imidazolidin-2-ylidenes (SIMes) yielded the molybdoradicals CpMo(CO)2(NHC) (NHC = IMes or SIMes). The carbonyl infrared stretching frequencies and the relative metal-to-NHC π-backbonding for IMes and SIMes complexes are compared. Reaction of the less bulky dimethyl-imidazol-2-ylidene (IMe) with Cp2Mo2(CO)4 yielded the Mo-Mo triple bond complex Cp2Mo2(CO)3(IMe) by CO substitution. This is the first example of an NHC-ligated metal-metal multiply bonded complex. Single crystal X-ray diffractometry of these new organomolybdenum and organodimolybdenum complexes is discussed.

In recent years, N-hetereocyclic carbenes (NHCs) have received considerable attention as organocatalysts due to their unusual ability to induce a reversal of reactivity (Umpolung) in aldehydes. Indeed, NHCs unique properties have been applied to the efficient and metal-free synthesis of organic compounds that have proven elusive using traditional approaches.<p> My Masters research program has been focused on the use of NHCs as organocatalysts in ring expansion reactions and their applications to cascade reactions.<p> During my Masters studies, an NHC-catalyzed efficient ring expansion of 4-, 5-, and 6-membered oxacycloalkane-2-carboxaldehydes to generate the corresponding lactone derivatives was developed. This reaction provides access to a variety of lactones using readily available NHCs under mild conditions.<p> Then, the ring-expansion lactonization has been successfully extended to an efficient lactamization using azacycloalkane-carboxaldehydes, which could provide functionalized lactams in moderate yields under mild conditions.<p> In addition, intrigued by the possibility of effecting the Umpolung of electron-poor dienes using NHC catalysts, the ring-expansion lactonization was applied to an attempted Diels-Alder-ring expansion cascade reaction. Though no cascade reactions were observed, some very interesting results were obtained, and those results will guide future investigations in this area.

In recent years, N-hetereocyclic carbenes (NHCs) have received considerable attention as organocatalysts due to their unusual ability to induce a reversal of reactivity (Umpolung) in aldehydes. Indeed, NHCs unique properties have been applied to the efficient and metal-free synthesis of organic compounds that have proven elusive using traditional approaches.<p> My Masters research program has been focused on the use of NHCs as organocatalysts in ring expansion reactions and their applications to cascade reactions.<p> During my Masters studies, an NHC-catalyzed efficient ring expansion of 4-, 5-, and 6-membered oxacycloalkane-2-carboxaldehydes to generate the corresponding lactone derivatives was developed. This reaction provides access to a variety of lactones using readily available NHCs under mild conditions.<p> Then, the ring-expansion lactonization has been successfully extended to an efficient lactamization using azacycloalkane-carboxaldehydes, which could provide functionalized lactams in moderate yields under mild conditions.<p> In addition, intrigued by the possibility of effecting the Umpolung of electron-poor dienes using NHC catalysts, the ring-expansion lactonization was applied to an attempted Diels-Alder-ring expansion cascade reaction. Though no cascade reactions were observed, some very interesting results were obtained, and those results will guide future investigations in this area.

The use of N-heterocyclic carbenes (NHCs) as ligands in catalysis is one of the most significant developments in modern catalysis and organometallic chemistry. One way to extend the scope of NHC ligand tuning is by means of annulation of carbocyclic and heterocyclic rings to the NHC backbone. The bis(imino)acenaphthene-supported N-heterocyclic carbene [IPr(BIAN)] has been synthesized and can be regarded as originating from the fusion of a naphthalene ring to an NHC. Several metal complexes of IPr(BIAN), including those incorporating copper(I), silver(I), gold(I), or iridium(I) have been synthesized and characterized, including single-crystal X-ray diffraction studies. The doncity of IPr(BIAN) was investigated using the Tolmen Electronic Parameter (TEP) method. A TEP value of 2042 cm-1 was calculated for the IPr(BIAN) ligand using the Ir(CO)2Cl complex which indicates that IPr(BIAN) is a relatively strong electron donating NHC ligand. The well-behaved redox chemistry of the BIAN ligand class rendered IPr(BIAN) an excellent candidate for exploration of the relationship between ligand charge and carbene donicity. The electrochemical reduction of IPr(BIAN) was studied by cyclic voltammetry (CV) in a THF solution and a reversible reduction wave was detected at - 1.79 V vs SCE. Spectroelectrochemical IR studies were also undertaken to further characterize the nature of the reduced state. IPr(BIAN) was found to be a stronger electron donating ligand in the reduced state in comparison with the neutral state of the ligand. IPr(BIAN) was also chemically reduced using potassium graphite and the resulting radical anion was studied by electron paramagnetic resonance (EPR) techniques. An isotropic EPR signal was observed at a g value of 2.0112. Due to the known antimicrobial activities of silver and gold NHCs, the activities of the silver and gold complexes of IPr(BIAN) and the imidazolium salts of several BIAN ligands were investigated using the minimum inhibitory concentration test. The silver(I) and gold(I) complexes of IPr(BIAN) were found to be moderately active. The most active compounds were found to be the imidazolium salts, with MIC values ranging between < 0.6 μg/mL and 78 μg/mL for the diisopropylphenyl(BIAN) and the mesityl(BIAN) imidazolium chlorides against S. aureas, B. subtilis, E. coli, and P. aeruginosa. The preparation of nanofibers impregnated with IPr(BIAN)AuCl by the process of electrospinning was also explored. The antimicrobial activities of the resulting nanofiber mats were determined on the basis of the inhibition zone test, and a localized antimicrobial activity was observed for the Gram-positive bacteria M. leuteus. / text

In an effort to develop broadly applicable photoswitchable catalysts, we have reported a method for modulating N-heterocyclic carbene (NHC) donicity using light by incorporating a photochromic diarylethene (DAE) into the backbone of a NHC scaffold. UV irradiation of 4,5-dithienylimidazolone or an analogous NHC-Ir(CO)₂Cl complex effected a photocyclization between the two thiophene rings, which led to a change in the electron donating ability of the NHC scaffold. Subsequent exposure to visible light reversed the photocyclization reaction. The concept of photo-modulating NHC donicity in this manner enabled photoswitchable NHC organocatalysis. The catalytic activity of a DAE-annulated imidazolium pre-catalyst in transesterification and amidation reactions was successfully switched between the active and nearly inactive states ([kappa]vis/[kappa]UV = 100) upon alternate UV ([lambda]irr = 313 nm) or visible ([lambda]irr > 500 nm) irradiation. The photoswitchable NHC organocatalysis was later extended to facilitating ring-opening polymerizations of cyclic esters, the rates of which were controlled via external light stimuli. Additionally, a photochromic dithienylethene-annulated N-heterocyclic carbene (NHC)-Rh(I) complex was synthesized and enabled photoswitching of the catalytic activity in series of hydroboration reactions. All of the examples demonstrate extremely rare instances of photomodulating a catalyst's activity by tuning its electronic properties. Furthermore, by taking advantage of the versatility of NHCs in both organo- and organometallic catalysis, we have developed novel photoswitchable catalysts for a variety of applicable transformations. Nanoparticles that can be transported in subsurface reservoirs at high salinities and temperatures are expected to have a major impact on enhanced oil recovery and electromagnetic imaging. We have developed an approach that will facilitate nanopaticle transport through porous media at high salinity by adsorbing or grafting rationally designed co-polymers on platform nanoparticles. Notably, co-polymers of acrylic acid with either 2-acrylamido-2-methylpropanesulfonate or styrenesulfonate have been electrostatically adsorbed or covalently grafted onto iron oxide nanoclusters. The presence of sulfonate groups on the iron oxide surface enabled long-term colloidal stability of the particles in extremely concentrated brine (8% wt. NaCl + 2% wt. CaCl₂) at elevated temperatures (90 °C) and minimized their adsorption on model mineral surfaces. / text

Despite previous extensive study of the widely-employed ruthenium-catalysed olefin metathesis reaction, the finer mechanistic details have not been elucidated. An area that is noticeably lacking is spectroscopic exploration of the relevant complexes. In this work, organometallic ruthenium complexes of importance to olefin metathesis have been investigated using Ru K-edge X-ray absorption spectroscopy. The lowest energy feature in the Ru K-edge spectrum has been unambiguously assigned as due to Ru 4d←1s transitions. These electric-dipole-forbidden transitions are extremely sensitive to geometry. For centrosymmetric complexes, the pre-edge feature has very low intensity because it is limited by the weak electric quadrupole mechanism. By contrast, non-centrosymmetric complexes exhibit a substantial increase in pre-edge intensity because Ru 5p-4d mixing introduces electric-dipole-allowed character to the Ru 4d←1s transitions. The energy of the edge feature in the Ru K-edge spectrum corresponds to ionisation of 1s electrons and is a good indicator of the charge on the metal centre. Unexpectedly, we found that the first-generation (L = PCy₃) Grubbs precatalyst (1) has a higher 1s ionisation energy than the second-generation (L = H₂IMes) complex (2). This effect provides a compelling rationale for the unexplained differences in phosphine dissociation kinetics for complexes 1 and 2: the phosphine dissociation rate of 2 is slower than 1 because the metal centre is more electron-deficient in 2. Density functional theory calculations confirm the charge differences and offer some insight into the nature of bonding in these complexes, particularly with regard to the N-heterocyclic carbene and trialkylphosphine ligands. On the basis of these results, we propose that, for this system, the NHC ligand is a weaker σ-charge donor than the phosphine ligand, and that the NHC accepts significant π-electron density from the metal; both interactions function to reduce the electron density on the ruthenium centre. An ultimate goal is to investigate reactive species in the olefin metathesis mechanism; accordingly, we have made considerable progress toward collecting XAS data for a metallacyclobutane species, and we are pursuing methods to trap the four-coordinate intermediate in the metathesis cycle.

The palladium-mediated cross-coupling of aryl electrophiles and amines (Buchwald-Hartwig amination) has become a widely used method of constructing arylamine frameworks. A crucial aspect of the advancement of this chemistry has been the design of ancillary ligands that are able to promote enhanced reactivity in challenging amination reactions. Despite significant ligand development within the field, challenges in this chemistry remain. Chemoselective aminations, wherein one amine substrate undergoes preferential arylation in the presence of multiple reactive amines has remained an underexplored area of Buchwald-Hartwig amination chemistry. This thesis describes the use of [Pd(cinnamyl)Cl]2 and N-[2-di(1-adamantylphosphino)phenyl]morpholine (Mor- DalPhos) in an extensive study of chemoselective Buchwald-Hartwig aminations, with 62 examples of structurally diverse di-, tri-, and tetraamines obtained in synthetically useful yields at reasonable catalyst loadings (1-5 mol % Pd). The coordination chemistry of [(Mor-DalPhos)Pd] species was also explored, as were complementary chemoselective aminations with the isomeric p-Mor-DalPhos ligand, leading to divergent product formation in some instances. The same [Pd(cinnamyl)Cl]2/Mor-DalPhos catalyst system used in the chemoselectivity study was also employed in a series of Buchwald-Hartwig aminations conducted under aqueous and solvent-free conditions, another underexplored area of this chemistry. A total of 52 amine products were isolated using these methodologies, moderate catalyst loadings (3 mol % Pd), and without the use of any additional additives, co-solvents, or rigorous exclusion of air. The synthesis of low-coordinate palladium complexes featuring both NHC and dialkylchlorophosphine ligands is also discussed herein. These complexes are prepared via a previously unreported and straightforward methodology involving an unusual net PCl bond reductive elimination, and represent a potential new class of pre-catalysts forpalladium-mediated reactions.