In this thesis, the synthesis and reactivity of metal complexes containing phosphido (PR2−) and phosphenium (PR2+) ligands for the hydrophosphination of alkenes were investigated. The mechanisms of hydrophosphination mediated by these M-PR2 fragments were explored. Based on previous work in the Rosenberg group, Ru(η5-indenyl) complexes were explored and developed as catalysts for hydrophosphination. It was determined that Ru-phosphido complexes are key intermediates in the hydrophosphination of electron-deficient alkenes. A detailed study on the mechanisms of hydrophosphination catalyzed by the phosphido complexes Ru(η5-indenyl)(PPh2)(L)(PPh3) (4a, L = NCPh; b, L = PPh2H; c, L = CO) was performed. Evidence for product inhibition was found for this catalyst system using Reaction Progress Kinetic Analysis. Product inhibition is consistent with the observed catalyst resting state of a complex containing product phosphines and the determination that substitution of the product phosphine from Ru is rate-limiting. The ancillary ligands (L) of 4 were found to influence catalytic activity by enabling catalyst deactivation (L = NCPh) or off-cycle processes including alkene telomerization (L = CO). Proposed mechanisms for catalysis were devised based on these findings. These results are important mechanistic insights that will be useful for designing new catalysts for hydrophosphination. The unprecedented viability of metal phosphenium complexes as intermediates in hydrophosphination was also explored. Three Mo phosphenium complexes were synthesized via P-H bond hydride abstraction from coordinated secondary phosphines, PR2H. These complexes were found to mediate the stoichiometric hydrophosphination of alkenes and ketones. In particular, trans-[Mo(CO)3(PPh2H)2(PPh2)]+ (13) mediates the hydrophosphination of a wide scope of alkenes that includes ethylene, propene and 1-hexene, which are challenging substrates for metal-catalyzed hydrophosphination. Preliminary attempts were conducted to render this synthetic phosphenium-mediated hydrophosphination catalytic. These results provide evidence for the putative steps of a hydrophosphination cycle utilizing metal phosphenium complexes as intermediates. The phosphenium complexes trans-[Mo(CO)4(PR2H)(PR2)] (12a R = Tolp2, b R = Ph) were also investigated as Lewis acid catalysts for hydrosilylation. A tentatively-assigned η1-HSiEt3 adduct of 12a, [Mo(CO)4(PTolp2H)(PTolp2{HSiEt3})] (20a), was observed by low temperature 31P{1H} NMR and was studied computationally. Complex 12b is proposed to behave as a Lewis acid catalyst for hydrosilylation. An off-cycle equilibrium is proposed that results in the formation of EtSi+. This work is a unique example of P(III) Lewis acid catalysis, of which there are few examples in the literature. / Graduate / 2020-07-29

Environmental concerns have birthed an awareness of how we conduct ourselves as citizens of this planet. To reduce environmental impact, we have learned that we must be responsible stewards in all ranges of life: from buying locally grown food to how scientific research and industrial processes are executed. In the realm of chemical research, "green chemistry" has initiated the development of new, sustainable methods that make use of atom economy, step economy, and utilize renewable materials to minimize waste and production of toxic by-products. The formation of carbon-carbon bonds lies at the very heart of organic synthesis, and traditional methods for forming such bonds generally require the use of at least one stoichiometrically preformed organometallic reagent. This corresponds to at least one equivalent of metallic waste byproduct. The in situ formation of alkyl metal nucleophiles for carbonyl additions via hydrogenation of [pi]-unsaturates represents an alternative to use of preformed organometallic reagents. Comprising nearly 90% of the atoms in the universe, hydrogen is vastly abundant and very cheap. The Krische group seeks to contribute new technologies which make use of catalytic hydrogenation and transfer hydrogenation in the reductive coupling of basic chemical feedstocks. / text

Controlling the absolute stereochemistry of molecules is a major challenge to contemporary chemists. Achieving high enantioselectivity with catalytic amounts of a chirality transfer (or inducing) agent, and the ease of regenerating such catalysts is yet another challenge. Due to the involvement of various transition metal complexes, the relatively young field of enantioselective catalysis has emerged as a powerful tool for organic chemistry. In our efforts towards the synthesis of a universal catalyst, O'Donnell Schiff base derived tetradentate ligands were shown to catalyze dialkylzinc additions to aldehydes in high selectivity. The three pot synthesis of bifurcated dipeptides in very good yields and the mechanistic aspects of diethylzinc additions to aromatic aldehydes are described in this dissertation. The chiral Lewis acidic behavior of these ligands was supported by a mechanistic study done examining the nonlinear effect. Unlike bidentate ligands such as (-)-3-exo-N,N-dimethylaminoisoborneol (DAIB), the tetradentate ligands in this study show strictly linear behavior. Also, the linear free energy relationships studied by observing the enantioselectivity with respect to electron donating or withdrawing substituents on the benzaldehyde substrates supported a Lewis acid role for the zinc complexes. A negative slope was obtained when ee's were plotted against sigma values of the substituted benzaldehydes. Since they bind to various bivalent transition metal cations, these ligands can be viewed as privileged structures, and may potentially become catalysts for various asymmetric reactions. As catalyst screening can be greatly facilitated by heterogeneous catalysis, solid phase ligands were synthesized using Wang and Merrifield resin supports. The synthetic methodology was developed using a diarylketimine linker with the aid of on-bead characterization techniques such as 13C NMR and UV-VIS spectroscopy. The ligands were shown to asymmetrically catalyze the alkenylzinc additions to aromatic aldehydes. In situ generation of alkenylzinc reagents by boron to zinc transmetalation followed by the addition to benzaldehyde in the presence chiral zinc complexes resulted in enantiomerically enriched allylic alcohols. The preliminary results for this transformation resulted in 3:1 selectivity in favor of S-isomer.

No description available.

Despite the great abundance of C–H bonds in readily available starting materials, their use in synthesis of functionalized molecules has been hampered by the high bond strengths, rendering them inert to common organic reagents. However, recent progress in the field has addressed this issue, enabling selective C–H bond functionalizations to be performed using catalytic transition metal mediated processes. Herein, the use of sulfonyl chlorides as versatile reagents for C–H bond functionalizations is reported. Using chelation assistance, the regioselective conversion of C–H bonds to either C–S, C–Cl or C–C bonds was achieved. The methodology development, substrate scope determination and mechanistic investigations will be discussed.

The use of sulfoxide ligands for transition metal catalyzed transformations has recently been brought to the forefront in organic chemistry. The synthesis of a series of tri- and disulfoxides will be presented, and their applications investigated. Their use in rhodium catalyzed 1,4-additions of phenylboronic acid to 2-cyclohexen-1-one result in enantioselectivities up to 80%. The incorporation of β-amino acid residues into polypeptides has resulted in new foldamers whose structures and enhanced stability provide interesting opportunities for new biological applications. A novel strategy for an iterative peptide synthesis involving β-amino acids will be proposed. Lastly, a hydroamidation-type strategy for the construction of β3-amino acids, or more specifically of β-(N-acylamino)acrylates, will be presented as preliminary work towards the goal of dipeptide synthesis.

Despite the great abundance of C–H bonds in readily available starting materials, their use in synthesis of functionalized molecules has been hampered by the high bond strengths, rendering them inert to common organic reagents. However, recent progress in the field has addressed this issue, enabling selective C–H bond functionalizations to be performed using catalytic transition metal mediated processes. Herein, the use of sulfonyl chlorides as versatile reagents for C–H bond functionalizations is reported. Using chelation assistance, the regioselective conversion of C–H bonds to either C–S, C–Cl or C–C bonds was achieved. The methodology development, substrate scope determination and mechanistic investigations will be discussed.

The use of sulfoxide ligands for transition metal catalyzed transformations has recently been brought to the forefront in organic chemistry. The synthesis of a series of tri- and disulfoxides will be presented, and their applications investigated. Their use in rhodium catalyzed 1,4-additions of phenylboronic acid to 2-cyclohexen-1-one result in enantioselectivities up to 80%. The incorporation of β-amino acid residues into polypeptides has resulted in new foldamers whose structures and enhanced stability provide interesting opportunities for new biological applications. A novel strategy for an iterative peptide synthesis involving β-amino acids will be proposed. Lastly, a hydroamidation-type strategy for the construction of β3-amino acids, or more specifically of β-(N-acylamino)acrylates, will be presented as preliminary work towards the goal of dipeptide synthesis.

Redox-active ligands deliver redox equivalents to impart multi-electron functionality at 3d metals that typically undergo to one electron redox events. It was proposed that 3d metals with redox-active ligands could form unusually well-defined catalysts for C-C bond forming reactions to mimic palladium-type reactivity. Therefore, several new complexes containing an iron or cobalt with redox-active ligands were synthesized and tested for their ability to form new C-C bonds. A bis(iminosemiquinone) iron (III) complex was able to homocouple aryl Grignards using dioxygen as the terminal oxidant. However, ligand redistribution prevented detailed mechanistic study of the C-C bond forming reaction and led to catalyst degradation. To address the challenges seen in the iron catalyst a new cobalt electron transfer (ET) series containing a pincer-type bis(phenolate) N-heterocyclic carbene ligand (CoNHC) was synthesized. Studies indicate the CoNHC ET series spans multiple-electrons by corporative metal and ligand redox. These complexes were evaluated for cross-coupling of alkyl halides and aryl Grignards. Mechanistic studies imply that the low cross-coupling yields were due to ligand degradation. However, CoNHC catalytically activate cross-couples ether nitriles and aryl Grignards via a novel C-O bond activation leading to a new C-C bond. Findings concerning redox-active ligands on iron and cobalt for C-C bond forming reactions and implications for future research are discussed.

The research investigations presented herein are concerned with the syntheses and applications of arylboronic acids and their derivatives; with a particular focus on their accessibility or utility in certain of the most significant modern transition metal-catalysed reactions to involve organoborons. Chapter 1 provides an introduction to the field of organoboron chemistry, from its roots employing borane and related highly reactive derivatives for uncatalysed hydroboration of olefins and acetylenes, to the modern classes of organoboron reagents of the greatest significance to the related contemporary transition metal-catalysed methodologies. Furthermore particular emphasis is placed on the discussion of arylboronic acids, their synthesis, and application to transition metal catalysis as a result of their propensity to undergo useful transmetallation events. Chapter 2 details the use of a commercially available sulfonated monophosphine ligand in the rhodium-catalysed 1,2-addition reaction employing aryl aldehydes and arylboronic acids in aqueous media. The high and continued activity of the catalytic complex is demonstrated by it being successfully recycled five consecutive times in the arylation reaction of an aryl aldehyde; as well as being active for the arylations of more sterically demanding aryl methyl ketone substrates. Chapter 3 details the design and synthesis of a novel bench-stable azidomethylene substituted arylboronate ester. The reactivity of this compound and a related analogue in both the coppercatalysed azide alkyne cycloaddition reaction and the Suzuki coupling reaction are detailed, culminating in the proof-of-concept use of such versatile synthetic building blocks in the synthesis of a drug-substance derivative. Chapter 4 details alternative synthetic approaches to that used in Chapter 3 in order to access bifunctional azidomethylene substituted arylboronate esters. In particular the application of Miyaura borylation of arylhalides bearing benzylic azides is addressed as a means to rapidly access substrates which are otherwise shown to be incompatible with classical s-block synthetic intermediates.