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.

Abstract Thirty phosphane ligands were prepared and characterized. Aryl groups of the phosphane ligands were modified through change in functionality. The side chains were the following: trifluoromethylphenyl, selenomethylphenyl, 9-anthryl, alkyl-substituted aryl groups, and pyridyl and alkyl groups. In addition, three chromium carbonyl complexes of potentially bidentate arylphosphanes containing nitrogen heteroatoms were prepared and characterized. Characterization of the isolated complexes verified the monodentate coordination from phosphorus and two bidentate coordination modes, (P,N)-bound and (N,N')-bound. Ligands and complexes were characterized by 1H, 13C{1H}, 31P{1H}, and two-dimensional NMR spectroscopy, X-ray crystallography, and mass spectrometry. The 13C{1H} and 31P{1H} NMR spectra, and calculated cone angles of the o-alkyl-substituted aryl- and arylalkylphosphane ligands provided valuable parameters, which could be plotted against catalytic results in the search for correlations between the structures and catalytic behavior of ligands. Correlations were found between the parameters and the catalytic behavior of Rh-catalysts modified with the o-alkyl-substituted phenylphosphanes. The research reported here was directed toward the preparation and characterization of phosphane ligands which would favor the formation of isobutanal in propene hydroformylation. The o-alkyl-substituted arylphosphanes, which were studied most throughly, gave the highest selectivity to isobutanal but at the cost of activity. Linear n-butanal was still the main product, though only barely. Alkyl substituents in meta position increased the activity of propene hydroformylation even up to the level with the reference ligand PPh3, but, the selectivity decreased simultaneously.

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. / Science, Faculty of / Chemistry, Department of / Graduate

This work discusses the C-H bond activation by transition metal complexes using various computational methods. First, we performed a DFT study of oxidative addition of methane to Ta(OC2H4)3A (where A may act as an ancillary ligand) to understand how A may affect the propensity of the complex to undergo oxidative addition. Among the A groups studied, they can be a Lewis acid (B or Al), a saturated, electron-precise moiety (CH or SiH), a σ-donor (N), or a σ-donor/π-acid (P). By varying A, we seek to understand how changing the electronic properties of A can affect the kinetics and thermodynamics of methane C–H activation by these complexes. For all A, the TS with H trans to A is favored kinetically over TS with CH3 trans to A. Upon moving from electron-deficient to electron-rich moieties (P and N), the computed C–H activation barrier for the kinetic product decreases significantly. Thus, changing A greatly influences the barrier for methane C–H oxidative addition by these complexes. Secondly, a computational study of oxidative addition (OA) of methane to M(OC2H4)3A (M = Ta, Re and A = ancillary ligand) was carried out using various computational methods. The purpose of this study was to understand how variation in A and M affects the kinetics and thermodynamics of OA. Results obtained from MP2 calculations revealed that for OA of CH4 to Re(OC2H4)3A, the order of ΔG‡ for a choice of ancillary ligand is B > Al > SiH > CH > N > P. Single point calculations for ΔG‡ obtained with CCSD(T) showed excellent agreement with those computed with MP2 methods. MCSCF calculations indicated that oxidative addition transition states are well described by a single electronic configuration, giving further confidence in the MP2 approach used for geometry optimization and ΔG‡ determination, and that the transition states are more electronically similar to the reactant than the product. Thirdly, a computational study of olefin polymerization has been performed on 51 zirconocene catalysts. The catalysts can be categorized into three classes according to the supporting ligand framework: Class I - Cp2ZrCl2 (ten catalysts), Class II - CpIndZrCl2 (thirty-eight catalysts), and Class III - Ind2ZrCl2 (three catalysts), Cp = η5-cyclopentaidenyl, Ind = η5-indenyl. Detailed reaction pathways, including chain propagation and chain termination steps, are modeled for ethylene polymerization using Class II catalysts. Optimized structures for reaction coordinates indicated the presence of α-agostic interactions in the transition states (TSs) for both the 1st and 2nd ethylene insertions as well as in the ethylene π-complex of the Zr-nPr cation. However, β-agostic interactions predominate in the cationic n-propyl and n-pentyl intermediates. The calculated relative Gibbs free energies show that the TS for insertion of ethylene into the Zr-CH3+ bond is the highest point on the computed reaction coordinates. This study, in concert with previous work, suggests that the type of ring attached to Zr (Cp vs. Ind) affects the reaction kinetics and thermodynamics less significantly than the type of substituents attached to the Cp and indenyl rings, and that substituent effects are even greater than those arising from changing the metal (Zr vs. Hf)

The morphology, crystallization and melting behaviors of a series of ethylene/alpha-olefin copolymers were investigated as a function of comonomer content, comonomer type and processing conditions, including crystallization temperature and time. This was achieved by using a combination of techniques such as Nuclear Magnetic Resonance Spectroscopy (NMR), Differential Scanning Calorimetry (DSC), Atomic Force Microscopy (AFM) and Fourier Transform Infrared Spectroscopy (FTIR). The results from the thermal analysis studies clearly indicated the existence of two distinct regions of crystallization, demarcated by a cross-over temperature, 𝑇*. The high temperature region (above 𝑇*) displayed cooling-rate dependence as well as significant hysteresis in crystallinity between cooling and heating processes, similar to that observed in linear polyethylene. This implied that the crystals associated with this region were formed via chain-folded lamellar growth. However, the lower temperature region (below 𝑇*) exhibited reversible changes in crystallinity between cooling and heating, and was found to be independent of the cooling rate. The temporal evolution of secondary crystallization in the copolymers was studied for times ranging from 100-106 min, at different crystallization temperatures (Tx). Two distinct melting endotherms were discerned at crystallization temperatures below 𝑇*. A higher melting endotherm that remained invariant with crystallization time (tx) was associated with lamellar crystals that were formed during primary crystallization. In contrast, both the positions as well as the magnitude of the lower temperature endotherm were found to vary systematically with log (tx). The peak positions of the low endotherm, i.e., the melting temperature of the secondary crystals, were found to consistently extrapolate to the crystallization temperature at very short times. Based on this and other considerations, the secondary crystals were associated with the melting of thin stacks of polymer chains aggregated in the form of "fringed-micelle"-like bundled crystals. The temperature dependence of the kinetic parameters (derived from Avrami and other analyses) above 𝑇* and their invariance below 𝑇*, suggested that the transformation in morphology from lamellar to bundled crystals was gradual and systematic, as the branch content was increased or as the crystallization temperature was lowered. Further verification of this result was obtained via AFM experiments. A systematic variation in morphology from lamellar to spot-like (lamellae were less clearly visible) was clearly discerned on increasing the comonomer content. Furthermore, a second morphological feature represented by bridge-like links between the lamellae, and approximately perpendicular to them, was also observed for some copolymers. This feature was correlated with the bundled crystals discussed above. The presence of an alternate crystal structure, in addition to the usual orthorhombic crystal form expected for linear polyethylene, was also established from the results of the FTIR studies. The relative proportions of the second crystal form in the copolymers as a function of branch content and temperature were modeled and estimated via mathematical deconvolution and curve-fitting processes. Comparing the results to those of the hexagonal rotator phase of n-paraffins, it was proposed that the second crystal structure in the copolymers could be assigned to a hexagonal type unit cell structure. Furthermore, the crystallization and melting behaviors of all three types of copolymers studied - ethylene/1-butene, ethylene/1-pentene and ethylene/1-hexene - were found to be identical to each other, suggesting that the crystallization process examined was independent of branch type for the ethyl, propyl and butyl branches examined. Since the lengthy butyl branch (in the ethylene-hexene copolymers) is not likely to be accommodated in the crystal, it was concluded that all three branch types were predominantly excluded from the crystal structure. Based on the results from these studies, a new model for the crystallization mechanism in these copolymers was proposed and could be further extended to other semicrystalline polymers such as PET, PEEK, PVC, PBT, i-PS and polycarbonate. In this model, the primary and secondary crystallization stages were redefined on the basis of the chain-folded lamellar growth process. According to the model, secondary crystallization involves the generation of the bundled crystals that may be viewed as physical cross-links in the amorphous phase. Therefore, it may provide a means of correlating the temporal evolution of secondary crystallization to the time and temperature dependence of the physical properties of semicrystalline polymers, above their glass transition temperatures. / Ph. D.

Thesis advisor: Amir H. Hoveyda / Chapter One: Stereoretentive Cross-Metathesis of Trisubstituted Olefins The development of stereoretentive olefin metathesis catalysts has solved a long-standing problem in the field, allowing for trisubstituted alkenes to be synthesized in high stereochemical purity and under kinetic control. E- as well as Z-isomers of trisubstituted alkenyl halides, nitriles, and allylic alcohols can be accessed through cross-metathesis of commercially available and easily accessible alkenes. Through the use of the same strategy, macrocyclic trisubstituted alkenes have been accessed in either isomeric form through stereoretentive ring-closing metathesis of the corresponding diene starting materials. Thus, for the first time, a wide range of E- and Z-trisubstituted alkenes can be obtained selectively through olefin metathesis, regardless of the underlying thermodynamic preferences. Chapter Two: Development of Catalytic Stereoretentive Cross-Metathesis of Trisubstituted Alkenyl Bromides We have introduced a general and widely applicable strategy for the synthesis of E- and Z-trisubstituted alkenyl bromides through cross-metathesis. The reaction is applicable to terminal, disubstituted, and trisubstituted olefins bearing a variety of functional groups including alkenes with α-, or β-branches. The requisite stereodefined cross-partners, E- and Z-2-bromo-2-butene are commercially available and can be synthesized with ease in one step from abundant starting materials. This represents a notable improvement over our previous approach, where the non-halogenated alkene starting material had to be prepared through cross-coupling in high stereochemical purity to ensure high stereoretention in the subsequent cross-metathesis. Catalysts derived from Mo monoaryloxide pyrrolide complexes, some of which are commercially available, are optimal for this transformation. The applicability of the approach is underscored through the formal synthesis of phomactin A with improved overall yield and step count. Chapter Three: Total Synthesis of Ambrein We have completed a total synthesis of ambrein, a terpenoid isolated from whale secretion, a much sought perfume ingredient. The approach involved joining two fragments through formation of the central trisubstituted alkene. Our route entailed a sequence of cross-metathesis of alkenyl bromides and cross-coupling, providing access to a previously difficult-to-access trisubstituted olefin with high efficiency and selectivity. One fragment was generated from a readily accessible enantiomerically enriched compound, sclareolide, and the other from inexpensive methylcyclohexenone. The stereogenic center of the latter was established through a NHC-Cu-catalyzed enantioselective allylic substitution, which was followed by differentiation of these alkenes through site-selective epoxidation. The total synthesis is more efficient and offers a more practical route to ambrein. Chapter Four: Stereoretentive Cross-Metathesis of Trisubstituted α,β-Unsaturated Carbonyl Compounds We have developed a strategy for the synthesis of Z- and E-Trisubstituted α,β-unsaturated carbonyl compounds through stereoretentive CM involving commercially available or easily accessible alkene substrates. The method is applicable to a variety of α,β-unsaturated esters, thioesters, and acyl fluorides. Furthermore, mono-, di-, and trisubstituted alkenes can be used as starting materials. Transformations may be carried out on gram scale and, in some cases, with commercially available Mo catalysts. The utility of the catalytic approach was highlighted through synthesis of previously accessed intermediates more directly and with improved efficiency. / Thesis (PhD) — Boston College, 2022. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

京都大学 / 新制・課程博士 / 博士(工学) / 甲第24625号 / 工博第5131号 / 新制||工||1981(附属図書館) / 京都大学大学院工学研究科材料化学専攻 / (主査)教授 松原 誠二郎, 教授 中尾 佳亮, 教授 杉野目 道紀 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

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