No description available.

Metal-Ligand Cooperation (MLC) has been under study over the past two decades as a powerful tool for small molecule activation and functionalization. However, more mechanistic details are needed in order to understand the detailed steps that are enabled by the bifunctional cooperation between ligand and metal. In this regard, the hydroboration reaction offers a useful platform through which to assess the details of bifunctional reaction pathways and catalyst speciation. This dissertation focuses on the synthesis, characterization, and catalytic activity of base-metal complexes with cooperative N-, S-, and O-donor ligands to explore reaction pathways that are a consequence of diverging from traditional phosphine-based ligands. In Chapter 1 concepts and examples of MLC, especially as applied to hydroboration catalysis, are presented. In Chapter 2, three new Zn(II)-(κ²-SNS)₂ complexes were synthesized to directly compare the bifunctional catalytic activity rendered by amido and thiolate SNS ligands. Although all three complexes catalyzed carbonyl hydroboration, a detailed catalyst speciation study showed that the Zn amido complex reacts with pinacolborane (HBpin) to generate Zn-H and an unbound borylamido ligand. Subsequent substrate-derived zinc alkoxide formation followed by a second equiv of HBpin generates the product, regenerating the Zn hydride catalyst. In contrast, the Zn thiolate complex adds HBpin to the ligand imine unit, followed by aldehyde deoxygenation to give a benzothiazoline heterocycle and [Zn](OBpin). Reaction of the latter with HBpin then gives pinBOBpin and Zn-H, leading to the same active catalyst as that derived from the Zn amido precatalyst! For these systems, then, the bifunctional N- and S-donors serve to activate the catalyst rather than participating in a bifunctional catalytic cycle. Dissociation of the borylamido SNS ligand in Chapter 2 led us to reinvestigate a previously reported Cu(I) amido complex Cu[(κ²-SNS)(IPr) that was proposed to hydroborate carbonyls via an outer sphere process [IPr = bis(2,6-diisopropylphenyl)imidazol-2-ylidene]. Indeed, we showed that this complex also undergoes ligand borylation-dissociation to form the active catalyst [CuH(IPr)]₂ which had been reported previously as a carbonyl hydrosilylation catalyst. To compare these complexes with their heavier Group 10 analogue, we prepared and structurally characterized the silver amido SNS complex. Interestingly, this complex was not able to serve as a carbonyl hydroboration catalyst. Then we sought to use the MLC catalyst activation strategy to prepare an especially active Zn hydride hydroboration catalyst. Using a bidentate amine-pyrollide ligand with an aryl ether side-group, the 5-coordinate Zn complex, Zn(κ²-ONN)₂(DDI) (2.11Zn) was prepared and structurally characterized (DDI = 4,5-dichloro-1,3-dimethylimidazol-2-ylidene). On treatment with excess HBpin, formation of ONN(Bpin)₂ [(Bpin)₂-L3] gave rise to the reactive NHC-stabilized ZnH₂ catalyst that effected the rapid hydroboration of nitriles and quinoline derivatives under ambient conditions with only 0.01 and 0.05 mol% catalyst loading, respectively. In Chapter 3, in an attempt to prepare a cobalt complex containing both amido and thiolate SNS ligands, we obtained instead the Co(II) dithiolate complex, Co(κ³-SNS)(DDI) (3.2Co). This complex showed a unique selectivity for aldehyde hydroboration, over other functional groups such as ketones, cyanides, nitriles and olefins. A DFT study, in collaboration with Prof. Erin Johnson from Dalhousie University, showed that 3.2Co bifunctionally assembles the HBpin and aldehyde substrates, with Co binding the aldehyde oxygen and sulfur binding the boron of HBpin. With aromatic aldehyde substrates, interesting aromatic-aromatic dispersion effects led to catalyst inhibition which could be reversed by simply rinsing off the product with hexane. These effects were not observed for catalytic hydroboration of aliphatic aldehydes. In Chapter 4 we focused on expanding our MLC investigation to include additional donors beyond N and S. First, a dimeric Zn(II)-(κ⁴-NSNO) complex (4.1Zn) was synthesized and evaluated as a catalyst for nitrile dihydroboration to compare aryloxide and amido donors for B-H bond activation. In fact, 4.1Zn successfully catalyzed dihydroboration of a range of different aromatic and aliphatic nitriles under neat condition. Mechanistic studies determined that the aryloxide donor activates the B-H bond in the first step and the mechanism then likely proceeds through an inner-sphere insertion. As detected by our kinetic study, at high turnovers the catalyst decomposes when Bpin also binds to the amido donor. To compare the potential of other donors for B-H bond activation, a series of divalent NiᴵᴵX(κ³-NNN) complexes were synthesized, with X = bromide (4.3Ni), phenoxide (4.4Ni), thiophenoxide (4.5Ni), 2,5-dimethylpyrrolide (4.6Ni), diphenylphosphide (4.7Ni), and phenyl (4.8Ni), and employed as precatalysts for nitrile dihydroboration. Superior activity of the phenoxy derivative (vs. thiophenoxy or phenyl) suggests that B-H bond activation occurs at the Ni-X (vs. ligand Ni-N_pyrrolide) bond. Furthermore, stoichiometric treatment of 4.3Ni-4.8Ni with a nitrile showed no reaction, whereas stoichiometric reactions of 4.3Ni-4.8Ni with pinacolborane (HBpin) afforded the same Ni-H complex for 4.3Ni, 4.4Ni and 4.6Ni. Considering that only 4.3Ni, 4.4Ni and 4.6Ni successfully catalyzed nitrile dihydroboration reaction, we suggest that the catalytic cycle involves a conventional inner sphere pathway initiated by substrate insertion into Ni-H. In summary, our investigations confirm the importance of mechanistic studies and catalyst speciation for studies involving potential bifunctional catalysis. In Chapter 5 we summarize the findings of this thesis, placing them in the context of the current state of the art and speculating on future investigations they may enable.

Boron reagents are known to be valuable in the field of organic chemistry due to their abilities to undergo a variety of transformations, resulting in useful pharmaceuticals and synthetic intermediates. It has also been shown that diboron reagents can act as reaction mediators due to the unique properties of the boron atom. To that end, this dissertation discloses three novel methods of employing boron reagents. Chapter 1 describes a method of utilizing a diboron reagent mediator in the palladium-catalyzed hydrogenation of allenes. In the presence of a palladium catalyst, tetrahydroxydiboron and stoichiometric water, allene semireduction proceeds in good yield. This semireduction is regioselective for the terminal alkene and results in the selective formation of Z-alkenes when used with unsymmetrical allenes (>80:20 Z:E). It is also compatible with more sterically hindered 1,1-diarylallenes, resulting in tri-substituted alkenes in good yields (63-88%). A borylation, defluorination of alpha-trifluoromethyl-alpha,beta-unsaturated esters is described in Chapter 2. The borylation is copper-catalyzed (10 mol %) and proceeds in the presence of stoichiometric bis(pinacolato)diboron and sodium tert-butoxide. The reaction affords compounds that contain two potentially useful functional handles: boronic esters and gem-difluoroalkenes. The products are obtained in moderate to good yield (up to 75%) with a large substrate scope including compounds with electron-donating, electron-withdrawing, heteroatom, and aryl substituents. In addition, the utility of the products in further transformation is demonstrated. A proposed reaction mechanism that provides rationale for the formation of products is described along with experimental evidence. Finally, Chapter 3 describes a transition-metal-free trans hydroboration of alkynoate esters and amides. The reaction is phosphine-catalyzed and proceeds with pinacolborane to afford (E)-beta-borylacrylates and (E)-beta-borylacrylamides in good to excellent yields. The reaction products are converted into novel oxaboroles through reduction with sodium borohydride. Theoretical calculations provide mechanistic insight for the transformation. The formation of a key phosphonocyclobutene intermediate is responsible for the observed stereoselectivity. / Doctor of Philosophy / Boron reagents are valuable in the field of organic chemistry due to their abilities to undergo and to facilitate a wide variety of chemical transformations. In some of these reactions, boron is transferred onto the final molecules. Compounds containing boron are valued both as pharmaceuticals and as intermediates toward the synthesis of other products. In other transformations, the diboron reagents act as reaction mediators. Often, incorporating diboron reagent mediators allows for replacement of less favorable reactants. This dissertation describes three novel uses for diboron reagents in the field of organic chemistry. The first method employs a diboron reagent mediator—replacing flammable hydrogen gas—in the hydrogenation of allenes. The second two methods are novel borylation reactions where boron is incorporated in the final molecules. These compounds are potentially useful in pharmaceuticals and organic synthesis.

No description available.

The generation and isolation of two novel borenium cations has been described. The observation that the reaction of the Lewis acid B(C6F5)3 and the Lewis base diazabicyclo[2.2.2]octane (DABCO) with pinacol borane (HBpin) resulted in the activation of the B–H bond of HBpin and formation of a stable borenium cation/borohydride salt. This stable salt was used as a catalyst in the hydroboration reaction. It was shown to catalytically reduce a wide array of substrates including imines, N-heterocycles, nitriles, and ketones using pinacol borane as the source of hydride. Another borenium ion, synthesized from trityl tetrakis-pentafluorophenyl borate, DABCO, and HBpin did not contain a nucleophilic borohydride counterion and it was isolated in the solid state. This salt was also found to reduce the same substrates with similar yields and reaction times. The mechanisms of both of these catalysts were investigated and were found to be proceeding by a similar borenium catalyzed process. Quantitative analysis of the initial rates of each catalyst under identical conditions yielded rate constants on the same order of magnitude which strongly suggested that both catalysts operated via similar mechanisms. Stoichiometric experiments and isotope labelling using deuterated pinacol borane demonstrated that the nucleophilic counterion was not a kinetically relevant reducing agent under the reaction conditions. Furthermore, these reactions and the use of an isolable iminium ion as a hydride acceptor indicated that the hydride delivery agent was a DABCO•HBpin adduct. The DABCO•HBpin adduct was observed spectroscopically at ambient and subzero temperatures. Lastly, the rate of reduction using pinacol borane and [d1]-pinacol borane were significantly different and produced a high kinetic isotope effect (KIE = kH/kD = 6.6 ± 0.2). This high KIE strongly indicates that hydride delivery is the rate limiting step in the catalytic cycle. With this knowledge an asymmetric model is discussed and the beginnings of the development of an asymmetric borenium cation catalyzed process are described. / Thesis (Master, Chemistry) -- Queen's University, 2012-10-13 08:48:37.506

Thèse de doctorat : Chimie : Le Mans : 2005. / Titre provenant de l'écran-titre.

A rhodium-catalyzed process has been developed in which mixtures of internal and terminal olefins are isomerized and hydroborated in one step yielding the corresponding terminal pinacolboronates. Homologation and subsequent oxidation regiospecifically affords the terminal aldehyde in what amounts to a one-pot CO free hydroformylation. Good overall yields are obtained in all substrates examined. In a related study, mechanistic aspects of the rhodium catalyzed hydroboration of vinyl arenes have been probed. A combination of substituent effects (Hammett study), deuterium labeling studies and heavy atom isotope effects has demonstrated mechanistic differences in the hydroboration of electron rich and electron poor substrates. The results of the study further demonstrate the differences in reaction mechansim for hydroborations mediated with catecholborane versus pinacolborane. The Corey-Chaykovsky reaction, in which an aldehyde and a sulfur ylide are coupled to yield an epoxide has proven to be a versatile and valuable method for the production of epoxides. The reaction between benzaldehyde and benzyldimethylsulfonium tetrafluoroborate has been subjected to a kinetic analysis. Activation parameters were determined for the reaction and a large negative ΔS‡ of -35 cal/mol/K was calculated for the epoxidation of benzaldehyde. A large carbon kinetic isotope effect of 1.026 and an inverse deuterium isotope effect of 0.93 were determined for the reaction. A large positive Hammett ρ of +2.50 was found for the epoxidation of various substituted benzaldehydes by competition experiments. These results aided in the identification of the rate limiting step as addition of the ylide species to benzaldehyde. In a separate, although related study, the mechanism of the collapse of hydroxysulfonium salts has been examined with regard for implications in the epoxidation of aldehydes. The anti-diastereomer reacted with complete retention of stereochemistry and no crossover, while the syn-diastereomer gave crossover products along with cis and trans epoxides. Deprotonation and re-protonation on the carbon of the alpha-hydroxy sulfonium ylide was responsible for production of the trans epoxide as demonstrated by deuterium labeling. / Thesis (Ph.D, Chemistry) -- Queen's University, 2007-08-29 17:56:36.642

The high temperature transformations of trialkylboranes were investigated in the range: 50- 200 ºC. The extent of dealkylation was found to be linked to temperature with ca. 10% octene liberation from tri-n-octylborane at 150 ºC in the absence of bulk solvent. Analysis of the oxidised samples from the dealkylation investigation shows that, whereas the control experiment shows no back-isomerisation of tri-n-octylborane at 150 ºC, the addition of 10 mol% of DMF, DMSO, HMPA and trimethoxyphosphate results in back-isomerisation of the alkyl chain. In general, the addition of Lewis base catalyst was found to enhance the extent of dealkylation. In a supporting 11B NMR spectroscopy study to understand the interaction of trialkylboranes and Lewis bases, the interactions of a series of oxygen and phosphorous donor Lewis bases with tri-n-butylborane were found to be favourable, as indicated by large negative binding enthalpy ( HBIND) and entropy ( SBIND) values. Only the trialkylamine Lewis bases were found to have unfavourable interactions with tri-nbutylborane, as indicated by positive HBIND and SBIND values. The results also show that the chemical shift of the adduct at infinite dilution ( 11 B = ) is not as reliable a measure of the interaction between the two species and that correlation of binding constant (logKBIND) at 25.0 ºC to GBIND defines a linear trend that orders the Lewis bases according to spontaneity of the interaction with the strength of the dative bond formed. The applicability of 11B NMR spectroscopy to the study of the reactions of boranes and alkylboranes was extended to the investigation of the reduction of nitriles by BH3.SMe2 in dichloromethane (15-30 ºC). Results from the kinetic study indicate that the overall reduction with BH3.SMe2 is associative ( Sactivation = -71 ± 10 J K-1 mol-1), with the dependence of kobs data on SMe2 concentration highlighting the importance of the dissociation of the SMe2 from BH3 to the reduction process. The lack of reaction with propionitrile and benzonitrile at 25 ºC can be attributed to lack of stability of their adducts with BH3 as demonstrated by the small equilibrium constants for the formation of their adducts with borane; as determined by 1H NMR spectroscopy and further illustrated by computational calculation of their energies at the B3LYP/6-31G* level of theory. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2008.

Thesis (MSc)--University of Stellenbosch, 2006. / Bibliography. Also available via the Internet.

Thèse numérisée par la Direction des bibliothèques de l'Université de Montréal.