Study of Diverse Chemical Problems by NMR and the Design of Novel Two Dimensional Techniques
Mishra, Sandeep Kumar
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The research work reported in this thesis is focused on the chiral analysis, quantification of enantiomeric composition, assignment of absolute configuration of molecules with chosen functional groups. The weak intra-molecular hydrogen bonding interactions are detected by exploiting several multinuclear and multi-dimensional techniques. Pulse sequences have been designed to manipulate the spin dynamics to derive specific information from the complex NMR spectra encountered in diverse situations. Broadly, the thesis can be classified in to three sections. The section I containing two chapters reports the introduction of new chiral auxiliaries and protocols developed for enantiomeric discrimination, measurement of enantiomeric contents, assignment of absolute configuration for molecules possessing specific functional groups using chiral solvating and derivatizing agents. The section II, reports NMR experimental evidence for the observation of the rare type of intramolecular hydrogen bonds involving organic fluorine in biologically important organic molecules, that are corroborated by extensive DFT based theoretical calculations. The section II also discusses the H/D exchange mechanism as a tool for quantification of HB strengths in organic building blocks. The section III reports the two different novel NMR methodologies designed for deriving information on the scalar interaction strengths in an orchestrated manner. The designed sequences are able to completely eradicate the axial peaks, prevents the evolution of unwanted couplings and also yields ultrahigh resolution in the direct dimension, permitting the accurate measurement of scalar couplings for a particular spin. The brief summary about each chapter is given below. Chapter 1 provides a general introduction to one and two dimensional NMR spectroscopy. The pedagogical approach has been followed to discuss the conceptual understanding of spin physics and the NMR spectral parameters. The basic introduction to chirality, existing approaches in the literature for discrimination of enantiomers and the assignment of absolute configuration of molecules with chosen functional groups and their limitations are briefly discussed. The brief introduction to hydrogen bond, experimental methods to obtain the qualitative information about the strengths of hydrogen bonds, and the theoretical approaches employed in the thesis to corroborate the NMR experimental findings have been provided. The mechanism of H/D exchange, the utilization of exchange rates to derive strengths of intra-molecular hydrogen bond in small molecules have also been discussed. This chapter builds the bridge for the rest of the chapters. Each of these topics are discussed at length in the corresponding chapters. Part I: NMR Chiral Analysis: Novel Protocols Chapter 2 discusses a simple mix and shake method for testing the enantiopurity of primary, secondary and tertiary chiral amines and their derivatives, amino alcohols. The protocol involves the in-situ formation of chiral ammonium borate salt from a mixture of C2 symmetric chiral BINOL, trialkoxyborane and chiral amines. The proposed concept has been convincingly demonstrated for the visualization of enantiomers of a large number of chiral and pro-chiral amines and amino alcohols. The protocol also permits the precise measurement of enantiomeric composition. The significant advantage of the protocol is that it can be performed directly in the NMR tube, without any physical purification. The structure of the borate complex responsible for the enantiodifferentiation of amines has also been established by employing multinuclear NMR techniques and DFT calculations. From DOSY and 11B NMR experiments it has been ascertained that there are only two possible complexes or entities which are responsible for differentiating enantiomers. From the combined utility of DFT calculations and the 11B NMR chemical shifts, the structure of the borate complex has been determined to be an amine-coordinated complex with the N atom of the amine. Chapter 3 discusses a simple chiral derivatizing protocol involving the coupling of 2-formylphenylboronic acid and an optically pure [1,1-binaphthalene]-2,2-diamine for the rapid and accurate determination of the enantiopurity of hydroxy acids and their derivatives, possessing one or two optically active centers. It is established that this protocol is not only rapid method for discrimination of enantiomers but also highly effective for assigning the absolute configuration of various chiral hydroxy acids and their derivatives. The developed protocol involves the coupling of 2-formylphenylboronic acid with (R)-[1,1-binaphthalene]-2,2-diamine, and 2-formylphenylboronic acid with (S)-[1,1-binaphthalene]-2,2-diamine as chiral derivatizing agents. The absence of aliphatic peaks from the derivatizing agent, large chemical shift separation between the discriminated peaks of diastereomers, and the systematic change in the direction of displacement of peaks for an enantiomer in a particular diastereomeric complex, permitted the unambiguous assignment of absolute configuration. Part II : Rare Type of Intramolecular Hydrogen Bonding In chapter 4 The rare occurrence of intramolecular hydrogen bonds of the type N–H˖˖˖F–C, in the derivatives of imides and hydrazides in a low polarity solvent, is convincingly established by employing multi-dimensional and multinuclear solution state NMR experiments. The observation of 1hJFH, 2hJFN, and 2hJFF of significant strengths, where the spin polarization is transmitted through space among the interacting NMR active nuclei, provided strong and conclusive evidence for the existence of intra-molecular hydrogen bonds. Solvent induced perturbations and the variable temperature NMR experiments unambiguously supported the presence of intramolecular hydrogen bond. The two dimensional HOESY and 15N–1H HSQC experiments reveals the existence of multiple conformers in some of the investigated molecules. The 1H DOSY experimental results discarded any possibility of self or cross-dimerization of the molecules. The results of DFT based calculations, viz., Quantum Theory of Atoms In Molecules (QTAIM) and Non Covalent Interaction (NCI), are in close agreement with the NMR experimental findings. In chapter 5 the rates of hydrogen/deuterium (H/D) exchange determined by 1H NMR spectra have been utilized to derive the strength of hydrogen bonds and to monitor the electronic effects in the site-specific halogen substituted benzamides and anilines. The theoretical fitting of the time dependent variation in the integral areas of 1H NMR resonances to the first order decay function permitted the determination of H/D exchange rate constants (k) and their precise half-lives (t1/2) with high degree of reproducibility. The comparative study also permitted the determination of relative strengths of hydrogen bonds and the contribution from electronic effects on the H/D exchange rates. Part III: Novel NMR Methodologies for the Precise Measurement of 1H-1H Couplings Chapter 6 describes two novel NMR methodologies developed for the precise measurement of 1H-1H couplings. Poor chemical shift dispersion and the pairwise interaction among the entire coupled network of protons results in the severely complex and overcrowded one dimensional 1H NMR spectra, hampering both the resonance assignments and the accurate determination of nJHH. The available two-dimensional selective refocusing (SERF) based experiments suffer from the evolution of magnetization from uncoupled protons as intense uninformative axial peaks. This creates ambiguity in the identification of peaks belonging to the coupled partners of a selectively excited proton, hindering the extraction of their interaction strengths. This challenge has been circumvented by designing two novel experimental technique, cited as “Clean-G-SERF” and “PS-Clean-G-SERF”. The Clean-G-SERF technique completely eradicates the axial peaks and suppresses the evolution of unwanted couplings while retaining only the couplings to the selectively excited proton. The method permits the accurate determination of spin-spin couplings even from a complex proton NMR spectrum in an orchestrated manner. The PS-Clean-G-SERF technique has been designed for the complete elimination of axial peaks and undesired couplings, with a blend of ultra-high resolution achieved by real time broad band homonuclear decoupling has been discussed in this chapter. The spin dynamics involved in both these pulse sequences have been discussed. The diverse applications of both these novel experiments have been demonstrated.