Structural Studies of Ylides and Other Organic Molecules by X-Ray Analysis

Duncanson, Fergus Dunbar (1975) Structural Studies of Ylides and Other Organic Molecules by X-Ray Analysis. PhD thesis, University of Glasgow.

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This thesis is divided into three parts. Part I discusses some theoretical and practical aspects of crystal structure analysis by X-ray diffraction, in particular direct methods of phase determination which were extensively used in this research. Part II deals with structural studies of stabilised ylides and some related compounds. Previous structure analyses of ylides have shown that detailed molecular geometries may be correlated with the expected stabilising bonding systems within the molecules and that the conformation, especially, may be indicative of delocalisation into an adjacent pi -electron containing group. In an attempt to establish the extent of competition between the two stabilising moieties (the 'onium species and the pi-electron-containing substituent) and to study further the relationship of conformation to the pattern of stabilisation when d-orbitals are available for bonding, the crystal structures of five ylidic compounds have been determined: two first-row ylides, trimethylammonium-N-(p-toluenesulphonyl)imine and a pyridyl analogue pyridiniura-N-(p-chlorobenzenesulphonyl)imine, two second-row ylides, a Wittig reagent 1-methoxycarbonyl-2-tert,butyloxycarbonylethyl-triphenylphosphorane and a sulphonium imine N-benzoyliminodimethyl-sulphur(lV), and an aromatic analogue of the latter compound, N-(p-nitrobenzoyl)-2-iminophenyldimethylsulphur(IV). A discussion of these structures and the trends they display in comparison with other relevant molecules is presented in the concluding section of Part II. In Part II are also described the structures of a diphenyl-phosphinamide and its N-methyl derivative. While both compounds are themselves ylides, they are the subject of additional interest on account of the marked acid lability of such compounds in comparison with carboxylic amides, which has led to their use as a protecting function for amines. The analyses have shown that the nitrogen atoms are pyramidal, although tending towards trigonal, suggesting that N-protonation may be the initial step in acid hydrolysis. In discussions of second-row ylides, in particular of sulphonium and sulphonyl-stabilised imines, the S-N bond in sulphamic acid is frequently referred to as a formal S-N single bond. In order to determine this bond length with a precision comparable to that of recent X-ray analyses, a new refinement of the structure using new three-dimensional intensity data has been carried out and is described here. In addition to the above series of related compounds, the structures of two separate unrelated compounds have been determined and are presented in Part III . The analysis of the p-bromobenzenesulphonate derivative of prieurianin, a tetranortriterpenoid natural product, has confirmed the E-lactone skeletal structure concurrently deduced from 13C n.m.r. spectra, and has yielded details of stereochemistry not otherwise obtainable. The analysis of 5,6-dihydrothiazolo [2,3-c] - [1,2,4] thiadiazol-3-one has identified the path of the condensation of chloroformylsulphur chloride with 2-aminothiasoline and similar compounds, as spectroscopic methods were unable to differentiate between the two plausible isomeric products.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Organic chemistry, Molecular chemistry, Physical chemistry
Date of Award: 1975
Depositing User: Enlighten Team
Unique ID: glathesis:1975-78692
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 30 Jan 2020 15:02
Last Modified: 30 Jan 2020 15:02

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