The Photoisomerisation of Some Chloro-Nitroso Terpenes

Hope, Alexander J. N (1955) The Photoisomerisation of Some Chloro-Nitroso Terpenes. PhD thesis, University of Glasgow.

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Investigation of the photochemical reactions attending light absorption of a number of chloro-nitroso terpene derivatives has shown a simple intramolecular rearrangement in the case of the following compounds: (i) (-)-2-chloro-2-nitrosocamphane; (ii) (+)-2-chloro-2-nitrosopinane; (iii) (-) -2-chioro-2-nitrosocarane; (iv) (+)-2-chloro-2-nitroso-p-menthane; (v) (-)-3-chloro-3-nitroso-p-menthane; (vi) i-2-chloro-2-nitroso-1-methyleyelohexane. When such photoisomerisation occurs by transformation at an asymmetric carbon atom, there is a resultant change in optical rotatory power and the process has been more accurately described as |photomutarotation'. The phenomenon was first observed with (-)-2-chloro-2-nitrosocamphane) Which has formed the parent member of the above series, the others having been structurally designed to illustrate the environmental influence of the neighbouring groups and ring system of the camphane skeleton on the chromophoric centre. Three distinct methods of investigation have been employed, studies of (a) optical rotatory power, (b) electronic absorption spectra, and (c) reaction kinetics. The first of these has found particular application in the case of (i) where spectral displacement and reversal of the Cotton effect on irradiation Indicates inversion of the chlorine and nitroso groups about the active centre. The second method has shown the batho-ehroiaic displacement of absorption characteristic of this change, and kinetic measurements illustrate a complex mechanism which has been successfully analysed to consecutive First Order component processes. Final evidence of the postulated rearrange ment has been provided by isolation of the isomeric (+)-2-chloro-2-nitrosocaphane form the product of irradiation of the (-) form. In all other cases, skeletal transformation of the normal → iso type has been found to take place, and occurs more readily than inversion of the asymmetric centre. Such a change causes a decrease in specific rotation without reversal of the original Cotton effect. The usual displacement of absorption is observed and kinetic studies have shown the overall mechanism to be similar to that of (i). Configurations have been assigned to each pair of stereo-isomeric compounds from considerations of their optical properties. These are based on the structures of (+)- and (-)-2-chloro-2-uiorosocamphsne which in turn have been derived from the established configurations of the closely related bornyl and isobornyl chlorides. It is concluded that isomerism proceeds through steric interaction of the photo-excited chromophore end vicinal alkyl group or ring system, whichever is more favourably situated. Photomutarotation results only when the molecular structure is rigidly fixed. In all other cases skeletal rearrangement is preferred.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Organic chemistry
Date of Award: 1955
Depositing User: Enlighten Team
Unique ID: glathesis:1955-79157
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 31 Mar 2020 09:09
Last Modified: 31 Mar 2020 09:09

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