| | 4 | Author
| D. Bryce-Smith, A. Gilbert, N. Al-Jalal, R. R. Deshpande, J. Grzonka, M. A. Hems, P. Yianni | Requires cookie* | | | Title
| Acid and Polarity Effects in Benzene Photoreactions with Alkenes and Alkynes  | | | | Abstract
| The effects of polar solvents and proton donors on various photoreactions of the benzene ring with electron-acceptor alkenes and alkynes are described and discussed in relation to the various electronic excitation mechanisms involved. Proton donors prove valuable as mechanistic probes for polar intermediates in such processes, and can in some systems both initiate and divert reaction pathways. In 1970 we observed that the course of the photo-chemical cycloadditions of certain ethylenic and acetylenic compounds to benzene, earlier reported by us [1, 2], was profoundly altered in the presence of proton donors, leading to a type of photochemical Friedel-Crafts reaction [3]. Since then, we have described, mostly in preliminary form, numerous further examples of acid catalysis in the photo-reactions of benzene, viz. with amines [4], ethers [5], a ketone [6], a carboxylic acid [6], and hexa-fluorobenzene [7]. Our objective in this present paper is to bring together the previous preliminary reports on acid catalysis in the photoreactions of ethylenic and acetylenic compounds with benzene in conjunction with unpublished recent findings, and to provide a mechanistic rationale together with relevant experimental details. But before we proceed to consider the acid-cata-lysed processes, it is desirable to summarise the main types of reaction that have been shown to occur in the absence of acids. Ethylenes and/or acetylenes photoadd to benzene in the following four main ways: (a) ortho cycloaddition; (b) meto-cycloaddi-tion; (c) £>ara-cycloaddition; and (d) para-ene addi-tion [8, 9]. The mode of addition varies with the addend, and in some cases more than one pathway may be followed. In the case of (a), the adducts (1) of ethylenes may be obtained as such when the ethylenic addend is not a strong thermal dienophile, e.g. acrylonitrile [10], 2,3-dihydropyran [11]. When it is a strong dienophile, as in the case of maleic * Reprint requests to Prof. D. Bryce-Smith. 0340-5087/83/0900-1101/$ 01.00/0 anhydride and certain maleimides, a second mole-cule may add thermally to give the remarkably | | | |
Reference
| Z. Naturforsch. 38b, 1101—1112 (1983); received May 2 1983 | | | |
Published
| 1983 | | | |
Keywords
| Photoreactions, Benzene, Alkenes, Alkynes, Acid Effects | | | |
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| default:Reihe_B/38/ZNB-1983-38b-1101.pdf | | | | Identifier
| ZNB-1983-38b-1101 | | | | Volume
| 38 | |
| 6 | Author
| H. Ernst3, M. Fechtelkord\, D. Fenzke3, D. Freude3, B. Knorr3, H. Kolley3, H. Pfeifer3 | Requires cookie* | | | Title
| E | | | | Abstract
| n h a n c e d S e n s itiv ity o f Z e r o -F ie ld N M R S p e c tro s c o p y a t H ig h M a g n e tic F ie ld s Dedicated to Professor Müller-Warmuth on the occasion of his 65th birthday After reviewing the basic principles of zero-field NMR spectroscopy at high magnetic fields (ZFHF NMR) as introduced by Robert Tycko, a 2-dimensional extension of this method is described which enhances its sensitivity by at least one order of magnitude. The essential point is a com bination of the original pulse program including sample rotation with a Carr-Purcell pulse se quence during the acquisition time. Experimental results are given for the proton resonance of meta-C6H2D4 dissolved in C6D6 at 245 K. | | | |
Reference
| Z. Naturforsch. 50a, 388—394 (1995); received December 24 1995 | | | |
Published
| 1995 | | | |
Keywords
| Zero-field NMR, High magnetic fields, 2-dimensional spectroscopy, Proton-proton distance, Benzene | | | |
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| default:Reihe_A/50/ZNA-1995-50a-0388.pdf | | | | Identifier
| ZNA-1995-50a-0388 | | | | Volume
| 50 | |
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