The reaction of aromatic nitro-compounds with tervalent phosphorus reagents
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As a synthetic procedure, the deoxygenation of aromatic nitro-compounds by tervalent organophosphorus compounds has been extended to include new routes to the anthranil and phenothiazine nuclei. Synthesis of anthranils from 2-nitrophenyl ketones has been exemplified by the preparation of 3-Phenylanthranil, 3-styrylanthranil and 5-chloro-3-methylanthranil from 2-nitro-benzophenone, 2 -nitrochalcone and 5-Chloro-2-nitroacetophenone respectively. The preparation of phenothiazines from 2-nitro-biaryl sulphides has been studied more extensively. It was found that when successful, this ring closure gave the cyclised product in about 55% yield. Rather more than half the sulphides used, however, gave no products or only a very low yield, and no rational means of predicting the success of a given ring closure could be found. Triethyl phosphite was the only phosphorus compound found to be capable of effecting this cyclisation. The use of several other phosphorus compounds was investigated, but although deoxygenation of the nitro-group undoubtedly took place, as evidenced by the large amounts of tar formed, no cyclised products could be isolated. Although a comprehensive investigation was not undertaken, the results of two experiments suggested that a considerable improvement in the yields of the phenothiazines could be achieved by carrying out the deoxygenations in a solvent (as opposed to pure phosphorus compound) and thus, reducing the extent of tar-forming side reactions. In addition, an unidentified product has been obtained from the reaction of 3-(o-nitrophenyl)coumarin with triethyl phosphate. Several unsuccessful attempts were made to synthesise a seven membered ring system. As part of an examination of the mechanism of the deoxygenation, the rate of reaction of a series of phosphorus compounds with 2-nitrobiphenyl was studied. It was found that phosphorus compounds with electron donating groups attached to the phosphorus atom reacted more rapidly, suggesting that the rate determining step involved a nucleophilic attack by phosphorus. No adequate explanation could be advanced for the finding that of all the phosphorus compounds studied, diethyl methylphosphonite reacted most rapidly. While the reactions of triethyl and triisopropyl phosphites were first order with respect to nitrobiphenyl, the reactions of hexaethyl phosphorous triamide and diethyl methylphosphonite appeared to be second order. A detailed kinetic study would be necessary to confirm this observation and investigate the fundamental change in mechanism which it implies. The possibility of the formation of nitroso-compounds as intermediates in the deoxygenation of the nitro-compounds has been discussed, and an unsuccessful attempt to trap such an intermediate has been made. Further attempts ought to be made in this direction and should be concentrated on finding a compound which will react more rapidly with the nitroso-compound than does triethyl phosphite. Such a trapping agent must also be capable of operating in the presence of an excess of nitro-compound. Much of the work in this thesis has been directed towards establishing whether an electron deficient nitrene intermediate is formed during the course of the deoxygenation of an aromatic nitro-compound. Although no one experiment could be regarded as conclusive, the overall weight of evidence argued convincingly in favour of this type of intermediate.
Thesis, PhD Doctor of Philosophy
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