Group VI Derivatives of Tertiary Phosphines and Arsines

Brown, Douglas Hugh (1978) Group VI Derivatives of Tertiary Phosphines and Arsines. PhD thesis, University of Glasgow.

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Some radical reactions between certain tertiary phosphines and organic derivatives of sulphur, selenium and tellurium were investigated. The product mixture arising from the photochemically-initiated chain reaction between a 2:1 mixture of Ph2PCH=CH2 and HSC3H6SH, consisting mostly of Ph2P(S)C2H4 SC3 H7 rather than the expected product Ph2PC2H4SC3H6SC2H4-PPh2, was accounted for by the occurrence of thiyl radical (RS) attack at P(III) to form an intermediate phosphoranyl radical (which decomposed to give the P(Y) sulphide and a carbon-centred radical) as well as the desired free-radical addition of RS to the double-bond. The major product, Ph2P(S)C2H4SC3H7, was thought to arise from a sterically favourable intramolecular ring closure from the S of Ph2PC2H 4SC3H6S, because of the close proximity of S to its own P(III). Analogous photochemical reactions of some thiols with a phosphine and an olefin indicated that certain thiols react preferentially with an olefin whereas others favoured attack at the P(III) of the phosphine. The relative ease of elimination of the carbon-centred radical R from the intermediate phosphoranyl radi- Et > Me >Ph , consistent with the operation of a radical chain mechanism involving the radicals RSe and R13P(SeR) as chain carriers. Although cleavage of Se-C bonds is known to be important in the u. v. irradiation of R2Se2 alone, Se-Se cleavage aeemed to be the more important initiation step in the presence of tertiary phosphines. The variation in reaction rates for the different organic groups, R, was consistent with increasing energy of radical formation, indicating that the rate determining step might be the elimination of R from the phosphoranyl radical intermediate, R31P(SeR). The alpha-scission of this selenophospho-ranyl radical was found to occur, to a limited extent, only in those reactions where beta-scission was relatively slow, or where the strength of the usually strong P-C bond in the tertiary phosphines, Ph2P(SeR)R1 (R1 = Ph,Me), was reduced, when R=CH2PPh2, to a point where P-C cleavage was as facile as. C-Se cleavage. Ultraviolet irradiation of solutions of R2Te2 produced R2Te and Te quantitatively. In the presence of tertiary phosphines, R13P, the reactions were accelerated via the formation of R31PTe which decomposed to liberate Te and reform R13P. All the reactions could be satisfactorily interpreted in terms of an initiation step involving photochemical cleavage of Te-C bonds. The ease of cleavage of element-carbon bonds by u. v. irradiation therefore increases in the series R2S2 < R2Se2<R2Te2, whereas the ease of cleavage of element-element bonds is the reverse. Some synthetic routes to tertiary arsine sulphides were studied. The reaction of the tertiary arsines, R3As, (R=Ph,Me,CH2AsPh2, and C2H4As-Ph2) with elemental sulphur gave good yields of R3AsS only when heated at ca. 150

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Organic chemistry
Date of Award: 1978
Depositing User: Enlighten Team
Unique ID: glathesis:1978-78796
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 30 Jan 2020 14:54
Last Modified: 30 Jan 2020 14:54

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