Einfache photochemische Synthese von Benzylphosphanen / Facile Photochemical Synthesis of Benzylphosphines

Abstract

Facile large-scale synthesis of the benzyl-dibromo-phosphines RPBr2 (R = C6H5CH2, 3,5-Me2C6H3CH2, 2-BrC6H4CH2) is accomplished by the photochemical reaction of toluene, mesitylene, and 2-bromotoluene, respectively, with PBr3 in the absence of solvent. If the reactions are carried out up to 50% completion the amount of by-products formed is remark­ ably low, rarely exceeding 10% of the total amount of products formed. The selectivity of the photoreaction may be improved up to 98% if it is carried out in a modified Soxhlet apparatus which allows the predominant irradiation of only the starting materials. The appar­ ent limitations imposed by the low turnovers required to achieve good product selectivity are by far outweighed by the large amounts of desired products formed in a short period of time and by the easy recovery of unreacted starting material by distillation. Thus in the case of 2-BrC6H4CH2PBr2, in a typical run 94 g (261 mmol) of product may be isolated after 2.5 h of irradiation of 970 mmol of 2-BrC6H5CH3 and 1.39 mol of PBr3. In this case the turnover is 32%. the isolated yield 27%, and the by-product 2-methylphenyl-dibromo-phosphine only amounts to less than 2%. The benzyl-dibromo-phosphines are easily methylated with two equivalents of MeMgCl to give the respective benzyl-dimethyl-phosphines in 79-93% yield. Tertiary benzylphosphines MePRR′ (R = 2-BrC6H4CH2, R′ = Me3SiCH2; R = R′ = 2-BrC6H4CH2) are obtained in easy one-pot syntheses by consecutive reactions of 2-BrC6H4CH2PBr2 with Me3SiCH2MgCl and MeMgCl, and with 2-BrC6H4CH2MgBr and MeMgCl, respectively.