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.
Controlling Product Selectivity with Nanoparticle Composition in Tandem Chemo-Biocatalytic Styrene Oxidation
