Trimethylsilyl- and trimethylstannyldimethylphosphane--convenient and versatile reagents for the synthesis of polyfluoroaryldimethylphosphanes.
Trimethylsilyldimethylphosphane (Me3SiPMe2) and the corresponding tin compound (Me3SnPMe2) were used as reagents for the substitution of fluorine by the Me2P group in polyfluoroarenes C6F5X (X = F, H, Cl, CF3) and C5NF5. The reactions occur even under mild conditions (T = 0-20 C), either in benzene or without solvent, to give as a rule 4-X-1-(dimethylphosphano)tetrafluorobenzenes (XC6F4PMe2, 1-4) and 4-(dimethylphosphano)tetrafluoropyridine (C5NF4PMe2, 5), respectively, in yields between 75 and 95%. In the case of C6F6, double substitution is also observed, which affords 1,4-bis(dimethylphosphano)tetrafluorobenzene (6). A very efficient route to the compounds XC6F4PMe2 (X = F, H, Cl, CF3) and C5NF4PMe2 was developed as a one-pot reaction of the corresponding fluoroarenes with tetramethyldiphosphane (P2Me4) and trimethyltin hydride (Me3SnH) at moderate temperatures. This process was tested for C6F6 and perfluorobiphenyl which gave C6F5PMe2 (1) and 4,4'-bis(dimethylphosphano)octafluorobiphenyl (7), respectively. The results, which included kinetic measurements that used the intensities of the 31P signals, revealed the influence of the substrate type on the rate of reaction in the sequence: C5NF5>C6F5CF3> C6F5Cl, C6F5PMe2>C6F5H>C6F6>> C6H5F. Ab initio calculations were carried out on the model reactions of pentafluoropyridine with silylphosphane, phosphane or phosphide to discriminate between possible reaction mechanisms. The novel phosphanes were characterised by spectroscopic investigations (NMR, MS), by preparation of the related thiophosphanes ArFP(=S)Me2 (8-14), their spectroscopic and analytic data and single crystal X-ray diffraction studies on five of these derivatives.
Metal hydride reduction of some σ- and π-allyl and benzyl derivatives of mercury and palladium
The reductionof 2-butenyl-mercuric (crotyl-mercuric) systems, RHgX where X = Cl, Br, OCOCH3, by sodium borohydride and trimethyltin hydride has been investigated and under all conditions examined, 1-butene is the sole organic product. Crotylmercuric hydride seems likely to form and then to undergo either intramolecular transfer of hydrogen from mercury to carbon in an SNi′ manner, or SN2′ displacement of mercury by some hydride species leading to 1-butene, or reactions producing crotyl-Hg1 or crotyl-Hg- which are cleaved by water or methanol to again yield 1-butene. π-Crotylpalladium chloride on reduction yields only trans-2-butene, but in the presence of increasing concentratins of triphenylphosphine, which promotes the π ⇄ σ equilibrium of allylpalladium chloride systems, substantial amounts of 1-butene are formed. In aqueous tetrahydrofuran, sodium borohydride converts venzylmercuric chloride cleanly to divenzylmercury. Reduction by tetramethylammonium borohydride in benzene, yields toluene (70%) and dibenzylmercury(30%) whereas toluene was the only product from trimethyltin hydride reduction.
Tourism in the Amazon: identifying challenges and finding solutions
Purpose – The purpose of this paper is to provide an overview of the tourism sector in the Amazon regions of Bolivia, Brazil, Colombia, Ecuador, Guyana, Peru, and Suriname and then discuss the manner in which tourism activity assists the protection of the Amazon rainforests. The paper also describes the manner in which the first World Hospitality and Tourism Themes Roundtable on Tourism in the Amazon is organized in 2009.Design/methodology/approach – Teams of researchers from the Ministries of Tourism, the private sector and academia in the Member Countries of Treaty for Amazon Cooperation collaborated to address, in ten‐page papers, the question “Does sustainable tourism offer solutions for the protection of the Amazon rainforest?”Findings – The paper provides valuable information on the current state of tourism policy and practice in the Amazon Member Countries. It also articulates the challenges that attend the development of sustainable tourism as a mechanism for the protection of the Amazon.Practical...