Trifluoromethanesulfonate Esters from Dibromoalkane Methatheses with Silver Triflate: Mechanistic and Synthetic Aspects

Article abstract of DOI:10.1021/jo00370a009

The methathesis reaction between silver triflate and bromoalkanes potentially offers an attractive synthetic complement to the well-known alcohol condensation with triflic anhydride for organic triflate esters.Dibromoalkanes can further give difunctional triflate intermediates and could provide convenient routes to asymmetrically substituted derivatives.Certain shorter members of the α,ω-dibromoalkane homologous series display a unique reactivity and product selectivity over higher homologues and corresponding primary monobromoalkanes.Triflate products from monobromoalkanes and α,ω- dibromoalkanes greater than 1,4-dibromobutane can lead to benzene solvent alkylation or polymerization in CCl4, but the lower 1,2-through 1,4-dibromoalkanes produce desired monobromoalkyl triflate and alkanediyl ditriflate products under the same reaction conditions.These same lower α,ω-dibromoalkanes also resist product rearrangement to secondary triflate products while the higher homologous α,ω-dibromoalkanes and primary monobromoalkanes do not.The 1,2-trough 1,4-dibromoalkanes further offer selective synthesis routes to difunctional derivatives via sequential metathesis.The unique stability and selectivity of the lower α,ω-dibromoalkane homologues are apparently best explained with anchimeric assistance by a cyclic bromonium ion in the first metathesis step followed by a rare example of cyclic anchimeric stabilization by the triflate group in the second bromine displacement.Kinetic results further support this mechanism.This metathesis reaction is, however, very dependent upon the control of several reaction conditions: dibromoalkane chain length, solvent, temperature, reaction time, and type of bromine leaving group.The optimum conditions for obtained certain α,ω- alkanediyl ditriflates, ω-bromoalkyl triflates, and 1-butyl triflate are presented.