Abstract
(Perfluoroalkyl) tetramethylcyclopentenols (alkyl = n-butyl, n-hexyl, n-octyl) were dehydrated to a complex mixture of endo, endo-(perfluoroalkyl) tetramethyl-cyclopentadienes and their endo-, exo-isomers. It was found in preliminary screening experiments that the best reagent for this transformation, giving an 89% yield of isomeric product mixture, was P2O5 in benzene at 80-90 °C. Products were characterized on the basis of their mass spectra and retention time information, and some peaks in the mass spectra were identified from their molecular fragments. Structures were assigned to the three most abundant products of (perfluorohexyl)tetramethylcyclopentenol dehydration. Formal dehydration kinetics showed a second order reaction in benzene but zeroth order with induction period in chlorobenzene, suggesting mass transfer limitations in the more polar chlorobenzene. Some of the products were formed by consecutive isomerization of the others, as shown by the kinetic analysis.
Highlights
The current interest in fluorous biphase systems [1] in general and in fluorous biphase catalysis [1,2,3,4,5,6,7,8] in particular has led to the introduction nowadays of a broad spectrum of ligands soluble in Molecules 2011, 16 fluorous phases
The synthetic design of fluorous cyclopentadienes, surprisingly somewhat lagging behind the developments, e.g. in the area of fluorous phosphines [ref. 1, p. 247], brought so far several novel types of cyclopentadienes and cyclopentadienyl complexes made fluorophilic by the attachment of polyfluorinated chains – fluorous ponytails
These include cyclopentadienes with one [9], two [10,11,12] or more [13,14]ethyl chains, cyclopentadienes with silicon atoms directly bonded to the ring to separate electronic effects of the fluorous ponytail [15,16,17,18] and cyclopentadienes monosubstituted with perfluoroalkyl chains [9], which are difficult to handle or use for metal complexation
Summary
The current interest in fluorous biphase systems [1] in general and in fluorous biphase catalysis [1,2,3,4,5,6,7,8] in particular has led to the introduction nowadays of a broad spectrum of ligands soluble in Molecules 2011, 16 fluorous phases. The last of the so far known types of fluorous cyclopentadienes is represented by the (perfluoroalkyl)tetramethylcyclopentadienes prepared in our group [16,19,20,21] In the latter compounds the electrondonating effect of four methyl groups compensates for the opposite effect of the perfluoroalkyl chain bonded directly to the ring, providing stable precursors to ligands with electronic properties close to the unsubstituted cyclopentadienyl but steric properties comparable to pentamethylcyclopentadienyl. These ligands may be considered to be higher homologs of the Gassman ligand [22,23,24,25,26,27,28]. Because of the expected destabilizing effect of strongly electron attracting perfluoroalkyl groups on the intermediate carbocation, our work may be of general interest to organic chemists
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