Benzyne polymer-bound

I, was oxidized with lead tetraacetate (LTA), the characteristic Diels-Alder adduct with tetracyclone (TC) could be obtained even when the diene was added following completion of N2 evolution, indicating that the polymer-bound intermediate had survived for tens of seconds. Secondly, while LTA oxidation of the monomeric aminotriazole I-a, affords the corresponding biphenylenes in high yield, the only products formed by I-b, in the absence of TC, were the aryl acetates II and III (combined yield of 80%). Monomeric o-benzyne is known to dimerize at the diffusion limited rate in the gas phase, therefore, formation of the Diels-Alder product following delayed addition of diene requires a substantially slower encounter process for the polymer-bound analog. 

The diaryliodonium carboxylate route to benzynes (Type B2) has been adapted to the polymer-bound precursor 58, prepared from Merrifield s resin by standard methods". (An earlier example of a polymer-bound h nzyne has been described. ) The polymer afforded benzyne 59 when heated above 200 °C, as demonstrated by trapping with 2-furoic acid the initial cycloadduct decarboxylates spontaneously at that temperature, affording naphthols 60 in about 20% yield. The three-phase test involving transfer of aryne from 58 to a... 

The reactivity of functionalised crosslinked polymers may significantly differ from that of analogous soluble reagents. Polymer-bound benzyne, generated by Pb(OAc)4-oxidation of resin-bound 1-aminobenzotriazole 3, for example was converted into aryl acetates 4. This reaction was not observed if an analogous 1-aminobenzotriazole derivative 6 was oxidised in solution, where the formation of dimers (7 and 8) was predominant (Scheme 1.6.3). 

The practice of using an insoluble polymer to isolate and kinetic-ally stabilize a reactive intermediate has been addressed in several reports, most commonly using DVB cross-linked polystyrene as a support. In these cases, the three dimensional structure of the polymer and rigidity of the polymer backbone diminish intramolecular reactivity between two sites on the same polymer bead. Physical constraints preclude any significant reaction between two different polymer beads. Similar, less dramatic reduced intramolecular reactivity has also been noted for reactive intermediates bound to linear polystyrene. For example, o-benzyne bound to linear polystyrene has been shown by Mazur to have enhanced stability relative to non-polymer-bound -benzyne (35). In this case, o-benzyne was generated by lead tetraacetate oxidation of a 2-aminobenzotriazole precursor, 1. Analysis of the reaction products after cleaving the benzyne derived products from the polymer by hydrolysis showed a 60% yield of aryl acetates was obtained (Equation 11). In contrast, the monomeric aryne forms only coupled products under similar conditions. Further comparisons of the reactivity of -benzyne bound to insoluble 2% or 20%... 

DVB cross-linked polystyrene showed the intermediate o-benzyne had an even longer lifetime. Overall, the reactivity of -benzyne bound to the soluble polymer was found to be intermediate between that of non-polymer bound benzyne and benzyne bound to 20% DVB cross-linked polystyrene. The 10 sec lifetime reported for benzyne in this case presumably reflects diffusiorjal constraints associated with the polymer chain whose was 10. ... 

Kinetic investigations of the lifetimes of polymer-bound benzyne (7.81 and of an N-deprotected amino acid active ester (2) put the concept of site isolation into voper po qi ve. Polymra -bound reactive intermediates have substantially longer lifetimes than the analogous micromolecular species, but they arc not completely isolated, and in time react with othra- polymer-bound species. Several reviews summarize the field as of 1978-82 (10-171. 

Benzyl phosphonium ions, polymer-bound, alkenes prepared, l65t Benzyne... 

Mazl979 Mazur, S. and Jayalekshmy, R, Chemistry of Polymer-Bound o-Benzyne. Frequency of Encounter between Substituents on Cross-Linked Polystyrenes, J. Am. Chem. Soc., 101 (1979) 677-683. 

Competitive kinetic experiments show that PS gels increase the lifetimes of reactive intermediates from seconds to minutes. Generation of benzyne in solution produces the dimer, biphenylene, if no reagent is present trap the benzyne. The polymer-bound benzyne in Scheme 30 either dimerizes or is trapped by tetraphenylcyclopentadienone. Sequential addition of iodobenzene diacetate and tetraphenylcyclopentadienone to the polymer gave yields of the tetraphenylnaphthalene that decreased as the time between addition of the two reagents increased. The lifetime r of benzyne in the 2% cross-linked, DF 0.11 PS was established as 0.6[Pg.881]

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