Abstract
We examined fragmentation reactions of (E)- and (Z)-2-methylbuten-1-yl(aryl)iodonium triflates (aryl = C6H5-, 4-(CF3)C6H4, 3,5-(CF3)2C6H4-) to afford aryl idodides and six enol triflates. Four of these vinyl triflates involve alkyl migrations followed by triflate trapping of secondary vinyl cations whereas two do not involve migrations. Fragmentation rates in dry, neutral CDCl3 were determined as were the distributions of enol triflate products. The ratios of rate constants for the (E)-/(Z)- isomers ranged between 5.0 and 8.5 and, in all salts, the rearranged enol triflate derived from migration of the alkyl moiety trans- to the aryliodonio- nucleofuge was observed in the greatest quantities. These data indicate that the fragmentation rates are significantly determined by the migratory aptitude of the trans-β-alkyl substituent and departure of the aryliodonionucleofuge occurs by anchimeric assistance. The ratios of inverted “unrearranged” enol triflate products were greater for the (Z)-isomers of the iodonium salt precursors indicating that steric effects play a role and implies that these inverted, unrearranged products are derived from inplane (σ*) SN2 reaction. The presence of the remaining, retained, unrearranged enol triflate can be explained by a ligand coupling mechanism (π* SN2) and the fragmentation mechanism(s) do not require the intermediacy of a primary vinyl cation.
Highlights
Over the last several years, we[1] as well as others[2] have discussed primary cation involvement in fragmentation processes of vinylic hypervalent iodine salts
Whereas Okuyama and coworkers have generally focused on chiral2b-c and β,β-disubstituted alkenyl(phenyl)iodonium tetrafluoroborate salts,2a we have investigated dialkyl substituted aryliodonium salts in which the aryl moieties commonly include one and/or two strongly electron-withdrawing trifluoromethyl groups.[1]
The fragmentation products of alkenyl(aryl)iodonium triflate salts include the aryl iodide and six enol triflate isomers;[6] all of these have been identified by comparison to authentic samples using GC MS (as shown for the (E)-isomer in eq 1)
Summary
Over the last several years, we[1] as well as others[2] have discussed primary cation involvement in fragmentation processes of vinylic hypervalent iodine salts. The phenyliodonio - moiety is an outstanding nucleofuge[3] and both β monosubstituted and β,β-disubstituted alkenyl(aryl)iodonium salts participate in both substitution and fragmentation reactions. Whereas Okuyama and coworkers have generally focused on chiral2b-c and β,β-disubstituted alkenyl(phenyl)iodonium tetrafluoroborate salts,2a we have investigated dialkyl substituted aryliodonium salts in which the aryl moieties commonly include one and/or two strongly electron-withdrawing trifluoromethyl groups.[1] The current study examines the differences in reactivity between stereoisomeric pairs of salts containing three different aryl moieties (Figure 1). –OTf (E)-1a-c (Z)-1a-c a: Ar = Ph b: Ar = 4-(CF3)C6H4c: Ar = 3,5-(CF3)2-C6H3-
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