Fragmentations of (E)- and (Z)-isomers of 2-methylbuten-1-yl(aryl) iodonium triflates: competing mechanisms for enol triflate formation
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-?-alkyl substituent and departure of the aryliodonio- nucleofuge 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 in-plane (?*) SN2 reaction. The presence of the remaining, retained, unrearranged enol triflate can be explained by a ligand coupling mechanism (?* SN2) and the fragmentation mechanism(s) do not require the intermediacy of a primary vinyl cation.