Photo- and thermal isomerization of some 1-methoxyallyl cations

The 1-methoxy-, 1-methoxy-3-rnethyl-, and 1-methoxy-2,3-dimethylallyl cations, 2, 5, and 9, were prepared by treatment of the dimethyl acetals of acrolein, crotonaldehyde, and tiglaldehyde, respectively, with FSO 3 H. Each of these ions was capable of existing in various stereoisomeric forms. The thermodynamically preferred conformation of 2, 5, and 9 all had a C1—O, E configuration and 5 and 9 an E configuration about the C2—C3 bonds. Irradiation of FSO 3 H solutions of these ions at −70 °C using 254 nm light caused them to be converted to the other stereoisomers. Relatively efficient photoisomerism about both the C1—O and C2—C3 bonds was observed. The rates of the reverse thermal conversion of the photoproducts to 2, 5, and 9 were measured and on the basis of medium and substituent effects it was concluded that thermal isomerism about the C1—O bond takes place by an inversion process while stereomutation about the C2—C3 bond involves the addition of base to C3.