Retention of Configuration and E/Z Stereoselectivity in the Photolysis of cis- and trans-Tetraalkyl-4-methylene-1-pyrazolines

The isomeric 4‐methylene‐1‐pyrazolines cis‐ and trans‐2 are synthesized by a Wittig reaction from the pyrazolin‐4‐ones cis‐ and trans‐1, respectively. Direct irradiation with 350‐nm light of cis‐2 affords the alkylidenecyclopropanes cis‐8, (E)‐ and (Z)‐9 (64: 17: 19) besides molecular nitrogen and small amounts of the 4‐methylenepyrazolidine cis‐7 as a result of photoreduction. Under the same conditions, trans‐2 is converted into trans‐8 as well as (E)‐ and (Z)‐9 (64: 14: 22). The configurations of cis‐ and trans‐8 have been established by means of a 1,3‐dipolar cycloaddition with mesityl nitrile oxide which furnishes two cycloadducts from cis‐8, e.g. cis‐14A and B, but only one cycloadduct from trans‐8, viz. trans‐14. The configurations of (E)‐ and (Z)‐9 have been assigned on the basis of homoallylic 1H,1H coupling constants and nuclear Overhauser experiments. The results demonstrate that, on the least‐motion paths leading to cis‐ and trans‐8, the configuration of the ring substituents in cis‐ and trans‐2 is completely retained. Of the non‐least‐motion products (E)‐ and (Z)‐9 the latter is favoured regardless of the configuration of the precursors cis‐ and trans‐2 though the extent of (Z) selectivity depends on that configuration. The results are interpreted in terms of bis‐orthogonal trimethylenemethane diradicals as primary intermediates from which irreversible paths bifurcate. Cyclization yields cis‐8 (from cis‐2) or trans‐8 (from trans‐2) and 90°‐bond rotations afford two diastereomeric mono‐orthogonal trimethylenemethane diradicals [(E)‐ and (Z)‐18] which eventually cyclize to the corresponding non‐least‐motion products (E)‐9 and (Z)‐9.