3.9.2.6. Category 6: Activation via Photo-isomerization of Azobenzene-Based Enediynes and Sulfones
Conformational changes can bring about significant perturbation in the kinetics of BC. Previously, König et al. have shown that (for a bipyridyl enediyne) a decrease in the distance between the two acetylenic arms undergoing covalent connection (c and d distance) upon complexation to mercury(II) brings about a remarkable increase in the reactivity toward BC.
It is thus reasonable to think that a similar conformational change might be achieved if a group capable of switching between E and Z configurations is incorporated in an enediyne moiety (Scheme 98). Thus, Basak et al. designed azo-based enediyne systems represented by the general structure A. These molecules should exist in the thermally stable E isomer. Photo-isomerization to the Z isomer B is expected to bring down the c and d distance (Scheme 98), which should lead to an increase in reactivity.
Scheme 98. Rationale behind triggering through E − Z isomerization and phototriggering of azo enediynes.
Scheme 99. Synthesis of nonaromatic azo enediyne. |
With this idea, the cyclic enediyne A, containing a stable E -azo moiety (azoenediynes) was synthesized and reported by Basak et al. The key step is the double N-alkylation to form the cyclic network (Scheme 99). The stable E -azoenediynes A and G upon irradiation with long-wavelength UV isomerize to the Z compounds F and H respectively, which can be thermally re-isomerized to the corresponding E compounds (Scheme 100). Reactivity studies toward BC using DSC predictably indicated higher reactivity for the Z isomers. These studies provide a novel way to modulate the reactivity of enediynes under thermal or photochemical conditions. With an appropriately sized enediyne, there could be a possibility of inducing BC upon irradiation under ambient conditions.
Scheme 100. Photochemical trans-cis isomerization of azo enediynes and thermal reactivity thereof.