Master of Science (MS)
Semester of Degree Completion
Edward M. Treadwell
Photochromic semicarbazones are an interesting group of compounds that show potential in a broad range of applications, including as a more environmentally friendly replacement for photochromic metal complex compounds. Previous work has shown the feasibility to synthesize photochromic cinnamaldehyde semicarbazones (CSCs) from a benzaldehyde and a protected acetaldehyde Wittig reagent. A new, quicker, and cheaper approach to constructing these CSCs from crotonaldehyde was explored in this research. A three reaction sequence would convert crotonaldehyde to a CSC; first, an allylic bromination of the semicarbazone with N-bromosuccinimide (NBS); second, an Arbuzov reaction to prepare the semicarbazone phosphonate; and finally a condensation of this phosphonate with a benzaldehyde. The first step of synthesizing crotonaldehyde semicarbazone was successfully achieved in a 97.2% yield (68.5% for the analogous thiosemicarbazone). However, over 20 attempts at the second step involving the allylic bromination of the semicarbazone were made, with various solvents, halogenation sources, radical initiators, reaction times and temperatures, but with no success. Each reaction yielded a complex mixture of several new compounds, none of which appeared to be the desired 4-bromocrotonaldehyde semicarbazone.
The bromination conditions were proved viable by reaction of 1-hexene and methyl or ethyl crotonate with NBS or dibromodimethylhydantoin (DBDMH) to successfully give the bromine substituted product, although the use of DBDMH also led to dibrominated addition across the double bond with the crotonates. The products were confirmed by both 1H and 13C NMR and GC/MS analysis. The results suggested that the problem was in the semicarbazide substrate and not the reaction conditions.
With this in mind, additional crotonaldehyde analogs with various imino groups were synthesized and subjected to the same bromination conditions. To this end, methyl (2-butenylidene)hydrazinocarboxylate (49% yield), crotonaldehyde dinitrophenyl-hydrazone (67% yield), and 2-cyclohexen-1-semicarbazone (44% yield) were synthesized. Bromination with NBS gave the desired product for the cyclohexenone substrate. However bromination of the methyl hydrazinocarboxylate gave a mixture of the allylic substitution product, the dibromide addition product, and a dimer. Reaction of the dinitrophenylhydrazone gave an allylic bromination product, but with the hydrazine group oxidized to an azo group and with migration of the double bond. These results suggest that the H off the imine carbon, and the NH in the hydrazine group, are responsible for preventing successful bromination of crotonaldehyde semicarbazone. Attempts were made to synthesize and brominate other crotonaldehyde analogs, such as the methylimine, the oxime (hydroxylimine), the cyclohexylimine, the hydrazone, and the 2-aminoethylimine. None of these starting materials were successfully prepared, either with traditional solution condition or with solventless techniques, and this is most likely due to the high solubility and instability of these compounds.
To further test the route, the third step, phosphorylation of an allylic bromide, was carried out on ethyl 4-bromocrotonate with high yield (91.7%). There are many examples in the literature that suggest the next proposed reaction, a Horner-Wadsworth-Emmons condensation with a benzaldehyde, would have yielded the cinnamaldehyde compound as desired. This provides evidence that the proposed synthetic route has great promise, if the bromination step could be achieved with a reasonable yield.
Ebbert, Byron R., "A Simpler Route to Building Photochromic Cinnamaldehyde Semicarbazones" (2015). Masters Theses. 1721.