Isocyanurates as versatile molecular platforms
Triazinane-2,4,6-triones, more commonly known as isocyanurates (see below), are cyclotrimeric molecules which have attracted considerable attention in polymer chemistry in regard of their numerous industrial applications. Alternatively, as discrete molecules, after proper functionalization, these molecular platforms have also given rise to various applications such as chloride anion receptors, drug delivery agents or tensioactive building blocks. When symmetrically substituted, these units are octupolar molecules and might therefore present enhanced nonlinear optical (NLO) properties (e.g. hyperpolarizabilities or two-photon absorption). We have thus developped various synthetic approaches toward selected classes of extended functional derivatives of this kind, such a triphenyl- and tristyryl- derivatives.
Most of these syntheses start from the tribromoaryl reactants, themselves accessible in good yields from the corresponding isocyanates, followed by Pd-catalyzed cross-coupling steps (threefold Sonogashira couplings). Beside opening access to these key precursors, these cyclotrimerization reactions also provide a simple access to the family of shorter functional derivatives.
These synthetic approaches are thus quite attractive from an atom-economy perspective, since both precursors can usually be obtained in good yields in a single step, most often from commercial isocyanates, by the use of ad hoc catalysts. Various families of isocyanurates were thus isolated and characterized:
If not commercial, the required isocyanate can usually be accessed by the Curtius reaction:
From the corresponding alkyne derivatives, we could also access a series of d6 transition metal-alkynyl derivatives, which usefully complemented the ferrocenyl derivative 5 previously isolated from 1-Br and ethynylferrocene via a Sonogashira coupling reaction.
The access to terminal alkyne derivatives such as 3-H can prove tricky, because depending on the deprotection method used, opening of the isocyanurate ring may also be observed as a competitive side-reaction. Deprotection conditions must therefore often be carefully optimized.
Finally, access to more unusual derivatives, such as azo-containing triphenyl isocyanurates, was recently performed from the tris-amino derivative 1-NH2, itself obtained from the tri-nitro precursor.
Thioisocyanurates
From the isocyanurates 1-X we also isolated the corresponding 1,3,5-triaryl-1,3,5-triazinane-2,4,6-trithiones 10-X (thioisocyanurates) using a thionation reagent. Thioisocyanurates have been previously mentioned in the literature, but an effective workup toward these derivatives had never be detailed. Furthermore, the comprehensive characterization of any of the few derivative cited in the literature had not been reported so far, apart from the X-ray structure of 10-H. Thus, these molecules were virtually new compounds.
Starting from 10-Br, the thioisocyanurate peripheral branches can be expanded via a threefold Sonogashira cross-coupling reaction using 2-fluorenyethynyl, as previously done for the corresponding isocyanurate 1-Br.
The previous set of new derivatives was therefore isolated and characterized, including partially thienylated derivatives such as 12 and 13. These two compounds, along with 1-Me and 10-Me, allow to monitor the impact of the stepwise oxygen-by-sulfur replacement in the central heterocycle on the electronic and optical properties of these octupoles.
For more information: Frédéric Paul, Olivier Mongin.
