Synthesis and Reactivity of Diimino-N-Heterocyclic Carbene Supported Tetrylones

Abstract

The work described in this thesis is focused on the preparation of a series of novel tetrylones stabilized by diimino-N-heterocyclic carbene followed by studies on their reactivity. In the pursuit of dimNHC-supported stannylone III-6, attempts to isolate free dimNHC ligand IV-19 before its reaction with a tin reagent proved challenging. Conventional deprotonation of imidazolium salt III-7 led to undesired rearrangement product III-10. Facing difficulties in isolating free dimNHC III-8, alternative routes were explored. An effective solution involved using bis(trimethylsilyl)amido chlorotin(II), SnCl[N(SiMe3)2], which generated dimNHC ligand III-8 in-situ. This process simultaneously incorporated the (Sn+-Cl) fragment at the central carbene position, forming cationic stannylene III-14. The subsequent reduction of III-14 with excess potassium graphite resulted in the desired stannylone III-6. The reactivity studies of dimNHC-supported germylone IV-6 were expanded. Germylone IV-6 reacts with azides N3R (R=SiMe3 or p-tolyl) to furnish the first examples of germanium π-complexes IV-10 and IV-11. Reaction of IV-10 with tetrachloro-o-benzoquinone results in the net transfer of a germanium atom and formation of the free diimino-guanidine ligand IV-13. Germylone IV-6 reacts with B(C6F5)3 to give the zwitterionic borate V-7. This compound can be converted into the hydroborate V-8 and further into germylene-borane V-4. Compound V-4 is a Ge/B analogue of Stephan’s FLP parent P/B compound (C6H2Me3)2P-C6F4-B(C6F5)2 but unlike the latter cannot split dihydrogen. Moreover, attempts to prepare a Ge/B analogue of the zwitterion (C6H2Me3)2HP-C6F4-BH(C6F5)2 by protonation of borate V-8 resulted in immediate elimination of H2. A synthetic approach for dimNHC-supported silylone VI-9 involved utilizing the adduct of dimNHC with ZnCl2 VI-11. The adduct VI-11 was prepared by trapping the in-situ formed dimNHC IV-19 with a Lewis acid (ZnCl2). Subsequent exchange between the dimNHC →ZnCl2 adduct VI-11 and the IPr→SiCl2 silylene VI-12 resulted in the formation of the novel dimNHC-supported silylene VI-13. Finally, VI-13 was successfully reduced to yield the desired dimNHC-supported silylone VI-9. Later, we found that dimNHC IV-19 could be directly prepared from imidazolium salt VI-10 by deprotonation with IPr, serving as a Lewis acid-free base. IV-19 is the first isolable diimino carbene. Upon heating IV-19 in toluene, it cleanly converted to the aromatic C-C bond activation product IV-17. With dimNHC IV-19, we established an alternative synthetic route to silylone VI-9 using ligand IV-19 and HSiCl3 as starting reagents.