Silylene: Application as an Alluring Ligand in Transition Metals and Catalysis

Abstract

Silylenes, previously only known as fugitive and transient laboratory curiosities in the 1960s, are now one of the most laboriously investigated classes of compounds in modish chemistry. The discovery of the first N-heterocyclic silylene by West and Denk in 1994, auguring the rising of a flourishing era in low-valent silicon chemistry by many research groups in academia and industry world-wide furiously pursuing their potential uses in catalysis, synthesis and stoichiometric transformations. As silicon is the non-toxic second most abundant element in Earth’s crust and given the bottleneck in metal-based resources, as well as the current energy crisis, alternative catalytic or stoichiometric transformations mediated by reactive silicon compounds represent a crucial scientific goal. The “holy grail” of silylene chemistry is metal-free catalysis, which is a target yet to be realised but given the rapid development in the field, it is likely that this goal might soon be achieved. Recently, the chemistry of transition-metal complexes bearing isolable NHSi ligands has introduced attractive and new synthetic methods with wide variety of properties that has largely influenced organic methodologies, in particular small-molecule activations and very limited number of organic transformation reactions. We have added some new feathers for the exploration of the versatility of the silylene in small molecule activation and catalysis. In the first part of the thesis, we have shown the reactivity of silylene with chalcogens to form Si=E (E= O, S, Se). Unlike ketones, their higher homologues are oligomeric or polymeric in nature due to the unfavourable overlapping between pπ(Si) and pπ(E) orbitals as well as their large electronegativity difference. Therefore, the isolation of such compounds has always been a tough job for the synthetic chemists. However, we are able to stabilize silylene chalcogenones of composition Si=E→LA (E = O, S, Se) using the Lewis acid−base adducts concept. The second and third part of the thesis deals with the design, systhesis, and thorough experimental investigations of a number of nearly naked Si(II)-coinage metal (Cu(I) and Au(I)) cationic complexes where the Cu(I) and Au(I) centres are weakly bound with arene ring such as benzene, hexamethyl benzene, toluene, and m-xylene. In this context, it is important to mention that we have successfully isolated and characterized the first elusive [Cu(η6-C6H6)]+ moiety stabilized by silylene, which is otherwise only been observed in the gas phase. The synthetic difficulties to isolate weakly bound Cu(I)/Au(I)-free arene constitutes a major thrust for the catalysis reaction by easy substitution of an arene with the substrates. Hence, we employed the above mentioned Si(II) supported Cu(I)/Au(I)-arene cationic complexes as catalysts in click reaction and in glycosidation reaction, respectively. Low catalyst loading with less reaction time is the important feature of Si(II)-Cu(I)-toluene catalyst to synthesize triazoles for a series of azides and alkynes. Conversely, well-defined [Au-arene]+ complexes using N-heterocyclic silylene, germylene, and carbene as a σ-donor ligand were probed first time for disaccharide synthesis and observed the superiority of Si(II)-Au(I) catalyst over Au(III) salt, even better than NHC- and phosphite-Au(I) complexes for the activation of propargyl 1,2-orthoester donors. Further isolation of branched pentamer which is the core of the HIV-gp120 envelope is the practical aspect of newly developed cationic Au(I) catalysts. Last part of the thesis, we concentrated on Buchwald-Hartwig amination reactions by using silylene/Pd(dba)2 system. We examined the potential application of a modified bidentate ligand (SiNP) bearing Si(II) and P(III) coordination site with Pd(dba)2 salt for Buchwald-Hartwig amination reactions for a series of extremely bulky primary anilines with electron-donating and withdrawing bulky aryl bromide by the conventional method as well as under the microwave technique (10-15 min) and found outstanding catalytic performance (~90-99%). Amination reaction of such bulky class of substrates are scarce. Hence, the study related to coordination and catalytic application of NHSi-TM complexes is still in the early stage and ‘miles to go’ before it reaches to mankind.