First reported less than a decade ago, the α,βunsaturated acyl azolium has emerged as a central reactive intermediate for reaction discovery using N-heterocyclic carbene catalysis. In this Perspective, an introduction to the four main reactivity patterns accessible from this intermediate is provided. The Perspective is handled in a largely chronological fashion, with an emphasis on alternate approaches to the key intermediate and first-in-class reaction cascades. Finally, a brief discussion of emerging trends in this field of catalysis is presented. Although not exhaustive, the Perspective provides an overview of this active area of research and serves as a guide for future investigations.

Readily prepared and bench-stable rhodium complexes containing methylene bridged diphosphine ligands, viz. [Rh(C(6)H(5)F)(R(2)PCH(2)PR'(2))][BAr(F)(4)] (R, R' = (t)Bu or Cy; Ar(F) = C(6)H(3)-3,5-(CF(3))(2)), are shown to be practical and very efficient precatalysts for the intermolecular hydroacylation of a wide variety of unactivated alkenes and alkynes with β-S-substituted aldehydes. Intermediate acyl hydride complexes [Rh((t)Bu(2)PCH(2)P(t)Bu(2))H{κ(2)(S,C)-SMe(C(6)H(4)CO)}(L)](+) (L = acetone, MeCN, [NCCH(2)BF(3)](-)) and the decarbonylation product [Rh((t)Bu(2)PCH(2)P(t)Bu(2))(CO)(SMePh)](+) have been characterized in solution and by X-ray crystallography from stoichiometric reactions employing 2-(methylthio)benzaldehdye. Analogous complexes with the phosphine 2-(diphenylphosphino)benzaldehyde are also reported. Studies indicate that through judicious choice of solvent and catalyst/substrate concentration, both decarbonylation and productive hydroacylation can be tuned to such an extent that very low catalyst loadings (0.1 mol %) and turnover frequencies of greater than 300 h(-1) can be achieved. The mechanism of catalysis has been further probed by KIE and deuterium labeling experiments. Combined with the stoichiometric studies, a mechanism is proposed in which both oxidative addition of the aldehyde to give an acyl hydride and insertion of the hydride into the alkene are reversible, with the latter occurring to give both linear and branched alkyl intermediates, although reductive elimination for the linear isomer is suggested to have a considerably lower barrier.

A Rh(I)-catalyzed method for the efficient functionalization of arenes is reported. Aryl methyl sulfides are combined with terminal alkynes to deliver products of carbothiolation. The overall process results in reincorporation of the original arene functional group, a methyl sulfide, into the products as an alkenyl sulfide. The carbothiolation process can be combined with an initial Rh(I)-catalyzed alkene or alkyne hydroacylation reaction in three-component cascade sequences. The utility of the alkenyl sulfide products is also demonstrated in simple carbo- and heterocycle-forming processes. We also provide mechanistic evidence for the course of this new process.

A comparative study of seven crystallographically characterized rhodium precatalysts, which contain a variety of chelating diphosphine ligands, for the hydroacylation of 1-octyne or 1-octene with 2-(methylthio)benzaldehyde has been undertaken. These studies show that the best performing catalyst for 1-octyne, [Rh(L)(η6-C6H5F)][BArF 4], L = iPr2PNMePiPr2, delivers alkyne selective hydroacylation with high efficiencies at low loadings (1 mol %, 2.0 M aldehyde, 25 °C, ToN = 100, 97% conversion in 5 min), and also shows high selectivity for the linear product. Experiments suggest that the alkyne selectivity arises from the alkyne being more competitive for metal binding compared to the alkene. Labeling experiments using the [Rh(tBu2PCH2PtBu2)(η6-C6H5F)][BArF 4] system, that gives the final product in a linear:branched ratio of 6:1, indicate that the pathway that produces the branched product operates via an irreversible hydride insertion. Intermediate acyl hydride complexes, [Rh(L)(H)(COC6H4SMe)(acetone)][BArF 4], have been characterized by low temperature NMR spectroscopy, as have their subsequent reductive decarbonylation products, one of which has also been crystallographically characterized: [Rh(iPr2PNMePiPr2)(SMePh)(CO)][BArF 4].

Amine for it! A cationic rhodium catalyst, which was assembled in situ from commercial components, promoted the reaction of a range of simple 2‐aminobenzaldehydes with terminal and internal alkynes in a series of intermolecular hydroacylation reactions. The products of this reaction, amino‐substituted enones, were efficiently converted into the corresponding dihydro‐4‐quinolones.

We have developed a general protocol for the interconversion of diverse protected boronic acids, via intermediate organotrifluoroborates. N-Methyliminodiacetyl boronates, which have been hitherto resistant to direct conversion to trifluoroborates, have been shown to undergo fluorolysis at elevated temperatures. Subsequent solvolysis of organotrifluoroborates in the presence of trimethylsilyl chloride and a wide range of bis-nucleophiles enables the generation of a variety of protected boronic acids.

The Rh(I)-catalysed coupling of aryl and alkenyl boronic acids with simple aryl and alkenyl methyl sulfides is reported. The process employs bench-stable Rh(I) precatalysts incorporating small bite-angle chelating phosphine ligands (R 2 PCH 2 PR 2 , R ¼ i Pr, Cy), shows good functional group tolerance, and proceeds under mild reaction conditions. Importantly, aryl bromide activating groups are inert to the reaction conditions, allowing selective reaction at either a methyl sulfide or bromide activating group, depending on catalyst (metal) choice. The scope of the coupling reactions, their combination with Rh-catalysed hydroacylation reactions in cascade processes, together with preliminary mechanistic studies, are all documented.

Poly(dibenzosilole)s have been increasingly reported as an alternative to polyfluorene in organic electronic materials. Poly(dibenzosilole)s show similar optical properties to polyfluorene, but with improved resistance to oxidation and thermal stability. Several poly(dibenzosilole)s and their co-polymers have been incorporated into organic electronic devices, such as light emitting diodes and solar cells. These materials have shown improved performance over their polyfluorene-based counterparts.