Joel F. Hooper

Aryl methyl sulfides as substrates for rhodium-catalyzed alkyne carbothiolation: arene functionalization with activating group recycling.

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.

Carbon–carbon bond construction using boronic acids and aryl methyl sulfides: orthogonal reactivity in Suzuki-type couplings

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 (R2PCH2PR2, R = iPr, 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.

2-Aminobenzaldehydes as versatile substrates for rhodium-catalyzed alkyne hydroacylation: application to dihydroquinolone synthesis.

Alkene and alkyne hydroacylation reactions are archetypal examples of simple addition processes that display excellent atom economy.1 Both reactions result in the formation of a new C–C bond and deliver synthetically useful carbonyl-containing products.2 In recent years, there has been considerable interest in converting these processes into synthetically useful transformations. Transition-metal-catalyzed variants represent the largest class of hydroacylation reactions, and amongst these, processes that involve some form of chelation control dominate. The need to employ a chelating substrate stems from the fact that the majority of the metal-catalyzed examples proceed through an inherently unstable acyl metal intermediate 1 (Scheme 1), which can lead to the formation of unwanted side products formed by decarbonylation. A limitation of the chelation-controlled strategy is that the coordinating group, which is present to stabilize the metal–acyl intermediate 2, will also be present in the product. If this group is not needed in the final product, then it must be removed or converted into an alternative functional group.3 Despite this limitation, the advantages of this chelation-controlled process, such as mild reaction conditions, control of enantio- and regioselectivity,4, 5 and broad substrate scope, have resulted in widespread applications of this approach. One strategy to overcome the innate limitation of a chelation-controlled approach is to develop catalytic methods that function without the need for such coordinating groups; although there are notable examples of success with this approach,2c, 6 significant limitations with regard to substrate scope and enantio- and regioselectivity remain. An alternative strategy is to consider the need for a chelating unit as an opportunity, and to expand the range of effective coordinating groups, so that a large variety of useful functional groups can act as the crucial chelating motif. As synthetic chemistry is generally concerned with the preparation of functionalized molecules, an approach that is tolerant of, or indeed benefits from, as many useful functional groups as possible should find wide application. Herein, we demonstrate that simple and readily available 2-aminobenzaldehydes are excellent substrates for intermolecular Rh-catalyzed alkyne hydroacylation, and in doing so add to the motifs available for use in these valuable processes. Furthermore, the products of these reactions, amino-substituted enones, were directly converted into a series of useful dihydroquinolone heterocycles.

Silicon Analogues of Polyfluorene as Materials for Organic Electronics

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.

Traceless Chelation‐Controlled Rhodium‐Catalyzed Intermolecular Alkene and Alkyne Hydroacylation

A new functional-group tolerant, rhodium-catalyzed, sulfide-reduction process is combined with rhodium-catalyzed chelation-controlled hydroacylation reactions to give a traceless hydroacylation protocol. Aryl- and alkenyl aldehydes can be combined with both alkenes, alkynes and allenes to give traceless products in high yields. A preliminary mechanistic proposal is also presented.

O-substituted alkyl aldehydes for rhodium-catalyzed intermolecular alkyne hydroacylation: the utility of methylthiomethyl ethers.

Combining α-methylthiomethyl (MTM) ether substituted aldehydes and 1-alkynes in the presence of [Rh(dppe)]ClO(4) results in efficient intermolecular alkyne hydroacylation to deliver α-O-MTM-substituted enone products. The product MTM ethers can be converted to the free hydroxyl group either in situ, by the addition of water to the completed reaction, or in a separate operation, by the action of silver nitrate.

Activating Group Recycling in Action: A Rhodium-Catalyzed Carbothiolation Route to Substituted Isoquinolines

A new rhodium(I) catalyst allows practical and efficient alkyne carbothiolation reactions to be achieved on synthetically useful ketone-bearing aryl methyl sulfides. The carbothiolation adducts, featuring a ‘recycled methyl sulfide’ activating group, are convenient precursors to highly substituted isoquinolines.

α-Amino Aldehydes as Readily Available Chiral Aldehydes for Rh-Catalyzed Alkyne Hydroacylation

Readily available α-amino aldehydes, incorporating a methylthiomethyl (MTM) protecting group on nitrogen, are shown to be efficient substrates in Rh-catalyzed alkyne hydroacylation reactions. The reactions are performed under mild conditions, employing a small-bite-angle bis-phosphine ligand, allowing for good functional group tolerance with high stereospecificity. Amino aldehydes derived from glycine, alanine, valine, leucine, phenylalanine, isoleucine, serine, tryptophan, methionine, and cysteine were successfully employed, as was an enantiomerically enriched α-OMTM-aldehyde derived from phenyllactic acid. The synthetic utility of the α-amino enone products is demonstrated in a short enantioselective synthesis of the natural product sphingosine.

A General Method for Interconversion of Boronic Acid Protecting Groups: Trifluoroborates as Common Intermediates

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.

Medium-Ring Effects on the Endo/Exo Selectivity of the Organocatalytic Intramolecular Diels–Alder Reaction

The intramolecular Diels-Alder reaction has been used as a powerful method to access the tricyclic core of the eunicellin natural products from a number of 9-membered-ring precursors. The endo/exo selectivity of this reaction can be controlled through a remarkable organocatalytic approach, employing MacMillans imidazolidinone catalysts, although the mechanistic origin of this selectivity remains unclear. We present a combined experimental and density functional theory investigation, providing insight into the effects of medium-ring constraints on the organocatalyzed intramolecular Diels-Alder reaction to form the isobenzofuran core of the eunicellins.