Thomas E. Storr

The Role of the Alcohol and Carboxylic Acid in Directed Ruthenium-Catalyzed C(sp3)H α-Alkylation of Cyclic Amines

A general directed Ru-catalyzed C(sp(3))-H α-alkylation protocol for piperidines (less-reactive substrates than the corresponding five-membered cyclic amines) has been developed. The use of a hindered alcohol (2,4-dimethyl-3-pentanol) as the solvent and catalyst activator, and a catalytic amount of trans-1,2-cyclohexanedicarboxylic acid is necessary to achieve a high conversion to product. This protocol was used to effectively synthesize a number of 2-hexyl- and 2,6-dihexyl piperidines, as well as the alkaloid (±)-solenopsin A. Kinetic studies have revealed that the carboxylic acid additive has a significant effect on catalyst initiation, catalyst longevity, and reverses the reaction selectivity compared with the acid-free reaction (promotes alkylation versus competing alkene reduction).

Palladium-catalyzed arylation of simple arenes with iodonium salts.

The development of an arylation protocol for simple arenes with diaryliodonium salts using the Herrmann-Beller palladacycle catalyst is reported. The reaction takes simple aromatic feedstocks and creates valuable biaryls for use in all sectors of the chemical industry.

Direct C-H/C-X Coupling Methodologies Mediated by Pd/Cu or Cu: An Examination of the Synthetic Applications and Mechanistic Findings

Organocopper chemistry has long been known to be powerful synthetic methodology leading to the generation of C-C, C-O, C-N, C-S and C-X bonds. Advances in catalytic and stoichiometric organometallic reactions for the formation of C-C bonds from unfunctionalised arenes, e.g. by activation-functionalisation of a C-H bond, provides diverse and applicable reaction conditions for direct functionalisation. This review covers the background, applications and recent advances made in the area of aromatic C-X/C-H-bond couplings promoted by copper and copper/palladium mixed systems. The efficacy and scope of these reactions are discussed and the different methodologies are placed into context. Mechanistic considerations and evidence are highlighted at appropriate junctures in the review.

Ruthenium‐Catalyzed α‐(Hetero)Arylation of Saturated Cyclic Amines: Reaction Scope and Mechanism

Transition-metal-catalyzed sp(3) C-H activation has emerged as a powerful approach to functionalize saturated cyclic amines. Our group recently disclosed a direct catalytic arylation reaction of piperidines at the α position to the nitrogen atom. 1-(Pyridin-2-yl)piperidine could be smoothly α-arylated if treated with an arylboronic ester in the presence of a catalytic amount of [Ru3(CO)12] and one equivalent of 3-ethyl-3-pentanol. A systematic study on the substrate and reagent scope of this transformation is disclosed in this paper. The effect of substitution on both the piperidine ring and the arylboronic ester has been investigated. Smaller (pyrrolidine) and larger (azepane) saturated ring systems, as well as benzoannulated derivatives, were found to be compatible substrates with the α-arylation protocol. The successful use of a variety of heteroarylboronic esters as coupling partners further proved the power of this direct functionalization method. Mechanistic studies have allowed for a better understanding of the catalytic cycle of this remarkable transformation featuring an unprecedented direct transmetalation on a Ru(II)-H species.

On the appearance of nitrite anion in [PdX(OAc)L2] and [Pd(X)(C^N)L] syntheses (X = OAc or NO2): photocrystallographic identification of metastable Pd(η1-ONO)(C^N)PPh3

Pd3(OAc)5NO2, an impurity in “Pd(OAc)2” {formally Pd3(OAc)6}, emerges as a serious issue in the synthesis of pure PdII complexes derived from Pd(OAc)2, for example in our C–H activation precatalyst, Pd(OAc)2(pip)2 (pip = piperidine). A previous proposal that nitrite anion can be formed by oxidation of CH3CN by metallic Pd and air, leading to cyclo(ortho)palladated complexes containing nitrite anion, e.g. Pd(NO2)(C^N)L (C^N = papaverine; L = CH3CN or DMSO) can be explained by Pd3(OAc)5NO2 acting as the nitrite source. Finally, photocrystallographic metastable linkage isomerisation and complete conversion to an oxygen-bound nitrito complex Pd(η1-ONO)(C^N)PPh3 has been observed.

Combining two-directional synthesis and tandem reactions. Part 21: Exploitation of a dimeric macrocycle for chain terminus differentiation and synthesis of an sp3-rich library

The application of a tandem condensation/cyclisation/[3+2]-cycloaddition/elimination reaction gives an sp(3)-rich tricyclic pyrazoline scaffold with two ethyl esters in a single step from a simple linear starting material. The successive hydrolysis and cyclisation (with Boc anhydride) of these 3-dimensional architectures, generates unprecedented 16-membered macrocyclic bisanhydrides (characterised by XRD). Selective amidations could then be achieved by ring opening with a primary amine followed by HATU-promoted amide coupling to yield an sp(3)-rich natural product-like library.

Palladium catalysed cross-dehydrogenative-coupling of 1,3,5-trialkoxybenzenes with simple arenes

Pd-catalysed cross-dehydrogenative coupling of 1,3,5-trialkoxybenzenes with simple aromatic hydrocarbons is reported. The method enables the coupling of two aromatic C-H positions to generate multi-ortho-substituted biaryls.

Expedient synthesis of an atypical oxazolidinone compound library

In order to address the current downturn in the drug discovery pipeline, initiatives are being undertaken to synthesise screening libraries of sp3-rich, low molecular weight compounds. As part of the European Lead Factory initiative, the synthesis and derivatisation of a simple hexahydrooxazolo[5,4-c]pyridin-2(1H)-one bicyclic carbamate has been achieved. The synthetic route employed involved a telescoped hetero-Diels-Alder/[2,3]-sigmatropic rearrangement/cyclisation sequence to deliver the desired core scaffold containing two points for further diversification. When applied, this synthesis was found to be robust and scalable which allowed the production of a 155 compound library.

Versatile C(sp2)−C(sp3) ligand couplings of sulfoxides for the enantioselective synthesis of diarylalkanes

The reaction of chiral (hetero)aryl benzyl sulfoxides with Grignard reagents affords enantiomerically pure diarylalkanes in up to 98 % yield and greater than 99.5 % enantiomeric excess. This ligand coupling reaction is tolerant to multiple substitution patterns and provides access to diverse areas of chemical space in three operationally simple steps from commercially available reagents. This strategy provides orthogonal access to electron-deficient heteroaromatic compounds, which are traditionally synthesized by transition metal catalyzed cross-couplings, and circumvents common issues associated with proto-demetalation and β-hydride elimination.

Cross-Dehydrogenative-Coupling of Alkoxybenzenes with Toluenes: Copper(II) Halide Mediated Tandem Halo/Benzylation of Arenes

A cross-dehydrogenative-coupling of alkoxybenzenes and toluenes with concomitant halogenation is reported. Conditions employed were the use of stoichiometric copper halide salts and dialkylperoxides to afford a range of bromoalkoxydi- and triarylmethanes. Preliminary mechanistic studies suggest that the in situ production of haloarenes (or dihaloarenes) followed by a copper-mediated coupling of a benzylic radical is operational.