Choon Boon Cheong

We report the successful development of a stable chiral pentanidium pyridinyl-sulphonamide ion pair as a nucleophilic organocatalyst for asymmetric Steglich rearrangement.

Dioxaphosphabicyclo[2.2.2]octanes (L1–L4) have been prepared in a one-pot reaction from tris(hydroxymethyl)phosphine and various α,β-unsaturated ketones. The non-volatile phosphines oxidise very slowly in air. They possess highly upfield ³¹P chemical shifts (−59 to −70 ppm), small cone angles (121–140°) and a similar electronic parameter to PPh₃. Reaction of L1 with [Rh(acac)(CO)₂] gave the complex [Rh(acac)(CO)(L1)] with a ν_(CO) of 1981.5 cm⁻¹, whereas reaction L1 with [Rh(CO)₂Cl]₂ gave [Rh(CO)(L1)₂Cl] with a ν_(CO) of 1979.9 cm⁻¹, remarkably similar to the CO stretching frequencies reported for analogous PPh₃ complexes. The cage phosphines were explored as ligands in rhodium catalysed hydroformylation of 1-octene. All of the ligands gave a linear selectivity to n-nonanal of 68%, regardless of the substituents. However the ligand substituents had a significant effect on the catalyst activity, with increased steric bulk around the coordination environment giving a three-fold increase in aldehyde yield. The phosphines undergo ligand subsitution with [Pd(MeCN)₂Cl₂] forming square planar trans-[Pd(L)₂Cl₂] complexes. Subsequent reduction with hydrazine furnishes homoleptic tetravalent [Pd(L1)₄] which was applied as a catalyst in Suzuki–Miyaura couplings, furnishing the C–C coupled products in moderate yields.

A vinyl cyclopropane rearrangement embedded in an iridium‐catalyzed hydrogen borrowing reaction enabled the formation of substituted stereo‐defined cyclopentanes from Ph* methyl ketone and cyclopropyl alcohols. Mechanistic studies provide evidence for the ring‐expansion reaction being the result of a cascade based on oxidation of the cyclopropyl alcohols, followed by aldol condensation with the pentamethyl phenyl‐substituted ketone to form an enone containing the vinyl cyclopropane. Subsequent single electron transfer (SET) to this system initiates a rearrangement, and the catalytic cycle is completed by reduction of the new enone. This process allows for the efficient formation of diversely substituted cyclopentanes as well as the construction of complex bicyclic carbon skeletons containing up to four contiguous stereocentres, all with high diastereoselectivity.

A hydrogen borrowing reaction employing secondary alcohols and Ph* (Me₅C₆) ketones to give β-branched carbonyl products is described (21 examples). This new C–C bond forming process requires low loadings of [Cp*IrCl₂]₂, relatively low temperatures, and up to 2.0 equiv of the secondary alcohol. Substrate-induced diastereoselectivity was observed, and this represents the first example of a diastereoselective enolate hydrogen borrowing alkylation. By utilizing the Ph* group, the β-branched products could be straightforwardly cleaved to the corresponding esters or amides using a retro-Friedel–Crafts reaction. Finally, this protocol was applied to the synthesis of fragrance compound (±)-3-methyl-5-phenylpentanol.

An iridium-catalyzed hydrogen-borrowing process has been developed whereby 2,6-di-tert-butylphenol can be alkylated at the C4-position by using a range of primary alcohols (11 examples, 40–93% yield). Following this, a selection of the products obtained underwent retro-Friedel–Crafts reactions to provide para-substituted phenols, which could potentially undergo further synthetic manipulations.

The application of an iridium-catalyzed hydrogen borrowing process to enable the formation of α-branched ketones with higher alcohols is described. In order to facilitate this reaction, ortho-disubstituted phenyl and cyclopropyl ketones were recognized as crucial structural motifs for C–C bond formation. Having optimized the key catalysis step, the ortho-disubstituted phenyl products could be further manipulated by a retro-Friedel–Crafts acylation reaction to produce synthetically useful carboxylic acid derivatives. In contrast, the cyclopropyl ketones underwent homoconjugate addition with several nucleophiles to provide further functionalized branched ketone products.

From a medicinal chemistry perspective, bicyclo[1.1.1]pentan-1-amine (1) has served as a unique and important moiety. Synthetically, however, this compound has received little attention, and only one scalable route to this amine has been demonstrated. Reduction of an easily available and potentially versatile intermediate, 1-azido-3-iodobicyclo[1.1.1]pentane (2), can offer both a flexible and scalable alternative to this target. Herein, we describe our scrutiny of this reportedly elusive transformation and report our ensuing success with this endeavor.