Insight into the mechanism of the arylation of arenes via norbornene relay palladation through meta- to para-selectivity

A theoretical insight was shown into the origin of site-selectivity in the arylation of arenes by a norbornene relay palladation through meta- to para-selectivity.

Understanding Cu(II)-based systems for C(sp3)-H bond functionalization: insights into the synthesis of aza-heterocycles

Herein we report the synthesis of imidazo[1,5-a]pyridine heterocycles via a Cu(II)-mediated functionalization of α'-C(sp³)-H bonds of pyridinylaldimines and subsequent cyclization. This strategy exploits the inherent directing ability of heteroleptic aldimine and pyridine groups in the substrate yielding the C-H functionalization of α'-methylene groups in a regioselective fashion over distant methyl or methylene groups in β or γ positions. The observed correlation between the nature of the anionic ligands (halide vs. carboxylate) bonded to copper and the chemoselectivity of the C(sp³)-H activation process points to a concerted metalation-deprotonation pathway prior to cyclization to furnish the corresponding imidazo[1,5-a]pyridine derivative. This copper-mediated C(sp³)-H bond functionalization reaction works for a variety of substrates incorporating linear alkyl chains (from 3 to 12 carbon atoms), and good functional group tolerance (aryl, ether and ester groups). Cu-Catalyzed C(sp²)-H cyanation on the imidazole ring can then take place selectively under oxidative conditions.

Modular Entry to Functionalized Tetrahydrobenzo[b]azepines via the Palladium/Norbornene Cooperative Catalysis Enabled by a C7-Modified Norbornene

Tetrahydrobenzo[b]azepines (THBAs) are commonly found in many bioactive compounds; however, the modular preparation of functionalized THBAs remains challenging to date. Here, we report a straightforward method to synthesize THBAs directly from simple aryl iodides via palladium/norbornene (Pd/NBE) cooperative catalysis. Capitalizing on an olefin-tethered electrophilic amine reagent, an ortho amination followed by 7-exo-trig Heck cyclization furnishes the seven-membered heterocycle. To overcome the difficulty with ortho-unsubstituted aryl iodide substrates, we discovered a unique C7-bromo-substituted NBE (N1) to offer the desired reactivity and selectivity. In addition to THBAs, synthesis of other benzo-seven-membered ring compounds can also be promoted by N1. Combined experimental and computational studies show that the C7-bromo group in N1 plays an important and versatile role in this catalysis, including promoting β-carbon elimination, suppressing benzocyclobutene formation, and stabilizing reaction intermediates. The mechanistic insights gained could guide future catalyst design. The synthetic utility has been demonstrated in a streamlined synthesis of tolvaptan and forming diverse pharmaceutically relevant THBA derivatives. Finally, a complementary and general catalytic condition to access C6-substituted THBAs from ortho-substituted aryl iodides has also been developed.

Installing the “magic methyl” – C–H methylation in synthesis

Following notable cases of remarkable potency increases in methylated analogues of lead compounds, this review documents the state-of-the-art in C–H methylation technology.

Compatibility Score for Rational Electrophile Selection in Pd/NBE Cooperative Catalysis

A mechanistically guided approach is developed to predict electrophile compatibility in the palladium/norbornene (Pd/NBE) cooperative catalysis for the ipso/ortho difunctionalization of aryl halides. A key challenge in these reactions is to identify orthogonal electrophile and aryl hali de starting materials that react selectively with different transition metal complexes in separate oxidative addition events in the catalytic cycle. We performed detailed experimental and computational mechanistic studies to identify the catalytically active Pd(II) intermediate and the substrate-dependent mechanisms in reactions with various types of carbon and nitrogen electrophiles. We introduced the concept of electrophile compatibility score (ECS) to rationally select electrophiles based on the orthogonal reactivity of electrophile and aryl halide towards the Pd(0) and Pd(II) complexes. This approach was applied to predict electrophile compatibility in the Pd/NBE cooperative catalysis with a variety of electrophilic coupling partners used in alkylation, arylation, amination, and acylation reactions.

Entry to 1,2,3,4-Tetrasubstituted Arenes through Addressing the "Meta Constraint" in the Palladium/Norbornene Catalysis

Arenes with four different contiguous substituents, i.e. 1,2,3,4-tetrasubstituted arenes, are commonly found in bioactive compounds, but they are nontrivial to access via conventional methods. Through addressing the "meta constraint" in the palladium/norbornene (Pd/NBE) cooperative catalysis, which is the difficulty of tolerating a sizable meta substituent in aryl halide substrates, here a modular and regioselective approach is realized for preparing 1,2,3,4-tetrasubstituted arenes. One key is the use of a C2-amide-substituted NBE, and a combined experimental and computational study reveals its role in promoting the NBE insertion and the ortho C-H metalation steps. The scope is broad: a variety of electrophiles and nucleophiles could be introduced to the ortho and ipso positions, respectively, with 1,4-disubstituted aryl halides, leading to diverse unsymmetrical contiguous tetrasubstituted arenes. Application of this approach has been demonstrated in streamlined syntheses of several bioactive compounds.