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Understanding how catalytic asymmetric reactions with racemic starting materials can operate would enable new enantioselective cross-coupling reactions that give chiral products. Here we propose a catalytic cycle for the highly enantioselective Rh(i)-catalysed Suzuki–Miyaura coupling of boronic acids and racemic allyl halides. Natural abundance 13C kinetic isotope effects provide quantitative information about the transition-state structures of two key elementary steps in the catalytic cycle, transmetallation and oxidative addition. Experiments with configurationally stable, deuterium-labelled substrates revealed that oxidative addition can happen via syn- or anti-pathways, which control diastereoselectivity. Density functional theory calculations attribute the extremely high enantioselectivity to reductive elimination from a common Rh complex formed from both allyl halide enantiomers. Our conclusions are supported by analysis of the reaction kinetics. These insights into the sequence of bond-forming steps and their transition-state structures will contribute to our understanding of asymmetric Rh–allyl chemistry and enable the discovery and application of asymmetric reactions with racemic substrates. A major drive in current chemistry research is to develop asymmetric versions of widely used carbon–carbon bond-forming reactions, such as Suzuki-Miyaura cross-couplings. Now, the origins of diastereo- and enantioselectivity in a Rh-catalysed cross-coupling of boronic acid and racemic allyl halides have been established.

Enantiomerically enriched allyl halides are rare due to their configurational lability. Here we report stable piperidine-based allyl chloride enantiomers. These allyl chlorides can be produced via kinetic resolution, and undergo highly enantiospecific catalyst-free substitution reactions with C, N, O and S-based nucleophiles. DFT calculations and experiments with deuterium-labelled chloro-tetrahydropyridine, selectively prepared using H/D primary kinetic isotope effect, were used to investigate the mechanisms of resolution and substitution reactions. The allyl chlorides may also serve as valuable mechanistic tools for probing stereoselective reaction pathways.

A mechanism for Rh(I)-catalyzed asymmetric Suzuki-Miyaura coupling with racemic allyl halides is proposed based on a combination of experimental studies and quantum chemical calculations.

The Pauson-Khand reaction is a powerful tool for the synthesis of cyclopentenones through the efficient [2 + 2 + 1] cycloaddition of dicobalt alkyne complexes with alkenes. While intermolecular and intramolecular variants are widely known, transannular versions of this reaction are unknown and the basis of this study. Macrocyclic enyne and dienyne complexes were readily synthesized by palladium(II)-catalyzed oxidative macrocyclizations of bis(vinyl boronate esters) or ring-closing metathesis reactions followed by complexation with dicobalt octacarbonyl. Several reaction modalities of these macrocyclic complexes were uncovered. In addition to the first successful transannular Pauson-Khand reactions, other intermolecular and transannular cycloaddition reactions included intermolecular Pauson-Khand reactions, transannular [4 + 2] cycloaddition reactions, intermolecular [2 + 2 + 2] cycloaddition reactions, and intermolecular [2 + 2 + 1 + 1] cycloaddition reactions. The structural and reaction requirements for each process are presented.

We report the kinetic resolution of racemic allyl chlorides via a copper-catalysed asymmetric allylic alkylation. Novel 3-alkyl substituted tetrahydropyridines and enantioenriched 3-chloro-1,2,3,6-tetrahydropyridines are formed in high enantioselectivities. The piperidine-based allyl halides undergo highly enantiospecific substitution reactions with C, N, O and S-based nucleophiles. Experiments with deuterium-labelled chloro-tetrahydropyridine and DFT calculations were used to investigate the mechanistic pathways of the reactions.

A concise and efficient synthesis of densely substituted novel pyrazoles with alkynyl, aryl and ferrocenyl functionalities is reported, providing a platform for biological studies. The general strategy involves Sonogashira and Suzuki–Miyaura cross-coupling reactions of easily obtainable 5-ferrocenyl/phenyl-4-iodo-1-phenylpyrazoles with terminal alkynes and boronic acids, respectively. The starting 4-iodopyrazoles were synthesized by electrophilic cyclization of α,β-alkynic hydrazones with molecular iodine. Sonogashira reactions have been achieved by employing 5 mol% PdCl2(PPh3)2, 5 mol% CuI, excess Et3N and 1.2 equiv. of terminal alkyne, relative to 4-iodopyrazole, in tetrahydrofuran at 65 °C, while Suzuki–Miyaura reactions have been accomplished using 5 mol% PdCl2(PPh3)2 and 1.4 equiv. of both boronic acid/ester and KHCO3, with respect to 4-iodopyrazole, in 4:1 dimethylformamide–H2O solution at 110 °C. Both Sonogashira and Suzuki–Miyaura coupling reactions have proven effective for the synthesis of alkynyl-, aryl- and ferrocenyl-substituted pyrazoles and demonstrated good tolerance to a diverse range of substituents, including electron-donating and electron-withdrawing groups. These coupling approaches could allow for the rapid construction of a library of functionalized pyrazoles of pharmacological interest. Copyright © 2016 John Wiley & Sons, Ltd.

A facile, efficient and general synthetic method for iodopyridines is described. When treated with molecular iodine in the presence of sodium bicarbonate, N-propargylic β-enaminones have underwent electrophilic cyclization to afford iodo-substituted pyridines in good to high yields. Iodocyclization has been found to be general for a broad range of N-propargylic β-enaminones and tolerated the presence of aliphatic, aromatic and heteroaromatic groups with electron-withdrawing and electron-donating substituents. The starting N-propargylic β-enaminones have been prepared by conjugate addition of propargylamine to α,β-alkynic ketones, followed by Sonogashira coupling with aryl iodides.

Triene substrates containing an (E)-dibromo-tetrasubstituted alkene moiety participate in selective ene-ene ring-closing metathesis reactions.

© 2015 American Chemical Society. Density functional theory calculations were performed on a set of 13 transannular Diels-Alder (TADA) reactions with 10-18-membered rings. The results were compared with those for bimolecular and intramolecular Diels-Alder reactions in order to investigate the controlling factors of the high TADA reactivities. The effects of tether length, heteroatoms, and alkynyl dienophiles on reactivity were analyzed. We found a correlation between tether length and reactivity, specifically with 12-membered macrocycles undergoing cycloaddition most readily. Furthermore, modifying 12-membered macrocycles by heteroatom substitution and utilizing alkynyl dienophiles enhances the reaction rates up to 105-fold.

© 2015 American Chemical Society. A new strategy to access macrocyclic enynes was developed. To block undesired ene-yne cyclization pathways, alkynes were protected via bromination and the resultant acyclic vic-(E)-dibromotrienes participated in selective ene-ene ring closing metathesis reactions. Zinc-promoted deprotection of (E)-dibromodienes provided macrocyclic enynes in high yields.

A facile, efficient and general synthetic method for iodopyridines is described. When treated with molecular iodine in the presence of sodium bicarbonate, N-propargylic β-enaminones have underwent electrophilic cyclization to afford iodo-substituted pyridines in good to high yields. Iodocyclization has been found to be general for a broad range of N-propargylic β-enaminones and tolerated the presence of aliphatic, aromatic and heteroaromatic groups with electron-withdrawing and electron-donating substituents. The starting N-propargylic β-enaminones have been prepared by conjugate addition of propargylamine to α,β-alkynic ketones, followed by Sonogashira coupling with aryl iodides.