Asymmetric α-allylic allenylation of β-ketocarbonyls and aldehydes by synergistic Pd/chiral primary amine catalysis

Catalytic Asymmetric 1,4-Hydrocarbonation of 1,3-Enynes via Photoredox/Cobalt/Chromium Triple Catalysis

A synergistic photoredox/cobalt/chromium triple catalysis system for regioselective, enantioselective, and diastereoselective 1,4-hydrocarbonation of readily available 1,3-enyne precursors was explored, providing a modular synthetic platform for various trisubstituted axially chiral allenes bearing an extra central chirality. The protocol features a broad substrate scope, good functional group tolerance, excellent selectivity, and mild reaction conditions. Furthermore, a possible reaction mechanism is proposed based on numerous control experiments and density functional theory calculations.

Diastereoselective and Enantioselective Hydrophosphinylations of Conjugated Enynes, Allenes and Dienes via Synergistic Pd/Co Catalysis

Different from the reported work focusing on the construction of single P- or C-stereocenter via hydrophosphinylation of unsaturated carbon bonds, the highly diastereo- and enantioselective hydrophosphinylation reaction of allenes, conjugated enynes and 1,3-dienes is achieved via a designed Pd/Co dual catalysis and newly modified masked phosphinylating reagent. A series of allyl motifs bearing both a tertiary C- and P-stereocenter are prepared in generally good yields, >20 : 1 dr, >20 : 1 rr and 99 % ee. The unprecedented diastereo- and enantioselective hydrophosphinylation of 1,3-enynes is established to generate skeletons containing both a P-stereocenter and a nonadjacent chiral axis. The first stereodivergent hydrophosphinylation reaction is also developed to achieve all four P-containing stereoisomers. The present protocol features the use of only 3-minutes reaction time and 0.1 % catalyst, and with the observation of up to 730 TON. A set of mechanistic studies reveal the necessity and roles of two metal catalysts and corroborate the designed synergistic process.

Total syntheses of the parvistemoline alkaloids enabled by stereocontrolled Ir/Pd-catalyzed allylic alkylation

The functionalized polycycle with densely contiguous tertiary stereocenters is a formidable challenge in synthesizing the parvistemoline family of Stemona alkaloids. We herein report their catalytic, asymmetric total syntheses in 13-14 steps from commercially available 2-(methoxycarbonyl)-pyrrole, featuring the development and deployment of an Ir/Pd-synergistically-catalyzed allylation of α-non-substituted keto esters with secondary aryl-substituted alcohols, stereodivergently accessible to four stereoisomers. Using chiral Pd-enolate and Ir π-allyl complex under neutral conditions, no epimerization occurs. Additionally, the other two adjacent stereogenic centers can be installed diastereoselectively by Zn(BH4)2-promoted reduction and Krische's Ir-catalyzed 2-(alkoxycarbonyl)allylation. Oxy-Michael addition delivered the fused tetrahydrofuran-γ-lactone scaffold. At the later stage, hydrogenation or oxidation of pyrrole moiety furnished groups of tetrahydropyrrole and pyrrolidone. Finally, vinylogous Mannich reaction of an in situ generated iminium ion or Krische's Ir-catalyzed 2-(alkoxycarbonyl)allylation of aldehyde installed the monocyclic lactone for parvistemonine (2) and didehydroparvistemonine (3), respectively.

Copper(I)-Catalyzed Asymmetric Nucleophilic Addition to Aldehydes with Skipped Enynes

The development of sustainable and novel strategies for constructing complex chiral molecules with versatile transformation potential is a long-term pursuit in the chemistry community. We report a copper(I)-catalyzed enyne addition to aldehydes under proton-transfer conditions, unlike previous examples which were limited to the use of preformed reactive nucleophiles containing allylic heteroatoms or electron-withdrawing groups. This protocol provides an efficient platform for installing chiral allylic alcohol moieties with a broad substrate scope and high regio-, stereo-, and enantioselectivity.

Stereodivergent Construction of 1,5/1,7-Nonadjacent Tetrasubstituted Stereocenters Enabled by Pd/Cu-Cocatalyzed Asymmetric Heck Cascade Reaction

The construction of chiral motifs containing nonadjacent stereocenters in an enantio- and diastereoselective manner has long been a challenging task in synthetic chemistry, especially with respect to their stereodivergent synthesis. Herein, we describe a protocol that enables the enantio- and diastereoselective construction of 1,5/1,7-nonadjacent tetrasubstituted stereocenters through a Pd/Cu-cocatalyzed Heck cascade reaction. Notably, a C=C bond relay strategy involving the shift of the π-allyl palladium intermediate was successfully applied in the asymmetric construction of 1,7-nonadjacent stereocenters. The current method allows for the efficient preparation of chiral molecules bearing two privileged scaffolds, oxindoles and non-natural α-amino acids, with good functional group tolerance. The full complement of the four stereoisomers of products bearing 1,5/1,7-nonadjacent stereocenters could be readily accessed by a simple combination of two chiral metal catalysts with different enantiomers.

Copper/ruthenium relay catalysis enables 1,6-double chiral inductions with stereodivergence

The dual catalysis strategy is an efficient and powerful tool to fulfill the stereodivergent synthesis of stereoisomeric products from the same set of starting materials. Great attention has been given to the construction of chiral compounds with two contiguous stereocenters. However, the synthesis of two remote noncontiguous stereocenters is more challenging and is less developed, despite the high demand for synthetic tactics. We herein developed an unprecedented example of the stereodivergent preparation of synthetically useful and biologically important chiral ζ-hydroxy amino ester derivatives containing remote 1,6-noncontiguous stereocenters and a unique β,γ-unsaturation moiety. This cascade dehydrogenation/1,6-Michael addition/hydrogenation protocol between readily-available ketoimine esters and racemic branched dienyl carbinols was rationally realized with bimetallic copper/ruthenium relay catalysis. The key features of the process were atom economy, step economy, and redox-neutrality. All four stereoisomers of chiral ζ-hydroxy amino ester derivatives were easily achieved by the orthogonal permutations of a chiral copper catalyst and chiral ruthenium catalyst. Importantly, a much more challenging stereodivergent synthesis of all eight stereoisomers of chiral peptide products containing three remote stereocenters was accomplished with excellent results through the cooperation of two chiral catalyst pairs and substrate enantiomers.

Ligand-Controlled, Nickel-Catalyzed Stereodivergent Construction of 1,3-Nonadjacent Stereocenters

In contrast to the asymmetric synthesis of molecules with a single stereocenter or 1,2-adjacent stereocenters, the simultaneous construction of acyclic 1,3-nonadjacent stereocenters via a single catalyst in an enantioselective and diastereoselective manner remains a formidable challenge. Here, we demonstrate the enantioselective and diastereodivergent construction of 1,3-nonadjacent stereocenters through Ni-catalyzed reductive cyclization/cross-coupling of alkene-tethered aryl bromides and α-bromoamides, which represents the major remaining stereochemical challenge of cyclization/difunctionalization of alkenes. Using Ming-Phos as ligand, a diverse set of oxindoles containing 1,3-nonadjacent stereocenters were obtained with high levels of enantio- and diastereoselectivity. Mechanistic experiments and density functional theory calculations indicate that magnesium salt plays a key role in controlling the diastereoselectivity. Furthermore, another set of complementary stereoisomeric products were constructed from the same set of starting materials using Ph-Phox as ligand.

Enantioselective Transformations by "1 + x" Synergistic Catalysis with Chiral Primary Amines

ConspectusSynergistic catalysis is a powerful tool that involves two or more distinctive catalytic systems to activate reaction partners simultaneously, thereby expanding the reactivity space of individual catalysis. As an established catalytic strategy, organocatalysis has found numerous applications in enantioselective transformations under rather mild conditions. Recently, the introduction of other catalytic systems has significantly expanded the reaction space of typical organocatalysis. In this regard, aminocatalysis is a prototypical example of synergistic catalysis. The combination of aminocatalyst and transition metal could be traced back to the early days of organocatalysis and has now been well explored as an enabling catalytic strategy. Particularly, the acid-base properties of aminocatalysis can be significantly expanded to include usually electrophiles generated in situ via metal-catalyzed cycles. Later on, aminocatalyst has also been exploited in synergistically combining with photochemical and electrochemical processes to facilitate redox transformations. However, synergistically combining one type of aminocatalyst with many different catalytic systems remains a great challenge. One of the most daunting challenges is the compatibility of aminocatalysts in coexistence with other catalytic species. As nucleophilic species, aminocatalysts may also bind with metal, which leads to mutual inhibition or even quenching of the individual catalytic activity. In addition, oxidative stability of aminocatalyst is also a non-neglectable issue, which causes difficulties in exploring oxidative enamine transformations.In 2007, we developed a vicinal diamine type of chiral primary aminocatalysts. This class of primary aminocatalysts was developed and evolved as functional and mechanistic mimics to the natural aldolase and has been widely applied in a number of enamine/iminium ion-based transformations. By following a "1 + x" synergistic strategy, the chiral primary amine catalysts were found to work synergistically or cooperatively with a number of transition metal catalysts, such as Pd, Rh, Ag, Co, and Cu, or other organocatalysts, such as B(C6F5)3, ketone, selenium, and iodide. Photocatalysis and electrochemical processes can also be incorporated to work together with the chiral primary amine catalysts. The 1 + x catalytic strategy enabled us to execute unexploited transformations by fine-tuning the acid-base and redox properties of the enamine intermediates and to achieve effective reaction and stereocontrol beyond the reach individually. During these efforts, an unprecedented excited-state chemistry of enamine was uncovered to make possible an effective deracemization process. In this Account, we describe our recent efforts since 2015 in exploring synergistic chiral primary amine catalysis, and the content is categorized according to the type of synergistic partner such that in each section the developed synergistic catalysis, reaction scopes, and mechanistic features are presented and discussed.

Synergistic Palladium/Copper-Catalyzed 1,4-Difunctionalization of 1,3-Dienes for Stereodivergent Construction of 1,5-Nonadjacent Stereocenters

The construction of two distal stereocenters through a single catalytic process is of great interest in organic synthesis. While there are some successful reports regarding stereodivergent preparation of 1,3- or 1,4-stereocenters, the more challenged 1,5-nonadjacent stereocenters have never been achieved in a stereodivergent fashion. Herein we describe a synergistic palladium/copper catalysis for 1,4-difunctionalization reactions of 1,3-dienes, providing access to 1,5-nonadjacent quaternary stereocenters. Because each of the two catalysts separately controlled one of the newly formed stereocenters, stereodivergent synthesis of all four diastereomers of the products could readily be achieved simply by choosing an appropriate combination of chiral catalysts. Experimental and computational studies supported a mechanism involving a Heck/Tsuji-Trost cascade reaction, and the origins of the stereoselectivity were elucidated.

Enantioselective Construction of Quaternary Stereocenters via Cooperative Photoredox/Fe/Chiral Primary Amine Triple Catalysis

The catalytic and enantioselective construction of quaternary (all-carbon substituents) stereocenters poses a formidable challenge in organic synthesis due to the hindrance caused by steric factors. One conceptually viable and potentially versatile approach is the coupling of a C-C bond through an outer-sphere mechanism, accompanied by the realization of enantiocontrol through cooperative catalysis; however, examples of such processes are yet to be identified. Herein, we present such a method for creating different compounds with quaternary stereocenters by photoredox/Fe/chiral primary amine triple catalysis. This approach facilitates the connection of an unactivated alkyl source with a tertiary alkyl moiety, which is also rare. The scalable process exhibits mild conditions, does not necessitate the use of a base, and possesses a good functional-group tolerance. Preliminary investigations into the underlying mechanisms have provided valuable insights into the reaction pathway.

Synergistic Pd/Cu-Catalyzed 1,5-Double Chiral Inductions

Much attention has been focused on the catalytic asymmetric creation of single chiral centers or two adjacent stereocenters. However, the asymmetric construction of two nonadjacent stereocenters is of significant importance but is challenging because of the lack of remote chiral induction models. Herein, based on a C═C bond relay strategy, we report a synergistic Pd/Cu-catalyzed 1,5-double chiral induction model. All four stereoisomers of the target products bearing 1,5-nonadjacent stereocenters involving both allenyl axial and central chirality could be obtained divergently by simply changing the combination of two chiral catalysts with different configurations. Control experiments and DFT calculations reveal a novel mechanism involving 1,5-oxidative addition, contra-thermodynamic η3-allyl palladium shift, and conjugate nucleophilic substitution, which play crucial roles in the control of reactivity, regio-, enantio-, and diastereoselectivity. It is expected that this C═C bond relay strategy may provide a general protocol for the asymmetric synthesis of structural motifs bearing two distant stereocenters.

Core Structure-Oriented Asymmetric α-Allenylic Alkylation of Amino Acid Esters Enabled by Chiral Aldehyde/Palladium Catalysis

Aiming at the reported chiral synthons leading to manzacidins A and D, here we report a highly efficient catalytic asymmetric α-allenylic alkylation reaction of NH2-unprotected amino acid esters that is promoted by combined chiral aldehyde/palladium catalysis. Fifty examples of unnatural α,α-disubstituted amino acid esters are reported with good-to-excellent yields and stereoselectivities. Based on this methodology, a key intermediate leading to manzacidin C and its other three stereoisomers is prepared accordingly.