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Advances in Asymmetric Organocatalysis Over the Last 10 Years ✉ Shao-Hua Xiang1,2 & Bin Tan 1

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Advances in Asymmetric Organocatalysis Over the Last 10 Years ✉ Shao-Hua Xiang1,2 & Bin Tan 1 COMMENT https://doi.org/10.1038/s41467-020-17580-z OPEN Advances in asymmetric organocatalysis over the last 10 years ✉ Shao-Hua Xiang1,2 & Bin Tan 1 Beyond esoteric interest, organocatalysis has now become one major pillar of asymmetric catalysis. Here, we discuss how new activation modes are con- 1234567890():,; quering challenging stereoselective transformations and the recent integration of organocatalysis with emerging photo- and electrocatalysis, as well as artificial intelligence. Impressed by the catalytic systems occurring in nature, chemists looked into imitating bioca- talytic processes. This led to the birth of organocatalysis in the late 1990s1,2 where chiral organic molecules bear the minimal functionalities to mimic biocatalysts and effect asymmetric reac- tions. Today, organocatalysis is one of the most thriving research domains in contemporary organic synthesis and covers a series of classic catalytic modes3–10 alongside numerous other valuable yet challenging chemical transformations. Although the green feature of organocatalysis is frequently disputed owing to difficulties with catalyst recycling and/or high catalyst loading, this field has evolved to parallel and complement transition-metal catalysis in assembling chiral molecules11. In the last decade, ~1500 publications on organocatalysis have been published each year. Prominently, the efforts to unify organocatalysis and photoredox catalysis by MacMillan in 2008 symbolize another milestone in enantioselective functionalization of otherwise difficult molecules12. These landmark discoveries refashion retrosynthesis and are revolutionizing other chemistry domains such as natural product synthesis, drug discovery, and fine chemicals pro- duction. Accordingly, enantioselective organocatalysis is elected as one of the 10 emerging technologies in Chemistry with potential to make our planet more sustainable by IUPAC on its 100th anniversary of foundation in 201913. Development of new catalytic strategies Despite the potentially green features of photocatalysis, practitioners in this area are often challenged by the achievement of full stereochemical control in a photochemical reaction due to the inherently high reactivity of the radicals. This scenario was overturned in 2008 after Mac- Millan and co-workers reported the inventive merging of enamine-mediated covalent catalysis and photoredox catalysis. In 2016, Melchiorre14,15 applied iminium activation in photocatalysis to deliver β,β-disubstituted cyclic enones 2 with very high enantiopurity in the asymmetric construction of quaternary carbon stereocenters (Fig. 1a, reaction i)16. The dual catalytic system with non-covalent organocatalysis was successfully implemented in 2013 by Knowles’ group17, who pioneered the use of chiral phosphoric acid (CPA)5,6,18 in asymmetric photocatalysis19. Harnessing this verified catalytic mode, Phipps’ group accomplished an intermolecular 1 Shenzhen Grubbs Institute and Department of Chemistry, Southern University of Science and Technology, Shenzhen 518055, China. 2 Academy for ✉ Advanced Interdisciplinary Studies, Southern University of Science and Technology, Shenzhen 518055, China. email: [email protected] NATURE COMMUNICATIONS | (2020) 11:3786 | https://doi.org/10.1038/s41467-020-17580-z | www.nature.com/naturecommunications 1 COMMENT NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-17580-z a : Merging of organocatalysis and photocatalysis i Generated by ii Ac R2 photocatalysis CO 1 O 2 R N O R - O 2 H + H + e pool R N 4 O O H N N 2 Chiral amine N R * P + O NHAc CPA O OHN O 1 R 3 NHAc Photocatalysis R 1 1 R1 R1 R Iminium activation Noncovalent H-bond acitivation Ac + H H N R2 - - O e holeH e hole 1 1 R R O O N N R2 R2 P H N N * – R H O OHN+ R R 5 NHAc NHAc R1 1 2 R1 R1 R b : Carbonyl catalysis and organotextile catalysis c : H-bond enhanced Lewis acid i + O OOTES Ph H CF3 – O P N R* OO Cl O Chiral aldehyde Ph NH O OH Ar P tBu TES OTf + + O tBu R* Ar H3N Ar N Ph N N OtBu Ph Ar OtBu O N H H NN CF3 6 7 8 NH N 2 OH Ar O O O H H S Ar = 1-pyrenyl O CF3 ii OTMS Anion abstraction 2 Textile R1 OR O O OMe 11 OR2 O H N Organotextile catalyst OH O R* R O R O O R1 OMe R2O R1 HN O S CF3 O N 12 R3 O n n H O O S More than 250 cycles n = 0 or 1 O N TMSO + [4+3] cycloaddition F C O 9 with excellent enantiocontrol 10 3 HN O 2 CF3 3 R O 13 R Ar Fig. 1 Development of new catalytic strategies. a Combination of organocatalysis with photocatalysis. b Carbonyl catalysis and organotextile catalysis. c Lewis acid enhancement by H-bond donor catalysis. decarboxylative cross-coupling of N-acetyl-L-phenylalanine 3 enantioselectivity preserved after >250 asymmetric catalytic and 4-methylquinoline 4 (Fig. 1a, reaction ii)20. Thereafter, a cycles. This impressive robustness of organotextile catalysts novel photosensitizer dicyanopyrazine-derived chromophore portends successful applications in industrial production. (DPZ) was developed by Jiang’s group as a substitute of expensive Asymmetric organocatalytic nucleophilic additions are usually iridium photocatalysts21. undertaken by hydrogen-bond catalysis. Given the modest acidity Activating carbonyl group with chiral secondary amine of chiral H-donors such as thioureas, squaramides, and guani- underpins the working principle of enamine catalysis for enan- dinium ions, the incompatibility of some substrate classes per- tioselective functionalization of aldehydes or ketones, whereas sists, which could be rectified by introduction of a Lewis acid as carbonyl catalysis represents the reversal of this strategy. By co-catalyst. Inspired by the reactivity enhancement through analogy to pyridoxal-dependent enzymes22 in carbonyl catalysis, interaction with more weakly coordinating ligands, an extremely Zhao’s group innovatively disclosed a class of organic small active Lewis acid catalyst generated by in situ silylation from molecule-carbonyl catalysts in 2018 (Fig. 1b, reaction i)23. This chiral C-H acids was designed by List and coworkers to promote well-defined strategy enabled asymmetric Mannich reaction of asymmetric Diels-Alder reaction26. One year later, a H-bond glycinate 6 with N-diphenylphosphinyl imines 7 in the presence catalyst-assisted Lewis acid enhancement strategy was inge- of an N-quaternized pyridoxal catalyst, furnishing synthetically niously utilized by Jacobson’s group with squaramide as a weak useful a,β-diamino acid esters 8 in high efficiency with remark- H-donor (Fig. 1c)27. Control experiments demonstrated the high able stereocontrol. Since then, substantial research efforts cul- reactivity of silyl triflate–squaramide ion pair and NMR analysis minated in a wide array of asymmetric transformations by shed light on the interaction between TESOTf (triethylsilyl tri- exploiting chiral aldehydes as the reliable amine activation fluoromethanesulfonate) and squaramide wherein the latter catalysts24. establishes an adequate spatial environment for stereoinduction. List group introduced organotextile catalysis built upon pho- These chiral active species readily mediate processes with tochemical fixation of organocatalysts on textile nylon via a heteroatom-stabilized cations to deliver Mukaiyama-type aldol radical course (Fig. 1b, reaction ii)25. The organocatalysts reaction and [4 + 3] cycloaddition of enolates 11 with furan exhibited excellent compatibility with polyamide to provide derivatives 12 in a highly stereoselective manner. heterogeneous organocatalysts harbouring Lewis bases, Brønsted acids and chiral cinchona alkaloid derivatives. Excellent stability and efficiency comparable to the corresponding homogeneous Target-oriented developments in organocatalysis catalysts were evidenced. This approach addressed the recycl- The catalytic enantioselective formation of fully substituted car- ability issue in classic organocatalysis with near-perfect bon stereocenters remains a challenging synthetic pursuit. 2 NATURE COMMUNICATIONS | (2020) 11:3786 | https://doi.org/10.1038/s41467-020-17580-z | www.nature.com/naturecommunications NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-17580-z COMMENT a : Enantioconvergent quaternary carbon stereocenters construction i + O OTMS Me OOChiral ion pair with OO AcO TMS tertiary carbocation TMS TMS Ar R* Ar 14 R* Ar15 R* Ar Me N N N N + N N H H H H H H Anion abstraction Me Ar SN1 process Me TMS O O O O + TMS O O 16 TMS S S S Ar O CF3 Ar O CF3 O CF3 ii Ph ‡ Ph Ph Ph Ph Ph R1 R1 Nu Chiral R1 N+ N Cation Ph Ph Br Nu Br Nu O NC Br Nu NC Br Nu R2 N N N R3 NC CO2Me NC CO2Me 2 X 2 CO2Me CO2Me R R Ar SK 1 R = 3,5-(tBu) PhCH racemic 17 18 Halogen-bonded S 2X process Chiral ion pair with 19 2 2 N R2 = 3,5-(CF ) PhCH ; X= Cl intermediate tertiary carbanion 3 2 2 b : Activation of unactivated alkene and arene i Me Ar Ar Strong Brønsted acid O OTES OH IDPi 6 6 O N O O 23 OTES O 20 21 P P O O O Strong NH N Ph H Ph H R Brønsted acid H 22 24 R R O OO O Ar Ar H S Single aldolizations of acetaldehyde enolates Ar S Ar N N R = 3,5-(CF3)2C6H3SO2 R = CF3SO2 up to 0.5 mol% catalyst loading O O P P * O N O * ii H H * O O H O O * CPA N N H NH2 N N R P N R P N R N R O O O O N XH HH2O 25 H 26 Binding-site for H-donor Conjugation transmission Axially chiral 27, NOBIN, X = O activating group of the electrophilicity compounds 28, BINAM, X = NH Fig. 2 Important target-oriented developments. a Chiral ion pair enabled quaternary carbon stereocenters construction in enantioconvergent manner. b Application in inactive alkene and arene activation. fi Contrary to a SN2-type nucleophilic substitution reaction, acidic and structurally well-de ned chiral organic acid, which can enantio-differentiating addition in SN1-type pathway is chal- functionalize this class of poorly active molecules is enthralling.
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