Electrophilicity Scales for Carbocations

Mayr, Herbert; Ofial, Armin R.

doi:10.1002/9780471678656.ch13

Cited by 53 publications

(9 citation statements)

References 79 publications

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“…The catalytic asymmetric alkylation of unactivated benzylic C-H bonds, leading to the formation of C-C bonds, is also challenging [33]. They are difficult to activate, and the generation of transient radicals requires harsh conditions [34].…”

Section: Transition Metal-catalyzed Enantioselective Cdcmentioning

confidence: 99%

New Trends in Enantioselective Cross-Dehydrogenative Coupling

2020

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The development of cross-dehydrogenative coupling in recent years has simplified the synthesis of many materials, as a result of facile C–H activation, which, together with its greater atom economy and environmental friendliness, has made an impact on modern organic chemistry. Indeed, many C–C and C–X (X = N, O, P, S, B, or Si) coupling reactions can now be performed directly between two C–H bonds or a C–H and an X–H bond, simply by adding catalytic amounts of a metal salt to a mixture of the two and an oxidant to accept the two hydrogen atoms released. Chiral organocatalysts or chiral ligands have been joined to promote enantioselective processes, resulting in the development of efficient reaction cascades that provide products in high yields and high levels of asymmetric induction through cooperative catalysis. In recent years, photochemical oxidation and electrochemistry have widened even more the scope of cross-dehydrogenative coupling (CDC). In this review, we summarized the recent literature in this subject, hoping that it will inspire many new synthetic strategies.

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Section: Transition Metal-catalyzed Enantioselective Cdcmentioning

confidence: 99%

New Trends in Enantioselective Cross-Dehydrogenative Coupling

2020

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“…1b , 1d , 6 Quantification of the electrophilicity of such ortho -carbonyl-substituted benzonitriles would avail the prediction of the scope and selectivities of these cascade reactions. However, our attempts to determine the electrophilicity of 2-acetylbenzonitrile by studying the kinetics of its reactions with carbanions of known Mayr nucleophilicity 7a were not conclusive. Nevertheless, these kinetic experiments indicated that the carbonyl group in 2-acetylbenzonitrile may well be accessible for reactions with α-halo-stabilized carbanions.…”

Section: Introductionmentioning

confidence: 96%

Base-Promoted Cascade Reactions for the Synthesis of 3,3-Dialkylated Isoindolin-1-ones and 3-Methyleneisoindolin-1-ones

et al. 2021

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Cascade reactions of ortho -carbonyl-substituted benzonitriles with ((chloromethyl)sulfonyl)benzenes as pronucleophiles led to new isoindolin-1-ones with a tetrasubstituted C-3 position or to ( Z )-3-(sulfonyl-methylene)isoindolin-1-ones. The reactions start from readily available materials, are carried out under mild conditions, and do not require metal catalysis. Promoted only by the cheap and environmentally benign K 2 CO 3 as the base, up to six elemental steps can be combined in a single pot. Hence, a sequential one-pot cascade/β-elimination/alkylation furnished useful intermediates for the synthesis of aristolactam natural products. The observed selectivity and the mechanism were investigated by DFT studies.

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“…10 Many examples of catalytic conjugate additions of enamine-based nucleophiles to electron-deficient alkenes are known (Figure 1A). 11 The absence of catalytic aldehyde enamine conjugate additions to conventional enoate esters likely stems from the low electrophilicity of these enoate esters, as established by Mayr et al, 12,13 along with the high electrophilicity of aldehydes, which leads to preference for homoaldol products. 14 The synthetic value of this unknown transformation arises from the potential utility of the products.…”

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confidence: 99%

“…16,17,20 The diminished electrophilicity of a ketone relative to an aldehyde curtails formation of homoaldol byproducts in these cases. 13,14 Therefore, achieving chemo- selective conjugate addition of a ketone-derived enamine to an enoate ester does not involve the challenges inherent in comparable aldehyde reactions. We report the identification of an amine−urea catalyst pair that enables efficient and stereoselective six-membered ring formation via intramolecular conjugate addition of an aldehyde-derived enamine to an α,β-unsaturated ester.…”

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confidence: 99%

“…24,25 However, the increased steric hindrance arising from the bulky substituent adjacent to nitrogen, relative to pyrrolidine itself, apparently inhibits reaction with an enoate ester, a weak electrophile. 12,13 We hypothesized that the desired conjugate addition would require electrophilic activation of the enoate ester in conjunction with enamine-based (nucleophilic) activation of the aldehyde. Brønsted acid and hydrogen bond donor additives have been employed to enhance carbonyl electrophilicity, 26,27 but use of such of such catalysts in our case could create a chemoselectivity problem.…”

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Catalytic Intramolecular Conjugate Additions of Aldehyde-Derived Enamines to α,β-Unsaturated Esters

et al. 2020

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We report a pairing of known catalysts that enables intramolecular conjugate additions of aldehyde-derived enamines to α,β-unsaturated esters. Despite extensive prior exploration of conjugate additions of aldehyde-derived enamines, catalytic conjugate additions to unactivated enoate esters are unprecedented. Achieving enantioselective and diastereoselective six-membered ring formation requires the coordinated action of a chiral pyrrolidine, for nucleophilic activation of the aldehyde via enamine formation, and a hydrogen bond donor, for electrophilic activation of the enoate ester. Proper selection of the hydrogen bond donor is essential for chemoselectivity, which requires minimizing competition from homoaldol reaction. Utility is demonstrated in a six-step synthesis of (−)-yohimbane from cycloheptene.Letter pubs.acs.org/OrgLett

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Electrophilicity Scales for Carbocations

Cited by 53 publications

References 79 publications

New Trends in Enantioselective Cross-Dehydrogenative Coupling

New Trends in Enantioselective Cross-Dehydrogenative Coupling

Base-Promoted Cascade Reactions for the Synthesis of 3,3-Dialkylated Isoindolin-1-ones and 3-Methyleneisoindolin-1-ones

Catalytic Intramolecular Conjugate Additions of Aldehyde-Derived Enamines to α,β-Unsaturated Esters

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