Cargando…

C(sp(3))-Arylation by Conformationally Accelerated Intramolecular Nucleophilic Aromatic Substitution (S(N)Ar)

[Image: see text] The asymmetric synthesis of heavily substituted benzylic stereogenic centers, prevalent in natural products, therapeutics, agrochemicals, and catalysts, is an ongoing challenge. In this Account, we outline our contribution to this endeavor, describing our discovery of a series of n...

Descripción completa

Detalles Bibliográficos
Autores principales: Wales, Steven M., Saunthwal, Rakesh K., Clayden, Jonathan
Formato: Online Artículo Texto
Lenguaje:English
Publicado: American Chemical Society 2022
Acceso en línea:https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9219115/
https://www.ncbi.nlm.nih.gov/pubmed/35620846
http://dx.doi.org/10.1021/acs.accounts.2c00184
_version_ 1784732039519404032
author Wales, Steven M.
Saunthwal, Rakesh K.
Clayden, Jonathan
author_facet Wales, Steven M.
Saunthwal, Rakesh K.
Clayden, Jonathan
author_sort Wales, Steven M.
collection PubMed
description [Image: see text] The asymmetric synthesis of heavily substituted benzylic stereogenic centers, prevalent in natural products, therapeutics, agrochemicals, and catalysts, is an ongoing challenge. In this Account, we outline our contribution to this endeavor, describing our discovery of a series of new reactions that not only have synthetic applicability but also present significant mechanistic intrigue. The story originated from our longstanding interest in the stereochemistry and reactivity of functionalized organolithiums. While investigating the lithiation chemistry of ureas (a “Cinderella” sister of the more established amides and carbamates), we noted an unexpected Truce–Smiles (T-S) rearrangement involving the 1,4-N → C transposition of a urea N′-aryl group to the α-carbanion of an adjacent N-benzyl group. Despite this reaction formally constituting an S(N)Ar substitution, we found it to be remarkably tolerant of the electronic properties of the migrating aryl substituent and the degree of substitution at the carbanion. Moreover, in contrast to classical S(N)Ar reactions, the rearrangement was sufficiently rapid that it took place under conditions compatible with configurational stability in an organolithium intermediate, enabling enantiospecific arylation at benzylic stereogenic centers. Experimental and computational studies confirmed a low kinetic barrier to the aryl migration arising from the strong preference for a trans arrangement of the urea N′-aryl and carbonyl groups, populating a reactive conformer in which spatial proximity was enforced between the carbanion and N′-aryl group, hugely accelerating ipso-substitution. This discovery led us to uncover a whole series of conformationally accelerated intramolecular N → C aryl transfers using different anilide-based functional groups, including a diverse range of urea, carbamate, and thiocarbamate-substituted anions. Products included enantioenriched α-tertiary amines (including α-arylated N-heterocycles) and alcohols, as well as rare α-tertiary thiols. Synthetically challenging diarylated centers with differentiated aryl groups featured heavily in all product sets. The absolute enantiospecificity (retention versus inversion) of the reaction was dependent on the heteroatom α to the lithiation site: the origin of this stereodivergence was probed both experimentally and computationally. Asymmetric variants of the rearrangement were realized by enantioselective deprotonation, and connective strategies were developed in which an intermolecular C–C bond-forming event preceded the anionic rearrangement. Substrates where the N′-nucleofuge (at the aryl ipso position) was tethered to the migrating arene allowed us to use the rearrangement as a ring expansion method to generate 8- to 12-membered medium-ring N-heterocycles from very simple precursors. Stabilized carbon nucleophiles such as alkali metal enolates also readily promoted intramolecular N → C aryl transfer in N′-arylureas, opening up access to biologically relevant hydantoins, and enabling a “chiral memory” approach for the (hetero)arylation of chiral α-amino acids with programmable retention or inversion of configuration. Collectively, our studies of electronically versatile T-S rearrangements in anilide-based systems have culminated in a practical and general strategy for transition metal-free C(sp(3))-arylation. More broadly, our results highlight the power of conformational activation to achieve unprecedented reactivity in the construction of challenging C–C bonds.
format Online
Article
Text
id pubmed-9219115
institution National Center for Biotechnology Information
language English
publishDate 2022
publisher American Chemical Society
record_format MEDLINE/PubMed
spelling pubmed-92191152022-06-24 C(sp(3))-Arylation by Conformationally Accelerated Intramolecular Nucleophilic Aromatic Substitution (S(N)Ar) Wales, Steven M. Saunthwal, Rakesh K. Clayden, Jonathan Acc Chem Res [Image: see text] The asymmetric synthesis of heavily substituted benzylic stereogenic centers, prevalent in natural products, therapeutics, agrochemicals, and catalysts, is an ongoing challenge. In this Account, we outline our contribution to this endeavor, describing our discovery of a series of new reactions that not only have synthetic applicability but also present significant mechanistic intrigue. The story originated from our longstanding interest in the stereochemistry and reactivity of functionalized organolithiums. While investigating the lithiation chemistry of ureas (a “Cinderella” sister of the more established amides and carbamates), we noted an unexpected Truce–Smiles (T-S) rearrangement involving the 1,4-N → C transposition of a urea N′-aryl group to the α-carbanion of an adjacent N-benzyl group. Despite this reaction formally constituting an S(N)Ar substitution, we found it to be remarkably tolerant of the electronic properties of the migrating aryl substituent and the degree of substitution at the carbanion. Moreover, in contrast to classical S(N)Ar reactions, the rearrangement was sufficiently rapid that it took place under conditions compatible with configurational stability in an organolithium intermediate, enabling enantiospecific arylation at benzylic stereogenic centers. Experimental and computational studies confirmed a low kinetic barrier to the aryl migration arising from the strong preference for a trans arrangement of the urea N′-aryl and carbonyl groups, populating a reactive conformer in which spatial proximity was enforced between the carbanion and N′-aryl group, hugely accelerating ipso-substitution. This discovery led us to uncover a whole series of conformationally accelerated intramolecular N → C aryl transfers using different anilide-based functional groups, including a diverse range of urea, carbamate, and thiocarbamate-substituted anions. Products included enantioenriched α-tertiary amines (including α-arylated N-heterocycles) and alcohols, as well as rare α-tertiary thiols. Synthetically challenging diarylated centers with differentiated aryl groups featured heavily in all product sets. The absolute enantiospecificity (retention versus inversion) of the reaction was dependent on the heteroatom α to the lithiation site: the origin of this stereodivergence was probed both experimentally and computationally. Asymmetric variants of the rearrangement were realized by enantioselective deprotonation, and connective strategies were developed in which an intermolecular C–C bond-forming event preceded the anionic rearrangement. Substrates where the N′-nucleofuge (at the aryl ipso position) was tethered to the migrating arene allowed us to use the rearrangement as a ring expansion method to generate 8- to 12-membered medium-ring N-heterocycles from very simple precursors. Stabilized carbon nucleophiles such as alkali metal enolates also readily promoted intramolecular N → C aryl transfer in N′-arylureas, opening up access to biologically relevant hydantoins, and enabling a “chiral memory” approach for the (hetero)arylation of chiral α-amino acids with programmable retention or inversion of configuration. Collectively, our studies of electronically versatile T-S rearrangements in anilide-based systems have culminated in a practical and general strategy for transition metal-free C(sp(3))-arylation. More broadly, our results highlight the power of conformational activation to achieve unprecedented reactivity in the construction of challenging C–C bonds. American Chemical Society 2022-05-27 2022-06-21 /pmc/articles/PMC9219115/ /pubmed/35620846 http://dx.doi.org/10.1021/acs.accounts.2c00184 Text en © 2022 The Authors. Published by American Chemical Society https://creativecommons.org/licenses/by/4.0/Permits the broadest form of re-use including for commercial purposes, provided that author attribution and integrity are maintained (https://creativecommons.org/licenses/by/4.0/).
spellingShingle Wales, Steven M.
Saunthwal, Rakesh K.
Clayden, Jonathan
C(sp(3))-Arylation by Conformationally Accelerated Intramolecular Nucleophilic Aromatic Substitution (S(N)Ar)
title C(sp(3))-Arylation by Conformationally Accelerated Intramolecular Nucleophilic Aromatic Substitution (S(N)Ar)
title_full C(sp(3))-Arylation by Conformationally Accelerated Intramolecular Nucleophilic Aromatic Substitution (S(N)Ar)
title_fullStr C(sp(3))-Arylation by Conformationally Accelerated Intramolecular Nucleophilic Aromatic Substitution (S(N)Ar)
title_full_unstemmed C(sp(3))-Arylation by Conformationally Accelerated Intramolecular Nucleophilic Aromatic Substitution (S(N)Ar)
title_short C(sp(3))-Arylation by Conformationally Accelerated Intramolecular Nucleophilic Aromatic Substitution (S(N)Ar)
title_sort c(sp(3))-arylation by conformationally accelerated intramolecular nucleophilic aromatic substitution (s(n)ar)
url https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9219115/
https://www.ncbi.nlm.nih.gov/pubmed/35620846
http://dx.doi.org/10.1021/acs.accounts.2c00184
work_keys_str_mv AT walesstevenm csp3arylationbyconformationallyacceleratedintramolecularnucleophilicaromaticsubstitutionsnar
AT saunthwalrakeshk csp3arylationbyconformationallyacceleratedintramolecularnucleophilicaromaticsubstitutionsnar
AT claydenjonathan csp3arylationbyconformationallyacceleratedintramolecularnucleophilicaromaticsubstitutionsnar