Carbonate-promoted C–H carboxylation of electron-rich heteroarenes

Porter, Tyler M.; Kanan, Matthew W.

doi:10.1039/D0SC04548A

Title: Carbonate-promoted C–H carboxylation of electron-rich heteroarenes

Abstract

C–H carboxylation is an attractive transformation for both streamlining synthesis and valorizing CO₂. The high bond strength and very low acidity of most C–H bonds, as well as the low reactivity of CO₂, present fundamental challenges for this chemistry. Conventional methods for carboxylation of electron-rich heteroarenes require very strong organic bases to effect C–H deprotonation. Here we show that alkali carbonates (M₂CO₃) dispersed in mesoporous TiO₂ supports (M₂CO₃/TiO₂) effect CO₃^2- promoted C–H carboxylation of thiophene- and indole-based heteroarenes in gas–solid reactions at 200–320 C. M₂CO₃/TiO₂ materials are strong bases in this temperature regime, which enables deprotonation of very weakly acidic bonds in these substrates to generate reactive carbanions. In addition, we show that M₂CO₃/TiO₂ enables C3 carboxylation of indole substrates via an apparent electrophilic aromatic substitution mechanism. No carboxylations take place when M₂CO₃/TiO₂ is replaced with un-supported M₂CO₃, demonstrating the critical role of carbonate dispersion and disruption of the M₂CO₃ lattice. After carboxylation, treatment of the support-bound carboxylate products with dimethyl carbonate affords isolable esters and the M₂CO₃/TiO₂ material can be regenerated upon heating under vacuum. Our results provide the basis for a closed cycle for the esterification of heteroarenes with CO₂ and dimethyl carbonate.

Authors:

Porter, Tyler M. ^[1];

^[1]

Department of Chemistry, Stanford University, Stanford, USA

Publication Date:: 2020-11-11

Sponsoring Org.:: USDOE Office of Science (SC), Basic Energy Sciences (BES)

OSTI Identifier:: 1675014

Alternate Identifier(s):: OSTI ID: 1817112

Grant/Contract Number:: SC0020394

Resource Type:: Published Article

Journal Name:: Chemical Science

Additional Journal Information:: Journal Name: Chemical Science Journal Volume: 11 Journal Issue: 43; Journal ID: ISSN 2041-6520

Publisher:: Royal Society of Chemistry (RSC)

Country of Publication:: United Kingdom

Language:: English

Subject:: 37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY

Citation Formats


                    Porter, Tyler M., and Kanan, Matthew W.. Carbonate-promoted C–H carboxylation of electron-rich heteroarenes.  United Kingdom: N. p., 2020. 
Web.  https://doi.org/10.1039/D0SC04548A.

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                    Porter, Tyler M., & Kanan, Matthew W.. Carbonate-promoted C–H carboxylation of electron-rich heteroarenes.  United Kingdom.  https://doi.org/10.1039/D0SC04548A

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                    Porter, Tyler M., and Kanan, Matthew W.. Wed .  
"Carbonate-promoted C–H carboxylation of electron-rich heteroarenes".  United Kingdom.  https://doi.org/10.1039/D0SC04548A.

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@article{osti_1675014,

  title        = {Carbonate-promoted C–H carboxylation of electron-rich heteroarenes},

  author       = {Porter, Tyler M. and Kanan, Matthew W.},

  abstractNote = {C–H carboxylation is an attractive transformation for both streamlining synthesis and valorizing CO2. The high bond strength and very low acidity of most C–H bonds, as well as the low reactivity of CO2, present fundamental challenges for this chemistry. Conventional methods for carboxylation of electron-rich heteroarenes require very strong organic bases to effect C–H deprotonation. Here we show that alkali carbonates (M2CO3) dispersed in mesoporous TiO2 supports (M2CO3/TiO2) effect CO32- promoted C–H carboxylation of thiophene- and indole-based heteroarenes in gas–solid reactions at 200–320 C. M2CO3/TiO2 materials are strong bases in this temperature regime, which enables deprotonation of very weakly acidic bonds in these substrates to generate reactive carbanions. In addition, we show that M2CO3/TiO2 enables C3 carboxylation of indole substrates via an apparent electrophilic aromatic substitution mechanism. No carboxylations take place when M2CO3/TiO2 is replaced with un-supported M2CO3, demonstrating the critical role of carbonate dispersion and disruption of the M2CO3 lattice. After carboxylation, treatment of the support-bound carboxylate products with dimethyl carbonate affords isolable esters and the M2CO3/TiO2 material can be regenerated upon heating under vacuum. Our results provide the basis for a closed cycle for the esterification of heteroarenes with CO2 and dimethyl carbonate.},

  doi          = {10.1039/D0SC04548A},

  journal      = {Chemical Science},

  number       = 43,

  volume       = 11,

  place        = {United Kingdom},

  year         = {2020},

  month        = {11}

}

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Title: Carbonate-promoted C–H carboxylation of electron-rich heteroarenes

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