Cross-dehydrogenative coupling reactions between arenes (C-H) and carboxylic acids (O-H): a straightforward and environmentally benign access to O-aryl esters

Abstract

Transition-metal catalyzed cross-dehydrogenative-coupling reactions encompass highly versatile and atom economical methods for the construction of various carbon-carbon and carbon-heteroatom bonds by combining two C(X)-H (X = heteroatom) bonds. Along this line, direct acyloxylation of C-H bonds with carboxylic acids has emerged as a powerful and green approach for the synthesis of structurally diverse esters. In this focus-review we will describe recent progress in direct esterification of aromatic C-H bonds with special emphasis on the mechanistic features of the reactions. Literature has been surveyed until the end of February 2019.

Fig. 1. Some ester-containing commercialized drugs.

Fig. 2. Direct esterification of aromatic C–H bonds with carboxylic acids.

Scheme 1. Rh-catalyzed direct benzoxylation of 2-arylpyridines 1 with benzoic acids 2.

Scheme 2. Mechanistic proposal for the formation of ortho-acyloxylated 2-arylpyridines 3.

Scheme 3. Some reported examples on the Rh-catalyzed ortho-selective acyloxylation of arene substrates 4 bearing coordinating directing groups with aliphatic carboxylic acids 5 reported by Xu and Li.

Scheme 4. Ru-catalyzed synthesis of ortho-benzoxylated acetanilides 9 from acetanilides 7 and benzoic acids 8.

Scheme 5. Cross-dehydrogenative coupling of N-methyl benzamides 10 with aromatic carboxylic acids 11 catalyzed by [{RuCl₂(p-cymene)}₂].

Scheme 6. Suggested mechanism for the formation of ortho-benzoxylated N-alkyl benzamides 12.

Scheme 7. (a) Ackermann's synthesis of ortho-benzoxylated sulfoximines 15; (b) plausible mechanism for the formation of ortho-benzoxylated sulfoximines 15.

Scheme 8. Ru-catalyzed ortho-directed aromatic amines 16 with carboxylic acids 17.

Scheme 9. Ru-catalyzed site-selective acyloxylation of indolines 19 with carboxylic acids 20.

Scheme 10. ortho-Selective acyloxylation of azoarenes 22 with carboxylic acids 23 reported by Kianmehr.

Scheme 11. Pd-catalyzed cross-dehydrogenative coupling of O-methyl aryloximes 25 with carboxylic acids 26.

Scheme 12. Coupling of 2-arylpyridines 28 with carboxylic acids 29 catalyzed by Pd(OAc)₂.

Scheme 13. Plausible mechanism for the Pd-catalyzed acyloxylation of 2-arylpyridines 28 with carboxylic acids 29.

Scheme 14. Synthesis of lactones 32 from the corresponding 2-aryl carboxylic acids 31.

Scheme 15. Pd-catalyzed ortho-acyloxylation of 2-substituted 1,2,3-triazoles 33 with different carboxylic acids 34.

Scheme 16. (a) Pd-catalyzed dehydrogenative cross-coupling of simple benzene with carboxylic acids 36; (b) Pd-catalyzed para-benzoxylation of anisole derivatives 38 with simple benzoic acid.

Scheme 17. Proposed mechanism for the reaction in Scheme 16a.

Scheme 18. Liao-Ji's synthesis of mono-acyloxylated 4-aryl-thiazoles 42.

Scheme 19. (a) Pd-catalyzed direct esterification of 2-substituted indoles 43 with carboxylic acids 44; (b) Pd-catalyzed direct esterification of 2,3-unsubstituted indoles 46 with carboxylic acids 47.

Scheme 20. Mechanistic explanation for the synthesis of acyloxylated indoles 48.

Scheme 21. Co-catalyzed cross-dehydrogenative coupling of N-aryl-2-pyridylcarboxamides 49 with carboxylic acids 50.

Scheme 22. Possible mechanism for the Co-catalyzed oxidative coupling between an arene 49 and carboxylic acid 50.

Scheme 23. Coupling of N-(quinolin-8-yl)benzamides 52 with carboxylic acids 53 catalyzed by Co(acac)₂.

Scheme 24. Cu-catalyzed intramolecular C–H oxygenation reaction of 2-arylbenzoic acid derivatives 55.

Scheme 25. Zhang's synthesis of mono-acyloxylated of N-(quinolin-8-yl)benzamides 59.

Scheme 26. (a) Cu-catalyzed C–H acyloxylation of 2-arylpyridines 60 with carboxylic acids 61; (b) Cu-catalyzed C–H acyloxylation of indolines 63 with carboxylic acids 64.

Scheme 27. Au-catalyzed oxidative acyloxylation of simple arenes 66.

Scheme 28. Proposed mechanism for the reaction in Scheme 27.

Scheme 29. Xu's synthesis of lactones 70.

Scheme 30. Ag-catalyzed intramolecular C–O cross-coupling of biaryl-2-carboxylic acids 71 through C–H acyloxylation.

Sattar Arshadi

Alireza Banaei

Aazam Monfared

Saeideh Ebrahimiasl

Akram Hosseinian