Rhodium-Catalyzed Dehydrogenative Coupling of Phenylheteroarenes with Alkynes or Alkenes

Abstract

Benzo-fused tri- to heptacyclic heteroarenes were effectively constructed by the rhodium-catalyzed dehydrogenative coupling of phenylheteroarenes with alkynes. Using alkenes as coupling partners, dehydrogenative alkenylation took place selectively on the phenyl moiety of phenylheteroarenes. Several experiments with deuterium-labeled substrates indicated that double C-H bond cleavages take place even in the reaction with alkenes.

Theoretical Investigation of Regioselectivity in the Rh‐Catalyzed Coupling Reaction of 3‐Phenylthiophene with Styrene

“…Therefore, the most rational reaction mechanism leading to the 5‐membered rhodacycle intermediate B is via first‐stage C–H cleavage at the 2‐position of the thienyl group and second‐stage C–H cleavage at the 2′‐position of the phenyl group. This is consistent with the our previously‐reported deuterium‐labeling experiment, in which deuteration at the C2 position showed a meaningful primary isotope effect of K H / K D = 2.3 as described above …”

“…Finally, the coupling product E is obtained by reductive elimination from D . This catalytic cycle is partly supported by the results of a couple of deuterium‐labeling/incorporation experiments: (a) The reaction of C2‐deuterated 3‐phenylthiophene ( A ‐ d 1 ) with styrene gave the non‐deuterated E ‐ d 0 with the recovery of A ‐ d 1 in the early stage. (b) Treatment of A with styrene‐ d 8 gave the corresponding C2‐deuterio‐C2′‐stylyl‐ d 7 compound E ‐ d 8 .…”

“…Cp*Rh(III) complexes have been proved to be among highly versatile catalysts for the C(sp 2 )–H activation and a variety of catalytic direct aromatic transformation reactions using the Rh(III) species have been developed in the last decade , . In the context of our continuous study of direct aromatic C–H coupling, we recently demonstrated that the Cp*Rh‐catalyzed reactions of 3‐phenylthiophene with styrene and its derivatives as well as with acrylate esters proceed exclusively at the 2′‐position of the phenyl group,, as shown in Scheme , although the C–H bonds at the 2‐ and 5‐position of thiophene are normally more reactive than those of the phenyl group due to the electron‐rich nature of the thiophene moiety and the enhanced acidity of the heterocyclic H2 and H5 hydrogens.…”

“…Based on the experimental results, the reaction mechanism shown in Scheme was proposed for this intriguing coupling reaction . First, the two C–H bonds at the 2‐position of the thienyl group and at the 2′‐position of the phenyl group in 3‐phenylthiophene ( A ) are cleaved by the Rh catalyst to produce a 5‐membered rhodacycle intermediate B .…”

“…lyzed reactions of 3-phenylthiophene with styrene and its derivatives as well as with acrylate esters proceed exclusively at the 2′-position of the phenyl group, [14,15] as shown in Scheme 1, although the C-H bonds at the 2-and 5-position of thiophene are normally more reactive than those of the phenyl group due to the electron-rich nature of the thiophene moiety and the enhanced acidity of the heterocyclic H2 and H5 hydrogens. Scheme 1.…”