Modular Synthesis of Phenanthro[9,10‐c]thiophenes by a Sequence of CH Activation, Suzuki Cross‐Coupling and Photocyclization Reactions

Abstract

A total number of 15 different 3,4-diarylthiophenes were synthesized, which bear a chlorine atom in ortho-position of one of the aryl substituents. One aryl group was introduced by an oxidative cross-coupling reaction, involving a CH activation at C4(3) of the thiophene core. The other aryl group was in most cases introduced by a Suzuki cross-coupling reaction, which succeeded the oxidative cross-coupling step. Photocyclization reactions of the 3,4-diarylthiophenes were performed in a solvent mixture of benzene and acetonitrile (50:50 v/v) at λ=254 nm and proceeded to the title compounds in yields of 60-82 %. The selectivity of the photocyclization was determined at the ortho-chloro-substituted aryl ring by the position of the chlorine substituent. At the other ring, a single regioisomer was observed for phenyl and para-substituted phenyl groups. For 2-naphthyl and ortho-substituted phenyl rings a clear preference was observed in favor of a major regioisomer, while meta-substitution in the phenyl ring led to a about 1:1 mixture of 5- and 7-substituted phenanthro[9,10-c]thiophenes. Mechanistically, the photocyclization is likely to occur as a photochemically allowed, conrotatory [(4n+2)π] process accompanied by elimination of HCl. It was shown for two phenanthro[9,10-c]thiophene products that they can be readily brominated in positions C1 and C3 (74-77 %), which in turn allows for further functionalization at these positions, for example, in the course of halogen-metal exchange and polymerization reactions.

New Arylating Agents in Pd-Catalyzed C–H Bond Functionalization of 5-Membered Ring Heteroarenes

“…Bach et al reported in 2011 that thiophenes substituted at C3 by CH 2 PO(OEt) 2 or CH 2 COOEt undergo a regioselective oxidative coupling reaction at C4 with aryl boronic acids in the presence of Ag 2 O, cesium trifluoroacetate [Cs(tfa)], benzoquinone (BQ), and Pd (tfa) 2 catalyst in trifluoroacetic acid as the solvent (Scheme 2, middle) [28]. They recently extended this procedure to a broad range of substrates [29,30]. Studer and Itami also described a procedure for the β-arylations of thiophenes with arylboronic acids under Pd/TEMPO catalysis (Scheme 2, bottom) [31].…”

Synthesis of Phenanthro[9,10‐c]thiophenes by 2,3‐Dichloro‐5,6‐dicyano‐1,4‐benzoquinone (DDQ)‐Promoted Cyclo‐Oxidation

“…The most straightforward approach to the phenanthrothiophenes is by oxidative cyclization of diaryl thiophenes ( 2 , Scheme ); this route also benefits from the synthetic accessibility of diaryl thiophenes with a variety of substituents . However, Schnapperelle and Bach attempted the cyclo‐oxidation of compound 2 under a wide variety of conditions, and found no reaction . To overcome this problem, they prepared the more complicated desymmetrized chlorodiarylthiophene 3 and successfully converted it to phenanthrothiophene 1 by irradiation with 254‐nm light.…”

“…Diaryl thiophene derivatives can be synthesized by a variety of methods . Dang and Chen reported the efficient synthesis of diaryl thiophenes in three steps from the bromoacetophenone ( 4 ) .…”

Kinetic Studies on the Palladium(II)‐Catalyzed Oxidative Cross‐Coupling of Thiophenes with Arylboron Compounds and Their Mechanistic Implications

“…Apart from synthetic applications of this method to the synthesis of new materials,8 we have also investigated the mechanistic course of the reactions. Preliminary results had indicated that an electrophilic palladium species could be the reactive intermediate, which attacked position C5 prior to CC bond formation at C4 5.…”