Skeleton Reorganization of Substituted Benzocyclobutenols through Rh-Catalyzed C–C Bond Cleavage Manipulated by Hydrogen Transfer

Abstract

Although transition-metal-catalyzed C–C bond activation has been investigated extensively, C–C bond cleavage manipulated by hydrogen transfer has been unexplored. In this work, we disclose a skeleton reorganization of alkene-tethered benzocyclobutenols through Rh-catalyzed C–C bond cleavage coupled with intra- and intermolecular hydrogen transfer. The reaction pathway was well-tuned by the catalytic systems. As a result, divergent benzofurans bearing 4-β-hydroxy or 4-β-keto moieties were synthesized under pH- and redox-neutral conditions.

Cobalt/Rhodium-Catalyzed Diversified Amidation of Benzocyclobutenols via C–C Cleavage under Catalyst and Condition Control

“…For example, Shi and Song et al recently realized Rh(I)-catalyzed cyclization of alkene-tethered benzocyclobutenols followed by hydrogentransfer, which results in the formation of benzofurans bearing a 4-β-hydroxy or 4-βketo moiety (Scheme 1a-3). 13 In the third category, Wang reported in 2014 a Rh(I)-catalyzed [4 + 1] ring expansion reaction of benzocyclobutenol, which utilizes diazoesters as a C 1 precursors to afford indanols with an all-carbon quaternary center (Scheme 1a-4). 14 Most reported results in the C−C activation of benzocyclobutenols have focused on C(sp 2 )−C(sp 3 ) or C(sp 2 )−C(sp 2 ) bond formation.…”

“…Dong and co-workers used the “cut-and-sew” strategy, which involved metal-catalyzed C–C bond functionalization by oxidative addition and coupling with unsaturated reagents . As a direct derivative of benzocyclobutanone, benzocyclobutenol also reacted in high activity and regioselectivity in intermolecular reactions via selective cleavage of the C­(sp 2 )–C­(sp 3 ) bond. − To date, metal-catalyzed C–C activation of benzocyclobutanols has been carried out in three reaction modes (Scheme ). First, the Orellana group reported Pd-catalyzed regioselective C­(sp 2 )–C­(sp 2 ) coupling of benzocyclobutenols with aryl bromides (Scheme a-1) .…”

“…This type of coupling has also been extended to intramolecular systems by taking advantage of the resulting ketone moiety. For example, Shi and Song et al recently realized Rh­(I)-catalyzed cyclization of alkene-tethered benzocyclobutenols followed by hydrogentransfer, which results in the formation of benzofurans bearing a 4-β-hydroxy or 4-β-keto moiety (Scheme a-3) . In the third category, Wang reported in 2014 a Rh­(I)-catalyzed [4 + 1] ring expansion reaction of benzocyclobutenol, which utilizes diazoesters as a C 1 precursors to afford indanols with an all-carbon quaternary center (Scheme a-4) …”

“…A catalytic cycle is proposed in Scheme 6 on the basis of the above reaction results and related reports. 13 In conclusion, we disclosed diversified amidation reactions between benzoclobutenol and dioxazolone via Co(III)-or Rh(III)-catalyzed C−C bond cleavage. Three classes of C−Ncoupled products have been selectively obtained in moderate to excellent efficiency via β-carbon elimination of the benzocyclobutenol.…”

Regiocontrolled Annulation of Benzocyclobutenols with Alkynes

“…The development of such methodologies would offer the opportunity to synthesize novel ring systems under mild conditions due to the C–C bond-cleaving pathway of benzocyclobutenols distinct from that of benzocyclobutenones. Following our continuous interest in the intramolecular C–C bond functionalization of benzocyclobutenols, we report herein a tandem reaction that involves an unprecedented Rh-catalyzed intramolecular annulation of benzocyclobutenols with alkynes and the subsequent ZnCl 2 -promoted dehydration of the resulting alcohols (Scheme c), offering an efficient approach to naphtho-fused rings under near pH- and redox-neutral conditions. Encouraged by our previous work that showed the ether group could be removed after C–C bond functionalization, oxygen was chosen as the tethering group with the aim of addressing the regioselectivity issue of internal alkynes.…”