Rhodium(I)‐Catalyzed Cycloisomerization of Benzylallene‐Alkynes through CH Activation

Abstract

The efficient Rh(I)-catalyzed cycloisomerization of benzylallene-alkynes produced the tricyclo[9.4.0.0(3,8)]pentadecapentaene skeleton through a C(sp2)-H bond activation in good yields. A plausible reaction mechanism proceeds via oxidative addition of the acetylenic C-H bond to Rh(I), an ene-type cyclization to the vinylidenecarbene-Rh(I) intermediate, and an electrophilic aromatic substitution with the vinylidenecarbene species. It was proposed based on deuteration and competition experiments.

Construction of Hexahydrophenanthrenes By Rhodium(I)‐Catalyzed Cycloisomerization of Benzylallene‐Substituted Internal Alkynes through C−H Activation

“…While investigating the Rh I ‐catalyzed cyclization of allene‐alkynes, we observed a novel cycloisomerization for the benzylallene‐substituted terminal alkynes 5 (R 1 =H, R 2 =alkyl) that proceeds through C(sp 2 )−H bond activation on the benzene ring to produce 6 . This reaction was assumed to proceed via the unique vinylidene carbene Rh intermediate 6′ .…”

Creation of Novel Cyclization Methods Using <i>sp</i>-Hybridized Carbon Units and Syntheses of Bioactive Compounds

“…Next, we succeeded in developing another type of C-H activation of benzylallene-alkynes using this concept. 50) Treatment of 93 with Rh I -catalyst effected novel cycloisomerization to produce tricyclo[9.4.0.0 3,8 ]-pentadecapentaene derivatives 94 through C sp 2 -H bond activation. Based on deuteration and competition experiments, a reaction mechanism was proposed, which might involve a vinylidenecarbene-Rh intermediate 95 50) (Chart 21).…”

Construction of Hexahydrophenanthrenes By Rhodium(I)‐Catalyzed Cycloisomerization of Benzylallene‐Substituted Internal Alkynes through C−H Activation

“…[10] This reaction was assumed to proceed via the unique vinylidene carbene Rh intermediate 6' '.H erein, we describe another type of C À Ha ctivation of benzylallene-alkyne species 5 (R 1 ¼ 6 H, R 2 = alkyl);t he replacement of the terminal alkyne with an internal one dramatically changed the ring-closing mode to furnish the hexahydrophenanthrene skeleton 7 in high yields (Scheme 2). [10] This reaction was assumed to proceed via the unique vinylidene carbene Rh intermediate 6' '.H erein, we describe another type of C À Ha ctivation of benzylallene-alkyne species 5 (R 1 ¼ 6 H, R 2 = alkyl);t he replacement of the terminal alkyne with an internal one dramatically changed the ring-closing mode to furnish the hexahydrophenanthrene skeleton 7 in high yields (Scheme 2).…”

“…While investigating the Rh I -catalyzed cyclization of allene-alkynes,w eo bserved an ovel cycloisomerization for the benzylallene-substituted terminal alkynes 5 (R 1 = H, R 2 = alkyl) that proceeds through C(sp 2 )ÀHbond activation on the benzene ring to produce 6. [10] This reaction was assumed to proceed via the unique vinylidene carbene Rh intermediate 6' '.H erein, we describe another type of C À Ha ctivation of benzylallene-alkyne species 5 (R 1 ¼ 6 H, R 2 = alkyl);t he replacement of the terminal alkyne with an internal one dramatically changed the ring-closing mode to furnish the hexahydrophenanthrene skeleton 7 in high yields (Scheme 2).…”

“…Our initial study employed 5a,w hich has ad imethyl group at the benzylic position so that unfavorable b-hydride elimination of the possible rhodacyclic intermediate A is avoided ( Table 1). As olution of 5a in toluene was heated to reflux in the presence of 5mol %[RhCl(CO) 2 ] 2 (an effective catalyst for the cycloisomerization of 5 when R 1 = H) [10] for 0.2 ht oproduce hexahydrophenanthrene derivative 7a in 77 %y ield. [11] Theb icyclo[4.3.0]nonadienone derivative 8a was isolated in 19 %yield as abyproduct (entry 1, method I).…”