Rhodium(I)‐Catalyzed Cycloisomerization and Cyclotrimerization Reactions

Abstract

IntroductionHighly efficient catalytic processes for the synthesis of natural and unnatural compounds of medicinal interest or functional materials play a central role in modern organic synthesis. The transformation of simple starting materials into monocyclic, bicyclic, and polycyclic scaffolds using transition metal-catalyzed cyclization reactions is one of the most powerful approaches to such methodology. Among the transition metal catalysts used in these reactions, rhodium complexes are particularly important because of their high efficiency and broad functional group tolerance.The carbocyclizations of enynes, ynals, dienes, and diynes, as well as enediynes, dienynes, diynals, and triynes, are extremely important and useful reactions for the syntheses of a variety of carbocyclic compounds. Cascade carbocyclization is a powerful method, providing a rapid access to polycyclic skeletons in a single step. The term "carbocyclization" has been used to describe an annulation process involving carboncarbon bond formation via carbometallation, which distinguishes it from radical cyclization as well as from thermal and photochemical cycloadditions. When the carbocyclization process incorporates carbon monoxide, this process is categorized as carbonylative carbocyclization, which provides efficient routes to cyclic and fused cyclic ketones. Excellent reviews have been published on transition metal-mediated carbocyclizations [1] and cycloadditions [2] covering the major advances in this field up to 1996. Metalassisted cycloaddition reactions [3] and their applications to polysubstituted benzene derivatives [4] as well as to medium-sized ring syntheses [5], have also been reviewed. A review on the reactions involving hydrosilane, carbon monoxide, and unsaturated substrates promoted by transition-metal catalysts was published in 1996 [6]. This chapter describes recent advances in the rhodium-catalyzed carbocyclization processes, including various cascade-and carbonylative carbocyclizations. Carbocyclization 7.2.1Cycloisomerization of Enynes 7.2.1.1 Cycloisomerization Transition metal-catalyzed intramolecular cycloisomerization of enynes represents one of the most useful carbocyclization reactions; specifically, the rhodium-catalyzed cycloisomerization of 1,6-enynes that provide 1,3-and 1,4-dienes with high degrees of regioand stereoselectivity has been extensively studied. Cyclization of 1,6-enyne 1 catalyzed by RhCl(PPh 3 ) 3 gives 1-exo-methylene-2-cyclohexene 2 regioselectively, via a 6-exo-trig mode in 83% yield (Eq. 1) [7]. Substitution in the alkene moiety suppresses the cyclization significantly, while substitution of the terminal triple bond shuts down the reaction. 7 Rhodium(I)-Catalyzed Cycloisomerization and Cyclotrimerization Reactions 130 Scheme 7.1 Silylcarbocyclization (SiCaC and CO-SiCaC)In the first rhodium-catalyzed carbonylative silylcarbocyclization (CO-SiCaC), which was reported in 1992 [12,13], silylcyclopentenone 9 was isolated as a minor product in the silylformylation of 1-hexyne 8 (Scheme 7.4). Un...

Rhodium-catalysed tetradehydro-Diels–Alder reactions of enediynes via a rhodium-stabilized cyclic allene

“…9 We hypothesized that a rhodium-catalysed cycloaromatisation reaction could be realized by harnessing the well-known oxidative cyclization process observed in Rh-catalysed 1,6-enyne reactions (Scheme 1c). [10][11][12][13][14][15][16] The triple bond of an enediyne opens up the possibility of forming a cyclic vinyl-rhodium species A via oxidative cyclization of ene-diyne 1-Rh. Isomerization of the oxidative cyclization product A should form Bproviding a direct route to the intriguing allenyl-rhodium intermediate B and an alternative route to a cyclic allene cf.…”

Rhodium-Catalyzed Asymmetric [2 + 2 + 2] Cycloaddition of Unsymmetrical α,ω-Diynes with Acenaphthylene

“…The transition-metal-catalyzed asymmetric [2 + 2 + 2] cycloaddition of α,ω-diynes with alkenes is one of the most convenient methods for the synthesis of chiral bicyclic 1,3-cyclohexadienes. − In this transformation, the use of 1,2-disubstituted cyclic alkenes is capable of affording the corresponding chiral multicyclic 1,3-cyclohexadienes. , As such, our research group recently reported that the cationic rhodium­(I)/( R )-Difluorphos complex-catalyzed asymmetric [2 + 2 + 2] cycloaddition of α,ω-diynes 1 with indene ( 2a ) affords the corresponding tetracyclic 1,3-cyclohexadienes 3a in high yields with high regio- and enantioselectivity under mild conditions (Scheme , top) . In this reaction, enantioselectivity highly depended on the structures of α,ω-diynes 1 .…”

Carbonylative and Hydrosilylative Carbocyclization of 1,5-Diynes for the Synthesis of Cyclic Dienones

“…Transition metal-catalyzed silylcarbocyclization is an established synthetic protocol for the construction of various ring systems containing silyl groups, in which the carbometalation initiated by the silylation of unsaturated motifs, such as alkynes and alkenes, is a key step . Silylcarbocyclization was first documented in Tamao’s report, where a nickel complex catalyzed the reaction of 1,7-diynes with hydrosilanes to give 1,2-dialkylidenecyclohexanes .…”