Rhodium-catalyzed ring-opening reaction of cyclopropenes. Control of regioselectivity by the oxidation state of the metal
摘要:
The Rh(II)-catalyzed reaction of unsymmetrically substituted cyclopropenes gives substituted furans derived from cleavage of the less substituted sigma-bond. In contrast, Rh(I) catalysis results in cleavage of the more substituted sigma-bond producing a regioisomeric set of furans.
Rhodium-catalyzed ring-opening reaction of cyclopropenes. Control of regioselectivity by the oxidation state of the metal
摘要:
The Rh(II)-catalyzed reaction of unsymmetrically substituted cyclopropenes gives substituted furans derived from cleavage of the less substituted sigma-bond. In contrast, Rh(I) catalysis results in cleavage of the more substituted sigma-bond producing a regioisomeric set of furans.
Cyclization Reactions of Rhodium Carbene Complexes. Effect of Composition and Oxidation State of the Metal
作者:Albert Padwa、Jamal M. Kassir、Simon L. Xu
DOI:10.1021/jo962271r
日期:1997.3.1
Treatment of o-(1,7-octadiynyl)benzoyldiazoethane with rhodium(II) octanoate in pentane resulted in a double internal/internal alkyne insertion reaction producing a labile bicyclo[4.1.0]hept-1(7)ene derivative which readily undergoes a Diels-Alder reaction with diphenylisobenzofuran. Changing the solvent from pentane to CH2Cl2 afforded a 2:1 mixture of cis- and trans-alkenyl-substituted indenones. Stepwise cyclization involving a set of dipolar intermediates occurs in CH2Cl2 whereas metallocyclobutenes are involved when pentane is used as the solvent. The rhodium(II) carboxylate catalyzed reaction of unsymmetrically substituted cyclopropenes gives substituted furans derived from cleavage of the less substituted beta-bond. Thus, treatment of 3-benzoyl-3-methyl-1-(n-butyl)cyclopropene with Rh2OAc4 afforded a 26:1 mixture of 2-phenyl-3-methyl-4-(n-butyl)- and 2-phenyl-3-methyl-5-(n-butyl)furan. In contrast, the [ClRh(CO)(2)](2)-catalyzed reaction resulted in cleavage of the more substituted sigma-bond producing only 2-phenyl-3-methyl-5-(n-butyl)furan. Both reactions involve electrophilic attack of the rhodium metal on the less substituted carbon atom of the cyclopropene pi-bond to give the most stabilized cyclopropyl carbocation. Ring opening followed by rapid electrocyclization to the furan occurs with the Rh(IH) catalyst. With the Rh(I) catalyst, the ring-opened species preferentially cyclizes to a metallocyclobutene intermediate which then equilibrates with the thermodynamically more stable isomer prior to furan formation. The Rh(I)-catalyzed reaction of 3-benzoyl-1-propylcyclopropene with various terminal alkynes gives 4-alkyl-4-propyl-7-phenyloxepins in good yield. These reactions involve electrophilic attack of the rhodium metal on the more substituted carbon of the cyclopropene; pi-bond to give a rhodium carbene complex, This metallo carbenoid undergoes a subsequent [2 + 2] cycloaddition with terminal acetylenes, The resulting rhodacycle rearranges by a formal 1,5-sigmatropic shift, and this is followed by reductive elimination of rhodium to produce the observed oxepin.
Rhodium-catalyzed ring-opening reaction of cyclopropenes. Control of regioselectivity by the oxidation state of the metal
作者:Albert Padwa、Jamal M. Kassir、Simon L. Xu
DOI:10.1021/jo00025a005
日期:1991.12
The Rh(II)-catalyzed reaction of unsymmetrically substituted cyclopropenes gives substituted furans derived from cleavage of the less substituted sigma-bond. In contrast, Rh(I) catalysis results in cleavage of the more substituted sigma-bond producing a regioisomeric set of furans.