CYCLIZATION REACTIONS OF RHODIUM CARBENE COMPLEXES - EFFECT OF COMPOSITION AND OXIDATION-STATE OF THE METAL

Treatment of o-(1,7-octadiynyl)benzoyldiazoethane with rhodium(II) oct anoate in pentane resulted in a double internal/internal alkyne insert ion reaction producing a labile bicyclo[4.1.0]hept-1(7)ene derivative which readily undergoes a Diels-Alder reaction with diphenylisobenzofu ran. Changing the solvent from pentane to CH2Cl2 afforded a 2:1 mixtur e of cis- and trans-alkenyl-substituted indenones. Stepwise cyclizatio n involving a set of dipolar intermediates occurs in CH2Cl2 whereas me tallocyclobutenes are involved when pentane is used as the solvent. Th e rhodium(II) carboxylate catalyzed reaction of unsymmetrically substi tuted 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-phen yl-3-methyl-4-(n-butyl)- and 2-phenyl-3-methyl-5-(n-butyl)furan. In co ntrast, the [ClRh(CO)(2)](2)-catalyzed reaction resulted in cleavage o f the more substituted sigma-bond producing only 2-phenyl-3-methyl-5-( n-butyl)furan. Both reactions involve electrophilic attack of the rhod ium metal on the less substituted carbon atom of the cyclopropene pi-b ond to give the most stabilized cyclopropyl carbocation. Ring opening followed by rapid electrocyclization to the furan occurs with the Rh(I H) catalyst. With the Rh(I) catalyst, the ring-opened species preferen tially cyclizes to a metallocyclobutene intermediate which then equili brates with the thermodynamically more stable isomer prior to furan fo rmation. The Rh(I)-catalyzed reaction of 3-benzoyl-1-propylcyclopropen e with various terminal alkynes gives 4-alkyl-4-propyl-7-phenyloxepins in good yield. These reactions involve electrophilic attack of the rh odium metal on the more substituted carbon of the cyclopropene; pi-bon d to give a rhodium carbene complex, This metallo carbenoid undergoes a subsequent [2 + 2] cycloaddition with terminal acetylenes, The resul ting rhodacycle rearranges by a formal 1,5-sigmatropic shift, and this is followed by reductive elimination of rhodium to produce the observ ed oxepin.