Rh^I‐Catalyzed [6+2] Cycloaddition of Alkyne–Allenylcyclobutanes: A New Entry for the Synthesis of Bicyclo[6.m.0] Skeletons

Rh^I-Catalyzed Cycloaddition between Allenyl π-Bonds and C-C Triple Bonds

“…1n The smooth cleavage of the unfunctionalized cyclopropane ring activated by the adjacent allenyl moiety prompted us to examine the Rh I catalyzed intramolecular [6 2] cycloaddition reaction between alkyne and the one carbon homologated allenylcyclobutane (1, n 2) leading to the formation of the bicyclo[6.4.0]dodecatriene frameworks 3 (n 2, Scheme 2). 12 Our initial evaluation was carried out as follows; a solution of the N tosyl derivative 1a 18 2 ] 20 afforded the best result (84%) (entry 3). Finally, the highest yield (87%) of 3a was attained when the reaction was performed in 0.1 M dioxane solution in the presence of 5 mol% [RhCl(dppp) 2 ] for 0.2 h (entry 4).…”

“…Encouraged by the successful development of the C sp3 C sp3 bond activation of the simplest unactivated cyclobutane described above, 12 we moved on to investigate the application of this methodology to the analogous cyclopentane derivatives.…”

“…The novel spiro [2.4]heptane derivative 15a was isolated as well in 38% yield (Table 4, entry 1). RhCl(dppp) 2 was the most suitable catalyst for ring opening of allenylcyclobutane, 12 and exposure of 13a to 10 mol% RhCl(dppp) 2 resulted in increased production of 14a, formed in 39% yield along with 15a in 52% yield (entry 2). However, in this case RhCl(PPh 3 ) 3 was found to be the most effective catalyst.…”

“…12 Initial coordination of 13 with Rh I would form the intermediate E, which should collapse to the intermediate F via the oxidative ring closing reaction (Scheme 4). This intermediate F could activate the inert C C bond in the cyclopentane with low strain energy (6.3 kcal/ mol, Figure 2), 11 which result in β C elimination 3d h to form the 10 membered rhodacycle G. Subsequent reductive elimination of rhodium from G would give the nal product 14.…”

“…1, n 1). 12 This combination, namely the reaction of allenylcycloalkanes with alkynes, could be applied to allenylcyclopentane derivatives; allenylcyclopentane alkynes underwent intramolecular [7 2] cycloaddition producing the corresponding bicyclo [7.4.0] compounds (eq. 1, n 2).…”