Cyclopentanone, formation

Wenkert and co-workers were the first to disclose the use of Rh2(OAc)4 for C-H insertion leading to cyclopentanone formation. Then the versatility of this methodology was developed by Taber, Stork, and Nakatani. The first reported example of asymmetric induction was by McKervey and co-workers in 1990 and involved the Rh2(Y-BSP)4-catalyzed decomposition of cr-diazo-/3-ketosulfone 80 (Equation (69)).212 Cyclopentanone 81 was obtained in an excellent yield as a mixture of cis- and trans-isomers, although with poor enantioselectivity (12% ee). 

In order for C-alkylation to occur, the p orbital at the a carbon must be aligned with the C—Br bond in the linear geometry associated with the SN2 transition state. When the ring to be closed is six-membered, this geometry is accessible, and cyclization to the cyclohexanone occurs. With five-membered rings, colinearity cannot be achieved easily. Cyclization at oxygen then occurs faster than does cyclopentanone formation. The transition state for O-alkylation involves an oxygen lone-pair orbital and is less strained than the transition state for C-alkylation. 

High diastereoselecivities are normally observed in the alkylation of five-membered ring enolates (eqnations 7 and 8), implying that the alkylation is mainly controlled by steric factors. Thus, with 3-substituted cyclopentanones formation of only the frawi-product (23 and 25, respectively) is preferred. Representative examples are the alkylations of lithium enolates derived from ketones 22 and 24 . Before the electrophiles were added, the enolate solutions were stirred at higher temperature for a longer time to secure the... 

Diara et al. carried out interesting applications of the cyclopentanone formation. Stereoselective synthesis of a-substituted cyclopentanone 54 with a quaternary carbon stereocenter was carried out by the reaction of the allenylcyclobutanol 52 with 4-iodoanisole (53) [9]. Intramolecular reactions of the stereoisomers 55 and 57 in the presence of silver salt in toluene afforded different products depending on their stereochemistry. The macrocyclic dimeric product 56 was obtained from... 

Irradiation of (methyl acrylate)iron tetracarbonyl in the presence of excess methyl acrylate at 20° forms a ferracyclopentane product. Thermal reaction of the metallocyclopentane with carbon monoxide or triphenyl-phosphine affords a cyclopentanone derivative (Grevels et al., 1974). Similar ferracyclopentane complexes may be involved as intermediates in the well-known cyclopentanone formation from iron carbonyls and strained olefins (for example, Grandjean et al., 1974 Mantzaris and Weissberger, 1974). 

---