Cycloisomerization methylenecyclopentane

Cycloisomerization of 1,6-diene 25 is effected by a number of transition metal catalysts. For example, both rhodium trichloride [22, 23] and Wilkinson s catalyst [24, 25] promote this reaction efficiently to give methylenecyclopentane 26 (Scheme 7.12). In the latter case, the active catalyst species is beheved to be [Rh(PPh3)2HCl2]. A mechanism proposed for this cycloisomerization is shown in Scheme 7.13. Coordination of a diene to [Rh(PPh3)2HCl2] and insertion of one of the olefin moieties of the diene into the [Rh]-H bond gives complex II.3a. Carbocychzation affords alkyl-[Rh] intermediate II.3i,. Subsequent reductive ehmination of the methylenecyclopentane regenerates the active catalyst species. 

Cycloisomerization of 1,6-dienes. Cyclopentenes are formed when the dienes are exposed to catalytic amounts of (MeCNj PdCl and AgBF. However, in the presence of 2 equiv of the silver salt (presumably forming a dicationic Pd complex) the methylenecyclopentane isomers are obtained. 

Cycloisomerization. The photoinduced formation of methylenecyclopentane derivatives from a-(4-pentynyl)acetoacetic esters represents a new way to prepare these substances. 

Representative procedure for the gold-catalyzed cycloisomerization of enynoic acids. Dimethyl 3-(tetrahydro-2-methyl-5-oxofuran-2-yl)-4-methylenecyclopentane-l,l-... 

Using the catalyst system based on [Ni(allyl)(cod)l [BARF] and Wilke s aza-phospholene ligand, several 1,6-dienes including diene 48 were converted to exo-methylenecyclopentanes 49 through the asymmetric cycloisomerization with enantiomeric excess of up to 91% (Scheme 28) (66). 

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