Methylenecyclopentanes chiral

Scheme 68 illustrates cyclopolymerization of 1,5-hexadiene catalyzed by a homogeneous chiral zirconocene complex to form optically active poly(methylenecyclopentane), whose chirality derives from configurational main-chain stereochemistry (757). This polymer is predominantly isotactic and contains predominantly trans cyclopentane rings. 

Considering the industrial importance of cyclopropanes in the pesticide field, it is not surprising that chiral ferrocenylphosphines have been applied as control ligands for the palladium-catalyzed enantioselective formation of cyclopropanes from the dicarbonate of 2-butene-1,4-diol and malonates, leading to 70% ee (Fig. 4-32e) [179]. Ferrocenylphosphines also induce chirality in the reaction of sulfonyl-substituted propenyl carbonates and acrylic esters to methylenecyclopentanes (up to 78% ee (Fig. 4-321)) [180], with potential applications in natural product synthesis. These examples show that the synthetic potential of chiral ferrocene derivatives is not yet fully exploited, and one may look forward to new applications. 

The atom-transfer cyclizations of 2-iodo-6-heptyne amides substituted with a chiral sultam derivative yield E Z mixtures of vinyl iodides38. The reductive dehalogcnation with tributyltin hydride and subsequent desilylation gives two diastereomeric methylenecyclopentanes in a ratio of 90 10. 

These reactions proceed smoothly at 20-40 °C with high regio- and stereoselectivity. For example, dimethyl maleate forms largely the cw-fused cyclopentane. This process has been used to excellent effect in the synthesis of optically active, methylenecyclopentanes by employing chirally modified acrylate acceptors (sec Section 1.6.1.2.3.3). 

The latter transition state, 3, leads to methylenecyclopentane as the final product. This transition state retains Cs-symmetry, the 2-fold internal rotor, and 3-fold internal rotation about the carbon nitrogen bond, for a net symmetry number transition state is chiral, the reaction path degeneracy is (18/6) = 3. 

An intramolecular Heck cyclisation of secondary ethers was revealed recently. The reaction starts from chiral secondary benzylic ethers and enan-tioenriched methylenecyclopentanes are obtained in high yield and enantiospecificity under catalysis of Ni(PCy3)2Cl2 (Scheme 14.34). The reaction using electrophilic substrates with pendant 1,2-disubstituted olefins... 

---