Kinetics of Z–E isomerization of spiroindolinopyrans

Abstract

The kinetics of the Z–E isomerization of a series of 3′-nonsymmetrically substituted spiroindolinopyrans bas been studied by nmr in different solvents.The rate-determining step in nonpolar solvents was found to be the cleavage of the C—O bond whereas in polar solvents it was the C(2)—C(3) bond rotation.A strong steric hindrance of substituents at carbon 3 was observed. It was found that the rate of the isomerization is more sensitive to the bulkiness of the substituent at C(3′) than at the N(1′). Protic solvents enhance the reaction more than nonprotic polar solvents.

Three-component spiropyran synthesis via tandem alkylation-condensation

“…17 The unusual substituent pattern possessed by structures such as 3 -in which inequivalent substituents occupy the indole 3-position -influences the stereoselectivity of spiropyran ring-closure and hence has relevance in the development of sensors for chiral substrates and asymmetric catalysts. [18][19][20] Moreover, the sterically-congested nature of such spiropyrans impacts upon spiropyran-merocyanine isomerisation and enables control of equilibrium position on the basis of steric effects -a complementary approach to more common electronic strategies. 21 Scheme 1 -Microwave promoted alkylation-condensation in spiropyran synthesis.…”