Effects of alkyl chain length, solvent and tandem Claisen rearrangement on two-dimensional structures of noncyclic isobutenyl compounds: scanning tunnelling microscopic study

Abstract

A series of isobutenyl compounds possessing various alkyl chain lengths (C_n-1) with a carbon number of n = 14–21 were synthesized and their two-dimensional (2D) structures were systematically studied using scanning tunnelling microscopy (STM) at a highly oriented pyrolytic graphite (HOPG)/solvent interface. Two kinds of solvent, such as 1-phenyloctane (PO) and 1-phenylnonane (PN), were selected to examine the 2D structures by changing the alkyl chain length of the isobutenyl compounds. At the HOPG/PO interface, C_n-1 molecules with shorter alkyl chains (n = 14–17) showed the same zig-zag shaped 2D structure regardless of the alkyl chain length, whereas an odd–even effect was recognized in C_n-1 compounds with longer alkyl chains (n = 18–21) displaying the wavy and tripod structures, alternately. This odd–even effect was also observed at the HOPG/PN interface rather more distinctly. These results suggest that there is a specific alkyl chain length range that shows the odd–even effect in the present 2D system. After a tandem Claisen rearrangement (TCR), the 2D structures of all the C_n-2 compounds formed were converged into the same linear structure, i.e. the odd–even effect was cancelled by the conformational limitation induced by the TCR.