Ten-Membered Cyclodecatetraene Derivatives Including Two Gallium Atoms: Experimental and Theoretical Studies on Synthesis, Structures, and Their Transformations to Nine- and Five-Membered Gallacycles

Abstract

The chemistry of medium-sized ring compounds, including group 13 elements, has scarcely been investigated. Herein, we report that 5,10-digallacyclodeca-1,3,6,8-tetraene derivatives Cl2Ga2C8R8 (R = Me (5a), Et (5b)) can be obtained by the reaction of GaCl3 and zirconacyclopentadiene Cp2ZrC4R4 (R = Me (2a), Et (2b), Cp: η5-C5H5) in toluene. X-ray crystal structure analysis revealed that compound 5b has a 10-membered cyclodecatetraene structure composed of two butadiene skeletons and two GaCl fragments. X-ray crystal structure analysis and IR measurement confirmed that compound 5b forms Ga2Cl2 four-membered rings via intermolecular Cl to Ga donor–acceptor interactions, which result in the formation of oligomeric chain structures in the solid state and toluene solution. The treatment of compound 5b with 2 equivalents of 4-(dimethylamino)pyridine (DMAP) proceeded via the coordination of DMAP to the Ga center and the ring contraction to give DMAP-coordinated five-membered gallole Cl(DMAP)GaC4Et4 (4bDMAP). The reaction of compound 5a with 2 equivalents of MesLi (Mes: 2,4,6-Me3C6H2) in toluene to form a 9,10-digallabicyclo[4.3.1]decatriene derivative (MesGaC8Me8)GaMes (6a) caused ring contraction. Theoretical investigation revealed that the 10-membered ring derivative, Cl2Ga2C8H8 (GaH-10), is formed by the dimerization of two gallole molecules, ClGaC4H4 (GaH-5), via the addition of two Ga–C bonds without an energy barrier. The immediate formation of GaH-10 was attributed to the transannular electronic interaction between the expanded empty p-orbital on the Ga atom and the electron-rich sp2 carbons of the C═C double bond in GaH-5. Moreover, GaH-10 and one of the isomer, nine-membered ring derivative (ClGaC8H8)GaCl, were found to be thermodynamically more stable, as compared to GaH-5.