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 Cl 2 Ga 2 C 8 R 8 (R = Me (5a), Et (5b)) can be obtained by the reaction of GaCl 3 and zirconacyclopentadiene Cp 2 ZrC 4 R 4 (R = Me (2a), Et (2b), Cp: η 5 -C 5 H 5 ) 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 Ga 2 Cl 2 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)GaC 4 Et 4 (4bDMAP). The reaction of compound 5a with 2 equivalents of MesLi (Mes: 2,4,6-Me 3 C 6 H 2 ) in toluene to form a 9,10-digallabicyclo[4.3.1]decatriene derivative (MesGaC 8 Me 8 )GaMes (6a) caused ring contraction. Theoretical investigation revealed that the 10-membered ring derivative, Cl 2 Ga 2 C 8 H 8 (GaH-10), is formed by the dimerization of two gallole molecules, ClGaC 4 H 4 (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 sp 2 carbons of the C�C double bond in GaH-5. Moreover, GaH-10 and one of the isomer, nine-membered ring derivative (ClGaC 8 H 8 )GaCl, were found to be thermodynamically more stable, as compared to GaH-5.