Versatile Reactivity of the Germene Mes2GeCR2 toward Esters and Related Carbonyl Derivatives

Dimesitylfluorenylidenegermane, Mes2GeCR2 (1; Mes = 2,4,6-trimethylphenyl, CR2 = fluorenylidene), undergoes a [2 + 4] cycloaddition with the CCCO system of α-ethylenic esters such as methyl acrylate, dimethyl maleate and dimethyl fumarate, and diethyl fumarate, leading to the 1,2-oxagerma-5-cyclohexenes 2 and 3a,b, respectively. With diethyl oxalate, maleic anhydride, and cyclopent-2-ene-1,3-dione [2 + 2] cycloadditions are observed between the GeC double bond and the carbonyl function, with formation of 1,2-oxagermetanes 4−6, respectively. In the last case, a double [2 + 2] cycloaddition on both CO groups to form a tricyclic compound occurs with excess germene. Germene 1 reacts exclusively by an ene reaction with 2-methyl-1,3-pentadione to give the corresponding (germyloxy)cyclopentenone 8.