Synthesis, structure, and reactivity of a 2,3-dialuminated indole derivative

1-Methylindole was dimercurated upon treatment with mercuric acetate (2 equiv) in 1:1 ethanol-diethyl ether to afford 2,3-bis(acetoxymercurio)-1-methylindole (88%). 2,3-Bis(acetoxymercurio)-1-methylindole was converted to 2,3-bis(chloromercurio)-1-methylindole (92%) by refluxing with excess lithium chloride in methanol for 48 h. Treatment of 2,3-bis(chloromercurio)-1-methylindole with trimethylaluminum (4 equiv) in hexane afforded dimeric 2-(chloromethylaluminia)-3-(dimethylaluminio)-1-methylindole in 93% yield. The structure of dimeric 2-(chloromethylaluminio)-3-(dimethylaluminio)-1-methylindole was established by NMR, IR, and microanalysis and by an X-ray structure determination. Dimeric 2-(chloromethylaluminio)-3-(dimethylaluminio)-1-methylindole shows hindered rotation of the dimethylaluminio groups, with a barrier to rotation of 14.2 +/- 0.2 kcal/mol. The molecular structure of dimeric 2-(chloromethylaluminio)-3-(dimethylaluminio)-1-methylindole was determined by X-ray crystallography and consists of a 2,3-dialuminated indole connected by a C4Al2 ring in a chair conformation. The C4Al2 ring is composed of two chloromethylaluminio groups bridging between the 3-position of one indole fragment and the S-position of another. A dimethylaluminio group is bonded to the carbon at the 3-position in each indole ring, and a bridging chloride is situated between the dimethylaluminio and chloromethylaluminio units in each half of the dimeric structure.