Synthesis and Characterization of (−)-1-Menthyl-4,7-dimethylindene and Its Main Group Metal Compounds with Lithium, Sodium, Potassium, and Tin

(−)-3-Menthyl-4,7-dimethylindene and (−)-4,7-diisopropyl-3-menthylindene were prepared in 63 and 48% yield, respectively, through a 1,1‘-bis(diphenylphosphino)ferrocene-palladium-catalyzed cross-coupling reaction of menthylzinc with 3-(4,7-dimethylindenyl) trifluoromethanesulfonate or 3-(4,7-diisopropylindenyl) trifluoromethanesulfonate. Several other methods including addition of menthyl Grignard to 1-indanone, addition of (indenyl)lithium to menthone, or coupling of 3-bromoindene with menthyl Grignard for preparing 1-(or 3-)menthylindene were inferior to direct substitution of menthyl sulfonate esters by (indenyl)lithium. (−)-3-Menthyl-4,7-dimethylindene could be deprotonated to form a single diastereomeric lithium salt that was characterized by X-ray crystallography. (−)-4,7-Diisopropyl-3-menthylindene was deprotonated to form a lithium salt. Formation of the sodium and potassium salts of (−)-3-menthyl-4,7-dimethylindene was possible, but the products were not stereochemically characterized. 1-Trialkyltin compounds with (−)-3-menthyl-4,7-dimethylindene were prepared as nearly 1:1 diastereomeric mixtures from the lithium salt.