Synthesis of chiral organotin reagents: synthesis of enantiomerically enriched bicyclo[2.2.1]hept-2-yl tin hydrides from camphor. X-Ray crystal structures of (dimethyl)[(1R,2S,4R)-1,7,7-trimethylbicyclo[2.2.1]hept-2-yl]tin chloride and methyl(phenyl)bis[(1R,2S,4R)-1,7,7-trimethylbicyclo[2.2.1]hept-2-yl]stannane

Abstract

2-Iodo-1,7,7-trimethylbicyclo[2.2.1]hept-2-ene 27 was prepared in two steps from camphor 23. Halogen–metal exchange using butyllithium followed by addition of the appropriate tin halide gave the corresponding bicyclo[2.2.1]hept-2-en-2-ylstannanes 26, 35–38 and the (diphenyl)bis[1,7,7-trimethylbicyclo[2.2.1]hept-2-en-2-yl]stannane 48. Reduction of the 1,7,7-trimethylbicyclo[2.2.1]hept-2-en-2-ylstannanes 35–38 using diimide took place predominantly from the exo-face to give the endo-1,7,7-trimethylbicyclo[2.2.1]hept-2-ylstannanes 18, 43–45, endo–exo = ca. 80 ∶ 20 in all cases. The methyl(phenyl)bis[endo-1,7,7-trimethylbicyclo[2.2.1]hept-2-yl]stannane 51 was prepared from the diphenyl(methyl)[endo-1,7,7-trimethylbicyclo[2.2.1]hept-2-yl]stannane 40 by selective removal of one of the phenyl groups using iodine to give the dialkyl(phenyl)tin iodide 49 which was treated with the alkenyllithium reagent generated from the vinyl iodide 27 to give the bicyclo[2.2.1]hept-2-en-2-yl(dialkyl)phenylstannane 50, as a mixture of epimers at the tin. Reduction using diimide then gave the methyl(phenyl)bis[endo-1,7,7-trimethylbicyclo[2.2.1]hept-2-yl]stannane 51 whose structure was established by X-ray crystallography.

The major (trimethyl)[1,7,7-trimethylbicyclo[2.2.1]hept-2-yl]stannane 39 was shown to be the endo-isomer by an X-ray crystal structure determination of the tin chloride 46 prepared by treatment of the trimethylstannane 39 with tin tetrachloride. The configurations of the other stannanes 40–42 were established by analogy and by comparison of their ¹H NMR spectra with those of 39. The dimethyl[1-dimethylaminomethyl-7,7-dimethylbicyclo[2.2.1]hept-2-enyl](phenyl)stannane 56 was similarly prepared from the parent ketone 52. The stannanes 41/44 and 51 were converted into the tin hydrides 59 and 61, but these gave only very modest enantiomeric excesses when used to reduce the bromoketone 62.