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
作者:Madeleine Helliwell、Eric J. Thomas、Linda A. Townsend
DOI:10.1039/b200317c
日期:2002.5.10
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.
2-碘-1,7,7-三甲基双环[2.2.1]庚-2-烯27是从樟脑23经过两个步骤合成的。使用丁基锂进行卤素-金属交换后,加入相应的锡卤化物,得到了相应的双环[2.2.1]庚-2-烯-2-基锡烷26、35–38,以及(苯基)双[1,7,7-三甲基双环[2.2.1]庚-2-烯-2-基]锡烷48。使用二氮减去1,7,7-三甲基双环[2.2.1]庚-2-烯-2-基锡烷35–38主要从外面(exo-face)展现还原,得到内源-1,7,7-三甲基双环[2.2.1]庚-2-基锡烷18、43–45,内源–外源比例约为80:20。通过选择性去除一个苯基,使用碘从二苯基(甲基)[内源-1,7,7-三甲基双环[2.2.1]庚-2-基]锡烷40制备了甲基(苯基)双[内源-1,7,7-三甲基双环[2.2.1]庚-2-基]锡烷51,得到的二烷基(苯基)锡碘化物49与从乙烯碘化物27生成的烯烃锂试剂反应,得到双环[2.2.1]庚-2-烯-2-基(双烷基)苯基锡烷50,作为锡的混合立体异构体。随后用二氮还原,得到结构已通过X射线晶体学确立的甲基(苯基)双[内源-1,7,7-三甲基双环[2.2.1]庚-2-基]锡烷51。主要的(三甲基)[1,7,7-三甲基双环[2.2.1]庚-2-基]锡烷39通过对三甲基锡烷39与四氯化锡的反应制备的锡氯化物46的X射线晶体结构测定显示为内源异构体。其他锡烷40–42的构型通过类比及与39的1H NMR光谱比较确立。二甲基[1-二甲氨基甲基-7,7-二甲基双环[2.2.1]庚-2-烯基](苯基)锡烷56同样是从母体酮52制备的。锡烷41/44和51转化为锡氢化物59和61,但在用于还原溴酮62时,仅获得了非常适度的对映体过量。