Construction of Fused Polycyclic System via Tandem Cationic Cyclization by Using the Properties of Fluorine

氟具有 (i) 作为氟离子 (F^-) 的离去基团能力和 (ii) 由于其孤对电子而具有 α-阳离子稳定作用。我们对利用这两个特性的兴趣促使我们研究从 1,1-二氟烯烃生成含氟碳阳离子，这将允许通过分子内捕获不饱和基团的阳离子来构建环，然后进行 F^-消除。因此，我们通过氟化双键的区域选择性质子化产生的α-氟碳正离子，完成了带有两个芳基的1,1二氟-1-链烯的氟促进弗里德尔-克来福特环化，从而构建了稠合多环系统。此外，这种环化已被证明甚至可以通过过渡金属络合物而不是强质子酸来实现。4-芳基-2-三氟甲基-q-丁烯和苄基阴离子的SN''反应得到在适当位置具有两个芳基的1,1-二氟-1-烯烃。用FSO_3H/SbF_5或Pd(BF_4)_2(MeCN)_4处理由此获得的二氟烯烃(CF_3)_CHOH (HFIP) 很容易促进分子内弗瑞德-克来福特反应，成功地进行串联环化，得到稠合多环化合物，其在活性炭上用氢氧化钯脱氢，得到高产率的[4]螺旋烯。反应顺序：SN2'反应、串联阳离子环化和脱氢，仅用三个步骤即可得到稠合多环化合物，例如螺旋烯。由于大多数螺烯是通过二苯乙烯部分的光环化制备的，因此开发一种提供大量螺烯的方法一直是一个理想的目标，本文公开的结果开辟了一种制备螺烯（包括官能化螺烯）的新方法。

Fluorine possesses (i) leaving-group ability as fluoride ion(F^-) and (ii) α-cation-stabilizing effect due to its lone pairs. Our interest in using these two properties prompted us to investigate the generation of fluorine-containing carbocations from 1,1-difluoroalkenes, which would allow ring constructions by intramolecular trapping of the cations with unsaturated groups, followed by F^-elinination. Thus, we have accomplished fluorine-promoted Friedel-Crafts cyclization of 1,1difluoro-1-dalkenes bearing two aryl groups via α-fluorocarbocations, generated by the regioselective protonation of the fluorinated double bonds, leading to the construction of fused polycyclic systems. Moreover, this cyclization has proved to be effected eve by transition metal complexes instead of strong protic acids.The SN"' reaction of 4-ary-2-trifluoromethyl-q-butenes and benzyl anions affords 1,1-difluoro-1-alkenes with two aryl groups at appropriate positions. Treatment of thus obtained difluoroalkenes with FSO_3H/SbF_5 or Pd(BF_4)_2(MeCN)_4 in (CF_3)_CHOH (HFIP) readily promotes intramolecular Friedel-Crafts reactions. The tandem cyclizations successfully proceed to afford fused polycyclic compounds, whose dehydrogenation with palladium hydroxide on activated carbon leads to [4]helicenes in high yield.The sequence of reactions: the SN2' reaction, the tandem cationic cyclizations, and the dehydrogenation affords fused polycyclic compounds such as helicenes in only three steps from the 1-trifluoromethyl vinyl compounds. Since most of helicenes have been prepared via photo cyclization of stilbene moieties, developing a methodology for supplying large amount of helicenes have been a desirable goal. The results disclosed herein open a new approach to helicenes including functionalized ones.