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

氟离子具有(1)作为氟离子（F^-）的离基能力和(2)由于其孤对而具有α-阳离子的稳定作用。我们对利用这两种性质的兴趣促使我们研究从1,1-二氟烯烃中生成含氟碳阳离子，这将允许通过分子内不饱和基团捕获阳离子，然后进行F^-消去来构建环。因此，我们通过α-氟化位点，通过氟化双键的区域选择性质子化，完成了含两个芳基的1,1二氟-1-烯烃的氟促进Friedel-Crafts环化，从而构建了融合多环体系。此外，这种环化被证明是由过渡金属配合物而不是强质子酸影响的。4-烯-2-三氟甲基-q-丁烯与苄基阴离子的SN′′反应生成了在适当位置上具有两个芳基的1,1-二氟-1-烯烃。用FSO_3H/SbF_5或Pd(BF_4)_2(MeCN)_4在（CF_3）_CHOH （HFIP）中处理得到的二氟烯烃，容易促进分子内的Friedel-Crafts反应。串联环化反应成功地得到了熔融多环化合物，其在活性炭上与氢氧化钯脱氢得到了高收率的[4]螺旋烯。反应的顺序：SN2'反应、串联阳离子环化和脱氢，从1-三氟甲基乙烯基化合物仅需三步就能得到螺旋烯等融合多环化合物。由于大多数螺旋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旋烯的方法是一个理想的目标。本文所披露的结果为包括功能化螺旋蛋白在内的螺旋蛋白的研究开辟了新的途径。

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.