窗烷

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窗烷,从左至右为:一般结构、[4,4,4,4]窗烷、[5,5,5,5]窗烷

窗烷是一种有机化合物,属烷烃类,其中心结构是由一个碳原子的四个共价键延伸出4个共边的碳氢环(稠碳氢环)[1],状如窗户而得名,可以看成是一类螺烷烃。于1972年由化学家Vlasios GeorgianMartin Saltzman[2]提出,其英文名字Fenestrane来自拉丁文的“窗户”。

正常情况下,中心碳原子的sp3杂化导致其共价键呈正四面体结构。窗烷的四环构型则需要这些共价键更倾向于平面结构,因此不容易合成。

理论上,最小的窗烷包含4个3员环(环丙烷),称为[3,3,3,3]窗烷。下一个成员[4,4,4,4]窗烷包含四个稠合的环丁烷体系,如同窗户一般。

命名窗烷时,只需要将各环中原子数目数出,以逗号相隔,并用方括号括起来,后面接上“窗烷”就可以了。窗烷也有系统命名,如[4,4,4,4]窗烷的系统命名是:四环[3.3.1.03,9.07,9]壬烷

在某些极端情况下正四面体结构的碳彻底变成平面四边形结构。平面四边形甲烷分子轨道图像表明,三个sp2杂化轨道中的两个轨道分别与两个原子成普通的σ键,第三个杂化轨道与两个剩余的氢原子成三中心两电子键,仅利用两个氢的电子。而碳的两个未成对电子则被挤进垂直的p轨道。由于共振,四个碳氢键是完全等同的。电脑模拟表明完成这样的过程需要95-250kcal/mol的能量。

张力很大的[4,4,4,5]窗烷已被合成出来。X射线衍射表明中心碳原子键角大约为130°,并且键长较短(149pm于普通的159pm)。

第一个合成的窗烷是[4,5,5,6]窗烷[3]

FenstraneSynthesis1972.png

氮杂[4,5,5,5]窗烷也于最近被合成出来,以便结晶被X射线衍射分析[4][5]

AzaFenestraneSynthesis.png

氟硼酸盐中 N-C-C 键角为126°。


参考资料[编辑]

  1. ^ (Eng.)Fenestranes and the flattening of tetrahedral carbon Bhaskar Rao Venepalli and William C. Agosta Chem. Rev.; 1987; 87(2) pp 399 - 410; DOI:10.1021/cr00078a007
  2. ^ (Eng.)Syntheses directed toward saturated “flat” carbon Vlasios Georgian Martin Saltzman Tetrahedron Letters Volume 13, Issue 42 , 1972, Pages 4315-4317 DOI:10.1016/S0040-4039(01)94304-7
  3. ^ (Eng.)The first step in this reaction sequence is an adaptation of the Stork enamine alkylation reacting cyclopentanone with 3-bromo-1-butene through an imine derivative with pyrrolidine and forming a magnesium salt with ethyl magnesium bromide. The next step is a regular Stork enamine reaction followed by an aldol condensation forming the cyclohexenone ring. The final step is a photolytic [2+2]cycloaddition.
  4. ^ (Eng.)Synthesis, X-ray Crystallography, and Computational Analysis of 1-Azafenestranes Scott E. Denmark, Justin I. Montgomery, and Laurenz A. Kramps J. Am. Chem. Soc.; 2006; 128(35) pp 11620 - 11630; (Article) DOI:10.1021/ja0632759
  5. ^ (Eng.)In step 1 the alkyl halide 1-iodo-3-butene 1 is converted to a cyanozinc cuprate 2 (by transmetalation of the organozinc iodide with copper cyanide) which reacts in the next step with 1-nitro-cyclopentene 3 in a nucleophilic addition whereby the nitronate 4 is captured by phenylselenenyl bromide to the selenium intermediate 5. Hydrogen peroxide oxidation of 5 yields the nitroalkene 6 as a mixture of syn and anti isomers. A [4+2]cycloaddition with n-butyl-enol ether in presence of trimethyl aluminum gives the nitronate 7 and a second [3+2]cycloaddition by heating in presence of potassium carbonate gives the nitroso acetal 8. Hydrogenation with Raney nickel gives the diol 9 which on a double Mitsunobu reaction (with an amine proton donor) gives the azafenestrane 10 as the borane salt.