Abstract
Diels-Alder and ene reactions of germaethene Me2Ge=C(SiMe3)2 (2) with butadienes
respectively, take place regioselectively, as well as stereoselectively. They are accelerated by an increasing tendency of substituents in butadiene or propene to donate electrons (e.g. 2-methylbutadiene > butadiene; 2-methylpropene > propene), and retarded by an increasing bulkyness of substituents in 1,4- or 1,3-positions (e.g. 1-methylbutadiene > 2-methylbutadiene; 1-vinylpropene > propene). It is concluded from these studies that Diels-Alder and ene reactions of 2 occur - like those of Me2Si=C(SiMe3)2 (1) or of ethenes >C=C< - in a concerted way and are HOMOdiene-LUMOdienophile and HOMOene-LUMOenophile controlled. Thus 2 and 1 behave as carbon analogues. With regard to a specific diene or ene (e.g. anthracene; toluene), 2 is the better dienophile or enophile than 1 or ethenes. With a fixed diene/ene mixture (e.g. butadiene/propene), 2 acts as the better dienophile, while 1 is the better enophile. These results can be explained by the π/π*- energy difference and the double bond polarity decreasing in the direction 1 > 2. Only cis-piperylene gives a [2+2] cycloadduct with 2. besides two [4+2] cycloadducts, and an ene reaction product
摘要:Germaethene Me
2Ge=C(SiMe
3)
2 (2)与
丁二烯和
丙烯发生Diels-Alder和ene反应,分别具有区域选择性和立体选择性。它们被
丁二烯或
丙烯中取代基电子给予性的增加(例如,
2-甲基丁二烯>
丁二烯;
2-甲基丙烯>
丙烯)加速,而被1,4-或1,3-位置取代基体积的增加(例如,
1-甲基丁二烯>
2-甲基丁二烯;
1-乙烯基
丙烯>
丙烯)减缓。从这些研究中可以得出结论,2的Diels-Alder和ene反应-像Me
2Si=C(SiMe
3)
2(1)或
烯烃的反应一样-以协同的方式进行,并且是HO
MOdiene-LU
MOdienophile和HO
MOene-LU
MOenophile控制。因此,2和1的行为类似于
碳类似物。关于特定的二
烯或
烯烃(例如
蒽或
甲苯),2比1或
烯烃更好的偶联物或
烯烃。对于固定的二
烯/
烯烃混合物(例如
丁二烯/
丙烯),2作为更好的偶联物,而1则是更好的
烯烃。这些结果可以通过π/π*能量差和双键极性在1>2方向上降低来解释。只有顺式-
戊二烯与2形成[2+2]环加成物,还有两个[4+2]环加成物和一个ene反应产物。