Synthesis and ligand properties of stable five-membered-ring allenes containing only second-row elements
Synthesis and ligand properties of stable five-membered-ring allenes containing only second-row elements
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DOI：
10.1002/anie.200801176
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发表时间：
2008-01-01
影响因子：
16.6
通讯作者：
Bertrand, Guy
中科院分区：
文献类型：
--
作者：
Lavallo, Vincent;Dyker, C. Adam;Bertrand, Guy
艾伦烯是具有两个连续碳-碳双键的化合物。因此，它们具有带有正交取代基对的线性 CCC 骨架。[1]由于其刚性，丙二烯框架只能困难地容纳成环，并且计算预测，随着从环状丙二烯主链上连续去除一个碳原子[从 1, 2-环辛二烯 (12kcalmol-1) 到 1, 2-环丁二烯 (90 kcal mol-1)]，环应变显着增加。 [2]即使是全碳环丙二烯的低温核磁共振波谱表征也仅限于含有七个以上碳原子的环丙二烯；[3]唯一的例外是1,2,4,6-环庚四烯A2，它被Warmuth和Marvel囚禁在分子容器中。[4]动力学保护的1, 2-环辛二烯A3（计算的CCC角：1588），[5]含三硅[6]和二磷[7]六环A4-A6（晶体学观察到的CCC角分别为：166、161和1568；Mes=mesityl），以及最近分离的五元含半鎓烯化合物A7（CCC角：1568）[8]是迄今为止分离出的最弯曲的环状丙二烯。由于存在较重元素，A4-A7 中的丙二烯片段并不比八元环 A3 中的丙二烯片段扭曲得多。张力较大的环状丙二烯，例如 1, 2-环戊二烯（A1；计算出的 CCC 角：1148）仅被称为反应中间体，这与预测它们具有强双自由基特征的计算一致。 [2]我们最近表明[9]，推-推取代模式[10]可能会导致与无环丙二烯的经典几何形状的严重偏差。事实上，单晶 X 射线衍射研究表明，丙二烯 A8 的 CCC 键角为 134.88，两个 NCN 平面彼此不垂直，而是扭转了 698°。此外，Tonner 和 Frenking [11] 对四氨基丙二烯的理论研究预测，这些推-推取代的丙二烯，他们将其描述为“碳二碳烯”（具有二价碳 (0) 中心的化合物） [12-13]具有两个N-杂环卡宾（NHC）配体），具有高度灵活性。基于这些结果，推-推取代模式似乎能够合成限制在相对较小的环系统内的丙二烯。在此，我们报告了第一个仅由第二行元素组成的五元环丙二烯的锂盐加合物和无盐衍生物的合成和X射线晶体学表征。基于铑 (I) 二甲酰氯配合物的红外分析，还初步说明了这些物质的配体性质。容易获得的热稳定性和空气稳定性 3, 5-二氨基吡唑鎓盐 2a [14] 是具有四个 π-供体氨基的五元环丙二烯的合理前体（方案 1）。当 2a 在 788C 下用正丁基锂处理时，反应混合物的 1HNMR 光谱显示完全去质子化，如由于吡唑鎓环上的氢原子导致 d= 5.4 ppm 处的信号消失所示。 13C NMR 谱显示在 d= 114 和 176 ppm 处有两个不同的共振，它们在无环弯曲丙二烯 A8 的中心和末端 CCC 碳原子（分别为 d= 110.2 和 144.8 ppm）观察到的范围内。对 2a 去质子化所得化合物的 X 射线衍射研究 [15] 表明，它不是所需的环状丙二烯，而是其四氟硼酸锂加合物 3a（橙色晶体，产率 26%；图 1）。众所周知，会形成一些高碱性化合物，例如双（二异丙氨基）亚环丙烯[16]和磷叶立德[17]……
Allenes are compounds with two contiguous carbon–carbon double bonds. They consequently feature a linear CCC skeleton with orthogonal pairs of substituents.[1] Because of their rigidity, allene frameworks can only be accommodated into a ring with difficulty, and calculations predict that the ring strain increases considerably with each successive removal of a carbon atom from the backbone of cyclic allenes [from 1, 2-cyclooctadiene (12kcalmolÀ1) to 1, 2-cyclobutadiene (90 kcal molÀ1)].[2] Even the low-temperature NMR spectroscopic characterization of all-carbon cyclic allenes is limited to those containing more than seven carbon atoms;[3] the only exception is the 1, 2, 4, 6-cycloheptatetraene A2, which was incarcerated in a molecular container by Warmuth and Marvel.[4] The kinetically protected 1, 2-cyclooctadiene A3 (calculated CCC angle: 1588),[5] the trisilicon-[6] and diphosphorus-containing [7] hexacycles A4–A6 (crystallographically observed CCC angles: 166, 161, and 1568, respectively; Mes= mesityl), and the recently isolated five-membered halfnium-containing allenoid A7 (CCC angle: 1568)[8] are the most bent cyclic allenes isolated to date. Because of the presence of the heavier elements, the allene fragments in A4–A7 are not significantly more distorted than in the eightmembered ring A3. More-strained cyclic allenes, such as 1, 2-cyclopentadiene (A1; calculated CCC angle: 1148), are only known as reaction intermediates, in line with calculations that predict that they have a strong diradical character.[2] We showed recently [9] that a push–push substitution pattern [10] can induce a severe deviation from the classical geometry of acyclic allenes. Indeed, a single-crystal X-ray diffraction study revealed that allene A8 has a CCC bond angle of 134.88, with the two NCN planes not perpendicular to one another but twisted by 698. Moreover, theoretical studies by Tonner and Frenking [11] on tetraaminoallenes predict that these push–push-substituted allenes, which they described as “carbodicarbenes”(compounds featuring a divalent carbon (0) center [12–13] with two N-heterocyclic carbene (NHC) ligands), are highly flexible. On the basis of these results, it seemed likely that a push–push substitution pattern could enable the synthesis of allenes confined within relatively small ring systems. Herein we report the synthesis and X-ray crystallographic characterization of the first lithium-salt adduct and salt-free derivative of five-membered ring allenes consisting only of second-row elements. A preliminary account of the ligand properties of these species is also given on the basis of infrared analysis of a rhodium (I) dicarbonyl chloride complex. The readily available, thermally and air-stable 3, 5-diaminopyrazolium salt 2a [14] was a logical precursor to a fivemembered ring allene with four π-donor amino groups (Scheme 1). When 2a was treated with n-butyllithium at À788C, 1HNMR spectroscopy of the reaction mixture revealed clean deprotonation, as shown by the disappearance of the signal at d= 5.4 ppm due to the hydrogen atom on the pyrazolium ring. The 13C NMR spectrum showed two distinct resonances at d= 114 and 176 ppm, which are in the range of those observed for the central and terminal CCC carbon atoms of the acyclic bent allene A8 (d= 110.2 and 144.8 ppm, respectively). An X-ray diffraction study [15] of the compound obtained from the deprotonation of 2a revealed that it was not the desired cyclic allene, but its lithium tetrafluoroborate adduct 3a (orange crystals, 26% yield; Figure 1). It is known that some highly basic compounds, such as bis (diisopropylamino) cyclopropenylidene [16] and phosphorus ylides,[17] are formed …