Thermal valence isomerization of 2,3-diborata-1,4-diphosphoniabuta-1,3-dienes to bicyclo[1.1.0]butanes and cyclobutane-1,3-diyls
Thermal valence isomerization of 2,3-diborata-1,4-diphosphoniabuta-1,3-dienes to bicyclo[1.1.0]butanes and cyclobutane-1,3-diyls
复制标题
DOI：
10.1002/anie.200701578
复制
发表时间：
2007-01-01
影响因子：
16.6
通讯作者：
Bertrand, Guy
中科院分区：
文献类型：
--
作者：
Bourg, Jean-Baptiste;Rodriguez, Amor;Bertrand, Guy
在他的诺贝尔奖演讲中，题为“无机和有机化学之间的桥梁”，霍夫曼总结说：“等瓣类比是一个模型。我们的科学工艺有责任将其推向极端，而仅仅是一个模型，它肯定会在某个地方失败。[1]丁二烯（C_4H_6）的不同价异构体与其等瓣类似物的相对稳定性和相互转化率的比较，很好地证明了Hoffmann的结论。在碳化学中，s-trans-buta-1，3-diene（1C）是最稳定的价态异构体，环丁烯（2C）和双环[1.1. 0]丁烷（3C）的能量分别高出14.8和31.1千卡摩尔/升（方案1，左）。[2]单重态环丁烷-1，3-二基（4C）仅被预测为3C反转的过渡态。[3，4]这些不同价异构体1C-4C的相互转化已经在实验和理论上进行了广泛的研究。[5]已发现2C [5a-c]和3C [5d，e]的热分解都导致丁二烯1C，丁二烯1C的光分解返回2C。[5 f-h]当骨架的部分或全部碳原子被重元素取代时，价态异构体的稳定性顺序及其相互转化的模式可能相似或非常不同。[6]例如，已经表明1，4-二磷杂丁二烯1 PC可以进行闭环形成2 PC，[7]但仅限于热（方案1，右）。此外，双自由基4PC不仅是势能面上的最小值，而且甚至可以是孤立的，只要它们具有正确的取代基。[8]4PC的光分解得到双环衍生物3 PC，随后由其热分解得到丁二烯1 PC。[8c]对于具有各种杂原子的体系，已经观察到异构体1-4之间的大多数可能的相互转化。[5-8]然而，仅有一个丁二烯（1Ca）光化学异构化为双环-[1.1. 0]丁烷（3Ca;方案2，顶部）[9，10]，并且在热条件下没有该过程的明确证据。[11]这种情况促使我们详细研究了1，3-二硼酸-2，4-二膦酰双环-[1. 1。0]丁烷3BP和1，3-二硼酸-2，4-二膦酰基环丁烷-1，3-二基4 BP。[12我们假设瞬时2，3-二硼酸-1，4-二膦酰基丁-1，3-二烯1BP参与（方案2，底部）。然而，通过阳离子三元杂环6 BP的第二种途径也可以解释3BP和4 BP的形成。本文报道了1BP型衍生物的合成和单晶X-射线衍射研究，以及其热价异构化成相应的双环衍生物3BP。强有力的证据表明，丁二烯1BP也可以热异构化成双自由基4 BP。此外，它表明，在光解活化下，杂双环[1.1。0]丁烷3BP经历伴随着1，2-位移的开环反应，该过程类似于对全碳类似物3Ca观察到的过程。[9]为了探索推定的6 BP型三元BBP杂环的形成，我们首先用1当量的二苯并噻吩锂处理1，2-二叔丁基-1，2-二氯二硼烷。
In his Nobel lecture entitled “Bridges between Inorganic and Organic Chemistry”, Hoffmann concluded:“The isolobal analogy is a model. It is the duty of our scientific craft to push it to its extremes, and being only a model it is certain to fail somewhere”.[1] Comparison of the relative stabilities and the interconversions of the different valence isomers of butadiene (C4H6) with those of its isolobal analogues provides a good demonstration of Hoffmann s conclusions. In carbon chemistry, s-trans-buta-1, 3-diene (1C) is the most stable valence isomer, cyclobutene (2C) and bicyclo [1.1. 0] butane (3C) being 14.8 and 31.1 kcalmolÀ1 higher in energy, respectively (Scheme 1, left).[2] The singlet cyclobutane-1, 3-diyl (4C) was predicted only as a transition state for the inversion of 3C.[3, 4] The interconversion of these different valence isomers 1C–4C has been studied extensively, both experimentally and theoretically.[5] It has been found that the thermolyses of both 2C [5a–c] and 3C [5d, e] lead to butadiene 1C, the photolysis of which gives back 2C.[5f–h] When some or all of the carbon atoms of the backbone are replaced by heavier elements, the order of stability of the valence isomers and their modes of interconversion may be either similar or very different.[6] For example, it has been shown that 1, 4-diphosphabutadienes 1PC can undergo a ring closure to form 2PC,[7] but only thermally (Scheme 1, right). Moreover, diradicals 4PC are not only a minimum on the potential energy surface, but can even be isolated, provided they have the right set of substituents.[8] Photolysis of 4PC affords the bicyclic derivatives 3PC, from which subsequent thermolysis gives butadienes 1PC.[8c] Most of the possible interconversions between isomers 1–4 have been observed for systems with various heteroatoms.[5–8] However, there is only one example of a photochemical isomerization of a butadiene (1Ca) into a bicyclo-[1.1. 0] butane (3Ca; Scheme2, top)[9, 10] and no definitive evidence for this process under thermal conditions.[11] This situation prompted us to study in detail the mechanism of formation of 1, 3-diborata-2, 4-diphosphoniabicyclo-[1.1. 0] butanes 3BP and 1, 3-diborata-2, 4-diphosphoniacyclobutane-1, 3-diyls 4BP.[12, 13] We postulated the involvement of transient 2, 3-diborata-1, 4-diphosphoniabuta-1, 3-dienes 1BP (Scheme2, bottom). However, a second pathway via a cationic three-membered heterocycle 6BP could also explain the formation of 3BP and 4BP. Herein, we report the synthesis and single-crystal X-ray diffraction study of a derivative of type 1BP, and its thermal valence isomerization into the corresponding bicyclic derivative 3BP. Strong evidence that butadienes 1BP can also isomerize thermally into diradicals 4BP is presented. Moreover, it is shown that, under photolytic activation, heterobicyclo [1.1. 0] butanes 3BP undergo a ring-opening reaction accompanied by a 1, 2-shift, a process similar to that observed for the all-carbon analogue 3Ca.[9] To probe the putative formation of a three-membered BBP heterocycle of type 6BP, we first treated 1, 2-di-tert-butyl-1, 2-dichlorodiborane with one equivalent of lithium diphe-