Air-stable, highly fluorescent primary phosphanes.
Air-stable, highly fluorescent primary phosphanes.
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DOI：
10.1002/anie.201108416
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发表时间：
2012-05-14
影响因子：
16.6
通讯作者：
Higham, Lee J.
中科院分区：
文献类型：
--
作者：
Davies, Laura H.;Stewart, Beverly;Harrington, Ross W.;Clegg, William;Higham, Lee J.
伯膦（RPH 2）是一种在空气中自燃的有毒化合物。[1]然而，我们最近已经证明，它们可以被稳定到空气氧化而不需要任何空间保护，只要足够的π共轭被并入有机R基团;因此，我们能够制备第一个空气稳定的手性伯膦。[2]从那时起，我们一直在开发一个基于DFT的工作模型，这表明，与流行的看法相反，如果分子含有高度共轭，许多初级膦将是空气稳定的（见下文）。[3]因此，该模型预测，将膦基基团结合到硼二吡咯亚甲基（Bodipy [4]）骨架上也会产生空气稳定的初级膦。这些磷烷应该提供一个高度通用的网关到一个广泛的荧光磷烷衍生物，这是目前严重不足，尽管磷烷在催化和生物医学应用的重要性。为了探索这种令人兴奋的可能性，我们开始了一项基于方案1所示策略的综合研究。采用一锅法合成了具有荧光性质的溴代芳香化合物Bodipy，1。[5]由于在后面的还原步骤中难以保留两个氟原子，[6]我们用两当量的苯基锂处理1，得到新的衍生物2a，其通过X射线晶体学表征（支持信息中的图S1）。2a与亚磷酸二乙酯的钯催化偶联反应产生了荧光膦酸酯3a，也通过X射线晶体学进行了分析（支持信息中的图S3）。然后通过使用氢化铝锂和三甲基氯硅烷的组合将膦酸酯3a定量还原为伯膦4a。正如模型预测的那样，发现4a对氧化是稳定的;当以固态和氯仿溶液暴露于空气中超过7天时，没有观察到分解。重要的是，当与母体芳基溴2a相比时，膦酸酯和膦基基团的掺入没有显著改变分子的物理化学性质（表1）。这一发现并不意外，因为我们的DFT计算结果表明，最高占据分子轨道（HOMO），直到2a的HOMO-6，不包含磷原子，尽管它们受到硼原子上苯环的影响（支持信息中的图S6）。值得注意的是，在1的芳基取代之后，芳基溴2a的量子产率（F）从0.65下降到0.079。为了保持1的高量子产率，但仍允许还原步骤，我们用两当量的甲基锂处理1，得到二甲基芳基溴衍生物2b，其也通过X射线晶体学表征（支持信息中的图S2）。用甲基取代氟原子对量子产率的不利影响较小（比较1、2a和2b的F值，表1）。然后，我们采用了以前的方法来编写相应的-
Primary phosphanes (RPH2) have a reputation as noxious compounds which are spontaneously flammable in air.[1] We have recently demonstrated, however, that they can be stabilized to air oxidation without any need for steric protection, providing sufficient π conjugation is incorporated into the organic R group; thus we were able to prepare the first air-stable chiral primary phosphanes.[2] Since then, we have been developing a working model based on DFT, which indicates that, contrary to popular belief, many primary phosphanes will be air-stable if the molecule contains a high degree of conjugation (see below).[3] As such, the model predicted that the incorporation of the phosphino group onto a boron dipyrromethene (Bodipy [4]) skeleton would also produce air-stable primary phosphanes. These phosphanes should provide a highly versatile gateway into a vast range of fluorescent phosphane derivatives, which are currently sorely underrepresented, despite the importance of phosphanes in catalytic and biomedical applications. To explore this exciting possibility, we commenced a synthetic study based on the strategy shown in Scheme1. The fluorescent aryl bromide derivative of Bodipy, 1, was synthesized in a one-pot reaction.[5] Following difficulties with retaining the two fluorine atoms in the later reduction step,[6] we treated 1 with two equivalents of phenyllithium to give the novel derivative 2a, which was characterized by X-ray crystallography (FigureS1 in the Supporting Information). A palladium-catalyzed coupling reaction of 2a with diethylphosphite yielded the fluorescent phosphonate 3a, which was also analyzed by X-ray crystallography (Figure S3 in the Supporting Information). Phosphonate 3a was then reduced quantitatively to the primary phosphane 4a by using a combination of lithium aluminum hydride and chlorotrimethylsilane. As predicted by the model, 4a was found to be stable to oxidation; when exposed to air both in the solid state and in chloroform solution over seven days, no decomposition was observed. Importantly, incorporation of the phosphonate and the phosphino group did not dramatically alter the photophysical properties of the molecules when compared to the parent aryl bromide 2a (Table 1). This finding was not unexpected, as the results from our DFT calculations show that the highest occupied molecular orbitals (HOMOs), up to the HOMO-6 of 2a, do not incorporate the phosphorus atom, although they are affected by the phenyl rings on the boron atom (Figure S6 in the Supporting Information). It is noteworthy that, after the aryl substitution of 1, the quantum yield (F) of aryl bromide 2a drops from 0.65 to 0.079. In an effort to maintain the high quantum yield of 1, but to still allow the reduction step, we treated 1 with two equivalents of methyllithium to give the dimethyl aryl bromide derivative 2b, which was also characterized by X-ray crystallography (Figure S2 in the Supporting Information). Substitution of the fluorine atoms by methyl groups has a less detrimental effect on the quantum yield (compare F values for 1, 2a, and 2b, Table 1). We then adopted the previous methodology to prepare the corre-
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