Relay catalysis enables hydrogen gas to participate in asymmetric organocatalytic hydrogenation.
Relay catalysis enables hydrogen gas to participate in asymmetric organocatalytic hydrogenation.
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DOI：
10.1002/anie.201204234
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发表时间：
2012-11
影响因子：
--
通讯作者：
F. Shi;L. Gong
中科院分区：
文献类型：
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作者：
F. Shi;L. Gong
使用清洁氢气的不对称加氢为制备大量手性分子提供了几乎无废弃物的方案，因此在合成有机化学中起着重要作用。这种转化的对映选择性取决于过渡金属配合物中激活氢气和不饱和化学键的手性配体。过渡金属催化反应是实现不对称氢化最可靠的方法。另一方面，高活性的氢化物供体，如Hantzsch酯（HEH），在有机催化的对映选择性还原中被用作外部还原剂，但在这些过程中氢气被认为是惰性底物。然而，这些传统的概念已经被周和同事最近关于N杂环的优雅不对称氢化的报告所驳斥。这些新方法利用了钌配合物/手性磷酸接力催化7]，其中氢气参与手性磷酸催化芳基杂环的加氢反应，取代活性氢化物供体作为末端还原剂。Rueping和同事报道了2-芳基喹啉类化合物和其他杂环化合物可以通过手性磷酸催化HEH进行高度对映选择性的转移氢化反应。Xiao等人证明了手性Ir催化剂和Brønsted酸的结合可以对无环酮胺进行高度对映选择性的加氢，其中氢气被用作还原剂。受这些发现的启发，Zhou等人提出了通过[{Ru（p-花香烃）I2}2]和以双酚为基础的磷酸衍生物（BPA；双酚= 1,1′-双-2-萘酚）的继电催化，对喹啉1进行不对称加氢，得到手性四氢喹啉3。在钌（II）存在下，喹啉1与氢气加氢有效地生成中间产物二氢喹啉2，然后由Brønsted酸（B* H）或钌配合物催化进行自转移加氢，与四氢喹啉3一起递送起始化合物1（方案1）。第一个rutheni-
Asymmetric hydrogenation using clean hydrogen gas provides an almost waste-free protocol for the preparation of a vast range of chiral molecules and thereby plays an important role in synthetic organic chemistry. The enantioselectivity of this transformation depends on the chiral ligands in the transition-metal complexes that activate both hydrogen gas and unsaturated chemical bonds. The transition-metalcatalyzed reaction is the established and most reliable strategy to realize asymmetric hydrogenation. On the other hand, highly active hydride donors, such as Hantzsch esters (HEH), are exploited as external reductants in organocatalytic enantioselective reductions, but hydrogen gas is considered to be an inert substrate in such processes. However, these traditional concepts have been refuted by recent reports by Zhou and co-workers on the elegant asymmetric hydrogenation of N heterocycles. These newer procedures take advantage of ruthenium complex/chiral phosphoric acid relay catalysis, 7] in which the hydrogen gas participates in the chiral phosphoric acid catalyzed hydrogenation of the aryl heterocycle and replaces the active hydride donor as the terminal reductant. Rueping and co-workers reported that 2-arylquinoxalines and other heterocycles could undergo a highly enantioselective transfer hydrogenation with HEH catalyzed by a chiral phosphoric acid. Xiao et al. demonstrated that the combination of chiral Ir catalysts and Brønsted acids afforded a highly enantioselective hydrogenation of acyclic ketimines, in which hydrogen gas was exploited as the reductant. Inspired by these findings, Zhou et al. proposed the asymmetric hydrogenation of quinoxalines 1 to give chiral tetrahydroquinoxalines 3 through the relay catalysis of [{Ru(p-cymene)I2}2] and binol-based phosphoric acid derivatives (BPA; binol = 1,1’-bi-2-naphthol). The hydrogenation of quinoxalines 1 with hydrogen gas in the presence of ruthenium(II) efficiently generated intermediate dihydroquinoxalines 2, which then underwent self-transfer hydrogenation catalyzed either by a Brønsted acid (B* H) or the ruthenium complex to deliver the starting compounds 1 along with tetrahydroquinoxalines 3 (Scheme 1). The first rutheni-