Air- and microwave-stable (C5H5)Ru catalysts for improved regio and enantioselective carroll rearrangements
Air- and microwave-stable (C5H5)Ru catalysts for improved regio and enantioselective carroll rearrangements
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DOI：
10.1002/anie.200704019
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发表时间：
2007-01-01
影响因子：
16.6
通讯作者：
Lacour, Jerome
中科院分区：
文献类型：
--
作者：
Constant, Samuel;Tortoioli, Simone;Lacour, Jerome
在允许构建复杂分子结构的各种各样的合成方法中，最好的记录之一是亲核试剂在烯丙基金属片段上的攻击，产生（几乎）对映体纯形式的手性烯丙基化合物。这种取代的好处之一是与不对称烯丙基底物反应的区域选择性可以通过金属催化剂来控制。[1]在这方面，几种基于铼的化合物，特别是Cp* Ru衍生物（Cp*= C5 Me 5），已经证明对于在更多取代的位置引入亲核试剂是非常有效的，导致支链（B）而不是直链（I）产物[Eq.①]。[2]典型的底物是烯丙基碳酸酯和烯丙基氯（伯或仲），并且已经开发了有效的烯丙基烷基化、芳基化、胺化和醚化反应。[3-6]虽然有用，但是这些反应通常用大量的钌催化剂进行（ca. 10摩尔%），并且活性络合物由于其对水分、氧气和分离条件的敏感性而在反应结束时不被回收。据我们所知，只有Bruneau、Renaud及其同事能够通过将阳离子Cp* Ru催化剂固定在离子液体相中来进行多个催化循环。[7]因此，1）空气不敏感，2）易于合成和纯化，3）催化活性，和4）可能可重复使用的环戊二烯基Ru衍生物将是该领域中重要的新颖性。在此，我们报告说，这样的物种确实可以通过使用的阴离子单齿配体。几个有益的方面，包括改进的区域和对映体选择性的卡罗尔重排的烯丙基酮酯在直接比较研究与经典的[PF 6]的类似物的催化物种。1型烯丙基-b-酮酯[Eq. (2)]已知的是，在双Ru部分和2，2 ′-联吡啶（bpy）的存在下，以高产率和优异的B/l比平稳地反应形成g，d-不饱和酮（eg 2）。[6]最近，开发了这种卡罗尔重排的对映选择性版本，其需要结合使用[CpRu（NCCH 3）3][PF 6]和对映纯的吡啶亚胺配体（Cp= C5 H5）。[8]这些烯丙基取代反应，使用相当大量的不可回收的钌催化剂（10mol%），从而构成了一个理想的框架，为目的的研究。最近，还描述了一种新的六配位磷阴离子TRISPHAT-N（3，图1），并证明其是一种有效的抗衡离子和配体，能够进入金属络合物的第一配位层。[9，10]此外，已表明衍生的两性离子金属络合物具有令人感兴趣的色谱、空气和水分稳定性。因此，3将与[CpRu-（NCCH 3）3][PF 6]和bpy的混合物形成稳定复合物的可能性对于卡罗尔重排特别有趣。通过溶解[CpRu（NCCH 3）3][PF 6][11]和bpy（各1.0当量）容易地评估该策略的可行性。
Among the wide variety of synthetic methods allowing the construction of complex molecular structures, one of the best documented is the attack of a nucleophile at an allyl metal fragment, yielding chiral allylic compounds in (virtually) enantiopure form. One of the benefits of such substitution is that the regioselectivity of the reaction with unsymmetrical allyl substrates can be controlled by the metal catalyst.[1] In this respect, several ruthenium-based compounds, in particular Cp* Ru derivatives (Cp*= C5Me5), have proven to be largely effective for the introduction of nucleophiles at the more substituted position, leading to branched (b) rather than linear (l) products [Eq.(1)].[2] Typical substrates are allyl carbonates and allyl chlorides (primary or secondary), and efficient allylic alkylation, arylation, amination, and etherification reactions have been developed.[3–6] Although useful, these reactions are, however, usually performed with large amounts of ruthenium catalysts (ca. 10 mol%), and the active complexes are not recovered at the end of the reaction owing to their sensitivity to moisture, oxygen, and isolation conditions. To our knowledge, only Bruneau, Renaud, and co-workers have been able to perform multiple catalytic cycles by immobilizing a cationic Cp* Ru catalyst in an ionic liquid phase.[7] Cyclopentadienyl Ru derivatives that would be 1) air-insensitive, 2) easily synthesized and purified, 3) catalytically active, and 4) possibly reusable would thus be important novelties in this field. Herein, we report that such species can indeed be made through the use of an anionic monodentate ligand. Several beneficial aspects are presented, including improved regioand enantioselectivity in the Carroll rearrangement of allyl-bketoesters in direct comparison studies with the classical [PF6] À analogues of the catalytic species. Allyl-b-ketoesters of type 1 [Eq.(2)] are known to react smoothly in the presence of cyclopentadienyl Ru moieties and2, 2’-bipyridine (bpy) to form g, d-unsaturated ketones (eg 2) in high yields and excellent b/l ratios.[6] Recently, an enantioselective version of this Carroll rearrangement was developed that required the combined use of [CpRu (NCCH3) 3][PF6] and enantiopure pyridine imine ligands (Cp= C5H5).[8] These allylic substitution reactions, utilizing rather large amounts of non-recoverable ruthenium catalysts (10mol%), thus constituted an ideal framework for the purposed study. Recently, a novel hexacoordinated phosphorus anion, TRISPHAT-N (3, Figure 1), was also described and demonstrated to be an efficient counterion and ligand altogether able to enter the first coordination sphere of metal complexes.[9, 10] Furthermore, it was shown that the derived zwitterionic metal complexes possess interesting chromatographic, air, and moisture stability. The possibility that 3 would form a stable complex with the mixture of [CpRu-(NCCH3) 3][PF6] and bpy was therefore particularly interesting for the Carroll rearrangement. The viability of this strategy was readily assessed by dissolving [CpRu (NCCH3) 3][PF6][11] and bpy (1.0 equiv each)