Palladium-catalyzed cross-coupling reactions of organosilanols and their salts: practical alternatives to boron- and tin-based methods.
Palladium-catalyzed cross-coupling reactions of organosilanols and their salts: practical alternatives to boron- and tin-based methods.
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DOI：
10.1021/ar800037p
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发表时间：
2008-11-18
影响因子：
18.3
通讯作者：
Regens CS
中科院分区：
文献类型：
--
作者：
Denmark SE;Regens CS
在形成碳-碳和碳杂原子键的现代合成方法中，过渡金属催化的有机金属亲核试剂与有机亲电试剂的交叉偶联占有突出的地位。这些反应的制备效用在很大程度上是各种各样的有机金属供体被征召投入使用的结果。这些试剂中最常见的是锡、硼和锌的有机衍生物，它们各自具有独特的优点和缺点。近年来，有机硅烷因其低成本、低毒性和高化学稳定性而成为传统试剂的可行替代品。然而，与不需要活化的锡基和锌基反应或只需要用温和碱加热的硼基反应不同，硅基交叉偶联反应通常需要在氟化物源存在的情况下加热；这极大地阻碍了有机硅烷的广泛接受。为了解决“氟化物问题”，我们引入了钯催化的硅基交叉偶联反应的新范式，该反应采用了有机硅醇，这是一种以前未充分利用的硅试剂。使用有机硅烷醇，无论是在br - nsted碱的存在下，还是作为它们的硅烷酸盐，代表了一种简单而温和的替代经典的氟基活化方法。有机硅醇很容易通过许多成熟的方法在烯烃、炔、芳烃和杂芳烃上引入碳硅键。此外，我们还开发了四种不同的方法来生成乙烯基、烯基、炔基、芳基和杂芳基硅烷酸酯的碱金属盐：(1)弱Brønsted碱的可逆去质子化，(2)强Brønsted碱的不可逆去质子化，(3)不可逆去质子化中盐的分离，以及(4)硅烷酸酯与二硅氧烷交换。我们已经为许多不同的耦合类展示了这些方法的优点。这个新工艺的定义特征是形成共价连接的硅酸钯物种，促进了关键的转化步骤。我们通过x射线分析，以及在无添加剂的情况下证明其在热交叉耦合中的能力，验证了含有关键Si-O-Pd键的关键物质的中间体。我们的结论与长期以来的教条相矛盾，即硅基交叉偶联反应需要在金属转化之前生成五配位硅酸盐。这一发现为发现涉及这一关键过程的新反应开辟了新的前景。
In the panoply of modern synthetic methods for forming carbon-carbon and carbon-heteroatom bonds, the transition metal-catalyzed cross coupling of organometallic nucleophiles with organic electrophiles enjoys a preeminent status. The preparative utility of these reactions is, in large measure, a consequence of the wide variety of organometallic donors that have been conscripted into service. The most common of these reagents are organic derivatives of tin, boron, and zinc, which each possess unique advantages and shortcomings. Because of their low cost, low toxicity, and high chemical stability, organosilanes have emerged as viable alternatives to the conventional reagents in recent years. However, unlike the tin- and zinc-based reactions that require no activation or the boron-based reactions that require only heating with mild bases, silicon-based cross-coupling reactions often require heating in the presence of a fluoride source; this has significantly hampered the widespread acceptance of organosilanes. To address the “fluoride problem”, we have introduced a new paradigm for palladium-catalyzed, silicon-based cross-coupling reactions that employs organosilanols, a previously underutilized class of silicon reagents. The use of organosilanols either in the presence of Brønsted bases or as their silanolate salts represents a simple and mild alternative to the classic fluoride-based activation method. Organosilanols are easily available by many well-established methods for introducing carbon-silicon bonds onto alkenes, alkynes and arenes, and heteroarenes. Moreover, we have developed four different protocols for the generation of alkali metal salts of, vinyl-, alkenyl-, alkynyl-, aryl-, and heteroarylsilanolates: (1) reversible deprotonation with weak Brønsted bases, (2) irreversible deprotonation with strong Brønsted bases, (3) isolation of the salts from irreversible deprotonation, and (4) silanolate exchange with disiloxanes. We have demonstrated the advantages of each of these methods for a number of different coupling classes. The defining feature of this new process is the formation of a covalently linked palladium silanolate species that facilitates the critical transmetalation step. We have verified the intermediacy of a critical species that contains the key Si-O-Pd linkage by its identification as the resting state in reaction mixtures, by X-ray analysis, and by demonstrating its competence in thermal cross-coupling with no additives. Our conclusions contradict the long-standing dogma that silicon-based cross-coupling reactions require the generation of a pentacoordinate siliconate prior to transmetalation. This revelation has opened a new vista for discovery of new reactions that involve this critical process.
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