钯催化的有机硅醇及其盐的交叉偶联反应:硼基和锡基方法的实用替代方法。
Palladium-catalyzed cross-coupling reactions of organosilanols and their salts: practical alternatives to boron- and tin-based methods.
作者信息
Denmark Scott E, Regens Christopher S
机构信息
Roger Adams Laboratory, Department of Chemistry, University of Illinois, 600 S. Mathews Avenue, Urbana, IL 61801, USA.
出版信息
Acc Chem Res. 2008 Nov 18;41(11):1486-99. doi: 10.1021/ar800037p.
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, which require no activation, or the boron-based reactions, which 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 reactions that involve this critical process.
在现代用于形成碳-碳键和碳-杂原子键的各种合成方法中,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据着卓越的地位。这些反应在制备方面的实用性,在很大程度上归因于已被投入使用的各种各样的有机金属供体。其中最常见的试剂是锡、硼和锌的有机衍生物,它们各自都有独特的优缺点。由于其低成本、低毒性和高化学稳定性,有机硅烷近年来已成为传统试剂的可行替代品。然而,与不需要活化的基于锡和锌的反应,或仅需与弱碱加热的基于硼的反应不同,基于硅的交叉偶联反应通常需要在氟化物源存在下加热;这严重阻碍了有机硅烷的广泛应用。为了解决“氟化物问题”,我们引入了一种新的钯催化的基于硅的交叉偶联反应模式,该模式采用有机硅醇,这是一类以前未充分利用的硅试剂。在布朗斯特碱存在下使用有机硅醇或使用它们的硅醇盐代表了一种简单温和的经典基于氟化物的活化方法的替代方法。通过许多成熟的将碳-硅键引入烯烃、炔烃、芳烃和杂芳烃的方法可以很容易地得到有机硅醇。此外,我们已经开发了四种不同的方法来生成乙烯基、烯基、炔基、芳基和杂芳基硅醇盐的碱金属盐:(1)用弱布朗斯特碱进行可逆去质子化,(2)用强布朗斯特碱进行不可逆去质子化,(3)从不可逆去质子化中分离盐,以及(4)与二硅氧烷进行硅醇盐交换。我们已经证明了这些方法对于许多不同偶联类型的优点。这个新过程的决定性特征是形成了一种共价连接的钯硅醇盐物种,它促进了关键的金属转移步骤。我们已经通过将其鉴定为反应混合物中的静止状态、通过X射线分析以及通过证明其在无添加剂的热交叉偶联中的能力,验证了含有关键Si-O-Pd键的关键物种的中间体性质。我们的结论与长期以来的教条相矛盾,即基于硅的交叉偶联反应在金属转移之前需要生成五配位硅酸盐。这一发现为涉及这一关键过程的反应的发现开辟了新的前景。