Stille Coupling Involving Bulky Groups Feasible with Gold Cocatalyst
Stille Coupling Involving Bulky Groups Feasible with Gold Cocatalyst
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DOI：
10.1002/anie.201209262
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发表时间：
2013-01-01
影响因子：
16.6
通讯作者：
Espinet, Pablo
中科院分区：
文献类型：
--
作者：
delPozo, Juan;Carrasco, Desiree;Espinet, Pablo
双金属催化是一种将两种或两种以上非主族金属催化剂串联起来的合成方法。[1]主族金属常作为亲核试剂。双催化的目的是在新的一锅法合成路线中利用每种催化剂的特定行为。对于Au/Pd对，已知Au/Pd金属转移，一个希望连接钯和金催化循环的步骤，在动力学上是可行的，[2，3]即使对于相当大的基团。[4]然而，到目前为止，只有少数Au/Pd催化的过程被报道，包括Sonogashira样的交叉偶联，[5]炔的carbometriquine，[6]和结合环化与交叉偶联步骤的过程。[7，8]在此，我们研究了Stille反应中Au/Pd对的电势，其中锡是参与该系统的第三种金属，从而提供了亲核试剂。经典的钯催化的Stille反应（方案1）是众所周知的、有效的和深入研究的方法。[9，10]该反应有时通过添加CuX盐来共催化。所谓的铜效应通常被认为是铜（I）介导芳基从有机锡转移到钯的结果。[11]然而，也已经表明，在某些情况下，观察到的动力学效应仅仅是因为铜（I）在溶液中螯合过量配体的能力。[12]Blum及其同事最近成功地使用金作为从锡到钯的乙烯基载体，用于炔的碳锡烷基化。[1b我们推测，使用AuXL络合物作为[PdR 1 L2]中间体的金属转移助催化剂（L在两种物种中是相同的），而不是CuX盐，我们可能产生由金（I）而不是铜（I）共催化的Stille反应。此外，如果发生这种共催化作用，我们可以避免L螯合或任何配体混乱（对于钯和金上的不同配体），从而观察到金催化剂对金属转移的非扰动效应。此外，该反应可能是一个很好的模型，检查钯和金作为助催化剂的兼容性，并探讨控制该体系的热力学和动力学参数。事实上，这些结果表明，金的中间性对于使涉及庞大锡烷的Stille反应可行至关重要。Au/Sn金属转移平衡的热力学是重要的。只有很少的报告孤立transmetalations从锡到金，反之亦然。已经报道了Ar基团从SnArMe 3（Ar= Ph、萘基或8-碘萘基）到[AuCl（EPh 3）]（E= P、As）络合物的金属转移，而SnPh（nBu）3的相同反应失败。[13]用SnAr 4或SnClAr 3（Ar= C6 F5、C6 F3 Cl 2、C6 Cl 5）进行Ar金属交换的尝试也不成功。[14]我们研究了试剂（X= Cl，I; R=乙烯基，芳基，炔基）和配体（L= PPh_3，AsPh_3）的不同组合的平衡，Keq值见表1。对于L= AsPh 3，在MeCN中测量平衡，对于L= PPh 3，在THF（络合物仅微溶于MeCN）中测量平衡，存在添加的L（L/Au= 2：1）以稳定金络合物。在室温下，大多数平衡在5分钟（对于L= AsPh 3）或24小时（对于L= PPh 3）内实现。对于具有大体积芳基的体系，需要更多的时间。数值得到了一些情况下，L= PPh 3，通过积分的两个金络合物的峰在…
There is a growing interest in bimetallic catalysis, a synthetic approach combining two or more non-main-group-metal catalysts working in tandem.[1] A main-group metal is often involved as the nucleophilic reagent. The aim of bimetallic catalysis is to take advantage of the specific behavior of each catalyst in new one-pot synthetic routes. For the Au/Pd couple it is known that the Au/Pd transmetalation, a step which is hoped to connect the palladium and the gold catalytic cycles, is kinetically feasible,[2, 3] even for fairly bulky groups.[4] However, so far only a few Au/Pd bimetallic catalyzed processes have been reported, including the Sonogashira-like cross-coupling,[5] the carbometallation of alkynes,[6] and processes combining cyclization with cross-coupling steps.[7, 8] Herein we examine the potential of the Au/Pd pair in a Stille reaction, where tin is the third metal involved in the system, thus providing the nucleophile. The classic palladium-catalyzed Stille reaction (Scheme 1) is a well-known, efficient, and deeply studied process.[9, 10] The reaction is sometimes cocatalyzed by addition of CuX salts. The so-called copper effect is frequently deemed to be the result of the copper (I) mediating the aryl transfer from organotin to palladium.[11] However, it has also been shown that in some cases the kinetic effect observed is simply because of the ability of copper (I) to sequestrate the excess ligand in solution.[12] Blum and co-workers have recently succeeded in using gold as a vinyl carrier from tin to palladium in the carbostannylation of alkynes.[1b, 6a] We speculated that using AuXL complexes as the transmetalation cocatalyst to [PdR1L2] intermediates (L is identical in both species), instead of CuX salts, we might produce a Stille reaction cocatalyzed by gold (I) instead of copper (I). Moreover, should this cocatalysis occur, we could avoid the L sequestration or any ligand scrambling (for different ligands on palladium and gold) and thus observe a nonperturbed effect of the gold catalyst on the transmetalation. Furthermore, this reaction might be a good model to check the compatibility of palladium and gold as cocatalysts, and to explore the thermodynamic and kinetic parameters controlling this bimetallic system. In fact the results show that the intermediacy of gold can be critical to making Stille reactions involving bulky stannanes feasible. The thermodynamics of the Au/Sn transmetalation equilibria are important. There are only very few reports of isolated transmetalations from tin to gold and vice versa. The transmetalation of Ar groups from SnArMe3(Ar= Ph, naphtyl, or 8-iodonaphtyl) to [AuCl (EPh3)](E= P, As) complexes has been reported, whereas the same reaction fails with SnPh (nBu) 3.[13] Attempts at performing Ar transmetalations with SnAr4 or SnClAr3 (Ar= C6F5, C6F3Cl2, C6Cl5) were also unsuccessful.[14] These results suggest that the thermodynamics of the Sn/Au transmetalation might be shifted in either direction, depending on the specific groups involved.We have studied the equilibrium for different combinations of reagents (X= Cl, I; R= vinyl, aryl, alkynyl) and ligands (L= PPh3, AsPh3), and the Keq values are shown in Table1. For L= AsPh3 the equilibria were measured in MeCN, and for L= PPh3 the equilibria were measured in THF (the complexes are only sparingly soluble in MeCN) in the presence of added L (L/Au= 2: 1) to stabilize the gold complexes. Most equilibria were achieved within 5 minutes (for L= AsPh3) or 24 hours (for L= PPh3) at room temperature. More time was needed for systems with bulky aryl groups. Numerical values were obtained for a few cases with L= PPh3, by integration of the peaks for the two gold complexes at …