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Two mechanisms have been proposed for the palladium-catalyzed silylation reaction, both of which account for the formation of reduced arene byproducts. Both the C-X bond of aryl halides and the Si-H bond of silanes are known to undergo oxidative addition to Pd(0) complexes. Thus, two logical mechanisms have been proposed, one with Ar-Pd(II)-X, and the other with R3Si-Pd(II)-H as the key intermediate, respectively. Scheme 63 illustrates a reasonable mechanism for the oxidative addition of the aryl halide to Pd(0), to form Ar-Pd(II)-X complex 132. Transmetallation with the silane forms four-centered palladium intermediate 133, which in turn undergoes reductive elimination to liberate aryl silane and regenerate the Pd(0) catalyst. Support for this mechanism was provided by Chatgilialoglu, who demonstrated that organic halides underwent oxidative addition to Pd(0) catalysts more readily than hydrosilanes.255 Additionally, it was shown that the mechanism did not involve the intermediacy of carbenium ions or free-radical species. A mechanism involving oxidative addition of the hydrosilane followed by σ-bond metathesis has been proposed by Kunai and Ishikawa for the exchange of the Si-H and C-I bonds of alkyl and aryl iodides and hydrosilanes in the presence of a Pd(0) catalyst (Scheme 64).258 The mechanism for silylation begins with formation of a Si-Pd bond by oxidative addition of Pd(0) to the silane. Silylation of the aryl halide results from σ-bond metathesis, for overall Si-H/C-I bond exchange and regeneration of the Pd(0) catalyst. Both mechanisms account for the formation of dehalogenated aryl halide substrate when ortho substituents are present on the arene. Reductive elimination of silylated arene (Ar-Si(OEt)3) from 4-coordinate Pd intermediate 133 requires that the –Si(OEt)3 and the –Ar ligands adopt a cis geometry, as shown above in Scheme 64. Steric bulk on the arene will disfavor the cis conformation, and isomerization of the complex reorients the two largest substituents to the trans geometry on the palladium complex (137, Scheme 65). As a consequence, the Ar- and H- ligands are placed preferentially cis, and Ar-H is formed upon reductive elimination from palladium. Similarly, for the proposed mechanism entailing σ-bond metathesis, steric bulk on the arene will disfavor the approach of Ar-X on the R3Si-Pd(II)-H intermediate which places the large arene adjacent to the bulky –SiR3 ligand, as shown in transition state 136 in Figure 13. Instead, the less sterically demanding approach 139 should be favored, and formation of Ar-H in lieu of the desired Ar-SiR 3 results. 翻译的通顺合理，金币再增加 |
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4楼2016-01-01 00:32:36
baoshanqiu
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lotuscsx: 金币+10, 翻译EPI+1, ★★★★★最佳答案 2016-01-01 08:10:59
lotuscsx: 金币+10, 翻译EPI+1, ★★★★★最佳答案 2016-01-01 08:10:59
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Two mechanisms have been proposed for the palladium-catalyzed silylation reaction, both of which account for the formation of reduced arene byproducts. Both the C-X bond of aryl halides and the Si-H bond of silanes are known to undergo oxidative addition to Pd(0) complexes. Thus, two logical mechanisms have been proposed, one with Ar-Pd(II)-X, and the other with R3Si-Pd(II)-H as the key intermediate, respectively. 钯催化的硅烷化反应已有人提出两种机制，二者均导致形成还原性芳烃副产物。已知卤代芳烃的C-X键和硅烷的Si-H键均会氧化加成到Pd(0)复合物。因此就提出了两种合理的机制，分别以Ar-Pd(II)-X和R3Si-Pd(II)-H为关键中间产物。 |
2楼2015-12-31 23:46:19
baoshanqiu
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lotuscsx: 金币+10, ★★★★★最佳答案 2016-01-01 08:11:10
lotuscsx: 金币+10, ★★★★★最佳答案 2016-01-01 08:11:10
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Scheme 63 illustrates a reasonable mechanism for the oxidative addition of the aryl halide to Pd(0), to form Ar-Pd(II)-X complex 132. Transmetallation with the silane forms four-centered palladium intermediate 133, which in turn undergoes reductive elimination to liberate aryl silane and regenerate the Pd(0) catalyst. Support for this mechanism was provided by Chatgilialoglu, who demonstrated that organic halides underwent oxidative addition to Pd(0) catalysts more readily than hydrosilanes.255 Additionally, it was shown that the mechanism did not involve the intermediacy of carbenium ions or free-radical species. 图63所示为卤代芳烃氧化加成到Pd(0)，形成Ar-Pd(II)-X复合物132的一个合理机制。硅烷的转金属作用形成四-中心钯中间产物133，后者反过来进行还原消除释放芳基硅烷并再生成Pd(0)催化剂。Chatgilialoglu为这种机制提供了支持，他证实有机卤较之含氢硅烷类255更容易氧化加成到Pd(0)催化剂。此外，已知该机制不涉及中间碳正离子或自由基。 |
3楼2016-01-01 00:15:48
5楼2016-01-01 00:48:09