This Review examines parts per million (ppm) palladium concentrations in catalytic cross-coupling reactions and their relationship with mole percentage (mol %). Most studies in catalytic cross-coupling chemistry have historically focused on the concentration ratio between (pre)catalyst and the limiting reagent (substrate), expressed as mol %. Several recent papers have outlined the use of “ppm level” palladium as an alternative means of describing catalytic cross-coupling reaction systems. This led us to delve deeper into the literature to assess whether “ppm level” palladium is a practically useful descriptor of catalyst quantities in palladium-catalyzed cross-coupling reactions. Indeed, we conjectured that many reactions could, unknowingly, have employed low “ppm levels” of palladium (pre)catalyst, and generally, what would the spread of ppm palladium look like across a selection of studies reported across the vast array of the cross-coupling chemistry literature. In a few selected examples, we have examined other metal catalyst systems for comparison with palladium.

中文翻译：

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Pd 催化的交叉偶联：关于催化剂数量描述符 mol % 和 ppm 的重要性

本综述研究了催化交叉偶联反应中的百万分之一 (ppm) 钯浓度及其与摩尔百分比 (mol %) 的关系。催化交叉偶联化学中的大多数研究历来都集中在（预）催化剂和限制试剂（底物）之间的浓度比，以 mol % 表示。最近的几篇论文概述了使用“ppm 级”钯作为描述催化交叉偶联反应系统的替代方法。这促使我们深入研究文献，以评估“ppm 水平”钯是否是钯催化交叉偶联反应中催化剂量的实际有用描述。事实上，我们推测许多反应可能在不知不觉中使用了低“ppm 水平”的钯（预）催化剂，通常，在大量交叉偶联化学文献中报告的一系列研究中，ppm 钯的分布会是什么样子。在几个选定的例子中，我们检查了其他金属催化剂体系以与钯进行比较。