涉及还原性自由基-极性交叉(RRPCO)的光催化反应产生具有碳阴离子反应性的中间体。这些提议的中间体中的许多类似于高反应性有机金属化合物。然而,它们的分离的有机金属形式通常不能容忍它们的形成条件,并且经常观察到不同的反应性。我们对它们在常用光催化条件下的性质和反应性的研究表明,这些中间体确实被最好地描述为游离的,能够去质子化常见极性溶剂的超碱性碳阴离子通常被认为是惰性的,如乙腈,二甲基甲酰胺,还有二甲基亚砜.它们的碱性不仅对溶剂,而且对亲电子试剂,如醛,酮,和酯类,与气相中分离的碳阴离子的反应性相当。以前不成功的转化被认为是由于缺乏反应性而导致的,原因是它们对溶剂的高反应性和反应伙伴的弱酸性质子。提供了对光催化产生的碳阴离子的作用方式的直观解释,这使得方法能够验证所提出的涉及RRPCO步骤的反应机制,并确定当前方法的限制的原因。
Photocatalytic reactions involving a reductive radical-polar crossover (RRPCO) generate intermediates with carbanionic reactivity. Many of these proposed intermediates resemble highly reactive organometallic compounds. However, conditions of their formation are generally not tolerated by their isolated organometallic versions and often a different reactivity is observed. Our investigations on their nature and reactivity under commonly used photocatalytic conditions demonstrate that these intermediates are indeed best described as free, superbasic
carbanions capable of deprotonating common polar solvents usually assumed to be inert such as acetonitrile, dimethylformamide, and dimethylsulfoxide. Their basicity not only towards solvents but also towards electrophiles, such as aldehydes, ketones, and esters, is comparable to the reactivity of isolated
carbanions in the gas-phase. Previously unsuccessful transformations thought to result from a lack of reactivity are explained by their high reactivity towards the solvent and weakly acidic protons of reaction partners. An intuitive explanation for the mode of action of photocatalytically generated
carbanions is provided, which enables methods to verify reaction mechanisms proposed to involve an RRPCO step and to identify the reasons for the limitations of current methods.