Reversibility of Ketone Reduction by SmI2–Water and Formation of Organosamarium Intermediates

The reduction of ketones by SmI2–water has long
been thought to proceed through a reversible initial electron transfer
with the formation of organosamarium intermediates in a follow-up
step. Kinetic experiments on the reduction of two model ketones and
structurally similar ketones with a pendant alkene are shown to be
consistent with a rate-limiting reduction by SmI2–water
through a proton-coupled electron-transfer (PCET). Literature values
for the rates of radical cyclizations and reduction of radicals by
SmI2 and thermochemical data for radical reduction by SmI2–water further support a rate-limiting initial step
for ketone reductions. These data suggest that discrete organosamarium
species may not be intermediates in ketone reductions by SmI2–water.

酮类化合物的SmI2-水催化还原反应长期以来被认为是通过可逆的初始电子转移进行，随后在后续步骤中形成有机铯中间体。对两种模型酮和具有附烯基的结构类似酮的还原动力学实验表明，这一过程与通过质子偶联电子转移（PCET）的SmI2-水催化的速率限制性还原相一致。关于自由基环化反应速率的文献值以及SmI2自由基还原的热化学数据进一步证实了酮还原的初始步骤为速率限制性步骤。这些数据表明，离散的有机铯物种可能不是SmI2-水催化酮还原反应的中间体。