The cyclization of an alkene-bearing cyclopentanone to a [2.2.1]-norcamphor ring system is described. The reaction is catalyzed by a combination of rhodium and Brønsted acid. Control experiments indicate that both are needed for acceptable yield. Control experiments with bulky base additives show that rhodium promotes alkene isomerization, likely the first step of this cascade reaction, and that rhodium alone does not promote cyclization. Cyclization is promoted by Brønsted acid in a Prins-type cyclization and carbocation rearrangement process. Trace Brønsted acid present in commercial samples of Rh(cod)₂OTf is likely responsible for the observed reaction. Indeed, the norcamphor product can be obtained simply with strong acid, presumably initiated by acid-promoted alkene isomerization. Since our initial motivation for this work was the development of rhodium catalysts for the activation of C–C bonds adjacent to ketones, this communication serves to identify other, perhaps less obvious, pathways for the reactions of unsaturated ketone compounds by the action of rhodium catalysts.

中文翻译：

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含烯烃的环戊酮的环化：铑和布朗斯台德酸的作用

描述了将带有烯烃的环戊酮环化为[2.2.1]-正樟脑环系统。铑和布朗斯台德酸的组合催化了该反应。对照实验表明，两者都需要达到可接受的产量。使用笨重的碱添加剂的对照实验表明，铑可促进烯烃异构化，这可能是该级联反应的第一步，而单独的铑则不会促进环化。布伦斯台德酸在Prins型环化和碳正离子重排过程中促进了环化。Rh（cod）₂商业样品中存在的痕量布朗斯台德酸OTf可能是所观察到的反应的原因。实际上，正樟脑产品可以简单地用强酸获得，大概是由酸促进的烯烃异构化引发的。由于我们开展这项工作的最初动机是开发铑催化剂来活化与酮相邻的C–C键，因此这种交流有助于确定其他（也许不太明显）通过铑催化剂的作用使不饱和酮化合物反应的途径。 。