Cyclopropenium Ion

Dedicated to my honorable mentor Prof. L. D. S. Yadav

Introduction

The cyclopropenium ion, an aromatic cation, is the smallest member of the Hückel aromatic systems. It was first prepared by Breslow in 1957 as triphenylcyclopropenium perchlorate.[ ¹ ] Despite the apparent inherent strain in the cyclopropene ring, cyclopropenium ions possess considerable thermodynamic stability and exhibit the dual properties of stability and ionic charge. Thus, they easily combine with an anion or a Lewis basic heteroatom leading to the reversible generation of the corresponding neutral carbocyclic species (Scheme [1]). Owing to their unique reactivity, cyclopropenium ions, especially 2,3-diarylcyclopropenium ions, have been strategically exploited for the activation of the hydroxyl group of alcohols, carboxylic acids, oximes, and diols in various synthetically useful dehydrative processes to afford alkyl chlorides,[2] [3] acid chlorides,[ ⁴ ] amides/lactams,[ ^5,6 ] and cyclic ethers, respectively.[ ⁷ ]

Scheme 1 Cyclopropenium ion

• References

• 1 Breslow R. J. Am. Chem. Soc. 1957; 79: 5318
• 2 Kelly BD, Lambert TH. J. Am. Chem. Soc. 2009; 131: 13930
• 3 Vanos CM, Lambert TH. Angew. Chem. Int. Ed. 2011; 50: 12222
• 4 Hardee DJ, Kpvalchuke L, Lambert TH. J. Am. Chem. Soc. 2010; 132: 5002
• 5 Srivastava VP, Patel R, Garima Yadav LD. S. Chem. Commun. 2010; 46: 5808
• 6 Vanos CM, Lambert TH. Chem. Sci. 2010; 1: 705
• 7 Kelly BD, Lambert TH. Org. Lett. 2011; 13: 740
• 8 Perkins WC, Wadsworth DH. Synthesis 1972; 205
• 9 Nogueira JM, Nguyen SH, Bennett CS. Org. Lett. 2011; 13: 2814