Unusual carbon shielding effects of cyclopropanes and double bonds in strained bicyclo[3.1.0]hexanes and cyclopentenes

✍ Scribed by Manfred Christl; Rainer Herbert

• Book ID: 102528274
• Publisher: John Wiley and Sons
• Year: 1979
• Tongue: English
• Weight: 291 KB
• Volume: 12
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/mrc.1270120305

✦ Synopsis

Abstract

Carbon‐13 shieldings and one‐bond ^13^CH coupling constants of bicyclo[2.1.1]hexane, bicyclo[2.1.1]hex‐2‐ene, tricyclo[3.1.1.0^2,4^]heptane and benzvalene are presented and compared to the data of related compounds. If a bicyclo[3.1.0]hexane system is part of a rigid skeleton, the cyclopropane ring exerts specific γ substituent effects of two kinds. In the case of the bicyclohexane boat form an upfield shift of the C‐3 signal is observed and in the case of the chair form a downfield shift of 15–20 ppm. Compared to the corresponding cyclopentanes the double bond in strained cyclopentenes causes downfield shifts of the C‐4 absorption. This effect increases with increasing strain, reaching a 45.9 ppm maximum in benzvalene. Hence it is the only known bicyclo[1.1.0]butane having a reversed order of carbon shieldings. The downfield shifts are explained by means of simple orbital interaction schemes.