Bicyclobutane bridgehead proton

Benzvalene (18) is a tricyclic benzene isomer containing a bicyclobutane ring system bridged by an ethylene moiety its radical cation is accessible by PET or radiolysis. CIDNP indicated negative hfcs for the alkene protons (H ), strong positive hfcs for the non-allylic bridgehead protons (Hy), and negligible hfcs for the... 

Another reactivity domain in bicyclobutane is the bridgehead proton. As can be predicted from the large s character of its bond to the bridgehead carbon, this proton is relatively highly acidic. As will be shown in Section V.I, this feature is often used to prepare certain bridgehead derivatives of bicyclobutane. 

A rough correlation between the relative rates of deprotonation and the Jq h for the bridgehead protons in the series 7, 8 and 9, was observed by Gloss and Larrabee. Taking the C-H coupling constant as a measure of the acidity of hydrocarbons, the acidity of the proton in bicyclobutane is expected to be between that of acetylene and ethylene (Jq-h values for acetylene, ethylene and bicyclobutane are 248,156 and 205 Hz respectively). 

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