Bredt's rule

https://doi.org/10.1351/goldbook.B00732
A double bond cannot be placed with one terminus at the bridgehead of a bridged ring system unless the rings are large enough to accommodate the double bond without excessive strain. For example, while bicyclo[2.2.1]hept-1-ene is only capable of existence as a transient, its higher homologues having a double bond at the bridgehead position have been isolated: e.g.
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Source:
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1077 (https://doi.org/10.1351/pac199466051077)