The Aluminum‐Bromide‐Catalyzed Adamantane Rearrangement of syn ‐ and anti ‐Tricyclo[4.2.1.1 2,5 ]decane

In the presence of AlBr 3 in CS 2 at temperatures below 0°, syn ‐tricyclo[4.2.1.1 2,5 ]decane ( 1 ) isomerizes exclusively to anti ‐tricyclo[4.2.1.1 2,5 ]decane ( 2 ) at a higher rate than the latter rearranges to 2‐ exo ,3‐ exo ‐trimethylene‐8,9,10‐trinorbornane ( 4 ). However, at temperature above 0°, the anti ‐isomer 2 isomerizes to 4 faster than 1 to 2 and 4 . As a consequence, hydride abstraction occurs at C(3) (→carbocation a , which rearranges to carbocation b ( anti ‐skeleton)) in the syn ‐isomer 1 , and more readily at C(9) (→carbocation c ) than at C(3) (→carbocation b ) in the anti ‐isomer 2 .