Relative migratory aptitudes of doubly and triply bonded groups in a cycloheptatriene system

Abstract

In cyclopentadiene systems, 1,5-shift of triply bonded groups is much slower than that of doubly bonded groups, whereas in the cycloheptatriene systems (34) and (29), as well as in (21) and (19), CN and CHO groups have very similar migratory tendencies. In compound (31) the CCCOPh group migrates to give (38; X = CCCOPh, Y = H) at 60 °C, whilst the related olefin (20) fails to rearrange at 80 °C, and undergoes intramolecular Diels–Alder reaction to afford tetracycle (40) at 100 °C. The simple acetylene (23) rearranges to a mixture of products (24) and (39; X = CCH, Y = H) at 145 °C. The order of migratory aptitude CHO > COMe > CO₂Me observed in indenes is preserved in the benzocyclo-heptene system. The aldehyde (19) rearranges to (38; X = CHO, Y = Br) at 114 °C, the acetyl compound (22) gives a little of the 1,5-shift product (38; X = COCH₃, Y = Br) but mainly the naphthalene (43) at 175 °C, whilst the ester (17) gives mainly tricycle (44) rather than a product of 1,5-CO₂Me shift at 205 °C. The overall migratory order for the cycloheptatriene system; CHO > CN, CCH > COMe > CO₂Me > alkyl, shows a promotion of the triply bonded groups consistent with variations of strain in the bridged transition states (1) compared with (2) and (3) compared with (4).