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Rearrangement of the tricarbonyl(cycloheptatriene)iron derivative 1 to 2 and 3 took place very readily in 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) at room temperature. This finding is in striking contrast to the stability of 1 in MeOH under otherwise similar conditions. The isomerization processes which give rise to an equilibrium among 1, 2 and 3 must involve zwitterionic intermediates of the type 4, whose formation, consequently, is highly favored by HFIP. Heating 9 in HFIP brought about its rearrangement to 15. We suggest that formation of the zwitterionic intermediate 12 triggers a rearrangement of the carbocyclic moiety which can take place as a result of the high polarity and low nucleophilicity of HFIP. The rearrangement 9 → 15 could not be achieved by using other polar solvents such as methanol or nitromethane. This observation once again demonstrates how suitable HFIP is as a medium for isomerizations via zwitterionic intermediates. The role of traces of free acid, present even in purified HFIP, is discussed.

3,4-Dihydroisoquinoline-N-oxide 1 reacted readily with both acyclic (Z)- and (E)-disubstituted alkenes bearing electron-attracting substituents (methoxycarbonyl, cyano, phenylsulphonyl and benzoyl groups) and with cyclic derivatives (e.g. maleimides) to give mixtures of the two possible diastereoisomers. Similar amounts of endo - and exo -adducts were formed in the reactions of (Z)-cyano, methoxycarbonyl and benzoyl derivatives whereas exo -addition clearly won over its endo -counterpart in the case of (Z)-(phenylsulphonyl) and cyclic derivatives. High exo -selectivity was also observed in the sluggish reactions of 1 with electron-rich alkenes [(Z)-stilbene, vinylene carbonate, acenaphthylene etc.] Our results, which revise previous literature data, clearly show that an “ endo -rule” does not hold for the reactions of 1 with (Z)-1,2-disubstituted alkenes. We conclude that in these reactions repulsive steric interactions either counteract efficientlt or clearly win over stabilizing secondary orbital overlaps in controlling endo/exo -selectivity. These reactions were found reversible under mild conditions so that relative formation rates of related pairs of (Z)- and (E)-dipolarophiles in cycloreversion processes of isoxazolidines could be determined; as a rule (E)-alkenes are extruded faster then (Z)-isomers. These results provide unambiguous experimehntal evidence that increase in steric compression between the substituents in (Z)-alkenes on there way toward transition state, is not a major factor in dertermining their lower reactivity with respect to (E)-isomers in 1,3-dipolar cycloadditions.