Q6(b). Acetolysis of threo- and erythro-3-phenyl-2-butyl tosylates

Introduction. The phenyl group on C-3 is anti-periplanar to the leaving OTs on C-2 and acts as a neighbouring group: its π electrons push out TsO⁻ from the back (anchimeric assistance) and form a bridged phenonium ion. Acetic acid then opens the three-membered ring from the back at C-2 or at C-3. Two inversions mean overall retention at the carbon attacked (Cram).

Scheme: Threo: the two opening paths give the two enantiomers, 50: 50: racemic threo acetate.

Threo isomer

1. In the conformation where Ph and OTs are anti, the two methyls of threo-tosylate end up on the same side of the phenonium ring: the bridged ion has a mirror plane (it is meso-like, achiral).
2. AcOH attacks C-2 or C-3 with equal probability (the two carbons are mirror images).
3. Attack at C-2 gives threo acetate with retention; attack at C-3 gives its enantiomer. An optically active threo tosylate therefore gives racemic threo acetate (Cram: about 96% threo, racemic; only about 4% erythro).

Erythro isomer

Scheme: Erythro: both opening paths give the same molecule: optically active erythro acetate, retention.

1. Here the methyls are on opposite sides of the phenonium ring; the ion has a C₂ axis and no mirror plane: it is chiral.
2. Attack at C-2 or at C-3 gives the same compound (the C₂ axis makes the two carbons equivalent).
3. Product: erythro acetate, optically active, with retention of configuration.

Evidence for the pathway

Without phenyl participation (open carbocation or SN2) the configuration at C-2 would be lost or inverted and erythro/threo mixtures would form. The retained erythro/threo identity, racemisation of threo but not of erythro, are explained only by the bridged phenonium ion. Some alkenes (2-phenylbut-2-ene) form by elimination as side products.

Conclusion. Threo tosylate → achiral phenonium ion → racemic threo-3-phenyl-2-butyl acetate. Erythro tosylate → chiral phenonium ion → optically active erythro acetate (retention). Both result from double inversion through the phenyl-bridged ion.

Application. Phenonium and other bridged ions explain retention and scrambling in solvolysis of β-aryl systems and in biosynthetic aryl migrations (NIH shift).