Q1(b). Solvolysis of anti- and syn-7-norbornenyl tosylates

Introduction. Both are secondary 7-norbornenyl tosylates; they differ only in the side of the C-7 bridge that carries OTs. In M the OTs is anti to the C2=C3 double bond (pointing away from it); in N it is syn (over the double bond). Acetolysis is SN1-like, and its rate depends on which neighbouring group can help push out TsO⁻ from the back side.

Reaction 1: M (anti) and π participation

The filled π orbital of C2=C3 lies exactly behind the C7–OTs bond. As TsO⁻ leaves, the π electrons attack C-7 from the back (anchimeric assistance), giving the bishomocyclopropenyl (non-classical) cation: two electrons shared over C-2, C-3 and C-7. Ionisation is the RDS and is strongly accelerated: M solvolyses about 10¹¹ times faster than saturated 7-norbornyl tosylate. Acetic acid can attack C-7 only from the face away from C2=C3, which is the face from which OTs left, so the product is anti-7-norbornenyl acetate with retention of configuration.

Figure: Reaction 1: the C=C attacks C-7 from the back; acetate enters on the same face that OTs left.

Reaction 2: N (syn) and σ participation

In N the double bond is on the same side as OTs, so it cannot reach the back of C-7. The bond anti-periplanar to C7–OTs is now the C1–C6 σ bond. It shifts to C-7 as TsO⁻ leaves (a Wagner–Meerwein type shift), so C-1 becomes cationic, and this cation is allylic because C-1 is next to C2=C3. The skeleton rearranges to a bicyclo[3.2.0]heptenyl cation, and acetate captures it at the allylic end to give bicyclo[3.2.0]hept-2-en-4-yl acetate (the printed product). The help from a σ bond is much smaller: N reacts only about 10⁴ times faster than the saturated tosylate.

Figure: Reaction 2: the C1–C6 bond, anti to OTs, migrates; the allylic ion takes up acetate.

Conclusion. M is about 10⁷ times more reactive than N. In M the C=C lies anti to the leaving group and gives strong π assistance (non-classical ion, retention); in N only the C1–C6 σ bond can assist, giving a slower, rearranged (bicyclo[3.2.0]) product.

Application. Winstein's 7-norbornenyl rates are the classic proof of neighbouring-group participation, the same effect that controls rearrangements of bicyclic terpene cations (camphor and borneol chemistry).