Q5(c). Simmons–Smith reaction directed by an allylic OH

Introduction. CH₂I₂ and zinc–copper couple form the Simmons–Smith carbenoid ICH₂ZnI. It transfers a CH₂ group to a C=C in one step, forming a cyclopropane. An allylic OH coordinates the zinc and delivers CH₂ to its own face.

Scheme: Product: the CH₂ bridges C-1 and C-8a on the same face as the OH.

1. Carbenoid: CH₂I₂ + Zn(Cu) → ICH₂ZnI (an organozinc, not a carbene itself).
2. Coordination: the Lewis-acidic Zn binds the hydroxyl oxygen; this holds the reagent on the face of the ring that carries OH.
3. Butterfly TS: the CH₂ group bonds to both alkene carbons at the same time while ZnI₂ leaves: a concerted, cheletropic transfer.
4. Both new C–C bonds form on the same face: syn (cis) addition, stereospecific (alkene geometry kept) and diastereoselective (OH-directed, cis to OH, even though that face may be the more hindered one).

Conclusion. Product: the cyclopropane fused at C-1/C-8a with the new CH₂ cis to OH (4a,8,8-trimethyl-octahydro-1H-cyclopropa[i]naphthalen-2-ol type). The addition is a concerted syn (cis), stereospecific, hydroxyl-directed cyclopropanation.

Application. Hydroxyl-directed Simmons–Smith cyclopropanation is used to make cyclopropane terpenes (thujopsene, carene) and cyclopropyl drug intermediates with controlled stereochemistry.