Question

The major product formed when the \(\,o\)-(\(\mathrm{Me_3Si}\))-aryl triflate (a Kobayashi benzyne precursor) is treated with Ph–substituted cyclohexene in the presence of \(\mathrm{CsF}\) in \(\mathrm{CH_3CN}\) at r.t. is (A)–(D) as shown in the figure. 

 

Hint:

\(o\)-Silyl aryl triflate + F\(^{-}\) \(⇒\) benzyne (Kobayashi conditions).
With allylic H alkenes, expect an ene reaction: H transfer + C–C bond formation + alkene migration.
Choose the site that best stabilizes positive charge in the TS (often the \(\mathrm{Ph}\)-substituted allylic carbon) to predict geminal vs vicinal outcomes.

Answer 1

Step 1: \(\mathrm{CsF}\) activates the \(o\)-silyl aryl triflate by forming a strong \(\mathrm{Si\!-\!F}\) bond and expelling \(\mathrm{OTf^-}\), generating benzyne (\(o\)-aryne).
Step 2: Benzyne reacts with alkenes bearing an allylic H via an ene reaction (not a Diels–Alder in this case). In the concerted TS, (i) the allylic \(\mathrm{C_{allyl}-H}\) migrates to one benzyne carbon, (ii) a new C–C bond forms between the allylic carbon and the other benzyne carbon, and (iii) the alkene shifts position.
Step 3: Regiochemistry is controlled by stabilization of the developing cationic character at the allylic carbon in the ene TS. Bond formation at the allylic carbon already substituted with Ph is favored because the benzylic/aryl group stabilizes the partial positive charge by resonance/induction. Thus the new aryl group (from benzyne) is introduced at the same carbon, giving a geminal diaryl center.
Step 4: The resultant product therefore bears two phenyl groups on the same allylic carbon with the C=C bond migrated within the cyclohexene ring—exactly the connectivity drawn in option (A). Alternatives (B) and (D) (1,3- or 1,2-diphenyl) arise from disfavored regiochemical courses, and (C) does not match the ene-reaction connectivity.