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.
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\(o\)-Silyl aryl triflate + F\(^{-}\) \(\Rightarrow\) benzyne (Kobayashi conditions).
With allylic H alkenes, expect an \textbf{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 \emph{geminal} vs \emph{vicinal} outcomes.
Step 1: \(\mathrm{CsF}\) activates the \(o\)-silyl aryl triflate by forming a strong \(\mathrm{Si\!-\!F}\) bond and expelling \(\mathrm{OTf^-}\), generating \emph{benzyne} (\(o\)-aryne). Step 2: Benzyne reacts with alkenes bearing an allylic H via an \emph{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 \emph{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 \emph{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.
