= Solution
The <carbamate> <carbonyl> <oxygen> attacks electrophilic <sulfur> of <triflic anhydride>, with cleavage of the anhydride S–O bond to release <triflate>. Donation of the <nitrogen> <lone pair> toward the <carbonyl> <carbon> then gives the indicated imidoyl-type cation: the former <carbonyl> <oxygen> now bears Tf, and the N–H <nitrogen> participates in a C=N$^+$ bond. This converts a relatively unreactive <carbamate> <carbonyl> into a strongly electrophilic <carbon> bearing an excellent leaving group.
The aromatic pi bond para to ethoxy attacks that <carbon>. The C=N pi <electrons> move to <nitrogen> as the new aryl–<carbon> bond forms. Loss of the aromatic <proton> restores <aromaticity>; <nitrogen> donation and departure of the O-triflylated group restore the imine bond. <proton> transfers during the sequence and carbonate workup remove the remaining N–H <proton>. The surviving five-membered ring is an oxazoline: \b[the product is 2-(4-ethoxyphenyl)-<2-Oxazoline>]. An equivalent arrow order can eliminate the triflyloxy leaving group before arene capture; these are limiting descriptions of the activated <electrophile>, not a need to postulate an unactivated <carbamate> undergoing ordinary Friedel–Crafts acylation.
Ethoxy donates a <lone pair> into the ring, stabilizing the ortho and para <Wheland intermediates>. The para pathway avoids the crowding of an incoming heterocycle beside ethoxy, giving the observed position. Mechanistic experiments on this transformation include in situ <carbon> <NMR> of the activated reagent, competition experiments and isotope-effect measurements supporting <electrophilic aromatic substitution>; they do not directly image every intermediate.<Simple and Efficient Aromatic C–H Oxazolination by Qiu Shi and others>[2023, Figure 7 and supporting information, product 3a and electrophilic-substitution evidence]{ref}
\Image[/past-exam-of-the-chemistry-b-course-of-the-university-of-cambridge/2024/paper-1-oxazoline.png]
{title=Activated cyclic carbamate and para-substituted oxazoline products}
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