= Solution
For the three elementary transformations just proposed, a qualitative free-energy profile has \b[three <transition states> and two intermediates]. The sugar <substituents> and <indole> C3 <substituent> stay unchanged, so the structural shorthand below suppresses them while retaining the reacting atoms.
\Image[/past-exam-of-the-chemistry-b-course-of-the-university-of-cambridge/2021/paper-2-glycosylation-energy.png]
{title=Qualitative three-step free-energy profile with intermediate and transition-state connectivity}
The first <transition state> has a partially broken anomeric C1–O(<leaving group>) bond and developing oxocarbenium character, $[\mathrm{O_{ring}-C_1^{\delta+}\cdots O_{LG}}]^\ddagger$. Its following minimum is the <glycosyloxocarbenium ion> plus counterion. The second <transition state> has a forming C1–C2 bond, $[\mathrm{O_{ring}-C_1\cdots C_2(H)_{ind}}]^\ddagger$, and developing positive charge in the <indole>. Its following minimum is the covalently joined <arenium ion> with an intact C2–H bond. The third <transition state> has simultaneous C2–H cleavage, B–H formation and restoration of the <indole> <pi bond>, $[\mathrm{B\cdots H\cdots C_2-C_3}]^\ddagger$. It leads to the aromatic C-linked product and protonated base.
The vertical separations are illustrative because no <activation free energies> or <equilibrium constants> are supplied. In particular, the highest barrier cannot be identified experimentally from this drawing alone. <protonation> of the <leaving group> is treated as an initial rapid equilibrium in the chosen mechanism; explicitly resolving it would add the corresponding proton-transfer state and intermediate.
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