= Solution
With an N-trityl substituent, the <carbene> attacks a nearby trityl phenyl ring. Addition across an arene bond gives a norcaradiene-like cyclopropane, followed by electrocyclic opening to the seven-membered cycloheptatriene: this is a <Buchner ring expansion>. Intramolecular trapping explains the fused product rather than an intermolecular <carbene> reaction.
With N-tert-butyl, there is no pendant arene. Instead the <carbene> inserts into a C-H bond of one tert-butyl methyl group. The new bond between the <carbene> carbon and that methyl carbon closes the five-membered N-containing ring; H is transferred to the <carbene> carbon in the same insertion. The remaining two methyl groups stay on the carbon attached to N.
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