Mapping the Potential Energy Surface of the Tolylcarbene Rearrangement in the Inner Phase of a Hemicarcerand.

Photolysis of p-tolyldiazirine (6) in the inner phase of a hemicarcerand with four butane-l,4dioxy linker groups (5) in C[sub 6]D[sub 5]CD[sub 3] at 77 K yields the 5-methylcycloheptatetraene hemicarceplex 5⊙3b in 41% together with innermolecular reaction products resulting from an insertion of transient p-tolylcarbene (1b) into an acetal C-H or linker C-O bond of 5 and from the addition of 1b to an aryl unit of 5. The yield of incarcerated 3b increased up to 67% if 6 is photolyzed inside a hemicarcerand with deuterated spanners and butane-1,4-dioxy linker groups (d[sub 48]-5). Hemicarceplex 5⊙3b is not formed if the photolysis is carried out in CDCI[sub 3]. Incarcerated 3b is stable at room temperature in the absence of oxygen and is characterized by 1D and 2D NMR spectroscopy. In the presence of oxygen, 3b reacts quantitatively to yield toluene and CO[sub 2]. Upon heating solutions of d[sub 48]-5⊙3b in C[sub 6]D[sub 5]CD[sub 3], 3b rearranges to 1b and m-tolylcarbene (18). Both tolylcarbenes immediately react with the surrounding host. From a product analysis and the measured rate constants for the thermal decomposition of d[sub 48]-5⊙3b in the temperature range 70-102 °C, the activation parameters for the 3b to 1b and 3b to 18 rearrangements are calculated (3b to 1b: ΔG[sub 373] = 27.3 ± 1.4 kcal/mol, ΔH[sub 373] = 26.4 ± 1.0 kcal/mol, TΔS[sub 373] = -0.9 ± 1.0 kcal/mol; 3b to 18: ΔG[sub 373] = 27.8 ± 1.4 kcal/mol, ΔH[sub 373] = 19.7 ± 1.0 kcal/mol, TΔS[sub 373] = 8.1 ± 1.0 kcal/mol). These values are compared with those calculated by Geise and Hadad at the B3LYP/6-311+G[sup **] level of theory (Geise, C. M.; Hadad, C. M. J. Org. Chem. 2002, 67, 2532-2540). The slightly higher inner phase activation free energy of the 3b to 18 rearrangement is explained through steric constraints imposed by the surrounding hemicarcerand on the transition state. The... [ABSTRACT FROM AUTHOR]