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, 128 (7), 2452-62

Experimental and Computational Studies on the Mechanism of N-heterocycle C-H Activation by Rh(I)

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Experimental and Computational Studies on the Mechanism of N-heterocycle C-H Activation by Rh(I)

Sean H Wiedemann et al. J Am Chem Soc.

Abstract

Evidence is presented for a proposed mechanism of C-H activation of 3-methyl-3,4-dihydroquinazoline (1) by (PCy(3))(2)RhCl. One intermediate (3), a coordination complex of 1 with (PCy(3))(2)RhCl, was identified along the path to the Rh-N-heterocyclic carbene product of this reaction (2). Isotopic labeling and reaction-rate studies were used to demonstrate that C-H activation takes place intramolecularly on the reaction coordinate between 3 and 2. Computational studies corroborate the proposed mechanism and suggest that the rate-limiting step is oxidative addition of the C-H bond to the metal center. The consequences of this mechanism for coupling reactions of N-heterocycles that occur via Rh-catalyzed C-H bond activation are discussed.

Figures

Figure 1
Figure 1
X-ray crystal structure (ORTEP diagram) of 2.
Figure 2
Figure 2
Calculated structure for 5 showing Rh-H “pre-agostic” interaction.
Figure 3
Figure 3
Plot of concentration vs. time for the species present during the reaction of 1 with Rh(I)/PCy3 at 36.7 °C. The solid curves depict kinetics simulations of the data (Path A: k1 = 1.0 ± 0.2 × 10−2 M−1s−1, k−1 = 2.0 ± 0.2 × 10−4 s−1, kd = 1.10 ± 0.05 × 10−5 s−1).
Figure 4
Figure 4
Plot of [2] vs. time from Figure 3, enlarged to show fits for two possible reaction pathways depicted in Scheme 3. (Path A: k2 = 1.809 ± 0.003 × 10−5 s−1).
Figure 5
Figure 5
Graph of k2 vs. [free PCy3] (at 62.7 °C). The curve depicts a linear least-squares fit k2 = a[PCy3] + b (a = 4.7 × 10−5 , b = 4.4 × 10−4). The filled point is provided for reference only (see Supporting Information for details), and is not included in the linear fit.
Figure 6
Figure 6
Eyring plot of 2 formation (k2) in the presence of 13 mM excess PCy3 between 42.2 °C and 72.9 °C. The curve depicts a linear least-squares fit to ln(k2/T) = a[1/T] + b (a = −1.31 ± 0.01 × 104, b = 18.6 ± 0.4).
Figure 7
Figure 7
Calculated reaction coordinates for C-H activation (zero-point calculations, 298 K).
Scheme 1
Scheme 1
The proposed mechanism for rhodium-catalyzed benzimidazole/alkene coupling.
Scheme 2
Scheme 2
Synthesis of isotopically labeled Rh-complexes.
Scheme 3
Scheme 3
Possible mechanisms for carbene formation via C-H activation.
Scheme 4
Scheme 4
Proposed mechanism for isotopic scrambling of C2-D-3.
Scheme 5
Scheme 5
Equilibria governing [(PCy3)2RhCl].
Scheme 6
Scheme 6
Proposed mechanism of C-H activation of 1.

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