Synthesis and reactivity of subvalent compounds: Part 11. Oxidation, hydrogenation and hydrolysis of stable diamino carbenes☆
Introduction
The structurally characterized imidazol-2-ylidenes reported by Arduengo et al. [1], [2] constitute the first [3] stable carbenes. They are stabilized electronically by an aromatic sextet [1], [2], [3], [4], [5] and, unlike the more recently obtained carbenes 2, [6], [7] do not require additional steric stabilization through bulky substituents on nitrogen (Scheme 1) [7].
Derivatives of 1 like the benzoanellated benzimidazol-2-ylidenes [8] and the 4-aza-imidazol-2-ylidene [9], and even the open-chain diamino carbene (iPr2N)2C: [10] have been described as stable, colorless solids. Stable aminooxy- and aminothio carbenes have been described by Alder et al. [11]. With one notable exception [1h], all stable carbenes have been described as air sensitive but the precise reason for this behavior has remained unclear.
Section snippets
Reactivity
In our previous studies on the synthesis, structure and bonding of carbenes 1 and 2 [4], [7] and the isostructural silylenes [4], [12], germylenes [4], [13], and phosphenium cations [14] the compounds were handled with inert gas techniques due to their apparent air sensitivity. It seemed obvious to assume that these subvalent compounds will readily be oxidized by O2 or other oxidizing agents. An investigation of the stable carbenes 1 and 2 has presented us with a different picture.
We found,
Computational studies
The structures (Table 2) and energies (Table 3) of the model compounds 1H–6H were calculated [24] at the B3LYP/6-31G(d) level. The final energies were corrected by the addition of zero point energies (ZPE) and, where indicated, thermal energies (298.15 K). The vibratory energies were scaled by 0.9804, the scaling factor recommended for B3LYP/6-31G(d) calculations [25].
The structures of 1H–4H are in good agreement with the experimental structures of 1–4. The equilibrium geometries of 1H–4H show
Conclusion
In the absence of catalysts, the carbenes 1 and 2 are inert towards triplet oxygen but can be oxidized to the corresponding ureas 3 and 4 in the presence of catalytic amounts of CuCl or with NO. At the B3LYP/6-31G(d) level, the oxidation of 1 and 2 is strongly exothermic: the oxygen affinity of the carbenes 1 and 2 is comparable to that of phosphines and phosphites and is substantially higher than that of CO. The air sensitivity of the stable diamino carbenes 1 and 2 is due to hydrolysis. At
Experimental
Melting points were recorded in sealed capillaries and are uncorrected. EI mass spectra were determined at an ionizing voltage of 40 eV or 70 eV as indicated. NMR-spectra were recorded on a Varian Gemini 200 (1H), a Bruker 500 MHz (13C) or a Varian 400 MHz (17O) NMR spectrometer at room temperature. The spectra are referenced versus TMS (1H and 13C, internal), H2O (17O, external) or CH3–NO2 (15N, external). IR spectra were recorded on a Nicolet FT-IR spectrometer. N,N′-di-tert
Supplementary material
Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre, CCDC nos. 140765 for compound 1, 140766 for compound 3, 141246 for compound 4 and 141247 for compound 8. Copies of this information may be obtained free of charge from The Director, Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge, CB2 1EZ, UK (Fax: +44-1223-336033; e-mail: [email protected] or www: http://www.ccdc.cam.ac.uk).
Acknowledgements
We thank the Natural Sciences and Engineering Research Council of Canada (NSERC) for support of this work through an operating grant. We thank Professor R. West and Dr S. Powell for the crystallographic investigation of 3 and 4 in the University of Wisconsin Crystallography Laboratory (supported by the National Science Foundation (NSF) through grant CHE-9105497).
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