By flanking a carbene carbon with two substituents, it is possible to synthesize persistent triplet carbenes and isolable singlet carbenes. Isolable singlet carbenes are among the most powerful tools in chemistry, and they have even found medicinal and materials science applications. Between the rich chemistry of disubstituted carbenes and the transient parent carbene are the monosubstituted carbenes that, so far, have only been observed in matrices at very low temperatures of just a few K. Herein, we describe the synthesis of a crystalline monosubstituted carbene. The key for isolating such a species was to design the correct substituent, namely a benzo[c]pyrrolidino heterocycle, which can single-handedly tame the intrinsic tendency of carbenes towards dimerization. The π-donor ability of the nitrogen atom, coupled with the steric bulk of chemically inert substituents at the two adjacent quaternary carbons, make these scaffolds very attractive for the isolation of a variety of other hitherto elusive electron-deficient species.
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Crystallographic data for the structures reported in this article have been deposited at the Cambridge Crystallographic Data Centre under deposition numbers CCDC 1814366 (2c), 1814371 (3c), 1814368 ((3c)Rh(cod)Cl), 1814370 (4b), 1835936 (5), 1814367 (7), 1814369 (8), 1814372 (9) and 1835937 (10). Copies of the data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif. All other data supporting the findings of this study are available within the article and its Supplementary Information, or from the corresponding author upon reasonable request.
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Thanks are due to the NSF (CHE-1661518) for financial support of this work, and the Japan Society for the Promotion of Science for a Postdoctoral Fellowship for Study Abroad (to R.N.). We are grateful to the W. M. Keck Foundation for funding the Keck II computer centre, and A. L. Rheingold, M. Gembicky and C. E. Moore (X-ray diffraction).
The authors declare no competing interests.
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Synthetic procedures, physical and spectroscopic properties for all compounds described in the manuscript, a copy of their NMR spectra, crystallographic data and structure refinements for compounds 2c, 3c, (3c)Rh(cod)Cl, 4b, 5, 7, 8, 9 and 10, and the Cartesian coordinates of computational structures
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Nakano, R., Jazzar, R. & Bertrand, G. A crystalline monosubstituted carbene. Nature Chem 10, 1196–1200 (2018). https://doi.org/10.1038/s41557-018-0153-1
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