N‐Heterocyclic Carbene Analogues of Thiele and Chichibabin Hydrocarbons

• Published in: Angewandte Chemie International Edition 2018-05, Vol.57 (20), p.5838-5842
• Main Authors: Rottschäfer, Dennis, Ho, Nga Kim T., Neumann, Beate, Stammler, Hans‐Georg, van Gastel, Maurice, Andrada, Diego M., Ghadwal, Rajendra S.
• Format: Article
• Language: English
• Subjects: Chichibabin's hydrocarbons; Computer applications; diradicaloids; diradicals; Energy gap; Hydrocarbons; N-heterocyclic carbenes; Nickel; Thiele's hydrocarbons
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.201713346

Stable N‐heterocyclic carbene analogues of Thiele and Chichibabin hydrocarbons, [(IPr)(C6H4)(IPr)] and [(IPr)(C6H4)2(IPr)] (4 and 5, respectively; IPr=C{N(2,6‐iPr2C6H3)}2CHCH), are reported. In a nickel‐catalyzed double carbenylation of 1,4‐Br2C6H4 and 4,4′‐Br2(C6H4)2 with IPr (1), [(IPr)(C6H4)(IPr)](Br)2 (2) and [(IPr)(C6H4)2(IPr)](Br)2 (3) were generated, which respectively afforded 4 and 5 as crystalline solids upon reduction with KC8. Experimental and computational studies support the semiquinoidal nature of 5 with a small singlet−triplet energy gap ΔES−T of 10.7 kcal mol−1, whereas 4 features more quinoidal character with a rather large ΔES−T of 25.6 kcal mol−1. In view of the low ΔES−T, 4 and 5 may be described as biradicaloids. Moreover, 5 has considerable (41 %) diradical character. Couple up: N‐heterocyclic carbene analogues of Thiele and Chichibabin hydrocarbons, (iii) and (iv), are accessed as stable crystalline solids by two‐electron reduction of (i) and (ii) (n=1, 2) with KC8. Calculations indicate that (iv) has considerable diradical character (41 %), while a close‐shell singlet state is the ground state for (iii) and (iv). Dipp=2,6‐diisopropylphenyl.