N‐Heterocyclic Carbene Analogues of Thiele and Chichibabin Hydrocarbons. (18th April 2018) - Rottschäfer, Dennis
Abstract: Stable N‐heterocyclic carbene analogues of Thiele and Chichibabin hydrocarbons, [(IPr)(C6 H4 )(IPr)] and [(IPr)(C6 H4 )2 (IPr)] (4 and5, respectively; IPr=C{N(2, 6‐ i Pr2 C6 H3 )}2 CHCH), are reported. In a nickel‐catalyzed double carbenylation of 1, 4‐Br2 C6 H4 and 4, 4′‐Br2 (C6 H4 )2 with IPr (1 ), [(IPr)(C6 H4 )(IPr)](Br)2 (2 ) and [(IPr)(C6 H4 )2 (IPr)](Br)2 (3 ) were generated, which respectively afforded4 and5 as crystalline solids upon reduction with KC8 . Experimental and computational studies support the semiquinoidal nature of5 with a small singlet−triplet energy gap Δ E S−T of 10.7 kcal mol −1, whereas4 features more quinoidal character with a rather large Δ E S−T of 25.6 kcal mol −1 . In view of the low Δ E S−T, 4 and5 may be described as biradicaloids. Moreover, 5 has considerable (41 %) diradical character.