Substituent effects on cyclonona‐3,5,7‐trienylidenes: a quest for stable carbenes at density functional theory level

Nine boat‐shaped cyclonona‐3,5,7‐trienylidenes are compared and contrasted with respect to their multiplicity, nucleophilicity, electrophilicity, band gap (Δ E HOMO − LUMO ), Natural bond orbital (NBO) atomic charge, force constant, as well as the aptitude for dimerization , and rearrangement through proper isodesmic reactions at B3LYP/AUG‐cc‐pVTZ and B3LYP/6‐311++G**//B3LYP/6‐31+G* levels of theory. The nine cyclic carbenes include unsubstituted (1 CH2 ) plus eight α‐cyclopropylcyclonona‐3,5,7‐trienylidenes, which are substituted with ά‐SiMe 2 , ά‐NMe, ά‐PMe, ά‐O, ά‐S, ά‐CH 2 , ά‐cyclopropyl, and ά‐CMe 2 (2 SiMe2 , 2 NMe , 2 PMe , 2 O , 2 S , 2 CH2 , 2 cyclopropyl , and 2 CMe2 , respectively). The latter eight species enjoy the stabilizing interaction of the occupied Walsh orbital of cyclopropyl with the vacant p π orbital of the carbene center ( Walsh cyclopropyl → p π carbene ). Among them, the singlet closed shell 2 NMe appears the most promising for exhibiting the highest relative singlet–triplet energy gap (Δ E s − t  = 27.1 kcal mol −1 ). In contrast, the least stable derivative is triplet 2 SiMe2 , which exhibits the lowest relative Δ E s − t of −5.5 kcal mol −1 . The overall trend of Δ E s‐t is 2 NMe  > 2 PMe  > 2 S  > 2 O  > 2 cyclopropyl  > 2 CMe2  > 2 CH2  > 1 CH2  > 2 SiMe2 . With one negative force constant, the unsubstituted 1 CH2 turns out to be a transition state, whereas the rest emerge as minima. Copyright © 2015 John Wiley & Sons, Ltd.