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2,2,9,9‐Tetramethylcyclonona‐3,5,7‐trienylidene vs . its heterocyclic analogues: A quest for stable carbenes at DFT
Author(s) -
Kassaee Mohammad Zaman,
Koohi Maryam,
Mohammadi Reza,
Ghavami Monireh
Publication year - 2013
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.3189
Subject(s) - chemistry , singlet state , isodesmic reaction , heteroatom , alkyl , crystallography , ground state , computational chemistry , proton affinity , stereochemistry , density functional theory , medicinal chemistry , protonation , organic chemistry , excited state , ion , physics , quantum mechanics , nuclear physics
Replacement of α‐methylenes with BH, AlH, CMe 2 , SiH 2 , NH, NMe, N t Butyl, NPh, PH, O, and S in non‐planar cyclonona‐3,5,7‐trienylidene (CH 2 ) alters its status from an unstable transition state to rather stable minima, at B3LYP/6‐311++G**//B3LYP/6‐31 + G* levels of theory. All species appear with singlet closed shell (S cs ) global minima, except for SiH 2 and CH 2 which exhibit triplet electronic ground states. The order of stability based on singlet–triplet energy gap (Δ E s–t / kcalmol −1 ) is: CMe 2 (45.8) > NH (35.8) > NMe (32.3) > O (31.5) > N t Butyl (27.7) ≥ NPh (27.5) ≥ BH (27.4) > S (21.9) > PH (17.0) > CH 2 (−4.4) > SiH 2 (−12.5). In contrast to many reports on N ‐heterocyclic carbenes, here alkyl groups appear to exert a higher stabilizing effect than heteroatoms, making CMe 2 the most stable. In addition bulky NMe, N t Butyl, and NPh appear more nucleophilic than their synthesized imidazol‐2‐ylidene congeners. Excluding SiH 2 , isodesmic reactions reveal that all substituents stabilize singlet state considerably more than the corresponding triplet. Finally, this work is hoped to pave the path for future matrix isolations and IR studies of these rather stable cyclic non‐planar carbenes. Copyright © 2013 John Wiley & Sons, Ltd.