From acyclic dialkylcarbene to the unsaturated cyclic heteroatom substituted ones: A survey of stability

In regard to the worldwide interest in synthesis and application of stable carbenes, ab initio and DFT calculations (CCSD(T)/6-311G//QCISD/6-31G* and B3LYP/AUG-cc-pVTZ//B3LYP/6-31 + G* levels) are employed to reach at a series of symmetrically substituted diheteroatom carbenes. In this way the multiplicity, stability, and reactivity of diethylcarbene and its saturated and unsaturated cyclic congeners are compared and contrasted to the corresponding disilyl-, diamino-, diphosphino-, dioxy-, and dithiocarbenes. The investigations reveal the enlargement of the singlet-triplet energy gaps (ΔE _S-T) in the order of (amino ≈ oxy) > thio > phosphino > alkyl > silyl. The observed trend is thoroughly analyzed applying appropriate isodesmic reactions which differentiate the substituent effects on each of the singlet and triplet states of all the carbenes. In contrast to previous reports, it is found that π-donor/σ-acceptor amino substituents stabilize not only the singlet but also the triplet states. The effects of cyclization and unsaturation are also probed in each series of the symmetrically substituted carbenes. The reactivity of the species is discussed in terms of nucleophilicity, electrophilicity, and proton affinity.