Electronic Stabilization of Ground State Triplet Carbenes

Abstract

Based on systematic ab initio (CCSD(T)/cc-pVDZ) studies of substituent effects, we present a concept for the construction of electronically stabilized triplet ground state carbenes with singlet-triplet energy separations (DeltaEST) exceeding that of methylene. Sterically demanding and conjugating substituents were excluded from the selection of model compounds under investigation, as these either destabilize both the singlet and the triplet states or delocalize unpaired spins away from the carbene carbon. Negative partial charges on the carbene center allow for the prediction of the electronic stabilization of substituted carbenes. To decrease carbene reactivity, we chose beta-substituents with strong polar bonds. Among them, highly electronegative elements such as fluorine and oxygen enlarge the DeltaEST value with respect to hydrogen, while chlorine does not due to p-orbital participation.

Persistent Triplet Carbenes

“…33 The group electronegativity of CF 3 is 2.98 Pauling unit, 43 which is larger than that of CH 3 (2.47 Pauling unit). Substitution of a CF 3 group on methylene increases ∆E ST from 10.5 to 13.5 kcal/mol.…”

“…42 Electronic stabilization of the triplet state has been examined recently by using ab initio [CCSD(T)/cc-pVDZ] methods for a series of carbenes (21-27) symmetrically substituted with electropositive atoms (Si, B, P, and S) at the R-position (Table 7). 33 ∆E ST values do not strictly follow the electronegativities of the R-atom, since contributions from other factors are also included. However, in accordance with Si's electronegativity, silylcarbenes (26) exhibit the largest splittings among the model carbenes.…”

“…Singlet-Triplet Energy Separation for Carbenes Symmetrically Substituted with Electropositive Atoms a,33 …”

“…Reaction Energies (∆E S for Overall Singlet, ∆E T for Overall Triplet Multiplicity) of Isodesmic Eq 6 at B3LYP/ 6-311++G(d,p)33 …”

Synthesis of azasilacyclopentenes and silanols via Huisgen cycloaddition-initiated C–H bond insertion cascades

“…25 For simple silicon-substituted carbenes like 44 , the triplet state is significantly stabilized relative to the singlet state. 27,28 1,2-Rearrangements to silenes are the most common outcomes for these compounds, 29 though cyclopropanations and C–H insertions have also been observed in suitable substrates (see 47 and 51 ). Intramolecular cyclopropanations appear to be possible with silyl carbenes of either a triplet or singlet nature if pendant olefins are present ( 47 and 49 , respectively).…”