Intermolecular Reactions of a Foiled Carbene with Carbonyl Compounds: The Effects of Trishomocyclopropyl Stabilization

Abstract

endo-Tricyclo[3.2.1.0(2,4)]oct-8-ylidene is a foiled carbene reaction intermediate. It was generated by thermolyzing Δ(3)-1,3,4-oxadiazoline precursors dissolved in benzaldehyde and acetophenone. The products appear to stem from direct insertion of the carbene's divalent C atom into the α-bonds of the carbonyl compounds; however, this is only superficial. The strict stereochemistry observed is due to the topologies of the reaction intermediates of the proposed two-step mechanism. Bimolecular nucleophilic addition generates bent 1,3-zwitterions. The neutral reaction intermediates undergo pinacolic rearrangements to form the observed adducts. Product ratios reflect the migratory aptitudes of the carbonyl compounds' α-substituents. The carbene reaction was modeled using DFT. The singlet carbene's bicoordinate C atom bends 31° toward the endo-fused cyclopropane bond, elongating it to r = 1.69 Å. The resulting trishomocyclopropyl HOMO{-1} is a three-center two-electron bond responsible for the electron-deficient carbene's nucleophilicity. Its calculated properties are consistent with this assertion: (1) singlet-triplet (ΔE(S-T)) energy gap of -25 kcal/mol, (2) gas-phase proton affinity (PA) value of 272 kcal/mol, (3) hard and soft acid and base (HSAB) ΔN value of -0.2 in its initial reaction with the carbonyl compounds, and (4) negative frontier orbital interaction values ΔΔE(PhC(O)H) = -4.38 eV and ΔΔE(PhC(O)Me) = -3.97 eV.

Tricyclo[2.1.0.02,5]pent-3-ylidene: Stereoelectronic Control of Bridge-Flapping within a Nonclassical Nucleophilic Carbene

“…A 2-D chart comparing carbene 1 with other alkenophiles is then assembled to reveal philicity trends (Table ). , They indicate that 1 is more nucleophilic than 4 , 2 , and dist - 1 -Br and almost as nucleophilic as the nonclassically bent carbene bicyclo[2.2.1]­hept-2-en-7-ylidene. ,,− …”

“…It is readily understood when one considers the intramolecular MO interaction between the carbene’s empty p orbital and its high-lying HOMO{−1} (Table ), which features bent C–C bonds having mixed σ/π-character (Figure b). Such donor–acceptor interactions are common within classical singlet carbenes (e.g., CCl 2 ) and nonclassical ones (e.g., bicyclo[2.2.1]­hept-2-en-7-ylidene) ,,− when the relevant orbitals are nearby, close in energy, and properly aligned. However, NHCs are such extreme examples of singlet carbenes that they are better described as ylides.…”

Bent Singlet Cyclobutylcarbene: Computed Geometry, Properties, and Product Selectivity of a Nonclassical Carbene

“…A stabilizing 3-center 2-electron (3C2E) interaction can develop if there is a small energy difference between a singlet carbene’s properly aligned donor and acceptor MOs (Figure ). The divalent C atom’s empty p orbital can accept electron density from a π bond, mixed σ/π “banana” bond, or a σ bond, , which is ostensibly strained in the case of 1 1 .…”

“…The structural dichotomy between a nonclassical singlet carbene and its classical triplet counterpart is due to 3C2E bonding in the former. , The divalent C atom of a bent :CH-group can acquire additional electron-density from the C1′–C2′ bond. This is possible only with a singlet carbene, which can be viewed as a “ gem -zwitterion.” The empty p orbital of the 1 (σ 2 p 0 ) electron configuration is available for a stabilizing intramolecular MO interaction.…”

“…The resulting C1···C2′ bond is nonclassical since the C1 and C2′ atoms are not adjacent in the classical Lewis bond description (Figure ). This phenomenon is further enhanced upon carbene protonation . For example, the + CH 2 -group of 1aH + , is bent toward the very elongated C1···C2′ “bond” by ω wag = 47° (Table ).…”