Ylide stabilized carbenes: a computational study: YLIDE STABILIZED CARBENES

A 2,2'-diphosphinotolane as a versatile precursor for the synthesis of P-ylidic mesoionic carbenes via reversible C-P bond formation

A metal-templated synthetic route to cyclic (aryl)(ylidic) mesoionic carbenes (CArY-MICs) featuring an endocyclic P-ylide is presented. This approach, which requires metal templates with two cis-positioned open coordination sites, is based on the controlled cyclisation of a P,P'-diisopropyl-substituted 2,2'-diphosphinotolane (1) and leads to chelate complexes coordinated by a phosphine donor and the CArY-MIC carbon atom. The C-P bond formation involved in the former partial cyclisation of 1 proceeds under mild conditions and was shown to be applicable all over the d-block. In the presence of a third fac-positioned open coordination site, the P-C bond formation was found to be reversible, as shown for a series of molybdenum complexes. DFT modelling studies are in line with an interpretation of the target compounds as CArY-MICs.

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

Tricyclo[2.1.0.0^2,5]pent-3-ylidene is a carbene foreseen to rearrange to pyramidane (c-C₄H₄)C, a highly strained molecule featuring an inverted C atom. Modeling of the carbene, using the (U)MPWB1K/cc-pVTZ//(U)MPWB1K/6-311G(d) theoretical model, indicated a large singlet-triplet energy gap (ΔE_S-T = -45 kcal/mol), a high gas-phase proton affinity (PA = 258 kcal/mol), and a preference for electron-poor alkenes. These properties are consistent with those of nucleophilic carbenes. Structural differences between the C_s-symmetric singlet (ω_flap = ±44 deg) and C_2v-symmetric triplet (ω_flap = 0 deg) stem from nonclassical electron delocalization in the former and the lack thereof in the latter. Degenerate bridge-flapping of the singlet's main bridge, which comprises the reactive divalent C3 atom, is computed to be slow due to a high activation barrier of the C_2v-symmetric transition state (TS) (E_a = 17 kcal/mol). The position of the conformeric equilibrium is subject to stereoelectronic control. 1-Substituted derivatives of the carbene (R ≠ H) are sensitive to σ inductive effects. A proximal conformation is preferred when R is electron-donating and a distal one is favored when R is electron-withdrawing. Finally, carbene rearrangements via 1,2-C atom shift or enyne fragmentation were computed. The C_2v-symmetric bridge-flapping TS has the proper geometry to initiate enyne fragmentation.

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

Ab initio computations of cyclobutylcarbene (c-C₄H₇CH) were performed using the UMP4(fc)/6-311++G(2df,2p)//UMP2(full)/6-311++G(d,p) theoretical model. The carbene's most striking feature is its :CH-group. It is markedly bent toward the elongated C1'-C2' bond in the singlet ground-state but not in the triplet state, which is at least 1.1 kcal/mol higher in energy. Nonclassical 3C2E bonding among the C1, C1', and C2' atoms is prominent in the HOMO{-1}. The electron-donating ability of the nonbonding HOMO is thereby enhanced. The intensified nucleophilicity of the singlet carbene is manifested in quantifiable ways. For example, its hard and soft acid and base (HSAB) hardness, HSAB absolute electronegativity, and gas-phase proton affinity rival those of ylide-stabilized N-heterocyclic carbenes. It is computed to act as a nucleophile toward alkenes with higher HSAB hardness values. Transition states from singlet cyclobutylcarbene to bicyclo[2.1.0]pentane, cyclopentene, and methylenecyclobutane were computed and confirmed by intrinsic reaction coordinate calculations. Activation energies depend on the singlet's conformation with regard to c-C₄H₇ ring-puckering, :CH-group rotation, and :CH-group bending. The singlet's bent :CH-group favors bicyclo[2.1.0]pentane and cyclopentene formation.

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

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.