Diastereoselective carbenes: Stereoelectronic control of bent singlet trans -2′-substituted cyclobutylcarbenes

Carbene Routes to Cyclopropatetrahedrane

The formation of cyclopropatetrahedrane (tetracyclo[2.1.0.01,3.02,4]pentane) via four different carbene reactions is computed using the (U)CCSD(T)(full)/cc-pVTZ//(U)ωB97X-D/cc-pVTZ + 1.3686(EZPVE) theoretical model. Intrinsic reaction coordinate plots confirm that each carbene is directly linked to cyclopropatetrahedrane via a unique cyclopropanation step. Each elementary step is assessed according to the structure and energy of its transition state.

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.

Generation and Rearrangement of (1-Hydroxycyclopropyl)- and (1-Hydroxycyclobutyl)carbene

Photolysis of exo-1-(1a,9b-dihydro-1H-cyclopropa[l]phenanthren-1-yl)cyclopropan-1-ol and exo-1-(1a,9b-dihydro-1H-cyclopropa[l]phenanthren-1-yl)cyclobutan-1-ol in benzene-d6 produces (1-hydroxycyclopropyl)- and (1-hydroxycyclobutyl)carbene respectively. It was observed that (1-hydroxycyclopropyl)carbene rearranges to cyclobutanone whereas (1-hydroxycyclobutyl)carbene forms cyclopentanone. Formation of both ketones is attributed to tautomerization of the corresponding enols that arise from ring expansion of the carbenes. Products assignable to intramolecular C–H insertions were not detected in the photolysates.