Cyclopropanes Walsh orbitals

No discussion about strained-ring radical cations would be complete without the valence isomers quadricyclane (15 +) and norbornadiene, (16 +) 15 features two adjacent rigidly held cyclopropane rings, whereas 16 contains two ethene n systems well suited to probe through-space interactions.Molecular orbital considerations suggest the antisymmetric combination of the ethene n orbitals (16) or cyclopropane Walsh orbitals (15) as respective HOMOs of the two parent molecules. The radical ions have different state symmetries and their SOMOs have different orbital symmetries. 

Ionization potentials of dispiro[2,2,2,2]deca-4,9-diene (531) have been measured, providing information about the conjugation of the cyclopropane Walsh orbitals with the diene orbitals. 

Conjugated chains, 14, 46 Correlation diagrams, 44, 50 Cyclobutadiene, 171 Cyclobutane, 47, 222 orbital ordering, 26 through-space interactions, 26 Walsh orbitals, 27 Cyclobutene, 200 Cyclohexane, 278 Cyclohexene (half-boat), 274 Cyclopen tadiene, 225 Cvclopen tadienone, 269 Cyclopentadienyl anion, 237 Cyclopentane, 254 Cyclopen ten e, 241 Cyclopropane, 41, 47, 153 construction of orbitals, 19, 22 Walsh orbitals, 22, 36, 37 Cyclopropanone, 48, 197 bond lengths, 38 Cyclopropen e, 49, 132 reactivity, 40... 

This notion is also snpported by the following experimental observations. Because substitution of a cyano gronp on the cyclopropane ring lowers the energy of the Walsh orbital of the cyclopropyl group, the resultant attennation of the interaction of the olefin orbital with the Walsh orbital, if this interaction is indispensable, would reduce the facial selectivity. However, substitution of a cyano gronp on the cyclopropyl group, as in ejco-cyano 59c and endo-cymo 59d, essentially does not modify the syn-preference in dihydroxylation and epoxidation, but even increases the syn preference (59c (98 2) and 59d (>99 <1)) in the case of dihydroxylation. 

Fig. 1. a) The highest occupied and lowest unoccupied Walsh orbitals of cyclopropane, b) Ground configuration (left) and excited configuration (right) of cyclopropane, and level shifts upon increase of a. 

The fragment of 1,4-PD is also present in 1,1-divinylcyclopropane (DVC), where the central methylene group is replaced by a three-membered ring. For this strained molecule a strong interaction between cyclopropane Walsh and vinyl 7r-orbitals was expected. The photoelectron spectra of DVC5 could be best understood with the assumption of optimal... 

In terms of MO theory the stabilization is described by a linear combination of a two-electron filled antisymmetric 3c-2e-Walsh orbital of the cyclopropane ring with the vacant p-orbital at the C +-carbon atom (Fig. 5). 

Strong shielding of the 0=0 carbon atom in 280 by endo-cyclopropane annelation was reported by Kessler and co-workers (362) and rationalized in terms of an interaction between a Walsh orbital and the n-orbital of the carbonyl oxygen via the intervening C-C bonds (281) an alternative explanation—spatial interference with the rr c—o orbital (282)—was mentioned as well (362). The double bond in 283 is polarized (A = 17.9) when 283 is transformed into 284... 

Cyclopropane bond angle, 16 bonding, 84-85 correlation diagrams, 207 electronic states, 207 hybridization, 16 point group of, 5 structure of, 16, 84 Walsh orbitals, 85 Cyclopropanes... 

Heilbronner and coworkers31 have shown that Walsh orbitals and Forster-Coulson-Moffitt orbitals are not equivalent. For example, only the Forster-Coulson-Moffitt orbitals but not the Walsh orbitals are part of the manifold of bonding cyclopropane orbitals. Therefore, a correct description of bonding in 1 in terms of Walsh orbitals is not possible. 

Kinetic studies on the decarbonylation of 113 and 114 are of interest in terms of the homoaromatic character of 105/106. It has been reported that the rate of decarbonylation of 113 was 1 x 10s times greater than that of 114247. It was suggested that this reactivity difference was due to partial opening of the cyclopropane in the transition state and overlap between the Walsh orbitals of the cyclopropane and the rehybridizing s-orbitals of the breaking bonds. Homoaromatic electron delocalization in a norcaradiene/cyclohepta-triene-like transition state is only possible with the anti-isomer 113. 

Fig. 1.42 A simplified version of the occupied Walsh orbitals of cyclopropane...

In hydrocarbon solvent. Values are for the most intense maximum. b Vertical ionization potentials obtained from He(I) photoelectron spectra. 0 IP assigned to Walsh orbital of cyclopropane ring. 

C. Walsh Orbitals as Aids in the Interpretation of Electrophilic Attack on Cyclopropanes... 

The Walsh orbitals for cyclopropane are particularly useful in visualiz-... 

---