“…The anti facial preference of the norbornane 41a was previously found in the additions of dichlorocarbene (syn:anti ) 44:56) 153,155 and 9-BBN (syn:anti ) 11:89). 153,155 The anti preference was also observed in the reactions of methylidenebicyclo[2.2.1]heptane (41b) bearing an endo-dimethylcyclopropyl group 153,155 with dichlorocarbene (syn:anti ) 34:66) and 9-BBN (syn:anti ) 5:95). Therefore, we can conclude that the anti preference, induced by a cyclopropyl group, is intrinsic to the 7-methylidenenorbornane 41a.…”
Section: E Effects Of Different Arrangements Of Composite Moleculesmentioning
confidence: 78%
“…This preference is in sharp contrast to the anti preference of 41a (syn:anti = 12:88), observed under similar dihydroxylation conditions with osmium tetroxide in pyridine. While the facial selectivity of 41a has been examined previously, − that of 40a has been little studied and the topology-dependent behavior described here has not previously been documented or characterized. …”
Section: E Effects Of Different Arrangements Of
Composite Moleculesmentioning
confidence: 97%
“…A cyclopropyl group is known to act as a strong π donor due to a high-lying occupied Walsh orbital, which is frequently regarded as an equivalent of a double bond. ,− Photoelectron spectra of the olefins 40a and 41a were previously measured and the signals assigned. ,− Vertical ionization potentials of the olefin π orbitals of 40a and 41a were found to be higher than those of the unsubstituted parent bicyclo[2.2.2]octene 35a (R 1 = R 2 = H) and 7-methylenenorbornane 28a (R 1 = R 2 = H), respectively, indicating a sizable interaction of the π orbital of the double bond with the occupied Walsh orbital of the fused cyclopropane ring. The previous account of the observed anti facial preference of 41a was based on this interaction, in particular, out-of-phase interaction of the relevant orbitals ( 4.36 ). − However, the corresponding out-of-phase interaction of the olefinic π orbital with the Walsh orbital of the cyclopropyl group ( 4.35 ) does not seem to be relevant to 40a because of the observed reverse syn preference. This view is consistent with the following observation.…”
Section: E Effects Of Different Arrangements Of
Composite Moleculesmentioning
confidence: 99%
“…The previous account of the observed anti facial preference of 41a was based on this interaction, in particular, out-of-phase interaction of the relevant orbitals (4.36). [153][154][155][156] However, the corresponding out-of-phase interaction of the olefinic π orbital with the Walsh orbital of the cyclopropyl group (4.35) does not seem to be relevant to 40a because of the observed reverse syn preference. This view is consistent with the following observation.…”
Section: E Effects Of Different Arrangements Of Composite Moleculesmentioning
“…The anti facial preference of the norbornane 41a was previously found in the additions of dichlorocarbene (syn:anti ) 44:56) 153,155 and 9-BBN (syn:anti ) 11:89). 153,155 The anti preference was also observed in the reactions of methylidenebicyclo[2.2.1]heptane (41b) bearing an endo-dimethylcyclopropyl group 153,155 with dichlorocarbene (syn:anti ) 34:66) and 9-BBN (syn:anti ) 5:95). Therefore, we can conclude that the anti preference, induced by a cyclopropyl group, is intrinsic to the 7-methylidenenorbornane 41a.…”
Section: E Effects Of Different Arrangements Of Composite Moleculesmentioning
confidence: 78%
“…This preference is in sharp contrast to the anti preference of 41a (syn:anti = 12:88), observed under similar dihydroxylation conditions with osmium tetroxide in pyridine. While the facial selectivity of 41a has been examined previously, − that of 40a has been little studied and the topology-dependent behavior described here has not previously been documented or characterized. …”
Section: E Effects Of Different Arrangements Of
Composite Moleculesmentioning
confidence: 97%
“…A cyclopropyl group is known to act as a strong π donor due to a high-lying occupied Walsh orbital, which is frequently regarded as an equivalent of a double bond. ,− Photoelectron spectra of the olefins 40a and 41a were previously measured and the signals assigned. ,− Vertical ionization potentials of the olefin π orbitals of 40a and 41a were found to be higher than those of the unsubstituted parent bicyclo[2.2.2]octene 35a (R 1 = R 2 = H) and 7-methylenenorbornane 28a (R 1 = R 2 = H), respectively, indicating a sizable interaction of the π orbital of the double bond with the occupied Walsh orbital of the fused cyclopropane ring. The previous account of the observed anti facial preference of 41a was based on this interaction, in particular, out-of-phase interaction of the relevant orbitals ( 4.36 ). − However, the corresponding out-of-phase interaction of the olefinic π orbital with the Walsh orbital of the cyclopropyl group ( 4.35 ) does not seem to be relevant to 40a because of the observed reverse syn preference. This view is consistent with the following observation.…”
Section: E Effects Of Different Arrangements Of
Composite Moleculesmentioning
confidence: 99%
“…The previous account of the observed anti facial preference of 41a was based on this interaction, in particular, out-of-phase interaction of the relevant orbitals (4.36). [153][154][155][156] However, the corresponding out-of-phase interaction of the olefinic π orbital with the Walsh orbital of the cyclopropyl group (4.35) does not seem to be relevant to 40a because of the observed reverse syn preference. This view is consistent with the following observation.…”
Section: E Effects Of Different Arrangements Of Composite Moleculesmentioning
“…For example, an anisotropy in the electron distribution about the double bond in 7-methylenenorbornane compounds 1 and 2 has been used to rationalise their stereoreactivity. 1,2 For the case of norbornene itself, Fukui and co-workers 3 have predicted this anisotropy by theoretical calculation. The exo selectivity of these systems has been rationalised on the basis of the predicted increased exo extension of the highest occupied molecular orbital (HOMO).…”
Addition reaction studies and ab initio calculations on methylenecyclohexane and 5-methylene-1,3dioxane systems suggest that two electronic factors contribute to the stereoselectivity of epoxidation and diimide reduction. These are respectively the spatial anisotropy of the HOMO with respect to the two faces of the double bond, common to both molecules, which is likely to be responsible for the overall axial stereofacial selectivity exhibited, and a similar anisotropy in the electrostatic potential field of the methylenedioxane caused by the oxygens; which also favours attack from an axial direction by polarisable electrophilic species. The anisotropy of the HOMO arises from the important topological difference between the contributions made to the HOMO by the periplanar C᎐H bonds and opposing C᎐O or C᎐C bonds. Catalytic reduction proceeds with equatorial face selectivity for both the cyclohexane and the dioxane systems and appears to be governed largely by steric effects. † There is some confusion here between the calculations of the Paquette group and those of Houk. Paquette speaks of a positively charged region, but talks of a charged electrophile being 'guided in' by the electron density, whereas Houk indicates that the predominant factor is a high electron density region, i.e. a negatively charged space on this side of the molecule. 11-13Paper 8/00999F
Sterically unbiased bicyclic and tricyclic olefins were reacted with electrophiles to assess the π‐facial stereoselectivity. The stereochemical preference of the reactions of these olefins with electrophiles is attributed to the HOMO, which is derived from the interaction between a higher‐lying π orbital and particular lower‐lying σ orbitals. The selectivity of electrophilic reactions of these olefinic molecules is thought to be controlled by two types of HOMO, which are formed by a combination of different orbitals, depending on the geometrical conditions. A novel interpretation of orbital interactions enabled us to account for the facial selectivity in a unified manner.
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