Cyclopropanes bonding

The direction of ring opening by homolytic cleavage of a cyclopropane bond is controlled by the stability of the diradical species formed. Upon heating of the mono-deuterated vinylcyclopropane 3, a mixture of the two isomeric mono-deuterated cyclopentenes 4 and 5 is formed ... 

A comparison of the rate constant for photoisomerization of the unsubstituted 3-phenyl derivative (kT = 3 x 1010 sec-1) to that of the 3-(p-methoxy phenyl) derivative (kr = 1.5 x 1010 sec-1) indicates that the presence of the p-methoxy groups imparts no special stability to the intermediate responsible for isomerization even though cleavage of a cyclopropane bond is predominant. Clearly these results are inconsistent with an intermediate possessing electron-poor or electron-rich species such as would be obtained from heterolytic cleavage of the cyclopropane. On the other hand, the results are consistent with a biradical species as intermediate. Further evidence consistent with this conclusion was obtained in a study of trans-3-p-cyanophenyl-/ra w-2-phenyl-1 -benzoylcyclopropane,<82)... 

The molecular structure of the parent compound was investigated in the vapor and in the solid phase using X-ray, XN and GED methods. The reported data are shown in Table 16. In both phases a clear bond length separation could be detected with a localized three-membered ring and its three adjacent double bonds. The symmetry-equivalent cyclopropane bonds are rather long in C3v-symmetric BUL (1.533-1.542 A), which can be explained by the common electron-withdrawing effect of the 7r-systems in a. svM-ciinal conformation. For comparison, the unaffected bonds in unsubstituted cyclopropane are 1.499 A in the crystal and 1.510 A in the gas phase. Therefore, the bond lengths in BUL... 

This is the case for the quadricyclane - to norbornadiene" reaction. Although the C2K reaction path provides an attractive interpretational tool for understanding the progress of this reaction, its highest point represents a conical intersection at which the two relevant states have the same energy at the same geometry. This point cannot be a transition state, so that lowering the symmetry in any direction leads to a stabilization. The result is an asynchronous reaction path in which one of the two cyclopropane bonds is broken first to form the biradical-like transition state la. The second bond can then break to form the norbornadiene radical cation 2. 

Electrophilic reagents undergo formal insertion into the cyclopropane bond and lead to ring-enlarged products. Reaction of 1 or 42 with dichlorocarbenes affords the corresponding dihalogenobenzocyclobutene (348). The same reaction has been realized with dicyclopropan hthalene (115). Reaction of 1 with SOj and SO3 affords the sultine 349 and the sultone 350, respectively. 4-Phenyl-1,2,4-tria-... 

Other systems studied include the spiroheptadiene system (13) in which the cyclopropane moiety lies in the nodal plane of the butadiene HOMO [1 2,9 10]bis-methano[2.2]paracyclophane (15), in which two cyclopropane moieties are joined with two benzene rings in such a way that tertiary-tertiary cyclopropane bonds lie parallel to the aromatic n system 7-methylenequadricyclane radical cation (16 ), in which a pair of cyclopropane groups lie orthogonal to an olefinic moiety and the 7-spirocyclopropanenorbomadiene radical cation (18 ), in which a pair of nonconjugated ethene groups and a cyclopropane moiety are joined. 

While the tertiary-tertiary cyclopropane bond of benzonorcaradiene (11) lies in a plane perpendicular to the aromatic n system, a parallel arrangement of these... 

The effects observed for 23 (Figure 8) are particularly clear-cut, since the spectrum is fully resolved." The key to the structure of 23 lies in the prominent enhanced absorption signals of H3 (5.2 ppm) and (-0.2 ppm) and the strong emission of (2,4 ppm), H4 (2.2 ppm), and H5 (1.5 ppm). This polarization pattern supports a spepies with spin density on C3 and C6, indicating the delocalization of spin and charge into the lateral cyclopropane bond. Weakly enhanced absorption observed for H2 (5.8 ppm), (0.8 ppm), and Hi (1.75 ppm), and weak emission for H5 further support this structure type. ... 

We rationalize the involvement of the less substituted lateral bond of 23 instead of its more highly substituted internal bond as reflecting the need for orbital overlap. The bicyclohexene system has limited mobility, allowing the lateral cyclopropane bond more significant orbital overlap with the alkene p-orbitals (23 ). The partici-... 

The bicycloheptene system 24 appears to be more flexible than 23, allowing either the internal or the lateral cyclopropane bond to aligrt with the alkene p-orbitals. Accordingly, delocalization of spin and charge into the more highly substituted bond is preferred (24 ). 

Substituted cyclopropane systems also undergo nucleophilic addition of suitable solvents (MeOH). For example, the photoinduced ET reaction of 1,2-dimethyl-3-phenylcyclopropane (112, R = Me) with p-dicyanobenzene formed a ring-opened ether by anti-Markovnikov addition. The reaction occurs with essentially complete inversion of configuration at carbon, suggesting a nucleophilic cleavage of a one-electron cyclopropane bond, generating 113. The retention of chirality confirms that the stereochemistry of the parent molecule is unperturbed in the radical cation 112 " ". 

The strongly electron-donating cyclopropane bonds in 15 should result in con-rotation being the dominant mode of coupled methylene rotation. However,... 

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... 

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