Cyclobutane electron density

It can be seen that the HOMO energy of cyclopropane is higher than that of cyclobutane or cyclohexane, and that the much more reactive bicyclo[1.1.0]butane has a much higher HOMO energy, which is close to that of propene. Another important factor is the polarizability, which reflects how easily the electron density may be shifted in the presence of an electric field (such as that developed by a proton). Here again, cyclopropanes have significantly higher polarizability than other cycloalkanes.52... 

Surface delocalization is not found for cyclobutane or larger cycloalkanes10. Furthermore, it does not appear for cyclotrisilane since in this case the overlap within the surface orbital is not sufficient to bring enough electron density into the centre of the ring71. 

Hybridization of the carbon to which a proton is attached also influences electron density. As the proportion of s character increases from sp to sp to sp orbitals, bonding electrons move closer to carbon and away from the protons, which then become deshielded. For this reason, methane and ethane resonate at 8 0.23 and 0.86, respectively, but ethene resonates at 8 5.28. Ethyne (acetylene) is an exception in this regard, as we shall see. Hybridization contributes to shifts in strained molecules, such as cyclobutane (8 1.98) and cubane (8 4.00), for which hybridization is intermediate between sp and sp. ... 

At this point we can gain additional information from perturbation theory, not easily available from orbital correlation diagrams. If we wish to join two olefin molecules together to form cyclobutane, it is clearly necessary to transfer electron density from a filled orbital of hx symmetry, to an empty orbital of bz symmetry (see Fig. 10). This is the only way in which we can break the carbon-carbon n bonds and convert them into suitable a bonds. Mango and Schachtschneider originally accomplished the necessary change by moving electrons from a filled b orbital (concentrated on the olefins) into the empty 61 orbital (concentrated on the metal), and from the filled bz orbital (concentrated on the metal) into the empty 62 orbital (concentrated on the olefins). 

Electron density contours of cyclobutane calculated from X-ray diffraction data. (Reproduced from reference 63.)... 

We have investigated the substituent effects of the olefin and enophile that lead to cyclobutane formation. Reaction occurs exclusively with the unsubstituted double bond of the NBD irrespective of the electronic nature of the substituent (Y) giving type I [2 + 2] adducts (Scheme 21). In addition, only two of the remaining isomers are observed in most cases. We find that cyclobutane formation is particularly facile when unreactive dienes are reacted with very reactive alkenes. We also observed a clear trend between the exo versus endo mode of reaction and the electron density on the remote, unreacting alkene of the NBD. The specific examples are outlined following. 

Fig. 1.1. Shapes of molecules represented by envelopes of constant electronic charge density. The envelope shown has the value of 0.001 au. The molecules are (a)-(f) the normal alkanes from methane to hexane (g) isobutane (h) neopentane (i) cyclopropane (j) cyclobutane (k) formaldehyde, H2OK) (/) acetone, (CH3)2C=0. The intersections of the zero-flux interatomic surfaces with the envelope are shown in some cases. They define the methyl, methylene, hydrogen, and carbonyl groups. The isobutane molecule (g), for example, exhibits three methyl groups topped...

---