LUMO energy, imine formation

In a study of imine formation from aldehydes in aqueous solution, formation constants have been correlated with three parameters the pK.d and HOMO energy of the amine and the LUMO energy of the aldehyde.23... 

In a further example, a genetically encoded alkene-containing protein in Escherichia coli was modified with a library of 2,5-diaryl tetrazoles under similar conditions (Song et al., 2008). Although the photoinduced formation of the nitrile-imine was very fast (2 min), the subsequent cycloaddition with the aUyl phenyl ether functional protein was considerably slower (the reaction was allowed to proceed overnight at 4 °C). By comparison, the second-order rate constant of the cycloaddition with allyl phenyl ether was determined to be 0.00202 0.00007 M s, whereas that for acrylamide (as was used in the previous example) was determined to be 0.15s. A reasonable explanation for this observation is the lower LUMO energy of acrylamide than that of allyl phenyl ether, which offers better orbital overlap with the HOMO of the nitrile imine. 

Dipolarophiles with electronegative heteroatoms such as carbonyl groups, imines and cyano groups also show an orientation in agreement with frontier orbital theory. Because heterodienophiles all have low-energy LUMOs, their reactions will usually be dipole-HO-controlled. The reaction of diazomethane with an imine giving the triazoline 6.240 and the final step in the formation of an ozonide... 

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