Diazomethane frontier orbitals

Mechanistically the 1,3-dipolar cycloaddition reaction very likely is a concerted one-step process via a cyclic transition state. The transition state is less symmetric and more polar as for a Diels-Alder reaction however the symmetry of the frontier orbitals is similar. In order to describe the bonding of the 1,3-dipolar compound, e.g. diazomethane 4, several Lewis structures can be drawn that are resonance structures ... 

Do the transition states of the 1,3-dipolar cycloadditions with diazomethane benefit from a stabilizing frontier orbital interaction Yes Computations show that the HOMOdia zomethm/LUMOethene interaction (orbital energy difference, -229 kcal/mol) stabilizes the transition state of the 1,3-dipolar cycloaddition to ethene (Figure 15.37) by about 11 kcal/mol. Moreover, computations also show that the HOMOethene/LUMOdjazomethane interaction (orbital energy difference, -273 kcal/mol ) contributes a further stabilization of 7 kcal/mol. 

Fig. 15.37. Frontier orbital interactions in the transition state of the 1,3-dipolar cycloaddition of diazomethane to ethene.

The reactions of diazomethane with C- and X-substituted alkenes are much slower, and consequently there are fewer known examples. The slower rate of reaction is explained easily by the larger energy separation in the frontier orbitals (10 and 9.8 eV, respectively, in Fig. 6.34). The regioselectivity, however, is the same A -pyrazolines like 6.225 and 6.227 with the substituent at C-3 are obtained with both C- and X-substituted dipolarophiles. This at first sight surprising observation can be explained by the change from dipole-HO-control in the cases of the Z- and C-substituted alkenes 6.223 and 6.224 to dipole-LU-control 6.226 in the case of the X-substituted alkene ethyl vinyl ether. 

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... 

By contrast, if the important frontier orbital in a cycloaddition is the LUMO of the fulvene, and if the fulvene is to react as n2 or n6 component, it will now react as a ic6 component, because the largest coefficients are on C-2 and C-6. Thus with diazomethane, the LU (fulvene)/HO (diazomethane) interaction is probably closer in energy than the HO(fulvene)/LU(diazomethane) interaction (Fig. 4-72), and the adduct actually obtained (374)298 is the one expected from these considerations. Although reaction with a simple diene ought also to be dominated by the LU.(fulvene)/HO(diene) interaction (Fig. 4-72), nevertheless the observed294 product (376) from the reaction of dimethyl-fulvene (375) with cyclopentadiene has the fulvene acting as a n2 component rather than... 

Fig. 4-72 Frontier orbital interactions for fulvene, diazomethane and butadiene...

The regiochemistry of the cycloadditions of allenes is not easily explained by these frontier orbitals. Penta-2,3-diene and acrylonitrile give the adducts Z- and -6.404 in which the central carbon of the allene, with the smaller HOMO coefficient, has bonded to the / carbon of the Z-substituted alkene.894 Equally unexplained is the regiochemistry of the reaction between allene itself and diazomethane, which gives more of the adduct 6.405 than of its regioisomer 6.406.893... 

The frontier orbital energies for diazomethane and a series of substituted ethenes as dipolarophiles result in two conclusions ... 

Reactions of simple fulvenes with dienophiles commonly take place at C(2) and C(5) and those with dienes at C(2) and C(3), i.e. the latter proceed by a [2 + 4] rather than a [6 + 4] cycloaddition. This, and other features of cycloaddition reactions involving fulvenes can be nicely explained in terms of frontier orbital theory [234,235]. In some reactions fulvenes may participate as 6ir rather than 27t components, for example with diazomethane ... 

Energetically this reaction can be expected to be dipole-HO-controlled ( Lumo - Fhomo = 15.5 eV), but this should have little regiocontrol since the LUMO of the allene is so little polarised. The LUMO(dipole)/ HOMO ene) interaction ( lumo - homo = 16.75 eV) might come into play, since the allene HOMO is polarised, but the orbitals do not match up to explain the high level of regioselectivity, since the larger coefficient in the LUMO of diazomethane is on carbon. Finally, the hydroboration of allenes gives mainly the product with the boron on the central carbon 6.407.895 With boron as the electrophilic atom, frontier... 

The rates of 1,3-dipolar cycloadditions of diazoalkanes to alkenes and alkynes have been determined electron-attracting substituents in the latter increase the rate, in accordance with frontier molecular orbital theory, which predicts that these reactions are controlled by the interaction of the highest occupied molecular orbital of the diazo-compound with the lowest unoccupied molecular orbital of the dipolarophile " the kinetics of the reactions of methyl diazoacetate or phenyl diazomethanesulphonate, on the other hand, give rise to U-shaped activity functions, which is also explained by the theory. Diazomethane or... 

For a discussion of the orbital interactions that control dipolar additions of diazomethane, see Fleming, I. Frontier Molecular Orbitals and Organic Chemical Reactions, Wiley New York, 1976 p 148. 

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