Bonding orbital allyl

Higuchi, J., J. Chem. Phys. 27, 825, (ii) Semi-localized bond orbital treatment of the allyl radical. Extension of VB. 

Although at first glance addition to the central carbon and formation of what seems like an allylic carbonium ion would clearly be preferred over terminal addition and a vinyl cation, a closer examination shows this not to be the case. Since the two double bonds in allenes are perpendicular to each other, addition of an electrophile to the central carbon results in an empty p orbital, which is perpendicular to the remaining rr system and hence not resonance stabilized (and probably inductively destabilized) until a 90° rotation occurs around the newly formed single bond. Hence, allylic stabilization may not be significant in the transition state. In fact, electrophilic additions to allene itself occur without exception at the terminal carbon (54). 

The group ofWalborsky probably has described one of the first true anionic/radi-cal domino process in their synthesis of the spirocyclopropyl ether 2-733 starting from the tertiary allylic bromide 2-730 (Scheme 2.161) [369]. The first step is a Michael addition with methoxide which led to the malonate anion 2-731. It follows a displacement of the tertiary bromide and a subsequent ring closure which is thought to involve a SET from the anionic center to the carbon-bromine anti bonding orbital to produce the diradical 2-732 and a bromide anion. An obvious alternative Sn2 halide displacement was excluded due to steric reasons and the ease with which the reaction proceeded. 

The new p orbital of the allyl radical is conjugated with those of the double bond => the allyl radical is a conjugated unsaturated system. 

A natural bond orbital-based CI/MP through-space/bond interaction analysis of the Sn2 reaction between allyl bromide and ammonia17 showed that allyl bromide reacted faster than propyl bromide because the a - n and n — a interactions stabilize the allyl bromide transition state equally. 

Other theoretical studies discussed above include investigations of the potential energy profiles of 18 gas-phase identity S 2 reactions of methyl substrates using G2 quantum-chemical calculations," the transition structures, and secondary a-deuterium and solvent KIEs for the S 2 reaction between microsolvated fluoride ion and methyl halides,66 the S 2 reaction between ethylene oxide and guanine,37 the complexes formed between BF3 and MeOH, HOAc, dimethyl ether, diethyl ether, and ethylene oxide,38 the testing of a new nucleophilicity scale,98 the potential energy surfaces for the Sn2 reactions at carbon, silicon, and phosphorus,74 and a natural bond orbital-based CI/MP through-space/bond interaction analysis of the S 2 reaction between allyl bromide and ammonia.17... 

In contrast to diazenes, the electronic structure of diazocompounds was not so intensively studied. The HOMO of diazomethane was calculated to be a bi(n) orbital corresponding to the non-bonding orbital of a tr-allyl system22 26-28 (Fig. 2). This orbital is responsible for the 1.3-dipolar reactivity of diazoalkanes. Typical ionisation energies are 9.0, 7.88 and 8.33 eV for diazomethane, 2-diazopropane and diazo-cyclopentadiene, respectively28. ... 

To get a clearer picture of what a resonance hybrid is—and, especially, to understand how resonance stabilization arises—let us consider the bond orbitals in the allyl radical. 

The allyl cation has it electrons only in the bonding orbital. The free radical has one electron in the non-bonding orbital as well, and the anion has two in the non-bonding orbital. The bonding orbital encompasses all three carbons, and... 

In the allyl system (Fig. 29.7) the third and fourth electrons go into a non-bonding orbital, whereas here they go into antibonding orbitals. As a result, the cyclopropenyl free radical and anion are less stable than their open-chain counterparts. For the cyclo-propeny] anion in particular, with two electrons in antibonding orbitals, simple calculations indicate no net stabilization due to delocalization, that is, zero resonance energy. Some calculations indicate that the molecule is actually less stable than if there were no conjugation at all. Such cyclic molecules, in which delocalization actually leads to destabilization, are not just non-aromatic they are u/i/iaromatic. 

More effective stabilization is provided by genuine conjugation with Jt or lone-pair electrons. The allyl cation has a filled (bonding) orbital containing two electrons delocalized over all three atoms and an important empty orbital with coefficients on the end atoms only. It s this orbital that is attacked by nucleophiles and so it s the end carbon atoms that are attacked by nucleophiles. The normal curly arrow picture tells us the same thing, the allyl cation curly arrows... 

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