Transition state orbital interaction

Because of the principle of microscopic reversibility it is appropriate to consider frontier orbital analysis of the reaction in either direction. The Hammond postulate dictates that the more exothermic the reaction the more the transition state will reflect the starting geometry, and frontier orbital analysis of reactant orbitals is expected to be a better predictor of relative transition state orbital interactions than for an endothermic or a less exothermic process. Conversely, frontier orbital analysis of product orbitals in exothermic reactions would be a poorer predictor of transition state energy. 

Hence Fukui s method is justified in the transition state, the FOs then being practically degenerate, with increased frontier coefficients at the reaction centers. All the other interactions are an order smaller.6 Unfortunately, transition state orbitals cannot be obtained easily,7 so we generally substitute the MOs of the reagents instead. This supplementary approximation,8 introduced for practical reasons, occasionally gives results which appear to violate the frontier orbital theory. We will examine such cases in more detail later (pp. 62, 76-78). 

MO representation of stabilization of substitution transition state through interaction with C=0 k orbital... 

Dipolarophiles which contain an electron-deficient substituent undergo smooth cycloaddition reactions with nitrile ylides. The relative reactivity of the nitrile ylide toward a series of dipolarophiles is determined primarily by the extent of stabilization afforded the transition state by interaction of the dipole highest-occupied (HO) and dipolarophile lowest-unoccupied (LU) orbitals. Substituents which lower the dipolarophile LU energy accelerate the 1,3-dipolar cycloaddition reaction. For example, fumaronitrile undergoes cycloaddition at a rate which is 189,000 times faster than methyl crotonate. Ordinary olefins react so sluggishly that their bimolecular rate constants cannot be measured. 

Theoretical work by the groups directed by Sustmann and, very recently, Mattay attributes the preference for the formation of endo cycloadduct in solution to the polarity of the solvent Their calculations indicate that in the gas phase the exo transition state has a lower energy than the endo counterpart and it is only upon introduction of the solvent that this situation reverses, due to the difference in polarity of both transition states (Figure 1.2). Mattay" stresses the importance of the dienophile transoid-dsoid conformational equilibrium in determining the endo-exo selectivity. The transoid conformation is favoured in solution and is shown to lead to endo product, whereas the cisoid conformation, that is favoured in the gas phase, produces the exo adduct This view is in conflict with ab initio calculations by Houk, indicating an enhanced secondary orbital interaction in the cisoid endo transition state . 

In summary, it seems that for most Diels-Alder reactions secondary orbital interactions afford a satisfactory rationalisation of the endo-exo selectivity. However, since the endo-exo ratio is determined by small differences in transition state energies, the influence of other interactions, most often steric in origin and different for each particular reaction, is likely to be felt. The compact character of the Diels-Alder activated complex (the activation volume of the retro Diels-Alder reaction is negative) will attenuate these eflfects. The ideas of Sustmann" and Mattay ° provide an attractive alternative explanation, but, at the moment, lack the proper experimental foundation. 

The regioselectivity benefits from the increased polarisation of the alkene moiety, reflected in the increased difference in the orbital coefficients on carbon 1 and 2. The increase in endo-exo selectivity is a result of an increased secondary orbital interaction that can be attributed to the increased orbital coefficient on the carbonyl carbon ". Also increased dipolar interactions, as a result of an increased polarisation, will contribute. Interestingly, Yamamoto has demonstrated that by usirg a very bulky catalyst the endo-pathway can be blocked and an excess of exo product can be obtained The increased di as tereo facial selectivity has been attributed to a more compact transition state for the catalysed reaction as a result of more efficient primary and secondary orbital interactions as well as conformational changes in the complexed dienophile" . Calculations show that, with the polarisation of the dienophile, the extent of asynchronicity in the activated complex increases . Some authors even report a zwitteriorric character of the activated complex of the Lewis-acid catalysed reaction " . Currently, Lewis-acid catalysis of Diels-Alder reactions is everyday practice in synthetic organic chemistry. 

In the lowest optieally excited state of the molecule, we have one eleetron (ti ) and one hole (/i ), each with spin 1/2 which couple through the Coulomb interaetion and can either form a singlet 5 state (5 = 0), or a triplet T state (S = 1). Since the electric dipole matrix element for optical transitions — ep A)/(me) does not depend on spin, there is a strong spin seleetion rule (AS = 0) for optical electric dipole transitions. This strong spin seleetion rule arises from the very weak spin-orbit interaction for carbon. Thus, to turn on electric dipole transitions, appropriate odd-parity vibrational modes must be admixed with the initial and (or) final electronic states, so that the w eak absorption below 2.5 eV involves optical transitions between appropriate vibronic levels. These vibronic levels are energetically favored by virtue... 

A more complete analysis of interacting molecules would examine all of the involved MOs in a similar wty. A correlation diagram would be constructed to determine which reactant orbital is transformed into wfiich product orbital. Reactions which permit smooth transformation of the reactant orbitals to product orbitals without intervention of high-energy transition states or intermediates can be identified in this way. If no such transformation is possible, a much higher activation energy is likely since the absence of a smooth transformation implies that bonds must be broken before they can be reformed. This treatment is more complete than the frontier orbital treatment because it focuses attention not only on the reactants but also on the products. We will describe this method of analysis in more detail in Chapter 11. The qualitative approach that has been described here is a useful and simple wty to apply MO theory to reactivity problems, and we will employ it in subsequent chapters to problems in reactivity that are best described in MO terms. I... 

Calculations at several levels of theory (AMI, 6-31G, and MP2/6-31G ) find lower activation energies for the transition state leading to the observed product. The transition-state calculations presumably reflect the same structural features as the frontier orbital approach. The greatest transition-state stabilization should arise from the most favorable orbital interactions. As discussed earlier for Diels-Alder reactions, the-HSAB theory can also be applied to interpretation of the regiochemistry of 1,3-dipolar cycloaddi-... 

Examine transition states leading to para and meta adducts 2-methoxybutadiene+acrylonitrile para and meta) as well as the adducts themselves (l-methoxy-4-cyanocyclohexene and l-methoxy-5-cyanocyclohexene). What should the kinetic product be Is this in line with your expectations based on orbital interactions Is this also the thermodynamic product ... 

An explanation for the stereoselectivity of the reaction involves optimal overlap of the 7t-orbital of the carbonyl with the developing electron rich p-orbital on C2 during the Sj,j2 displacement of the chloride by the alkoxide (24). Thus, orbital overlap imposes conformational constraints in the transition state that leads to nonbonding interactions disfavoring transition state 15P... 

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