Hiickel calculations acrolein

Z-Substituents. The simplest Z-substituent is the formyl group in acrolein 2.1. A simple Hiickel calculation gives Fig. 2.2 in which we are looking at the p orbitals from above. This figure gives us what we want, but no insight. 

The first calculations of the frontier orbitals for acrolein gave the HOMO coefficients on the C=C double bond of acrolein, with the a carbon having the larger coefficient. This failed to explain the regiochemistry, but only because the simple Hiickel theory that was used is notoriously weak in dealing with electron distribution in heteroatom-containing systems. Later calculations gave a better set of coefficients, as shown in Fig. 6.29. 

Selectivity is predicted by examination of the orbital coefficients of the HOMO and LUMO for both diene and alkene. The transition state for a typical reaction is shown for the reaction of methyl acrylate and 2-phenyl-1,3-butadiene (see 67), where the orbitals with largest coefficients (HOMOdiene-LUMOalkene) combine (the absolute value of the orbital coefficient is used, -0.625 0.69l and -0.475 1-0.471)) to predict the cycloadduct produced in the greatest amount (the para product, 68). Just as the HOMOgy-LUMOaikene is [0 - (-8.77) = 8.77], and it predicts the relative reactivity of these reactants. The magnitude of the orbital coefficients in each partner is important for predicting selectivity. In this case, the orbital coefficients are 0.065 and 0.004, respectively, and there is selectivity for the para product. Similarly, the orbital coefficients for 1-methoxy-1,3-butadiene and acrolein correctly predict the major product is the ortho adduct. For simple cases, the orbital coefficients can be estimated by a simple Hiickel molecular orbital calculation (a very low level calculation but one that gives a first approximation that is useful for estimating relative differences). 

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