Houk rule

The Houk Rule for Steric Effects in Electrophilic Attack on Open-Chain Alkenes. As in... 

A second point that needs to be clarified is why allowed reactions actually have a barrier at all. In fact, some so-called allowed reactions possess particularly high barriers so that they hardly proceed. One such case, discussed by Houk et al. (1979), concerns the allowed trimerization of acetylene to yield benzene. Despite being extremely exothermic (AH° = — 143 kcal mol ) this reaction appears to have an activation barrier in excess of 36 kcal mol"1. Since the orbitals of the reactants correlate smoothly with those of the products, in accord with the Woodward-Hoffmann rules, the origin of barrier formation in allowed reactions generally, needs to be clarified. 

Very few mechanistic studies have appeared on this subject and Houk group has studied the hetero-Diels-Alder reaction between thioformaldehyde and butadiene475, using ab initio calculations, to show that the reaction is concerted and nearly synchronous. In the case of unsymmetrical dienes, several rules are known to establish the regiochemistry of the cycloadducts1 471. 

The Diels-Alder reaction and related pericyclic reactions, which can be treated qualitatively by the Woodward-Hoffmann rules (Section 4.3.5), have been reviewed in the context of computational chemistry [39]. The reaction is clearly nonionic, and the main controversy was whether it proceeds in a concerted fashion as indicated in Fig. 9.5 or through a diradical, in which one bond has formed and two unpaired electrons have yet to form the other bond. A subtler question was whether the reaction, if concerted, was synchronous or asynchronous whether both new bonds were formed to the same extent as reaction proceeded, or whether the formation of one ran ahead of the formation of the other. Using the CASSCF method (Section 5.4.3), Li and Houk [40] concluded that the butadiene-ethene reaction is concerted and synchronous, and chided Dewar and Jie [41] for stubbornly adhering to the diradical (biradical) mechanism. 

With this model, we need only apply the method already used to derive the selection rules for electrocyclic reactions (p. 53). From the Coulson equations, we can deduce that in the in conrotatory cyclization of pentadiene, the MO generates a destabilizing C5-C4 secondary interaction, a stabilizing and Fg a destabilizing interaction. The absolute values of these contributions rise steadily because the terminal coefficients increase from Fg to Fg. Therefore, the sign of their sum is given by the HOMO contribution. If R is an attractor, the HOMO is Fg and rotation inwards is favored. If R is a donor, the HOMO is 4T and rotation inwards is disfavored. As the Coulson equations are valid only for polyenes, these conclusions are correct insofar as R can be modeled by a carbon 2p orbital. It follows that the Rondan-Houk theory works better for conjugative than for saturated substituents. 

Perhaps the most spectacular illustrations of the above rules come from the Houk group. For example, these authors were able to show that heating 3-r-butyl-3-methoxycyclobutene (12), or the 3-tri-methylsilyloxy analog (13) in CeDe (90-95 C) or CDCb (reflux), respectively, afforded only the dienes... 

The selection rules predict that the thermal ring opening should be conrotatory for cw-3-chloro-4-methylcyclobutene, but they do not distinguish between products 6-2 and 6-3. Both products are allowed by the selection rules, but experimentally only product 6-2 is formed. An explanation for this preference of one allowed process over the other is beyond the scope of this book however, possible explanations can be found in the following references Dolbier, W. R., Jr. Koroniak, H. Burton, D. J. Bailey, A. R. Shaw, G. S. Hansen, S. W. /. Am. Chem. Soc. 1984, 106, 1871-1872 Rondan, N. G. Houk, K. N. J. Am. Chem. Soc. 1985, 107, 2099-2111 Krimse, W. Rondan, N. G. Houk, K. H. J. Am. Chem. Soc. 1984, 106, 7989-7991. 

One of the most dramatic manifestations of an interference effect is the vanishing of a line or of an entire band that, on the basis of known Franck-Condon factors and inappropriately simple intensity borrowing ideas, should be quite intense (see Fig. 6.6). This effect can easily be mistaken as an accidental predissociation (Section 7.13). Yoshino, et al, (1979) have studied the valence Rydberg N2 b, E+ cVE+ perturbations. Abrupt decreases in emission intensity for c 4 — X1E+ (v = 1 and 4) and b — X (v = 4) bands had been attributed to weak predissociation rather than perturbation effects (Gaydon, 1944 Lofthus, 1957 Tilford and Wilkinson, 1964 Wilkinson and Houk, 1956). The b (v = 4) C4 (v = 1) and b (v = 13) C4 (v = 4) deperturbation models of Yoshino et al., (1979) provide a predissociation-ffee unified account of both level shift and intensity effects. Weak predissociation effects cannot be ruled out, but are not needed to account for the present experimental observations. 

Houk s rule, an electrophilic analogue of Cram s rule which predicts the transition state depicted as (36) to be favoured, has been tested quite extensively by Fleming and co-workers for the methylation and protonation of enolates. Although the... 

The philicity of a carbene directly depends on the structure of the transition state of an addition reaction. The rules of orbital symmetry conservation forbid the least-motion C2v-symmetry reaction path [41]. For electrophilic carbenes, characterized by predominance of the n — p interaction, preferable is the so-called 7r-approach (Fig. 8.3). In the case of nucleophilic carbenes, optimum conditions for the overlap between the (Tcxy 7r -orbitals are provided by the asymmetrical cr-approach (Fig. 8.3b). By making use of certain assumptions, Rondan, Houk, and Moss [44, 45] calculated the overlap integrals Sjj between the corresponding frontier orbitals of carbene and alkene for the n- and the (7-approaches. Then, having computed the energies of those orbitals, they obtained the energies of stabilization of the composite system arising in two... 

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