Diels-Alder reactions periselectivity

It is frequent but not invariable that where a longer conjugated system has a geometrically accessible and symmetry-allowed transition structure like that in 5.90, the longer system is used. Thus, the [8+2] and [6+4] cycloadditions on pp. 15 16, and the [14+2] cycloaddition on p. 44 take place rather than perfectly reasonable Diels Alder reactions, and the 8-electron electrocyclic reactions of 4.51 and 4.54 takes place rather than disrotatory hexatriene-to-cyclohexadiene reactions. This kind of selectivity is called periselectivity. 

Intramolecular Diels-Alder reaction (with high periselectivity and good yields) of conjugated carbodiimides, catalyzed by Lewis acids, affords a simple procedure for the construction of pyrido[2,3-h]indole and indolo[2,3-ft]quinoline ring systems (equation 176)631. This procedure is superior to the often mixed reactions that occur in the absence of the Lewis acid632-635. It is interesting to note that Lewis acids also improve yields and selectivity in intermolecular reactions of this type636. 

There is a special kind of site-selectivity which has been called periselectivity. When a conjugated system enters into a reaction, a cycloaddition for example, the whole of the conjugated array of electrons may be mobilized, or a large part of them, or only a small part of them. The Woodward-Hoffmann rules limit the total number of electrons (to 6, 10, 14 etc. in all-suprafacial reactions, for example), but they do not tell us which of 6 or 10 electrons would be preferred if both were feasible. Thus in the reaction of cyclopentadiene (355) and tropone (356), mentioned at the beginning of this book, there is a possibility of a Diels-Alder reaction, leading to 354, but, in fact, an equally allowed, ten-electron reaction is actually observed,121 namely the one leading to the adduct (357). The product is probably not thermodynamically much preferred to the... 

In this reaction, formation of exo adduct predominates. This is in contrast to the Diels—Alder reaction, which gives endo adduct as the major product. Moreover, the [6 + 4] cycloaddition takes place in preference to a Diels—Alder [4 + 2] cycloaddition (both are thermally allowed). Such a situation is known as periselectivity and is explained by the fact that the coefficients of the frontier molecular orbitals of the LUMO of the tropone are highest at atoms C-2 and C-7. It has been found that the ends of conjugated systems have the largest coefficients in the frontier orbitals, and in accordance with the orbital symmetry rules, pericyclic reactions make use of the longest part of such systems. However, such reactions have to be permissible by the geometry of the molecule. 

Asymmetric catalysis of ene reactions was initially investigated for the intramolecular examples, because intramolecular versions are much more facile than their inter-molecular counterparts. The first reported example of an enantioselective 6-(3,4) car-bonyl-ene cyclization employed a BINOL-derived zinc reagent [81]. This, however, was successful only when excess zinc reagent (at least 3 equiv.) was used. An enantioselective 6-(3,4) olefin-ene cyclization has also been developed which uses a stoichiometric amount of a TADDOL-derived chiral titanimn complex (Sch. 26) [82]. In this ene reaction, a hetero Diels-Alder product was also obtained, the periselectivity depending critically on the solvent system employed. In both cases, geminal disubstitution is required of high ee are to be obtained. Neither reaction, however, constitutes an example of a truly catalytic asymmetric ene cyclization. 

Styrene type [59a], As will be noted further on, the tendency of these initially formed vinylcyclobutane adducts to quickly rearrange via a cation radical mechanism to the cyclohexene (i.e., Diels-Alder) adducts is also more pronounced. In the case of a very highly electron rich substrate such as A-methyl-A-vinylacetamide, even the reaction with the rigidly s-cw-l,3-cyclohexadiene is CB periselective (Scheme 28) [61]. 

Periselectivity and Stereoselectivity.—The thermal stabilities of the Diels-Alder adducts (179) and (180), which are obtained as a mixture from reaction of 2,5-dimethyl-3,4-diphenylcyclopentadienone with ds-3,4-dichlorocyclobutene, are widely different in respect to cheletr opic loss of carbon monoxide. The major isomer (179) remains... 

---