Periselectivity tropones

The periselectivity of the tropone-diene cycloaddition is dependent on the reaction temperature. The exo [6 + 4] cycloadduct is considered to be the kinetic product, the endo [4 + 2] cycloadduct being the thermodynamic product291. 

The relative complexity of the extended ir-systems present in these reactants suggest that a variety cf potentially competitive pericyclic processes could be operative in the thermal reaction of a given diene with tropone. Consequently, the question of periselectivity in these reactions has been examined in some detail in recent years and a numl r of useful generalizations have been enumerated for predicting products in a variety of circumstances. 

The periselectivity of the reaction of a given diene with tropone is critically dependent on temperature. The variable course of the cycloaddition of ( )-l-trimethylsilyloxybutadiene with tropone as a function of reaction temperature is a Carnatic illustration of this phenomenon. At 80 C the major adduct is bicy-clo[4.4.1]undecenone (10), whereas in refluxing xylene the [4 + 2] cycloadduct (11) prevails as a mixture of regioisomers. A further example of the dichotomy between [6 + 4] and [4 + 2] reaction pathways can be seen with (Z)-l-acetoxybutadiene, which provides some [6 + 4] cycloadduct along with larger quantities of various [4 -i- 2] pr ucts as depicted in equation (1). In contrast to these results, dienes such as ethyl 2,4-hexadienoate and furan do not yield any [6 + 4] or [4 2] products when heated with tropone. The latter result, in particular, may be reflective of the reversibility of the furan cycloadducts. Various furan derivatives do provide modest yields of mixtures of endo and exo [4 + 2] cycloadducts with tropone when reacted together at 3 kbar and 130 C. ... 

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. 

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