Investigating endo-exo selectivity in Diels-Alder reactions

Gosse Welshman, Tammy Lynn (2010) Investigating endo-exo selectivity in Diels-Alder reactions. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

An important goal for synthetic organic chemists is the synthesis of the desired isomer of a target chemical compound exclusively and in high yield. The value and beauty of the Diels-Alder cycloaddition lies in its capacity for selectivity; that is, the isolation of a single stereoisomer in good yield is possible in many cases. A chemist's knowledge of the typical stereochemical outcomes of a variety of Diels-Alder reactions may be used to predict the product distribution for their system of interest. There are three types of selectivity generally observed in these cycloadditions; the reasons for the selectivity we see are not always clear. Endo-exo selectivity is the main focus of this thesis. -- In Chapter 2, the Diels-Alder reaction of 3,3-disubstituted cyclopropene with butadiene was determined to be exo selective for all of the substituents examined, as was predicted from a previous study of a 3-substituted cyclopropene / butadiene system. The addition of a second substituent at the C3-position resulted in a greater stabilization of the cyclopropene ring when compared to the 3-substituted cyclopropenes; this result can be attributed to the anomeric effect. The activation barriers for the 3,3-disubstituted cyclopropene / butadiene system were below those of the syn 3-substituted system (for both endo and exo addition), mostly due to the substantial relief of ring strain in the 3,3-disubstituted cyclopropene / butadiene transition state structures. -- Long-range and short-range substituent effects on the reactivity of cyclopentene dienophiles were considered in Chapter 3. Disubstitution at C4 (a long-range effect) had essentially no impact on the stabilization of the cyclopentene dienophiles, while disubstitution at C1 and C2 (the reaction center) resulted in significant changes in the lengths of the C1=C2 bonds. The Diels-Alder reactions of the various 4,4- and 1,2-disubstituted dienophiles with both butadiene and cyclopentadiene were examined and the most significant selectivities were observed for the 4,4-disubstituted cyclopentene / butadiene system (exoselective) and the 1,2-disubstituted cyclopentene / cyclopentadiene system (endo selective). Secondary orbital interactions are not possible in the transition state structures of these cyclopentene Diels-Alder systems and thus cannot play a role in determining endo-exoselectivity. The observed selectivities can be rationalized qualitatively by examining the favourable and unfavourable interactions between the diene and dienophile in the transition state structures. -- The investigation of the Diels-Alder reactions involving cyclopentadiene and its substituted analogs in Chapter 4 involved two main objectives: the examination of the endo-exo and facial selectivities for a large number of diene / dienophile combinations, as well as the in-depth analysis of the potential energy curve of the endo dimerization of cyclopentadiene. Endo addition was preferred for all of these cycloadditions, which is the most commonly observed preference. However, a characterization of the reaction coordinate revealed an occurrence considered to be rather atypical for Diels-Alder reactions: the existence of the bispericyclic transition state, first discussed in a 2001 communication by Caramella et al. This transition state leads to a lower energy Cope transition state later on the reaction coordinate. Vibrational frequency data along the curve were examined and product distribution was discussed. In the case of the Diels-Alder reactions of cyclopentadiene and its derivatives, the wealth of p-orbitals available for favourable secondary orbital interactions are likely the major contributing factor in the determination of the endo selectivity observed.

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