Strain and Reactivity: Electrophilic Addition of Bromine and Tribromide Salts to Cyclic Allenes

The kinetics and the products of the bromination of several cyclic allenes, from C9 to C13 (1 a–e), with tetrabutylammonium tribromide (TBAT) and Br2 have been investigated in 1,2‐dichloroethane (DCE) and methanol. The first product of the interaction between the allene and Br2 is a 1:1 π complex. T...

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Veröffentlicht in:Chemistry : a European journal 2002-02, Vol.8 (4), p.967-978
Hauptverfasser: Chiappe, Cinzia, De Rubertis, Antonietta, Detert, Heiner, Lenoir, Dieter, Wannere, Chaitanya S., Schleyer, Paul von R.
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Sprache:eng
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Zusammenfassung:The kinetics and the products of the bromination of several cyclic allenes, from C9 to C13 (1 a–e), with tetrabutylammonium tribromide (TBAT) and Br2 have been investigated in 1,2‐dichloroethane (DCE) and methanol. The first product of the interaction between the allene and Br2 is a 1:1 π complex. The stability constant of this complex, determined at 25 °C for allene 1 a, is 7.4 M−1. The comparison of this value with those reported for several alkenes and alkynes further support the hypothesis of the existence of sizeable structural effects on the stability of these complexes. The negative values of the apparent activation energy for the reaction of allenes 1 a–e with Br2 in DCE demonstrate the involvement of these complexes as essential intermediates along the reaction coordinate. Different stereochemical behavior was observed in the bromine addition on going from the strained 1,2‐cyclononadiene to the larger compounds. Furthermore, a solvent‐dependent stereochemistry has been observed for each compound. The kinetic and product distribution data have been interpreted in terms of the influence of the strain on the nature of the intermediate and by considering the competition between pre‐association and ion‐pair pathways on going from aprotic to nuclophilic solvents or when nucleophilic bromide ions are added. Ab initio (MP2/6‐311+G**) and density functional (B3LYP/6‐311+G**) computations of 1:1 Br2 complexes showed that the association energies of allene⋅Br2 and ethene⋅Br2 complexes are nearly the same but are greater than that of acetylene⋅Br2 complexes. Allene⋅2 Br2 complexes are more stable than their ethene⋅2 Br2 counterparts. Br2⋅allene⋅Br2 structures, in which the bromine molecules interact either with a single allene double bond or individually with both double bonds, are not preferred significantly over alternatives with Br2⋅⋅⋅Br2 interactions. As a result of the entropy, the association of bromine with unsaturated hydrocarbons is usually unfavorable in the gas phase (except at extremely low temperatures); complexes are observed in solution (under ambient conditions), since the entropy loss is reduced as a result of restricted translation and rotation and possible association to the solvent. The 1,2‐cycloheptadiene⋅Br2 > 1,2‐cyclononadiene⋅Br2 > 1,3‐dimethylallene⋅Br2 association energies increase with ring strain. The kinetics and the products of the bromination of several cyclic allenes (1 a–e) have been investigated in 1,2‐dichloroethane and met
ISSN:0947-6539
1521-3765
DOI:10.1002/1521-3765(20020215)8:4<967::AID-CHEM967>3.0.CO;2-I