Bergman reactions

Other researchers have reported that the cyclization step is believed to be rate determining in the cycloaromatization (Bergman) reaction of aliphatic enediynes." It has been found that the rate-limiting step is hydrogen abstraction by benzannelation. This effect should be attributable to the faster rate of retro-Bergman cyclization from the aromatic ring-condensed 1,4-didehydrobenzene diradicals and/or the slower rate of hydrogen abstraction by them. 

Interest in the aza-Bergman reaction and the influence of protonation on 2,5-pyridyne and its derivatives was (once more) directly stimulated by the search for less toxic antimmor drugs. One approach to increase the selectivity is to decrease the reactivity of the biradical intermediate in hydrogen-abstraction reactions.In this regard, Chen has established a simple model that correlates the reactivity of singlet biradicals with their singlet-triplet gap This model is based on the assumption that the reactivity of the triplet biradical is... 

Grafenstein, J. Hjape, A. M. Kraka, E. Crana, D. An accurate description of the Bergman reaction using restricted and unrestricted DFT stability test, spin density, and on-top pair density, 7. Phys. Chem. A 2000,104,1748-1761. 

Graefenstein, J., Hjerpe, A. M., Kraka, E., Cremer, D. An Accurate Description of the Bergman Reaction Using Restricted and Unrestricted DFT Stability Test, Spin Density, and On-Top Pair Density. J. Phys. Chem. A 2000,104, 1748-1761. 

On the other hand, the introduction of halogen atoms on the double bond was demonstrated to have an inhibitory effect on the Bergman reaction. " In particular, compound 38 has a half-life of 60 h at lOO C. Thus the two chlorine atoms may be considered efficient blocking devices. However, efficient triggering devices for such stabilized enediyne have not been yet devised. 

Transition metal catalysts can also be used in photochemical organic synthesis. For example, the photo-Bergman reaction (cycloaromatization see also Scheme 6.62) of (Z)-hex-3-en-l,5-diyne [(Z)-589], which is obtained by photostationary E Z isomerization (Section 6.1.1) from ( )-589, occurs via transition metal-catalysed cycloaromatization reaction followed by photochemical dissociation of the arene ligand from the complex 590 (Scheme 6.289).1530 The product, 1,2-di( -propyl)benzene (591), is obtained in 91% chemical yield. 

Table 18.2 Enthalpy of activation and heat of reaction of the Bergman reaction.

Scheme 1-5 The Bergman reaction. Calculated geometries (CCSD(T)/6-31G(d,p)) are taken from [116e]. Bond lengths in A, bond angles in degrees.

Many-body Brillouin- Wigner second-order perturbation theory an application to the autoaromatisation of hex-3-ene-1,5-dlyne (the Bergman reaction)... 

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