Thermal cyclization electronic effects

The Bergman cyclization is quite sensitive to substituent effects, and substituents at the alkyne termini are generally the most effective. It has been demonstrated that electron acceptor substituents, and more in particular cr-acceptors and/or Tr-donors. can facilitate the thermal cyclization, but apart from the work reported by Sander for the tetrafluoro-substituted enediyne 110 (which undergoes an exothermic Bergman cyclization), so far this approach has not produced particularly promising results (Scheme 19.30). 

Novel photochemical (and thermal) reactions of macrocyclic oxa-sila-acetylenic ring systems (expected to show unusual optical properties because of electronic effects arising from orbital overlap of the acetylenic n system with the silicon a bonds and the oxygen lone-pair electrons) were described. While thermolysis in the presence of a transition metal carbonyl compound gave cyclization to both benzenoid and fulvene species, photolysis in the presence of the transition metal carbonyl compound (which catalyzes 1,2-silyl shifts across a carbon-carbon triple bond) gave fulvene and vinylidene products, the latter being readily photolyzed to the fulvene 159 (equation 101). 

The effect of the number of n electrons upon the stereochemistry of a reaction is illustrated by the cyclization of a diene system compared to a triene system, as shown above. Although the methyl groups in both compounds have the E configuration, the products have different stereochemistry. Although both reactions are thermal, only the trans isomer results from the diene and only the cis isomer results from the triene. Thermal electrocyclic reactions of systems with 4 n electrons have the opposite stereochemistry to structurally related systems with 4 + 2 ti electrons. Furthermore, the stereochemistry of the thermal and photochemical pericyclic reactions is opposite. Photochemically initiated cychzation of the triene gives the trans isomer, whereas the ds isomer forms in the thermal cyclization. 

The type of motion the orbital of the terminal carbon undergoes in an electrocyclic reaction can be detected only if substituents are bonded to these atoms. Substituents move as orbitals move. The thermal cyclization of (2A,4Z,6T)-octatriene provides an example of this effect. We consider 713 because it is the HOMO. It contains the highest energy electrons, so it is the frontier molecular orbital. As outlined above, disrotatory motion is required for a O bond to form at the ends of a conjugated triene. Disrotatory motion of the terminal 2p orbitals causes simultaneous disrotatory motion of the C-1 and C-8 methyl groups and yields r-5,6-dimethyl-l,3-cyclohexadiene (Figure 25.6a). 

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