Cis-trans inversion

Properties. Ab initio calculations predict that the cis isomer of triazene is energetically less favorable than the trans isomer by 23 to 30 kJ/mol. Activation energies for the cis trans inversion in the range 200 to 380 kJ/mol were calculated for various transition states at the HF/6-31G level [9]. For an earlier theoretical study on this inversion, see [10]. 

Figure B2.4.2. Eyring plot of log(rate/7) versus (1/7), where Jis absolute temperature, for the cis-trans isomerism of the aldehyde group in fiirfiiral. Rates were obtained from tln-ee different experiments measurements (squares), bandshapes (triangles) and selective inversions (circles). The line is a linear regression to the data. The slope of the line is A H IR, and the intercept at 1/J = 0 is A S IR, where R is the gas constant. A and A are the enthalpy and entropy of activation, according to equation (B2.4.1)...

High temperature searches of conformational space (see Quenched Dynamics on page 78), can produce unwanted conformational changes, such as cis-trans peptide flips, ring inversions, and other changes that you cannot reverse easily by geometry optimization. You can use restraints to prevent these changes. 

A quite different type of titanium catalyst has been used in an inverse electron-demand 1,3-dipolar cycloaddition. Bosnich et al. applied the chiral titanocene-(OTf)2 complex 32 for the 1,3-dipolar cycloaddition between the cyclic nitrone 14a and the ketene acetal 2c (Scheme 6.25). The reaction only proceeded in the presence of the catalyst and a good cis/trans ratio of 8 92 was obtained using catalyst 32, however, only 14% ee was observed for the major isomer [70]. 

The thermolysis of ladderanes has been studied in detail (Scheme 1). On heating, bicyclo[2.2.0]hexane and its derivatives exhibit skeletal inversion and cleavage to 1,5-hexadiene derivatives.26 The thermolysis of anti- and yyft-tricyclo[4.2.0.02,5]octanes and their derivatives gives cis,cis- and cis, trans-1,5-cyclooctadienes, cis- and trans-1,2-divinylcy clobutanes, and 4-vinylcyclohexene as ring-opening products.27-29 Furthermore, syn-tricyclo-[4.2.0.02,5]octane isomerizes to aw//-tricyclo[4.2.0.02,5]octane.29c,d The thermodynamic parameters and the reaction mechanisms for these thermal reactions have been discussed. 

The l3C-NMR spectrum of the ethyl-substituted derivative at — 80°C is consistent with the cis-cis-trans conformation and a complete line-shape analysis of the coalescence of the CH2N resonances at higher temperatures gives AG 11.76 0.2 kcal mol-1 at — 40°C for the inversion processes 357 358 359.302... 

The temperature-dependent PMR spectra of the Mn(MePhDtc)3 complex demonstrate that cis—trans and optical isomerization are slow on the PMR time scale below 60°C. For this complex the metal-centered inversion in CD2C12 solution is characterized by a AH of 9.8 1.0 kcal mole 1 and a AG of... 

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