Trans-Cis Interconversion

Aromatic compounds such as toluene, xylene, and phenol can photosensitize cis-trans interconversion of simple alkenes. This is a case in which the sensitization process must be somewhat endothermic because of the energy relationships between the excited states of the alkene and the sensitizers. The photostationary state obtained under these conditions favors the less strained of the alkene isomers. The explanation for this effect can be summarized with reference to Fig. 13.12. Isomerization takes place through a twisted triplet state. This state is achieved by a combination of energy transfer Irom the sensitizer and thermal activation. Because the Z isomer is somewhat higher in energy, its requirement for activation to the excited state is somewhat less than for the E isomer. If it is also assumed that the excited state forms the Z- and -isomers with equal ease, the rate of... 

Although the cis-trans interconversion of aikene isomers does not occur spontaneously, it can often be brought about by treating the aikene with a strong acid catalyst- If we interconvert ris-2-butene with fraus-2-butene and allow them to reach equilibrium, we find that they aren t of equal stability. The leans isomer is more stable than the cis isomer by 2.8 kj/mol (0.66 keal/moj) at room tempera-... 

It has been suggested that the elusive zwitterionic state [75], or a novel nucleophilic addition/elimination mechanism at the central carbon of the exocyclic bridge [79], or solvent-solute H-bonding interactions [76, 80] might play a role in modulating cis-trans interconversion. Cis-trans isomerization gives rise also to a remarkable intrinsic photochromism of HBI, as it can be easily and reversibly induced upon light absorption [74—76, 79, 80]. 

Substantial data exist in the literature to demonstrate that the metathesis reaction itself is a means for cis trans interconversions. For the purpose of this discussion, it is assumed that metathesis is the only pathway for cis trans isomerization. 

Three different isomers are formed consecutively on reacting [RuCl2(PPh3)3] with 4,6 -bis(pyrazol-l-yl)pyrimidine, bpzpm the final product is cis,trons-[RuCl2(PPh3)2(bpzpm)] (160). Nuclear magnetic resonance spectroscopy (NMR) shows facile cis < - trans interconversion for [H(dtbp)Ru(p-Cl)2Ru(dtbp)H], where dtbp = bis[di(t-butyl)phosphano-methane or -ethane. Crossover experiments implicate mononuclear intermediates in these isomerizations (161). 

An excellent synopsis of the influence of double-bond (and pseudo double bonds, such as the amide bond) geometry on biological activity has been published [8], The central role of the amide pseudo double bond on the architecture of proteins is illustrated when the trans preference of amide bonds is recognized. The enthalpic difference between the two conformers has been shown to lie between 0.5 and 2.5 kcal/mol with a barrier to cis-trans interconversion of approximately 61-22 kcal/mol [8] (Fig. 8). 

We have mentioned already (Section 22-3C) the large differences in nmr chemical shifts between the inside and outside hydrogens of [18]annulene —a substance which with 18 tt electrons should be aromatic by the 4n + 2 rule. These differences are observed only at low temperatures. The proton nmr spectrum of [18]annulene at room temperature is a single resonance, which indicates that the inside (Ha) and outside (Hb) hydrogens are equilibrating rapidly. This can take place only if cis-trans interconversion occurs about the double bonds (marked c and t) ... 

Studies of the kinetics of cis-trans interconversion in disilene 33 indicate that it takes place by dissociation to the silylene and recombination, rather than by Si=Si bond rotation as is the case for other disilenes187. This isomerization occurs even at 50 °C. 

After the TICT minimum is reached, the transition moment between charge-separated state and the ground state represented by A-B and AB, respectively, is expected to be fairly small owing to almost no overlap between part A and B. Therefore, the fluorescence intensity will be small and significant contributions most probably stem from neighboring geometries (6 90°) for which the emission from admixed locally excited states can occur. The return from Sj or Tj minimum to S0, which can proceed in radiative or radiationless manner, usually does not lead to formation of cis-trans isomers as one would expect from the assumed energy surfaces. This is due most probably to rapid thermal cis-trans interconversion in the S() state. So far in this Section electronic properties of the free molecules have been addressed. 

Kametani and co-workers (43, 44) have used 13C NMR to study the effect of C-1 substituents on the stereochemistry of the quinolizidine system in this group of alkaloids. In O-methylcapaurine (73), which has a methoxy group at C-l, the quinolizidine system was considered to be in the cis form. This conclusion was based on the upheld shift of C-6. Comparison of 73 with 71 and 72 revealed some interesting differences. C-5, C-6, C-8, and C-l3 were all shielded in the cis form of the 13-methyltetrahydroprotoberberines relative to the trans form but only C-6 and C-l3 were affected in 73. Carbon-14 in 73 was shifted upheld relative to 72 in a manner analogous to that found for C-l in compound 72, and this steric shielding is probably greater than any chemical shift change associated with cis-trans interconversion. C-5 and C-8 of 73 did not show any upheld shift as they did in the 13-methyl compounds. These results indicated a difference in the cis conformation of the two types of compounds and it has been proposed that this was caused by different cis-trans equilibrium. constants (28, 48). 

This reasoning was used by the author in 1961 to rationalize the ubiquitous photochemical cyclization of butadienes to cyclobutenes here it was noted that the excited state has a high 1,4-bond order. The same reasoning was applied 6,12) to understanding the key step of cyclohexadienone rearrangements (vide infra). Still another example is the decreased central bond order in the excited state of stilbene which, as Daudel has noted 13), is in accord with photochemical cis-trans interconversion. 

It may then be asked how the small amount of cis-trans interconversion sometimes observed in aquation can come about. It is quite possible that a fraction of the pyramidal intermediates, X, rearrange to a trigonal bipyramidal intermediate (XI) before reacting with a water molecule. As indicated in Figure 23-2, two of the three most likely positions of attack will result in a cis product, whereas the third will result in a trans product. 

From the very limited evidence available, it appears that when an octahedral substitution proceeds via direct displacement, there is only a minor amount of d,l or cis-trans interconversion—that is, configuration is primarily retained. This is the case for a large number of conversions of the complexes of Pt(IV), and a smaller number of conversions involving complexes of Co(III), Cr(III), Rh(IlI). and Ir(III)—all of these may not involve the direct displacement mechanism, but some almost certainly do. Thus, we see a marked contrast to substitution reactions of tetrahedral carbon, where every act of displacement results in inversion of configuration. 

Russian workers have reported that cis-l,2-bis-(l-napththyl)ethene in aqueous microsuspensions undergoes cis-trans interconversions and cyclization to... 

Chiral centers, more than one, lljf Chiral stereomer, 69 Cholesterol, chirality in, 81 Cinnamaldehyde, 328 Cis-trans interconversion, 111 Cis-trans isomerism, in cyclic compounds, 163 Claisen condensation, 394 rearrangement, 439 Cleavage, oxidative, 117 Clemmensen reduction, 219, 311 Coenzyme A, 354 Collins reagent, 264 Collision frequency, 39 Configuration, 72 relative, 76 Conformation, 51 Conformational diastereomers, 78 enantiomers, 78 stereomers, 78... 

At 14Q-150°C ds-1-phenyl-2-vinylcyclopropane (207) undergoes cis trans interconversion and above 200°C ring enlargement into 4-phenylcyclopentene (208) is observed (equation 140). ... 

N.m.r. spectroscopic evidence has been provided to suggest the dissociation of relatively simple 1,3,2,4-dithiadiphosphetane 2,4-disulphides (i.e. those with non-bulky groups) in solution this evidence comprises the observed inversion at one of the phosphorus atoms (i.e. cis/trans interconversion) and the exchange reactions... 

Even after conversion to the dienal, reversible conjugate addition of water would allow cis/trans interconversion. 

---