E!Z equilibration

In the latter example in Scheme 10, the enamide moiety provides an accessible E/Z-equilibration pathway (Scheme 11). 

A series of increasingly refined experiments were performed. The final example in the series is outlined in Scheme 12.18, where E/Z equilibration is used to monitor substrate (deca-2,8-diene) and product (but-2-ene) equilibration, and a pair of D3-labelled methyl... 

Re-examination of the benzene-sensitized isomerization of alkenes revealed that the photostationary EjZ ratio critically depends on the structure of alkenes and the triplet energy of sensitizers [69]. For instance, thermal E-Z equilibrations of 2-octene 19c and 3,4-dimethyl-2-pentene 20 give the thermodynamic EjZ ratios of 3.3 and 3.5, respectively, while the photostationary E Z ratios upon benzene sensitization are 1.1 and 1.6, respectively (Schs. 9 and 10). For practical runs, the use of /7-xylene or phenol, rather than benzene, in ether is recommended, as the yellowing of the irradiated solution, which retards the photoreaction, is substantially decreased. 

Acyclic semidiones (RC(0)=C(0 ) R) and their metal complexes exhibit rapid E — Z equilibration where the ElZ ratio is determined by the extent of ion pairing. The ElZ ratio changes directly with the alkali metal cation radius or the solvent dielectric constant. Both K+ and Rb+ dimethylsemidione prefer the ( )-conformation. The free energy of the ( ) rubidium species is lower by ca 2 kJmoH than that of the potassium species. As the size of the R group increases, the proportion of E increases . In DMSO in the presence of potassium ion, the ( )-dimethylsemidione is more stable than the (Z)-isomer by 10.5 kJmoH. For comparison, dimethylethylene (i.e. 2-butene) is more stable as the ( )-form by 4.3 kJmoH. ... 

The experiments reported by Malkov and Kocovsky [12h] and by Sun [lla,b,e,f], indicate that the Lewis base catalyzed imine reduction with trichlorosilane is not affected by isomeric nonhomogeneity of the starting imines. Thus, for example, imines 9h-9j, which exist as 5 2 to 5 3 E/Z mixtures, were reduced to the corresponding amines with 94 97% ee (Table 4.5, entries 8 10). Appar ently, traces of HCl, naturally present in the moisture sensitive Cl3SiH, trigger an E/Z equilibration of imines 6-10, which is faster than the reduction (Scheme 4.5). 

Same product distribution as for 7c, probably due to E/Z equilibration in the presence of the catalyst. b Overall yield from the lactonic precursor of7e. 

The enhancement of E selectivity by benzoic acid is especially striking. This phenomenon was first reported in 1974 (100b), but little is known regarding its origin. In the examples mentioned above, benzoic acid preferentially catalyzes the E-selective Wittig pathway and does not cause E/Z equilibration of the products (99). Thus, the enhanced ( )-enoate selectivity is the result of kinetic control. 

Remarkably, the same final enantiomer was obtained in the products even starting from Z enals, which is in agreement with a stereoconvergent catalytic system and a rapid E-Z equilibration, as detected by NMR sjiectroscopic studies. The mechanism is believed to occur via an iminium ion intermediate since salts of tertiary amines seem to be ineffective. 

Stereoconvergence owing to reactant or product equilibration We also saw several cases where the product composition was the same for stereoisomeric reactants, e.g., for E- and Z-allylic reactants. This can occur if there is an intermediate step in the mechanism that permits E- and Z-equilibration or if the final stereoisomeric product can attain equilibrium. 

In contrast, oxime ethers and esters are usually stable in solution but the E/Z isomerization can be induced by acids " or by irradiation ". Recently, Narasaka and colleagues"" "" studied the equilibration-isomerization of (E)-O-acyl oximes 239 in the presence of an acid in a nucleophilic solvent (equation 71). Isomerization probably proceeds via protonation of the oxime nitrogen followed by addition-elimination of a nucleophilic solvent until the equilibrium of E and Z isomers is achieved. The isomerization of the more labile 0-acyloximes occurs either by an Sjv2 substitution at the oxime nitrogen with acids and/or by acyl exchange through the formation of a mixed anhydride and the free oxime. 

Thus, the (3-ionone was smoothly deconjugated and ethynylated to give ethynyl-retro-ionol as a mixture of E/Z stereoisomers. Formation of the carbonate and its Pd-catalyzed rearrangement produced straightforward a mixture of aldehydes and a allene compound. After silica-gel chromatography, the allenic-aldehyde was conjugated with a catalytic amount of HBr in acetone. Retinal was obtained as a mixture of E and Z isomers (75/25), which could be converted into the all E isomer by simple equilibration, Fig. (33). 

The conversion of higher substituted to less substituted olefins is not favored thermodynamically. This obstacle can be overcome when the newly formed C=C bond gains stabilization by conjugation with another double bond, e.g. C=0, or an aromatic unit, or by equilibration into a more stable product in a consecutive reaction. The E-Z geometry of the starting olefin may have a significant effect on the configuration and enantioselectivity in the product. 

Geometrical stereoselectivity can often be achieved in the condensation of unsymmetrical ketones 8 with tosylhydrazine l,2 and this feature means Shapiro reactions direct from an unsymmetrical ketone 8 via E-9 lead to the less substituted vinyllithium 11. On the other hand, a sequential alkylation-Shapiro sequence from a starting symmetrical hydrazone 12 will reliably form the more substituted vinyllithium 14 via Z-9 Retention of Z stereochemistry in Z-9 is dependent on its re-use almost immediately on standing, for example, Z-9 (R = vinyl) equilibrates to an 85 15 ratio E Z-9J ... 

In the reaction of the lithium salt of 62 with acetone after a short lithiation time of 30 min, a 10% or 25% E Z mixture of 63 was obtained together with the addition product 64 in 65% yield (equation 42)65. The formation of only one addition product, and the quick disappearance of the -isomer, are due to a fast deprotonation process the -isomer compared with the Z-isomer, and the high rate of equilibration between the lithium compounds that greatly favors the syn anion, which reacts with acetone to give 64. These results point out that the formation of the syn lithium compounds is favored in oxime ethers for kinetic as well as for thermodynamic reasons. The kinetic preference, according to Ensley and Lohr65, is due to coordination between the lithium amide and the oxime oxygen. 

Tin enolate chemistry is quite sparse and thermochemical data generally lacking. One of the few examples arises from the addition of BusSnH to ketenes RR C=C=0 to form enolates. While the enthalpies of formation of few ketenes are known, and thereby provide little guidance as to the enthalpy of formation of these enolates, Z/ -equilibration of the final enolates where R 4 R tells us about isomers. With R = Me and R = Ph, the enolate MePhC=CHOSnBu3 is formed in a 7 3 Z/ ratio the Z isomer is almost 2 kJmol more stable than its E counterpart. 

It was found that the bisphosphine system gave a 4.5 1 E Z ratio of products, independently of the time of exposure to the catalyst. The 4.5 1 mix of products could be exposed to more Ru-2 without change. However, when the more active catalyst Ru-4 was used in the reaction, the products were equilibrated. At low conversions, the E Z ratio was near that obtained with Ru-2, but at higher conversions the ratio increased to 11.5 1. If the product mixture of 4.5 1 was reacted with Ru-4, it was equilibrated to the equilibrium mixture of 11.5 1 [25]. 

If E,E dienes like 163 are wanted, then such Wittig reactions are ideal as the mixed products can be equilibrated to the E,E diene by addition of small amounts of radical generators such as iodine or PhSH. The commercial (BASF) synthesis of Vitamin A involves all trans retinol 174 that can be made from two different allylic ylids derived from 175 and 178 with the appropriate aldehydes 176 and 177. In both cases E,Z mixtures are formed, but equilibration with iodine gives 174 with an all E side chain.46 A different synthesis of such compounds appeared in chapter 11. 

E Z (or trans cis) isomerization (Section 5.5) of alkenes, involving a 180° rotation about a C=C bond, is known to be initiated both thermally and photochemically.524 526,530,553 fter equilibrating, the thermal process produces a mixture of the two isomers in a ratio that reflects their relative thermodynamic stabilities. In contrast, photoisomerization of an alkene (Scheme 6.1) is governed by the ratio of the E —> Z and... 

Apart from photocyclization reactions, simple trienes undergo many types of other phototransformations, including E Z photoisomerization (Section 6.1.1), sigmatropic shifts (see later) or photoadditions (Section 6.1.4). For example, conformational control and possibly equilibration between the excited conformers result in a... 

The photochemistry of vitamin D (see also Special Topic 6.4 above and Scheme 6.8) has also played a central role in the development of modern organic photochem-istry.564,598,617 618 The concept of non-equilibration of excited rotamers (NEER Section 6.1.1) has been used to explain the excitation-wavelength dependence of E Z isomerization (Section 6.1.1) of previtamin D3 (41).619 Whereas the quantum yield for E Z isomerization decreases with increasing wavelength, the formation efficiencies of the 6jt-electron conrotatory ring-closure products, diastereomeric 7-dehydrocholester-ol (provitamin D) (64) and lumisterol (65) (Scheme 6.22), increase dramatically. This was found to occur on the basis of a participation of both the Si and S2 excited states in the photoreaction.620 For example, the quantum yields of 64 and 65 formation were [Pg.244]

---