E and Z configurations

The hydrogen bond formation decreases the frequency of the O—H bond valence vibration (see Section 4.2.3). Two configurations of tertiary hydroperoxides are known E- and Z-configurations. The activation barrier for transition from Z- to /i-configuration is found to be equal to 195 kJ mol 1 (quantum-chemical calculation [64]). 

Alkylamino crotonic esters (22) all show in solution the presence of the isomers of both E and Z configurations, and there is also some evidence for rotamers involving restricted rotation about the (C=C)-N and C-C02R bonds (64). In all cases studied, when only one crystal form was found the molecular configuration was Z about the double bond. For R = CH2Ph, R = CJI, two crystal forms were obtained, one with E and one with Z molecular configuration. 

The authors did not report on trisubstituted diphenylphosphine oxides, but apparently, the results are excellent with what is usually a difficult 1,1 unsubstituted pattern about the olefin. The investigation found near perfect selectivity and yields for most of the reported compounds when using 26 as a ligand. However, a very slow reaction was noted for the E olefin 59h while hydrogenation of E configured olefin 59j preceded smoothly, indicating a delicate balance between structure and reactivity. Other anomalous behavior of the substrates 59h-j is that both E- and Z-configured olefins produce the same enantiomer. 

It was found that benzaldehyde reacts with E and Z configured silyl ketene acetals to furnish identical aldol products [93] with high enantioselectivity. Neither diastere-oselectivity nor enantioselectivity were affected by double bond geometry of the silyl ketene acetal. This is an evidence for an acyclic transition state (Scheme 27). 

The E and Z configurations were assigned by determining the Jhh coupling constant between the two chemically equivalent olefinic protons from the satellites in the H P NMR spectrum, and has been confirmed by X-ray analysis of both isomers [19],... 

The first family is represented by peptides based on a, 3-dehydro a-amino adds in which a C=C bond confers rigidity and electronic Y-conjugation on the system (Section 11.1). Peptides rich in these amino acids are widespread in nature, particularly as antibiotics, and possess interesting conformational properties. 1-11 In addition to the E and Z configurational isomers, occurring when two different atoms (substituents) on CP are present, fully-extended, characteristically flat conformations or p-tums and 310/a-helices may be induced by u,p-dehydro a-amino acids. 

A combined experimental and computational approach has been undertaken to identify the origin of syn/anti diastereoselectivity in two types of crotylation reactions of aldehydes and ketones (i) multi-component crotylations of simple aldehydes/ketones and (ii) acetal substitution reactions of aldehyde dimethyl acetals with E- and Z-configured crotyltrimethylsilane.175 The stereochemical outcome is nearly identical in the two reactions, and the computational results suggest that this is due to near identical mechanisms an SN1 process involving attack of O-methyl-substituted carboxenium ions by crotylsilane. 

In addition to the concerns about the stability of 10-membered cycloalkenes under the reaction conditions, one has to keep in mind that RCM tends to give mixtures of the (E)- and (Z)-configured products when applied to compounds of this size (7). Because a reliable catalyst allowing to control the configuration of the newly formed double bond has yet to be found (14), recourse was taken to a... 

PNA occurs as both E- and Z-rotamers in un-complexed forms, but in duplexes it is uniformly oriented in the Z-form. Schutz et u/. have prepared olefinic PNA (OPA) in which the amide bond is replaced by the isostructural C-C double bond in both E- and Z-configurations (19, 20). Incorporation of either isomer into OPA resulted in a decrease in thermal stability against DNA with similar affinity. Homopolymers of OPA also did not form triplexes. OPA also binds preferentially in the parallel mode. A PNA oligomer to which has been attached diethylenetriamine (DETA) via a urea bond was shown to cleave an RNA target at micromolar concentrations under physiological conditions. ... 

The use of diethyl(phenylthio)aluminum or a mixture of chlorodiethylaluminum and triphenyl-phosphane leads to the usual Claisen products 2 but with E and Z configuration in a nearly 1 1 ratio342. Other catalysts, such as diethyl(diethylthio)aluminum. (tcrt-butylthio) diethylaluminum or diisopropyl(phenoxy)aluminum, are ineffective. 

In the 1,2 rearrangement of 1 [prepared in situ from either ( )- or (- )-(R)-3-buten-2-o]] chirality is transferred from an acyclic side chain to the ring449. The stereochemical outcome reveals a strong preference for a chairlike transition state with either the methyl group equatorial or axial thus leading to E- and Z-configurated products 3 and 2. 

A diene such as l-chloro-2,4-heptadiene has four configurational isomers because each of the double bonds can have either the or the Z configuration. Thus, there are E-E, Z-Z, E-Z, and Z-E isomers. The rules for determining the E and Z configurations were given in Section 3.5. Recall that the Z isomer has the high-priority groups on the same side. 

Both E- and Z-configured a-trifluoromethyl enones are accessible via a Meyer-Schuster-type rearrangement of propargylic alcohols (Scheme 13a) [43, 44]. Under copper catalysis, the is-configured enone is obtained. When adding a rhenium salt as co-catalyst, the Z-configured product forms preponderantly. A similar report shows... 

The enantioselective isomerization of a series of E- and Z-configured aromatic and aliphatic allylic alcohols including ( )-3-methyl-2-hepten-l-ol is catalysed by the complex [RuCl2 (5)-tol-binap (/ )-dbapen ] (60) (tol-binap=2,2 -bis(di-4-tolyl-phosphanyl)-l,l -binaphthyl, dbapen = 2-dibutylamino-l-phenylethylamine) in the presence of KOH and EtOH to the / -substituted aldehydes with 90% yield and >99% ee along with a small amount of the saturated alcohol with >99% ee. A plausible mechanism promoted by the hemilability of the dbapen/Ru chelate stmcture is also discussed. ... 

Later, it has been proven that the initial stage of the process is a 1,3-prototropic shift in O-vinyl oximes, primary intermediates of the reaction. Therefore, the early interpretation of regiodirectivity of the pyrrole synthesis, based on the idea about different reactivity of E- and Z-configurations of the starting ketoximes, needs correction. It is obvious that 1,3-prototropic shift in O-vinyl oximes will proceed easier with the participation of the methylene group and not the methyl moiety, because in the former case the disubstituted ethylene fragment of N,0-divinyl hydroxylamine A is known to be more thermodynamically stable in comparison with unsubstituted compound B (terminal vinyl group) (Scheme 1.104). 

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