Optical isomeric shift

The mathematical expression of the above concepts is virtually identical to that for the optical isomeric shift in electronic spectra, and as such is well documented [1-5]. One form of the theory is given in the following paragraphs. 

If -ONO2 group is close to other big groups, the asymmetric and symmetric bands of NO2 group will be blue shifted. Optical isomerism can be distinguished by this phenomenon. Because of the optical isomerism, both stretching vibrations of NO2 group will be split to two distorted peaks... 

In /cTf-butyl methyl ether, the optically active, bicyclic 2//-azepine 12 undergoes a suprafacial [1,5]-H shift to yield the isomeric, non-racemic 3/f-azepine 13.291... 

Another possible source of modification of the HBI optical properties arises from cis-trans (or, more properly, Z-E) isomerization around its exocyclic ethylene bridge (dihedral angle x as depicted in Fig. 3a) [74, 75]. The absorption spectrum of trans HBI in different solvents is red-shifted by 5-10 nm compared to that of the cis conformation [76]. While the trans conformation is thermodynamically unfavorable and contributes only a minor population at room temperature, cis-trans isomerization seems to take place regardless of the chromophore ionization state, and involves a relatively low energy barrier of about 50 kJ/mol [75], a value that appears significantly lower than initially predicted from quantum mechanics [77, 78]. 

As an extension of this work, Atkinson and co-workers (123) prepared l-dibenzylamino-l,2-dihydro-2-quinolone (78) and 1 -(/V-benzy l-N-carboxy-methyl)amino-l, 2-dihydro-2-quinolone (79). The benzylic protons of 78 showed an AB quartet that did not coalesce up to 180°C, and 79 was resolved into optical isomers. The E, for racemization was 26.2 0.4 kcal/mol. Various attempts were made to elucidate the possible pathways for isomerization in these quinolone derivatives (123). Radical dissociation, a sigmatropic shift followed by rotation, and restricted rotation about the S—N bond were excluded. The aforementioned authors (123) also excluded the possibility of nitrogen inversion and preferred restricted rotation about the N—N bond as an explanation for the existence of stereoisomers. They supported this explanation by examining the steric effects... 

In contrast to the isomerization of 39, the retention of optical activity for the conversion of 41 to 42 is very small. However, the loss of activity cannot be explained by a degenerate vinylcyclopropane rearrangement, since the recovered starting material ([(X]54g = +32 ) shows essentially complete retention of chirality. An alternative explanation involves competing stereoselective processes in a chiral intermediate (41a ). For 41a a hydride shift from the allylic position, C2, might compete with the migration from C6 to Cl. This possibility can be probed by... 

When the above-mentioned ring expansion with diazomethane 74) of trimethyl-dioxo[2.2]metacyclophane 65 (methylation was necessary to increase the inversion barrier to > 130 kJ) was performed in the presence of optically active alcohols at —60 °C, asymmetric induction occurred to an extent of ca. 40% ee (enantiomeric excess as determined by nmr-spectroscopy in the presence of chiral shift reagents)85). (+)-DibutyI tartrate favoured the dextrorotatory diketone 66 ([a]D 160° for the optically pure product) — the isomeric 67 was formed only with 3% ee (—)-ethyl lactate on the other hand led to an excess of (+)-67 ([a]D +240°) but gave (+)-66 with only 10% ee85). 

Cotton-Mouton effect), NMR chemical shift and coupling constants, the optical rotation of polarized light and correlation coefficients between different properties. Extensions to incorporate long-range interactions have also been elaborated11 and it has even been possible to adapt RIS theory for the description of the dynamics of transitions between rotational isomeric states.12,13... 

A special case of isomerization is the racemization of optically active compounds catalyzed, for example, by sulfuric acid or promoted A1C13. Thus, treatment of (+)-(5)-3-methylhexane at 60°C with 96% sulfuric acid yields a mixture of racemic 2- and 3-methylhexane68 (Scheme 4.4). At lower temperature (0 or 30°C), racemization occurs, but shift of the methyl group does not take place. It can be concluded that at 60°C methyl migration is faster than hydride abstraction to yield isomeric alkanes. At 0 or 30°C, hydride transfer occurs before methyl... 

In the hydroformylation of optically active alkenes, all aldehydes were shown to be formed with predominant retention of configuration.53,54 This was interpreted to prove, in accordance with the transformation of deuterium labeled alkenes,55 56 that stereoselective hydrogen shifts of the coordinated olefins occur without dissociation of the isomeric alkenes.30... 

An extension of the above work to fV-BOC deprotection of the amino ketones 86 gave the optically active 277-azepines 87 (e.g., 87, R1 = Me, Rz = Me 56%). Ready isomerization of 87 by a [1,5]-H shift to the corresponding 377-azepines was observed when the former were left in solution in organic solvents at 25 °C or on warming in CHCI3 solution (Equation 10) <1996T10883>. 

Reactions studied include dehydrations of alcohols, double bond shifts in olefins, isomerization of hydrocarbons, racemization of optically active compounds, etc.. In the literature a rather rigid separation is made between a Brested acid, which is actually a proton donor, and a Lewis acid, which works as a hydride abstractor. We may illustrate this difference by using the double bond shift in olefins as the model reaction. 

Silver(I) compounds are often used as promoters for substitution reactions of aliphatic halides with carbon nucleophiles. A cyclic (8-bromo ether 29 can be reacted with allyltrimethylsilane (30) imder the influence of AgBp4, yielding a mixture of ally-lated products 31 and 32 (Sch. 7) [15]. Product 31 is formed by direct substitution of the bromine atom in ether 29 by an allyl group and isomeric ether 32 arises from the carboxonium ion which is generated by debromination and subsequent [l,2]-hydrogen shift. A synthesis of optically active 4-allylazetidinone 33 (Ft = phthalimido) has also been achieved by employing the silver-promoted substitution reaction of 4-chloro-azetidinone 34 with allylsilane 30 [16]... 

X-ray analysis showed that oxaziridine 30 is (2R,3R)-2-(S)-l-phenylethyl-3-p-bromophenyloxaziridine. Furthermore, thermal isomerization at 120°C converted oxaziridine 33 into 30 and oxaziridine 32 into 31. Hence the absolute configurations of the chiral centers in 30-33 were established." Moreover, a rule analogous to Freudenberg s rule of shift was proposed that relates the value of the optical rotation of oxaziridines to the absolute configuration, and therefore predicts the latter in a number of oxaziridines. 

This 2,3-sigmatropic shift has been of substantial utility since Evans and Hoffmann realized that allyl alcohols can be obtained by intercepting the sulfenate with thiophilic agents. One possible way of preparing optically active allylic sulfoxides, which started to racemize even at 0 C was the isomerization of the corresponding vinylic sulfoxide followed by the sulfenate rearrangement in the presence of trimethyl phosphite (Scheme 51). ... 

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