Separation of the Cis and Trans Isomers

The crude product is dissolved in the minimum quantity of benzene and is placed on an alumina-filled column about 60 cm. in length and 5.5 cm. in diameter prepared by packing it, under benzene, with 60 g. of Merck alumina per gram of crude crystalline product. Elution with 1 1 (by volume) methylene chloride (dichloromethane)-benzene mixture removes a red band a second band is eluted from the column by methanol. Evaporation of the solvent from each of these fractions affords the red crystalline isomers, t 

Recrystallization apparently leads to considerable loss of material without appreciable gain in purity. From a 5.1-g. sample of the crude mixture subjected to both chromatography and recrystallization the checkers isolated 0.72 g. of cis isomer, m.p. 163.4 to 164.0 , and 2.25 g. of trans isomer, m.p. 169.2 to 169.6°. 

The isomers of tris(3-oxobutanaIato)chromium(III) are red crystalline compounds soluble in benzene and chloroform and insoluble in water. The purity of the isomers is best determined by chromatography. The infrared spectra of both isomers show major bands at 1585, 1500, 1430, and 1335 cm.  

The cis isomer is defined as that compound in which the three methyl keto groups are on adjacent corners of the octahedral structure the trans isomer has two of the methyl keto groups on opposite apexes of the octahedron. Both the cis and trans isomers should be di-racemates, but their resolution has not been reported. The structures of the isomers are assigned on the basis of their different chromatographic adsorption (the cis isomer adheres more strongly to alumina) and the ratio of isolated products (the trans isomer predominates) by analogy with the work of Fay and Piper on tris complexes of unsymmetrical 1,3-diketones. 

The crude mixture of cis and trans isomers has been nitrated, brominated, and thiocyanogenated at the central carbon of each chelate ring in the manner of a quasiaromatic system. 

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The synthesis of dithienyl photochromes containing a maleic anhydride fragment (a furan-2,5-dione fragment) as the ethene bridge was a difficult problem. Initially, vicinal dinitriles 160 and 162 were used as the starting compounds (Scheme 49). But the latter compounds give a maleic anhydride only in the cis conformations. The separation of the cis and trans isomers is an additional time-consuming laborious problem. 

The separation of the cis- and trans-isomers of l,2-bis(trimethylsilyl)-, 1-tri-methylgermanyl-2-trimethylsilyl- and l,2-bis(trimethylgermanyl)ethylene is possible by gas-solid chromatography on graphitised carbon black and by gas-liquid chromatography on Chromosorb W supporting Apiezon L, with helium as carrier gas and a thermal conductivity or flame ionisation detector445. 

In a study of the analysis of organophosphorus pesticides, fenchlorphos, cruformate and fenthion were hydrolyzed to the corresponding phenols which were reacted with dansyl chloride to form fluorescent derivatives. Separation of the three derivaties was achieved by adsorption chromatography on silica. Separation of the cis and trans isomers of three organophosphorus pesticides, dimethyl-... 

Chiral P-enamino ester 8 7 was prepared from 2-oxo-cyclopentanecarboxylates and (R)-a-methylbenzylamine [69, 99]. Reduction of 87 with sodium triacetoxyborohydride led to the amino ester 88 with diastereoselectivities of 85% (R=Et) and 67% (R=CH2Ph). After separation of the cis and trans isomers of 88 (R=CH2Ph), the benzyl groups were removed by hydrogenolysis, giving directly pure 89 with ( S,2R) configuration [99]. 

The four stereoisomers of dihydroxyprecocene I (66) have been prepared by monoesterification of the diol, separation of the cis and trans isomers, hydrolysis and resolution of the diastereoiso-mers as their bis(-)menthyloxyacetates. Study of the 4-dimethyl-aminobenzoates by circular dichroism enabled the absolute configuration of the diols to be determined.82... 

Figure 1. Chromatogram showing the separation of the cis and trans isomers of 3-hexen-l-ol (cis/trans 3-H) and a and p-naphthoflavone (NF) on a 25-cm p-CD column using 50% aqueous methanol as the mobile phase. [Conditions flow rate = 1.0 ml/min., 24 C, and chromatographic system II. The wavelength for detection of the 3-hexen-l-ols was 220 nm while that for the naphthof lavone s was 273 nm.]...

The new complexes II (n = 1, m = 2 n = m = 2) have been prepared from the reaction between two moles of [Mo(NO)L Cl2] and the appropriate diaminodibenzo-crown ether. The bis complex IV was also prepared from the reaction between the monoiodide derivative, [Mo(NO)L I NHC6H30CH2(CH20CH2)3CH20 ], and NH2QH30CH2(CH20CH2)3CH20 in the presence of sodium naphthalenide. These new complexes were characterised by elemental analysis and by infrared, H-NMR and mass spectroscopy. In the cases where n = m= and n= 1, m = 2 it proved possible to separate the cis and trans isomers of the diaminodibenzo-crown ether precursors and in these cases only one isomer of the bimetallic complex was synthesised and used in the subsequent studies. However, in the case where n = m = 2 separation of the cis and trans isomers was not possible and the complex characterised was a mixture of the two isomeric forms. Since the single isomers and the mixture exhibited similar spectral properties and behaved in a similar manner in the electrochemical experiments it would appear that there are no electro-chemically detectable differences between the cis and trans isomers. 

The S-bonded thiocyanate is too labile to permit separation of the cis- and trans-isomers. 

Chapter 8 mentions the preparation of the acetyihumulinic acids in 35% yield. The separation of the cis and trans isomers could not be achieved completely by CCD. It would be interesting to realize this separation by LC, to confirm the claimed yield and to find out what the rest of the reaction mixture consists of. 

Thiete sulfones may be74b converted to the corresponding saturated thietanes and followed by oxidation of the latter to the desired sulfoxides185 (equation 82). By chromatography, the mixture (207) can be separated to the cis and trans isomers. 

Co. and 20 mM nitrite in water, pH 3, 25 C, 60 min reaction], it was nitrosated to yield 3.62 mM 2,6-dimethyl-N-nitrosomorpho-line (DMNM), which was a 10% greater yield than that for a similar nitrosation of morpholine to give NMOR. This indicated a slightly larger rate constant for DMNM than for NMOR formation (3). Crude DMM is a 2 1 mixture of the cis and trans isomers ( 0). GC analysis of the product of the kinetic run showed that the 2 isomers were nitrosated at similar rates. Cis-DMNM [retention time (RT), 320 sec] was well separated from NMOR (RT, 430 sec), but trans-DMNM (RT, 405 sec) was not. Accordingly, we prepared pure cis-DMM, b.p. 133 C, by spinning-band fractional distillation of crude DMM and used it in the analytical procedure. The RT of N-nitrosopyrrolidine (NPYR) was 390 sec. 

Perinones. The most important pigment in this family is the orange perinone, Pigment Orange 43 [4424-06-0] which is obtained by reaction of naphthalene-1,4,5,8-tetracarboxylic dianhydride with tf-phenylenediamine. The result is a mixture of the cis- and trans-isomers. The commercial product is the orange trans-compound which must be separated from the dull, bluish red cis-isomer, and then conditioned for pigment use. The pigment is fairly weatherfast and heat stable, and is used primarily in plastics and fiber applications. 

In summary, therefore, after on-line chiral HPLC separation, NMR spectroscopy has been used to characterise compounds in terms of the cis- and trans-isomers and to identify the racemic pairs on the basis of their identical NMR spectra. In addition, HPLC-circular dichroism (HPLC-CD) was used to identify... 

An example of increasing the efficiency by decreasing the flow rate is shown on a pellicular silica gel column in Figure 6-5a. (This figure was shown previously as Fig. 4-15.) This is the separation of a cis and trans isomer of synthetically prepared juvenile hormone mimics which are separated in the normal-phase mode. (Juvenile hormone mimics are used to stop or retard the maturation process of insects and, hopefully, control the insect population.) The presence of benzene is probably residue from the reaction solvent. At 2 mL/min the first peak corresponds to 5000 plates and the last peak corresponds to 2300 plates. By lowering the flow rate to 0.5 mL/min, there is a corresponding increase in efficiency in the separation. With the additional efficiency at the slower flow rate, it is possible to observe a small, additional shoulder (peak) under the first peak. However this was done at the expense of increased time (a factor of 4). 

Let us examine separately the case of the cis and trans isomers of pentadiene. For the trans isomer the cis conformation is permissible so that one cannot assume a priori that this isomer won t coordinate to Ti by the two double bonds. This hypothesis, however, can be easily rejected by the following considerations. If the steric situation around Ti during the polymerization were to permit the coordination of the trans isomer of pentadiene by the two double bonds, in the cis conformation, butadiene or isoprene should also coordinate the same way. In this case, however, cis-1,4 units should be obtained both from butadiene and isoprene, and not 1,2 and 3,4, respectively, as observed. It seems reasonable to conclude, therefore, that the trans isomer of pentadiene coordinates to Ti by the vinyl group only, as butadiene or isoprene, before it is incorporated as a cis-1,4 unit. 

Suzuki coupling of 9,10-dibromoanthracene and boronic acids produces a separable mixture of the cis and trans isomers of 9,10-diarylanthracene 64 which independently behave as a molecular rotary switch on treatment with 02 via the endoperoxide (Scheme 46)... 

Condensation catalyst. Triethylamine serves as basic catalyst for the Perkin condensation of o-nitrobenzaldehyde with phenylacetic acid and acetic anhydride to give a-phenyl-trans-o-nitrocinnamic acid. A student experiment on the condensation of benzaldehyde with the same reactants includes a simple procedure for the separation of the cis- and /rans-isomers. ... 

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