Carbon, asymmetric tetrachloride

Dilongifolyl borane [77882-24-7] M 422.6, m 169-172 . Wash with dry Et20 and dry in a vacuum under N2. It has m 160-161 in a sealed evacuated capillary. It is sparingly soluble in pentane, tetrahydrofuran, carbon tetrachloride, dichloromethane, and chloroform but the suspended material is capable of causing asymmetric hydroboration. Disappearance of solid indicates that the reaction has proceeded. [J Org Chem 46 2988 1981.]... 

According to this correlation model, in which the principles of steric control of asymmetric induction at carbon (40) are applied, the stereoselectivity of oxidation should depend on the balance between one transition state [Scheme 1(a)] and a more hindered transition state [Scheme 1(6)] in which the groups and R at sulfur face the moderately and least hindered regions of the peroxy acid, respectively. Based on this model and on the known absolute configuration of (+)-percamphoric acid and (+)-l-phenylperpropionic acid, the correct chirality at sulfur (+)-/ and (-)-5 was predicted for alkyl aryl sulfoxides, provided asymmetric oxidation is performed in chloroform or carbon tetrachloride solution. Although the correlation model for asymmetric oxidation of sulfides to sulfoxides is oversimplified and has been questioned by Mislow (41), it may be used in a tentative way for predicting the chirality at sulfur in simple sulfoxides. 

This reaction is a formal asymmetric aldol addition following a modified Evans protocol. The enolate 26 is formed at 0 °C in the presence of one equivalent of titanium tetrachloride as Lewis acid and two equivalents diisopropylethylamine (Hunig s base) as proton acceptor. Selectively the Z-enolate is formed. The carbon-carbon bond formation takes place under substrate control of the Tvan.v-auxiliary, whose benzyl group shields the, v/-face of the enolate. 

Figure 5.7 The structure of carbon tetrachloride and its symmetric and asymmetric...

In 1965, Raban and Mislow [90] postulated that nuclei placed in an asymmetric magnetic field should show NMR nonequivalence. In 1966, Pirkle [91] first reported the validity of the prediction when it was shown that (5)-l-phenylmethylamine caused F-NMR nonequivalency of 2,2,2-trifluoro-1-phenylethanol in a carbon tetrachloride solution. In later studies, 2,2,2-trifluoro-l-(9-anthryl)ethanol, an NMR shift reagent, was used as a mobile-phase additive to separate 2,4-dinitrophenyl methyl sulfoxide on a silica gel column [92]. Later, one enantiomer of this fluoroalcohol was covalently attached to silica gel and used for resolution of a large number of solutes including sulfoxides, lactones, derivatives of alcohols, amines, amino acids, hydroxy acids, and mercaptans [93]. 

Our synthesis started from hydroxy ketone B (Figure 5.31), which was obtained by asymmetric reduction of prochiral diketone A with fermenting baker s yeast.38 The key step in the present synthesis was the ring formation by intramolecular alkylation of C to give D. To obtain C, the enfifo-hydroxy group of B was first epimerized via retro-aldol/aldol by treatment with p-toluenesulfonic acid in carbon tetrachloride. The tricyclic intermediate D was converted to (+)-pinthunamide (146), mp 187-189°C, [a]D215 = +60 (EtOH), which was identical with the natural product. Its absolute configuration was thus determined as depicted in 146.39... 

The photolysis of an optically active acetyl silane 1 (eq. [1]) implies the formation of an asymmetric radical 2 which retained its configuration upon trapping by carbon tetrachloride (17). The chlorosilane 3 was then reduced to the hydrosilane 4 with inversion of configuration. 

The compound precipitates from ethanol as minute, violet crystals. Larger, darker-colored crystals are obtained by recrystallization from water. The salt is very soluble in water but is only slightly soluble in methanol, ethanol, carbon tetrachloride, chloroform, acetone, ethyl ether, or benzene. The anion is asymmetric,2,5 the salt thus existing as a racemic mixture of dextro and levo forms. 

Ruthenium-catalyzed asymmetric addition of alkane- or arenesulfonyl chlorides to styrenes leads to optically active suifones22,23. When this conversion is conducted under mild conditions no C —C bond is formed, however, at higher temperatures (100 =C), enantioselective asymmetric carbohalogenation of styrene with trichloromethanesulfonyl chloride (accompanied by sulfur dioxide extrusion) can be achieved with tetrachlorotris[( + )- or (—)-Diop]diruthenium. Excellent yields (up to 100%), but only low asymmetric inductions (up to 13 %), arc observed12. Similar results are obtained with carbon tetrachloride. A mechanism with radical formation w ithin the metal coordination sphere ( radicaloid ) has been proposed. 

The key structural feature of POST-1 - the presence of dangling pyridine groups in the channels - affords a unique opportunity to perform asymmetric heterogeneous catalysis. Thus, potentially, any base catalyzed reactions (e.g., esterification or hydrolysis) can be performed with POST-1. Moreover, chiral pores should induce a degree of enantioselectivity in the final product mixture. The catalytic activity of POST-1 in the transesterification reaction was examined. Although the reaction of 16 and ethanol in the presence of POST-1 in carbon tetrachloride produced ethyl acetate in 11% yield, little or no transesterification occured without POST-1 or with the iV-methylated POST-1 (Sect. 2.2). The post chemical modification of the pyridine groups in POST-1 proves the role of free pyridine moiety in transesterification reaction. Transesterification of ester 16 with bulkier alcohols such as isobutanol, neopentanol, and 3,3,3-triphenyl-l-propanol occurs at a much slower rate under otherwise identical reaction conditions. Such size selectivity suggests that catalysis mainly occurs in the channels. 

In a molecule such as methane (Figure 6-7) or carbon tetrachloride, in which the four bonds are equivalent, the bond angles have the value 109°28. In an asymmetric molecule such as CHFClBr the angles differ somewhat from this value, but only by a few degrees. It has been found by experiment (x-ray diffraction, electron diffraction, microwave spectroscopy) that these angles usually lie between 106° and 113°, with the average value for the six bond angles close to 109°28. ... 

As shown in Figure 8.1, primary and secondary amines are distinctly different in the first overtone region near 6600 cm. Primary amines have a doublet, and secondary a single peak. Tertiary amines have no peak as they have no N-H functionality and are not shown in the figure. The asymmetric and symmetric NH-stretching peaks occur at 6553 and 6730 cm (1625 and 1486 nm), respectively, in butyl amine in carbon tetrachloride. The first overtone of secondary amines has only one band near 6530 cm (1530 nm). 

The N—H stretching vibration corresponds to an infrared band between 3500 and 3400 cm in the spectra of secondary aliphatic amines taken in dilute solution in carbon tetrachloride to avoid molecular association . Under these conditions primary aliphatic amines show two bands near 3500 and 3400 cm (asymmetrical and symmetrical vibrations) Among the vibration frequencies associated with ammonia two harmonic frequencies are found in approximately the same region (3506 and 3577 cm ) Therefore we can anticipate that the main force constant RR will be of the same order of magnitude for NHg, as well as for secondary and primary amines. This fact supports the hypothesis that the electronic structure of the N—bonds would not be very different in all these molecules. 

Explaining symmetry numbers is not easy. Just consider the following examples asymmetric molecules have = 1 sample rotational symmetry numbers are 3 for NH3, 2 for H2O, 4 for ethylene, 6 for ethane, 12 for benzene and methane. Molecules with the same symmetry have the same symmetry number, so methane and carbon tetrachloride both have cr = 12. 

Similar results have been obtained by polymerization of a or j3-naphthofuran (LXXIV) [200]. Like benzofuran, these monomers are not dissymmetric and it is only when the molecule has reacted that a sequence of asymmetric carbon atom is formed along the main chain they polymerize very easily in presence of aluminum chloride or titanium tetrachloride and j3-phenylalanine the use of c/-alanine or /-histidine as co-catalyst always leads to polymers with no optical activity both for benzo- and naphthofurans. Polynaphto-furan can be crystallized by thermal treatment, in contrast to polybenzofuran and its structure, at least partially, must be considered isotactic. This last work dates back to 1966. Since that time, no other results have been published in this field. 

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