Substituted alkanes, chirality

When there are many carbon atoms which might be asymmetrical, the solution is more complicated. Nevertheless, by methods that basically rely on Polya s Theorem, enumeration of compounds taking chirality into account can be carried out. For alkanes and mono-substituted alkanes see the paper [BalA76] for the chiral alkanes with some restrictions see [QuiL77,79]. See also [HarF75], [PalE77], and [WorNSl] for other problems in which chirality appears. 

The compound has four degrees of unsaturation (if it were a substituted alkane, its formula would be C,Hn,N), which indicates the presence of a phenyl ring, (A) dissolves in HCI and therefore is an amine. Since (A) releases with HONO, it is a 1° amine. Since (A) is optically active, it has a chiral C. The —NH cannot be on the ring since the two remaining C s cannot be positioned to give a chiral C. The —NH must be on the side chain. The compound is C H5CH(NH2)CH,. 

Give examples of a chiral substituted alkane and an achiral substituted alkane. 

Alkanes undergo substitution of hydrogen atoms when treated with halogens. Bromination of which of the following compounds could give rise to chiral monosubstituted products ... 

In regard to the stereoselectivity of the insertion process, Murray and coworkers have shown that the CH oxidation of substituted cyclohexanes by dioxiranes is, like the already discussed epoxidation, highly stereo-controUed . A specific case is c -decalin, which gives only the cis alcohol, as exemplarily displayed in equation 27. A similar stereoselective retention of configuration was also obtained for frawi-decalin and cis- and frawi-dimethylcyclohexanes"°. In fact, complete retention of configuration was demonstrated in the CH oxidation of chiral alkanes ". For example, the optically active (f )-2-phenylbutane was converted by either DMD or TFD" to (5 )-2-phenylbutan-2-ol (equation 28) without any loss of the enantiomeric purity (ep) in the product. 

Bromination of alkanes follows the same mechanism as chlorination. The only difference is the reactivity of the radical i.e., the chlorine radical is much more reactive than the bromine radical. Thus, the chlorine radical is much less selective than the bromine radical, and it is a useful reaction when there is only one kind of hydrogen in the molecule. If a radical substitution reaction yields a product with a chiral centre, the major product is a racemic mixture. For example, radical chlorination of n-butane produces a 71% racemic mixture of 2-chlorobutane, and bromination of n-butane produces a 98% racemic mixture of 2-bromobutane. 

Nucleophilic addition of sulfur ylides to C=0 double bonds is an important means of synthesis of epoxides [198], Because optically active epoxides are widely applied as versatile intermediates in the preparation of, e.g., pharmaceuticals, the asymmetric design of this sulfur ylide-based reaction has attracted much interest [199, 200, 212, 213], One aspect of this asymmetric organocatalytic process which has been realized by several groups is shown in Scheme 6.87A. In the first step a chiral sulfur ylide of type 204 is formed in a nucleophilic substitution reaction starting from a halogenated alkane, a base, and a chiral sulfide of type 203 as organocata-... 

Various other [3 + 2] cycloadditions, affording chiral, anellated C6o derivatives with stereogenic centers in the addends are reported in literature. The products were generally obtained as racemates and resulted from reaction of buckminsterfullerene with species like 2,3-disubstituted 2//-azirincs (via nitrile ylides [under direct irradiation] or via 2-azaallenyl radical cations [sensitization by photoinduced electron transfer]),365 1-substituted 5-diazopentane-1,4-diones (via cyclic carbonyl ylides),366 7-alkylidene-2,3-diazabicyclo[2.2.1] hept-2-ene (via a diradicaloid trimethylenemethane derivative),367 1-benzylpy-razolidine-3-ones in the presence of aldehydes (via pyrazolidinium ylides),368 2-trifluoromethyl-2,5-dihydro-l,3-oxazol-5-ones (via nitrile ylides),369 nitro-alkanes in the presence of triethylamine and trimethylsilyl chloride (via N-silyloxynitrones),370 or dv-HOCH2 CH=C H C H 2 OCO 2 H( in the presence of... 

The property of chirality is determined by molecular topology, and there are many molecules that are chiral even though they do not possess an asymmetrically substituted atom. Examples include certain allenes, spiranes, alkylidenecyclo-alkanes, and biaryls as well as other specific examples. Some specific molecules that have been isolated in optically active form are given in Scheme 2.2. The configuration of these molecules is established by subrules in the Cahn-Ingold-Prelog convention. We will not describe these here. Discussion of these rules can be found in Ref. 1. 

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