Chiral radical

Abstraction of a hydrogen atom from C3 of ( )-2-chloropentane produces a radical that is chiral (it contains a stereocenter at C2). This chiral radical can then react with chlorine at one face [path (a)] to produce (2lS,3.S)-2,3-dichloropentane and at the other face [path (b)] to yield (2 S, 3i )-2,3-dichloropentane. These two compounds are diastereomers, and they are not produced in equal amounts. Each product is chiral, and each alone would be optically active. 

The trick used in asyrmnetric inclusion polymerization is to perform the reaction in a rigid and chiral environment. With more specific reference to chirality transmission, the choice between the two extreme hypotheses, influence of the starting radical (which is chiral because it comes from a PHTP molecule), or influence of the chirality of the channel (in which the monomers and the growing chain are included), was made in favor of the second by means of an experiment of block copolymerization. This reaction was conducted so as to interpose between the starting chiral radical and the chiral polypentadiene block a long nonchiral polymer block (formed of isoprene units) (352), 93. The iso-prene-pentadiene block copolymer so obtained is still optically active and the... 

Stereoinduction is also possible in addition of chiral radicals to ethyl acrylate.4 Thus the radical formed from the bromo amide 6 derived from (R,R)-2,5-dimethyl-pyrrolidine reacts with ethyl acrylate to give the mono- and di-adducts 7 and 8. The monoadduct is formed with 36 1 stereoselectivity at C2. 

Abstraction of a hydrogen atom from carbon 4 yields a chiral radical intermediate. Reaction of this intermediate with chlorine does not occur with equal probability from each side, and the two diastereomeric products are not formed in 1 1 ratio. The first product is optically active, and the second product is a meso compound. 

This result argued for the alternate view of the EEC Cp mechanism sequence, involving initial face-selective attack on the chiral radical-cation intermediate by methoxyl radical (path A), followed by acetal cyclization and proton loss. A rationale for the increased chemical yields observed for the systems studied in Eqs. (44) and (45) was not addressed in this work, although intramolecular solvation and stabilization of the intermediate radical cation by the appended hydroxyether side chain was suggested as one of the possible factors involved in the observed stereodifferentiation [101,102]. 

Chiral auxiliaries have also been applied to the radicals themselves in the formation of chiral hydroxyalkyl radical equivalents [59]. Once again, stereocontrol is accessed through the use of chiral acetals, which are readily available in the form of sugars. Typical reactions of this type are shown in Eq. (13.47). First, the thiohydroxamate ester 148 is prepared so that radical intermediate 149 can be formed photolytically via Barton s radical decarboxylation protocol [60]. The chiral radical 149 can then be trapped by methyl acrylate in a 61% yield with an 11 1 diastereomeric preference for y-substituted 150. 

In some examples, the stereochemistry of radical reactions was controlled by chiral carbohydrate auxiliaries. As a radical counterpart to the ionic conjugate additions discussed above, Garner et al. [169] prepared carbohydrate linked radicals that were reacted with a,P-unsaturated esters. The radical precursor, the carboxylic acid 256, generated by the addition of ( Sj-methyl lactate to tri-O-benzyl-D-glucal and subsequent ester hydrolysis, was decarboxylated by Barton s procedure (Scheme 10.84) [170]. Trapping of the chiral radical 258 with methyl acrylate furnished the saturated ester 259 in 61% yield and with high diastereoselectivity (11 1). The auxiliary caused a preferential addition to the si-facQ of radical 258, probably due to entropic effects. The ester 259 was transformed in acceptable yield to the y-butyrolactone 261 by reductive removal of the thiopyridyl group followed by acid hydrolysis. 

Acetoxy-substitutcd chiral radicals are generated via halogen abstraction from 1-acetoxy-l-bromo-2,3,3-trimethylbutane11. Addition across the double bond of a variety of different alkenes proceeds with very similar diastereoselectivity. 

In nitrile- or alkyl-substituted chiral radicals there is no allylic strain and little or no diastereose-lectivity is observed18. 

This similarity of transition state geometries of oc-oxyalkyl radicals and carbonyl groups suggests that the Felkin-Anh rule should be applicable also in radical chemistry. This is actually the case. The transition state 50A of chiral radical 50 is about 1.5 kcal/mol lower in energy than SOB. 

Recently, Curran and coworkers had demonstrated that chiral radicals derived from camphor sultam give high levels of asymmetric addition in cyclization reactions with... 

An example of cyclization using chiral radicals derived from natural products is given in the 5-exo-dig radical cyclization of A-alkynyl sulfonamide 53 derived from 2(5)-serine to give 2(5)-4-exomethyleneproline 54 (equation 43). The exomethylene product has been recovered without any detectable directly reduced, noncyclized product or product derived from an endo cyclization. 

The most typical approach is the introduction of a chiral center into the end groups of mesogenic side chains. Some chiral terminal radicals are presented in Fig. 9. The first FLCP [9], as well as the first low-molar-mass ferroelectric liquid crystal [5], contains the simplest 2-(5)-methylbutyl chiral radical. Many related chiral 2-methylalkyl and 2-alkyl fragments have been tested to date [41-45]. To... 

FIGURE 9 Chiral radicals used as terminal fragments of side chains in ferroelectric LC polymers. 

In vinyl polymerization with the aid of chiral radical initiators, an enantiodifferentiation reaction sometimes occurs. In this case it can be assumed that once the reaction is initiated, the differentiating reaction proceeds following the equation below, by chirality transferred only to an S group of the growing end of the polymer. 

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