Retention of the optical activity

Under irradiation in diethyl ether, the dextrorotary enantiomer of 3,6-hexanooxepin-4-carb-oxylic acid rearranges to the oxabicyclo[3.2.0]heptadiene system with retention of the optical activity.36... 

The nature of the intermediate cationic species, whether it is a classical methyl cation or a nonclassical bridged cation, has also been determined. The deamination of optically active e c/o-2-aminobicyclo[2.2.1]hcplane-exo-2-carbonitrile gave 2-hydroxybicyclo[3.1. l]-heptane-1-carbonitrile (19) in 9% yield with 92% retention of the optical activity. Similarly, deamination of e.vo-2-trifluoromethylbicyclo[2.2.1]heptyl-e Jo-2-amine gave 1-trifluoromethyl-bicyclo[3.1.1]heptan-2-ol (20) in 11 % yield with 88% retention of the optical activity. These results are interpreted in terms of intermediate bridged cations.87... 

Intermediate diazonium ions as precursors of carbocations which can rearrange in the manner discussed are also formed in the photolyses of arylsulfonylhydrazones in basic medium. The photolysis of the optically active bicyclo[2.2.1]hept-5-en-2-one tosylhydrazone in 0.5 M sodium hydroxide gave bicyclo[3.1.1]hept-3-en-2-ol (25) in 6% yield with only 5% of retention of the optical activity, which indicates an achiral ally cation as the product-determining intermediate.84... 

Although the same product (exo-acetate 3) is obtained by acetolysis of exo- and enfifo-brosylates 2 there are two data that are clearly pointing to a different solvolysis mechanism for each epimer. The first one is the enhancement of the reaction rate in the case of the exo-isomer. The second is the complete retention of the optical activity of the exo-brosylate (99%) contrasting to the loss of optical activity observed for the e [Pg.271]

Mechanisms involving axial coordination of the optically active amine have also been invoked, and crystal structure data on RCo(DMG)2B complexes, where R is alkyl or (R)-l-(methoxycarbonyl)ethyl, and B is (R )-a-methylbenzylamine, were obtained (316, 317). Because deuteration of the (R)-methoxycarbonyl complex gave (5)-methylpropionate-2-d, it was concluded that Co—C bond cleavage occurred with inversion of configuration at the carbon (317). It would be useful to know the mechanistic details of this step, which could involve attack by D+, DCo(III), or coordinated D, as well as D2, for it is an unusual, if not unique, observation [contrast with the usual retention mechanism outlined in Eq. (39)]. 

The problem of retention of asymmetry of the formed free radical in the fast geminate recombination of radicals was studied by photolysis of the optically active azo-compound PhMeCH—N=NCH2Ph [88,89]. The radical pair of two alkyl radicals was initiated by the photolysis of the azo-compound in benzene in the presence of 2-nitroso-2-methylpropane as a free radical acceptor. The yield of the radical pair combination product was found to be 28%. This product PhMeEtCCH2Ph was found to be composed of 31% 5,5 -(-)(double retention), 48% meso (one inversion), and 21% R.R(+) double inversion. These results were interpreted in terms of the competition between recombination (kc), diffusion (kD), and rotation (kml) of one of the optically active radicals with respect to another. The analysis of these data gave kxo[Pg.126]

A similar vinylic cation was once claimed to be formed during the solvolysis of cyclohexylidenemethyl triflate in aqueous methanol at 140 °C (eq 9),13 but this reaction could occur via participation and SN2-type reaction. Solvolysis of the optically active 4-methyl-substituted triflate under the same reaction conditions took place with complete retention of the optical purity.14... 

Solvolysis of the optically active tosylate IV provided further data to substantiate the intermediate formation of V. If the N-nitroso group did not become involved in the solvolysis, then the product would show either inversion of configuration or race-mization. If, on the other hand, V was an intermediate then the product should show retention of configuration. This was indeed found to be the case, the product acetate had exactly the same configuration as the starting material IV. This result is summarized in the following scheme. 

The original activity referred to is the activity of the optically active sec-butylmercuric bromide used to make the dialkyl compound. The actual result was that, under several different sets of conditions, the product had one-half of the original activity, demonstrating retention of configuration. 

Cleavage of the optically active cyclopropyl compounds (I M = Li)5 by methanol in ether or ether/1,2-dimethoxyethane and (I M = SnMe3)6 by concentrated hydrochloric acid in carbon tetrachloride and by anhydrous hydrogen bromide in CC14 also take place with retention of optical activity and configuration, viz. 

Rearrangement of the acetate of the optically active cyanhydrin 463 proceeds stereoselectively to yield the oc,/ -unsaturated nitrile 464 with 89% ee [197]. Allyl trichloroacetimidate 465 is rearranged from O to N to give 466 at room temperature with retention of the stereochemistry [198]. 

Another effect of the coordination is that the benzylic cation is also stabilized [63]. This stabilization is explained by delocalization of the positive charge due to the interaction of the d-orbital of Cr with the 71-orbital of the benzylic carbon, caused by the coordination of Cr(CO)3. Facile stereospecific Friedel- Crafts-type cyclization of the complex of the optically active benzyl alcohol 248 gave the tetrahydrobenzazepine 249 with retention of the stereochemistry, and the free amine 250 with 98% ee was... 

The Favorski rearrangement normally goes with inversion at the terminus, as in 86 (semi-ezo-S) rather than 87 (semi-17). Reusch and Mattison (1967) have verified this for the pulegone oxides of which only one of the optically active forms is given in (181). It is of interest that the cyclopropanone intermediate opens abnormally, that is with retention. These authors note that their conditions are similar to those for electrophilic substitution with retention, namely frontside proton transfer from a polar aggregate (Cram, 1965). 

Haller-Bauer reaction.1 A study of the base-induced cleavage of the optically active tertiary a-phenyl ketone (l)2 shows that cleavage proceeds with retention with metal amides in benzene and alkoxides in an alcohol or benzene, but with... 

An entirely different result has been reported by Jacobus and Pensak. They found that the reduction of the optically active 1-bromo-l-methyl-2,2-diphenylcyclopropane (51) with sodium naphthalenide (NaN) in DME (0.5 m) yields the corresponding hydrocarbon 49 of 29% optical purity with net retention of configuration. This observation was interpreted to mean that the l-methyl-2,2-diphenylcyclopropyl (t radical was being captured by a second SET from sodium naphthalenide to give the sodium derivative (which transforms in DME to 49) at a rate faster than its inversion frequency (Scheme 14). 

Table 18 summarizes the net retentions of configuration of the cyclopropane 49 observed in the reactions of the optically active cyclopropyl halides 5(1-52 with alkali metal naphthalenides MN as the halide X, the gegenion M and the concentrations are varied. The effect of inverse versus normal addition is demonstrated in Table 19. The influence of the solvent is given in Table 20. Dicyclohexyl-18-crown-6 also affects the. amount of retention of cyclopropane 49 (Table 21).

Kinetics studies allowed us to evaluate k0 and k for the Fe(phen)32+/cyanide reaction in relation to the stereochemistry of the Fe(phen)2(CN)2 product (1 ). Although it is evident that the optically active product comes from the cyanide dependent k path, the percent of optically inverted Fe(phen)2(CN)2 product is not directly related to the percent which reacts by the cyanide-dependent path. (To illustrate, at 0.4 M cyanide almost 90% of the reaction proceeds through the cyanide dependent path but the optical activity is only about one-third of the optical activity observed for the 2.0 M cyanide reaction product.) On the other hand, a linear relationship does exist for a plot of (% by kx path)/(% optically active product) vs. 1/[CN-], where the % by the ki path is determined from kx[CN ]/(k0+k1[CN ]). Several possible mechanisms have been considered for the reaction, but the simplest mechanism considered which agreed with the results allowed a separation of retention and inversion paths. The strange linear relationship can be fit to the reaction scheme ... 

NiCl2 (229) (eq. [63]). Deuterium-hydrogen exchange was shown to occur with retention of configuration at the silicon atom of the optically active deuterio-silane 154 (229) (eq. [64]). 

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