Retention/inversion ratio

An interesting behavior of the retention/inversion ratio in dl- and meso-(CH3CHCl)2 was observed upon dilution with several compounds (Fig. 1) (91). This effect was assigned to the relative concentrations of... 

Page 115) Not only does steric hindrance predict the retention/inversion ratio, but also the nature of the leaving group. In halopropionyl halides the degree of inversion for 34mCl-for-X substitution is... 

The controlled potential electrolysis of endo-7-bromo-exo-7-chlorobicyclo[4.1.0] heptane (31) and exo-7-bromo-endo-7-chlorobicycyclo[4.1.0]heptane (30) resulted in a mixture of exo-32 and endo-7-chlorobicyclo[4.1.0] heptane (33) in which the retention-inversion ratio was 2.6 1 in each case. Overall retention of configuration is the usual observation However, this need not always be the case, since by changing the substituent at the reductive center from methyl in bromo-l-methyl-2,2-diphenyl-cyclopropane (51) to a carboxyl or carbomethoxyl group, the resulting product was still optically active (30-40%) but the configuration was inverted... 

The stereochemical results are dependent on similar reaction stereochemistry and rates for the mphen and phen chelates. The calculations are also valid if the retention inversion ratio for the second step is simply due to a k -j and (1-a) k i.e., kdi>>kai- Clearly, the A configuration is formed in the first step, but the C3 isomer clearly predominates in the optical activity. The slight A activity of the C2(trans)-Fe(mphen) (CN) isomer appears to be due to the large amount of A-C1-Fe(mphen)2-(CN)(H20)+ produced in the first step. Some reinverts either through the 1-a part of kd path and/or the kal- path. 

Compound mb) aq. EtOH (fcEtOH/ AcOH)Yb) fcOTs/feBr c) 80 % EtOH CH3/fcH d) Bromides 80 % EtOH Retention e) Inversion Ratios Azide Rate-Product Correlation 0... 

Alternatively, it is possible that the very short life-time of the car-banion is the product-determining factor. Since less rotation is required for it, retention would always be preferred, and inversion would be observed only if the conformer obtained after 60° rotation is by-passed either as a result of the stability of the carbanion, or if the rotation and the elimination rates are comparable. We will call this route in which the importance of retention, inversion and isomerization decrease in this order, the preferred retention mechanism . Discrimination between conformers due to steric interactions leading to them does not play a role in determining the product ratio, and the difference in the degree of retention for each member of the pair of isomers cannot be predicted. 

Fig. 1. Solvent effect on the stereochemical course (ratio retention/inversion) of 38C1-for-Cl substitution in me- and meso-2,3-DCB. Key rac-DCB-Br2 rac-DCB-CH3OH A rac-DCB-rc-CsHia O rac-DCB-o-C5Hi2 9 meso-DCB-Br2 meso-DCB-CH3OH A meso-DCB-n-C5H12 meso-DCB-o-C5H12. [Reprinted with permission from ref. (91). Copyright 1972 American Chemical Society.]...

Surprisingly, the Hunsdiecker reaction using the silver salts of exo- and endo-7-chlorobicyclo[4.1.0]heptanecarboxylic acids and bromine at 0°C did not result in the same ratio of products but instead showed a high retention to inversion ratio of 88 12 for the exo acid and 88 12 for the endo acid". This anomalous result may be a reflection of the bromine radical s ability to trap the cyclopropyl radical but this is unlikely. Altman and Baldwin as well as Ando and coworkers found that the reduction of each of the isomers of 7-bromo-7-chlorobicyclo[4.1.0]heptane, 30 and 31, respectively, by the excellent radical scavenger triphenyltin hydride resulted in an identical mixture (21 79) of exo-(32) and en io-7-chlorobicyclo[4.1.0]heptane (33). This ratio of products is, within experimental error, identical with that found in the thermal decomposition of exo- and endo-t-buiy 7-chlorobicyclo[4.1.0]heptane-7-percarboxylate in cumene. 

Herriott used the phosphorus epimers of 0-(-)-menthyl methylphenylphosphinoth-ioate to demonstrate the almost quantitative stereospecificity in the reaction with mCPBA by contrast, the use of trifluoroperoxyacetic acid was less successful, with a retention to inversion ratio of 21 79. A further feature in favour of mCPBA is the independence of its activity relative to solvent. Other extensive correlations were made by Inch and coworkers during investigations into the stereochemistry of displacement reactions at phosphorus in compounds based on a carbohydrate template 81 by means of structural assignments based on infrared absorption frequencies for the P=0 bond together with P NMR chemical shifts, they showed that mCPBA oxidation of thiophosphoryl or selenophosphoryl bonds (A or B = =S or =Se, B or A = Me or Ph) occurred highly stereoselectively with the formation of the phosphonate ester with retained stereochemistry ... 

Furthermore, the catalytic allylation of malonate with optically active (S)-( )-3-acetoxy-l-phenyl-1-butene (4) yields the (S)-( )-malonates 7 and 8 in a ratio of 92 8. Thus overall retention is observed in the catalytic reaction[23]. The intermediate complex 6 is formed by inversion. Then in the catalytic reaction of (5 )-(Z)-3-acetoxy-l-phenyl-l-butene (9) with malonate, the oxidative addition generates the complex 10, which has the sterically disfavored anti form. Then the n-a ir rearrangement (rotation) of the complex 10 moves the Pd from front to the rear side to give the favored syn complex 6, which has the same configuration as that from the (5 )-( )-acetate 4. Finally the (S)-( )-mal-onates 7 and 8 are obtained in a ratio of 90 10. Thus the reaction of (Z)-acetate 9 proceeds by inversion, n-a-ir rearrangement and inversion of configuration accompanied by Z to isomerization[24]. 

One interesting observation deduced from the data of Table 13 is that, despite the same diastereomeric ratio, the absolute configuration around the carbon atom of the major diastereomer changed with the electrophile used. In other words, the reactions of a-lithiobenzyl sulfoxides with 020 - and its carbonylation and carbonation proceeded with retention of configuration, while methylation with CHjl resulted in inversion . The stereochemical study of these reactions was conducted with optically active benzyl sulfoxides 42 and 39 as shown in Scheme i - - nd an interesting... 

The ratio of inversion (51ab) to retention (51d) in the product ketone would then be determined by the relative rate of rotation about Ci—Q> in (52c) compared with the rate of migration of Ph. 

Bromination of (-)-(5 )-benzyl methyl sulfoxide 34 with bromine in pyridine affords a mixture of two regioisomers a-bromomethyl benzyl sulfoxide 328 and a-bromobenzyl methyl sulfoxide 329, in a molar ratio of 3 2 (325). Oxidation of thp latter gives the corresponding sulfone (-)-(S)-330, the absolute configuration of which was determined by X-ray analysis. In this context, it is interesting to point out that the formation of the sulfoxide 328 is accompanied by retention at sulfur, whereas 329 is formed with inversion at sulfur. 

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