Threo-erythro interconversion

Iodide attack trans to the bridging phenyl at either carbon yields the threo isomer, but will yield the RS) or SR) isomer depending on which carbon is attacked. The basic assumption used in the interpretation of the erythro and threo isomer interconversions is that the configuration of the carbon a to the R group remains constant. Hence, if inversion occurs at both centers, it would not be detected. Thus, an observation interpreted as retention with the Ph(CHD)2-system should be viewed with suspicion nevertheless, racemization or inversion would be interpreted correctly. Studies with optically-active (CHD)Ph corroborate the observed inversion with bromination found with -(CHD)2f-Bu hence, care must be taken with interpretations using the (CHD)2Ph system, and it would appear that the butyl system provides a more reliable indication of processes at a primary carbon. 

Manganese.—Elimination of transition-metal hydride from metal alkyls and addition of metal hydrides to alkenes are usually considered to be cA-processes. Since acylmanganese compounds undergo stereospecific reversible decarbonylation, thermal decomposition of (eryrAro-2,3-dimethylpentanoyl)(pentacarbonyl)manga-nese(i) should allow the determination of the stereochemistry of elimination of [MnH(CO)8] (Scheme 4). However, both the erythro and a mixture of the erythro and threo acyl complexes decompose thermally to give the same mixture of cis- and trans-3-methylpent-2-ene and 3-methylpent-l-ene under conditions which do not isomerize these alkenes. It is suggested that the mechanism involves interconversion of... 

The stereospecificity of the allylation in Scheme 5.12 is rationalized by identifying the most favorable transition state of the retro-allylation (Scheme 5.14). Oxidative addition of Np-Br followed by bromide-alkoxide exchange with 4 generates intermediate 6 or 7. In the case of threo-4,6a should be the more favorable conformation than 6b according to the routine conformational analysis of chairlike transition states. The retro-allylation thus predominantly proceeds from 6a to yield ( )-8. Thanks to bulky tricyclohexylphosphine, the following reductive elimination should take place rapidly prior to o-jt interconversion to retain the E-stereochemistry. A similar picture is applicable to the reaction of erythro-4, where 7a is more favorable than 7b to afford the corresponding Z-product selectively. 

Mechanistically, this variation is similar to the classical Wittig reaction. The only departure stems from the physico-chemical nature of the intermediates. The addition of ylide 34 to carbonyl compound 3 occurs as before to set up a mixture of erythro 35 and threo 37 adducts. Now the betaines are more acidic, which facilitates the interconversion of 35 and 37. Deprotonation of 35, in situ, produces 36. Kinetic protonation occurs from the least sterically hindered face, as shown in 38. With the equilibration to... 

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