Walden cycle

The following Walden cycle has been carried out. Explain the results, and indicate where Walden inversion is occurring. 

In a second Walden cycle, germane S -3 was converted to the chloro derivative R-13, which was reduced by LiAlPLt to the enantiomeric germane R-32. The configuration of the chloride J -13 was assigned by mixture melting point with the known R sila analogue and from consideration of Brewster s rales. Thus, chlorination must proceed with retention and hydride reduction with inversion of configuration. 

Chlorination with CI2, CCI4, SO2CI2 or ClCH20Me also took place with predominant retention of configuration33. Reduction of the chlorogermanes with lithium aluminum hydride proceeded mainly with inversion to complete the Walden cycle (equation 15). Bromination, on the other hand, led to a nearly 1 1 mixture of cis and trans bromoger-manes. 

Michalski, M., Mikolajczyk, M., and Omelanczuk, ]., Stereochemistry of nucleophilic displacement reaction at thiophosphoryl centre. An example of the Walden cycle involving phosphorus, Tetrahedron Lett., 1779, 1965. 

Nucleophilic substitutions at optically active sulfur sites in sulfinic esters, sulfoxides, sulfoxonium salts, sulfilimines proceed with inversion. Much of the previous literature is cited by Cinquini et at. (1967), who gives an interesting Walden cycle in (124). The actual inversion step is presumed to be the attack of water on the sulfoxonium intermediate, which is isolable (Hogeveen et al., 1966). 

Figure 11.2 A Walden cycle interconverting ( + ) and (—) enantiomers of 1-phenyI-2-propanol. Chirality centers are marked by asterisks, and the bonds broken in each reaction are indicated by red wavy lines.

The stereochemistry of the homogeneous rhodium-catalyzed alcoholysis reaction has also been studied (77) (eq. [52]). The predominant stereochemistry was established through a Walden cycle, reduction of the alkoxysilane 142 occurring with almost complete retention at silicon. As shown in Table 31, the rhodium-catalyzed alcoholysis occurs with retention of configuration but low stereoselectivity. Moreover, when the alcohol was used as solvent, predominant inversion or racemization was observed. Predominant inversion was also found in alcoholysis of a substituted silacyclopentane (171), but concomitant epimerization yielding the equilibrium mixture of isomers was observed. 

Brook addressed this important mechanistic issue early, developing evidence for retention of configuration at silicon in the rearrangement. Based on the precedented assumption of inversion at asymmetric silicon in chloride displacement with either an organolithium or an alcohol, Brook and coworkers showed a stereochemical Walden cycle that implicated retention at silicon in the rearrangement step. Mechanistically, retention at silicon corresponds to a frontside-type displacement, consistent with the mechanism shown above. The displacement might implicate 10 as a pentacoordinate silicon intermediate rather than a transition state. 

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