Pseudorotation racemization

The reduction of optically active phosphonium salts by lithium aluminum hydride, which probably does involve 70 as an intermediate, affords racemic phosphines, presumably by pseudorotation in 70 before it decomposes 63). 

The stability of five-coordinate intermediates also makes possible the ready racemization of optically active silanes by catalytic amounts of base. The base can add readily to form a five-coordinate intermediate. The latter can undergo Berry pseudorotation with complete scrambling of substituents followed by loss of the base to yield the racemized silane. 

This molecule has an approximately TBP electronic structure and is chiral. However, potentially it could racemize via a series of Berry pseudorotations. 2 That it docs not do so readily, and is therefore the first optically active sulfuranc to have been isolated, has been attributed to the fact that all racemization pathways must proceed through a TBP with an apical lone pair. 3 As we have seen in the preceding chapter, there is a very strong tendency for the lone pair to seek an equatorial site. The reluctance of the lone pair to occupy an apical site appears to be a sufficient barrier to allow the enantiomers to be isolated. 

It was proposed by Sommer and Bauman that the methanol-catalysed racemization of 1-naphthylphenylmethylfluorosilane involves formation of a pentacoordinate intermediate followed by pseudorotation49. Three successive pseudorotations are required for racemization, followed by axial loss of the solvent. However, this mechanism is not consistent with the reaction being subsequently reported to be third order with respect to nucleophile46,47. 

Two mechanisms have been proposed for the racemization of AA-binuclear species in a homogeneous solvent one takes into account inversion of the coordination sphere when a new metal-to-ligand bond is formed and the other includes conversion to its opposite enantiomer by the pseudorotation. The second mechanism seems to be more feasable in homogeneous systems such as hexane solutions or liquid crystals. In hexane, enantiomeric [Cr(acac)3] undergoes recemization by photoisomerization, giving a mixture of the A,A-, A,A- and A,A-isomers. Spectral assignments were followed by changes in CD and UV-Vis spectra. 

Replacement of a CH2 group in cyclooctane by a CHX or CX2 group, where X is achiral, or by a heteroatom such as O or S, leads to a chemically achiral molecule. If the conformational preference of the group or heteroatom is for the 1 or 5 positions in the boat-chair, the molecule is also con-formationally achiral. However, if the preference is for any other positions, two enantiomeric conformers will result. In the latter case, dynamic nmr can be used to study the sequence of pseudorotations which leads to conformational racemization. 

The data collected on the racemization of arsonium ions are consistent with a mechanism involving Berry pseudorotation of a transient five-coordinate halogeno-arsenic(V) intermediate (equation 22). The more nucleophilic the halide the faster the racemization. 

The question might be asked Are there similar mechanisms for changing the configuration of molecules without breaking bonds in molecules with coordination numbers other than 3 and 5 The answer is yes. One of the most important series of inorganic compounds consists of six-coordinate chelate compounds exemplified by the tris(ethylenediamine)cobalt(III) ion. Because of the presence of the three chelate rings, the ion is chiral and racemization can take place by a mechanism that is closely related to atomic inversion or Berry pseudorotation (the mechanism for six-coordination is termed the Bailar twist see Chapter 13). 

Whereas base-catalyzed epimerization of 27 is faster than hydrolysis base-catalyzed hydrolysis of optically active 25 gives the corresponding P-phenyl phosphetane 1-oxide with retention of configuration The initial phosphoranes A or A, which result from a attack of hydroxide on (R)- or (S)-25, respectively, can in principle equilibrate with five diastereomeric phosphoranes, as shown in Fig. 10 for (7 )-25 However, A, C, and E are relatively high energy intermediates since a Z-butyl group is located in the a position As discussed above, racemization prior to hydrolysis requires that loss of benzyl is slow compared to pseudorotation and addition-elimination... 

The expansion of coordination at silicon. Pentacoordinated species seem to be quite intimately involved in many processes taking place at silicon. Expansion of coordination is the fundamental step not only in the nucleophilic induced racemization reviewed some years ago (13), but also in nucleophilic substitution activated by nucleophiles. A part of this review is devoted to the stereochemical and mechanistic aspects of nucleophilic activation. Furthermore, in connection with a possible isomerization of trigonal bipyramidal silicon by Berry pseudorotation, the dynamic stereochemistry of pentacoordinated silicon compounds is discussed. 

The intervention of one mole of nucleophile in the inversion process contrasts with the results obtained for racemization of chlorosilane where two moles of nucleophiles are involved (13). To account for their results, Martin and Stevenson proposed that inversion at tetracoordinate silicon arises from an isomerization of a pentacoordinated intermediate via Berry pseudorotation steps. Several pathways were proposed, the most interesting being a nucleophilic attack at a carbon-oxygen edge of the tetrahedral silicon compound. In the resulting intermediate, a single Berry pseudorotation with the nucleophile as pivot and equatorial departure of the nucleophile causes an inversion at silicon (eq. [110]). A... 

However, it has been pointed out81-83 that the high order with respect to nucleophile, as given in Table 9, is not compatible with such a mechanism. Indeed, Sommer reported the reaction to be third order in nucleophile but rationalized this in terms of ion-pair formation and solvation effects. Corriu has also pointed out that the energy of activation of racemization (Table 10) is smaller than that expected for pseudorotation (Table 7)82... 

Martin52 has shown that racemization by pseudorotation is important for 46. This reaction has a first-order dependence on the nucleophile and pseudorotation has been... 

Pseudorotation is then followed by loss of Nu to give back a similar molecule 71. However, if 72 underwent pseudorotation to 73 racemization would occur. Whilst this... 

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