Inversion and retention

A relatively few oxirane ring openings have been reported to give retention or a mixture of inversion and retention (see the general reviews on the mechanisms of oxirane ring opening, and the specialized one devoted to retention (68RCR448)). 

When this prior stereoi merization is accounted for, the rearrangonent is found to have resulted fixtm a mixture of all possible suprafacial, antarafacial, inversion, and retention combinations in reughly equal amounts, indicating that no stereochemical pathway is strongly preferred. Substituted systems, however, show higher stereoselectivity. Theoretical modeling of the reaction finds no intermediate, but tire titumtinn state is diradical in character. ... 

Fig. 13.7. Symmetry properties for [1,4] sigmatropic shifts with inversion and retention.

When the reaction step R—X R- takes place at a chiral carbon, racemization is almost always observed because free radieals do not retain configuration. Exceptions to this rule are found at cyclopropyl substrates, where both inversion and retention of eonfiguration have been reported, and in the reactions mentioned on page 899. 

The stereochemistry of the nonconcerted reaction has been a topic of considerable study. Frequently, there is partial stereorandomization, indicating a short-lived diradical intermediate. The details vary from case to case, and both preferential inversion and retention of relative stereochemistry have been observed. 

The stereochemistry of the reaction depends on the Lewis acid. Protic acids favor retention of configuration, as does TMSOTf. Most metal halides give mixtures of inversion and retention, but A1(CH3)3 gives dominant inversion.142 Inversion is suggestive of direct carbonyl group participation. 

See Shoppee (1972 362) and Ingold (1948 2730). "A great part of the mystery of Walden inversions is dissipated since we have related it to different mechanisms of substitution" (Ingold [1948] 30). Walden made his discovery in 1895 and listed more than twenty cycles of inversion and retention in Optische Umkehrserscheinungen (1919). 

Further examples of the effects of added cations on stereochemical pathways have been observed using 5-chloromethyl-5-methyl-2-oxo-l,3,2-dioxaphosphorinans. Under normal conditions, methanolysis of (107 R = SPh) leads to the methoxy-ester having the same configuration. In the presence of added cations, particularly Li+, both inversion and retention pathways are observed.79 Phenolysis of the phos-phorochloridate (107 R = Cl), normally proceeding with inversion, proceeds with complete retention of configuration in the presence of Li+.80... 

Inspection of the results in Table 2 shows that the predominant retention of configuration was observed in the cases of isopropanol, cyclopentanol, and cyclohexanol. These preliminary data may be rationalized in terms of the paraUel formation of two sulfurane intermediates (254 and 255), which are responsible for inversion and retention at sulfur, respectively. The relative stabilities of these inter-... 

Density functional calculations, incorporating clusters with and without solvent coordination to lithium and/or copper, reveal that the 5 n2 transition state always features inversion and retention at the electrophilic and nucleophilic centres, respectively. This transition state (100) is such that the carbons of the three alkyl groups are in a different electronic and spatial environment thus, the formation of RR, rather than RR, is governed by the transition state (101) for the reductive elimination reaction of the Cu(II) intermediate. 

Nucleophilic displacement on 1-chlorosilacyclopentanes is dominated by inversion, in contrast to silacyclobutanes (78JOM(154)203). For 2-oxa- and 2-thia-silacyclopentanes, however, both inversion and retention are observed. The stereochemistry is determined by the leaving group and the electronic nature of the nucleophile rather than ring strain... 

An explanation could be achieved by the consistent application of the mechanistic concepts used to explain data at tetravalent phosphorus compounds (48). Most of the available data on nucleophilic displacements are usually explained by an apical entry of the nucleophile displacement of the leaving group occurs from an apical position (Scheme 3). The first invoked TBP intermediate is the most stable one on the basis of the relative apicophilicity of the groups attached to phosphorus the more electronegative ligands prefer apical positions. Since both inversion and retention can proceed, the retention pathway has been rationalized on the basis of intramolecular ligand exchange by pseudorotation (Scheme 3). 

The valence orbitals around F are contracted. The o-g, F MO is centered on silicon (Scheme 12). These two factors favors retention. But because the Si—F bond is short [1.54 A (41)], the unfavorable X—Nu overlap increases. Both inversion and retention may be possible. In contrast, valence orbitals on sulfur are diffuse the o-gj SR MO has more p character (Scheme 13). These two factors favors inversion. But the Si—SR bond is long [2.14 A (41)]. The unfavorable SR—Nu overlap in a RN attack decreases and retention may be also possible. 

In the case of the six-membered ring (Tables XVII and XVIII), the angular value [zLCr-Si-Ci = 105° ( 5)] cannot explain why inversion of configuration is so unfavorable compared to retention. This value suggests that the angular strain at silicon is quite similar in intermediates like 28 and 29 (Scheme 25). Thus both inversion and retention would be expected. 

Substitution with inversion and retention of the pentacoordinate species 33 has been interpreted in terms of two competing dissociative mechanisms Si—O cleavage and Si—NMI cleavage65. A hexacoordinate intermediate is not implicated. 

Direct reductive cleavage by catalytic hydrogenolysis of a cyclopropane C-C-bond is only possible in exceptional cases, where the three membered ring is further activated by a phenyl or a vinyl group. With unpoisoned catalyst a subsequent reductive desil-oxylation occurs to afford esters like 124 (Eq. 48), whereas addition of small amounts of triethylamine allows isolation of the desired siloxy compounds (e.g. 125, Eq. 49). Interestingly, both reactions demonstrate that the cleavage of the cyclopropane bond proceeds non-stereoselectively with inversion and retention at C-l and C-2, respectively 79). 

Stereochemistry The SnI reaction involves a flat carbocation intermediate that can be attacked from either face. Therefore, the SN1 usually gives a mixture of inversion and retention of configuration. 

Predicting the outcome of substitutions and eliminations is only straightforward in certain cases. For primary halides, S 2 and E2 reactions are predicted. For tertiary halides, SnI, E2 andEl(to a certain extent) are the choices. The possibilities for secondary halides are more complicated. In addition, many reactions yield both substitution and elimination products, and both inversion and retention of configuration may occur in the same reaction. 

Summary The rich variety of the coordination chemistry of silicon is discussed and some theoretical issues are raised. In an attempt to understand further the underlying chemistry, some thermodynamic and kinetic parameters for the formation and substitution of pentacoordinate silicon compounds have been measured by NMR methods. Values of -31 3 kJ mol for SHand -100 10 J K mor for A5-were measured for the intramolecular coordination of a pyridine ligand to a chlorosilane moiety. A detailed kinetic analysis of a nucleophilic substitution at pentacoordinate silicon in a chelated complex revealed that substitution both with inversion and retention of configuration at silicon are taking place on the NMR time-scale. The substitution with inversion of configuration is zero order in nucleophile but a retentive route is zero order in nucleophile at low temperature but shows an increasing dependence on nucleophile at higher temperatures. These results are analysed and mechanistic hypotheses are proposed. Some tentative conclusions are drawn about the nature of reactivity in pentacoordinate silicon compounds. 

An even more striking example has recently been found in our laboratories, where the completely symmetrical all cfs-l,2,3-trimethyl-cyclopropane 18) has been found to ring open with a mixture of inversion and retention with the latter predominating (Eq. (12)). Thus all three possible stereochemical outcomes, complete retention, complete... 

Halogermane reductions by complex hydrides are efficient, preferred methods for germane synthesis " . The complex hydrides used are MBH (M = Li, Na, K) and LiAlH.,. Reduction of Ge—X bonds by this method can be used for any molecule that otherwise is unsusceptible to complex hydride reduction or reaction. Lithium tetrahydroaluminate reduction of chiral halogermanes and alkoxygermanes results in inversion and retention of configuration, respectively. The LiBH., and LiAlH reactions require aprotic solvents, such as EtjO, THF, n-Bu O or glyme ethers. Sodium and K... 

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