Strain-release Lewis acidity

Next, enantiopure silicon allylation reagent will be presented, which already inherits Lewis acidity. It is accepted that Lewis acidity of silicon, as well as its high tendency to expand valence shell, increases [59, 60] if it is tetravalent and incorporated into strained four- or five-membered ring systems (strain-release Lewis acidity) [61]. This corresponds to smaller energy gaps between sp and dsp orbitals of a strained system as compared to an acyclic species. 

Scheme 5.23) [37]. This unusual spontaneous aldehyde allylsilylahon in the absence of a Lewis acid promoter/catalyst was attributed to strain-release Lewis acidity. 

Scheme 7.5 The concept of strain-release Lewis acidity. ...

Pinacol rearrangement driven by the release of the ring strain of a cyclobutane ring has been employed in an extremely efficient manner to form cyclopentanone derivatives. Experimentally. the Lewis acid mediated aldol condensation of benzaldehyde with l,2-bis(trimethyl-siloxy)cyclobutcne at —78 C gave the pinacol 1 in its silylated form.35,36 Subsequent treatment of this pinacol with trifluoroacetic acid at room temperature afforded 2-phenyl-cyclopentane-l,3-dione (2) in 97% yield.35,36... 

Another approach uses silacyclobutanes. These small-ring silanes offer enhanced Lewis acidity because angle strain is released when the silicon transitions from tetrahedral to pentavalent, which favours the activation. 

Strained siletanes may also be used in the Tamao Oxidation instead of halosilanes these intermediates offer a comparable Lewis acidity because coordination of the fluoride ion releases angle strain. 

The chemistry of epoxides is dominated by the basicity of the oxygen lone pairs and the release of ring strain as the three-membered ring opens. Many of the reactions of epoxides are acid or Lewis acid catalysed. The catalyst may coordinate with the oxygen, increasing the polarity of the C -O bond and the sensitivity of the carbon atom to reaction with a nucleophile. The reactivity of epoxides are summarized in Box 2.5. 

The reactivity of epoxy groups towards nucleophilic and electrophilic species can be explained through the release of ring strain in the three member oxirane group. Nucleophilic curatives such as amines or mercaptans attack the secondary ring carbon while electrophilic curatives behave as Lewis or Bronsted acids. The epoxy ring can be opened by hydroxyl or other epoxy group aided by tertiary amines, Lewis acids or coreactants such as primary amines, mercaptans and dicarboxylic acids ... 

Organic boron molecules with multidentate Lewis acidic sites such as (2) and (3) bind strongly with fluoride ion so as to form a pyramidalized chelate (4) (Scheme 3.32). The incorporation of the anion into a medium-sized cyclic structure releases ring strain via the pyramidalization of planar boron centers. It is interesting to see the binding energies of... 

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