Ab Initio MO Study of the Reaction of Pentacoordinate Allylsilicates with Aldehydes

Abstract

Transition state structures for the reaction of allyltetrafluorosilicate with formaldehyde as well as structures of the silicate and related compounds were located with ab initio MO calculations. The origin of the unique reactivity of pentacoordinate allylsilicates observed experimentally is discussed on this basis.

Lewis Base Catalysis in Organic Synthesis

“…[293] Whereas the degree of induced polarization, evidenced by increased electron density at C g (q(C g )) and decreased electron density at the silicon atom (q(Si)), is small in changing from the tetracoordinate ground state to the (C g =À0.05, Dq(Si) = À0.02), the change upon binding of a second fluoride ion is dramatic (Dq(C g )) = À0.07, Dq(Si) = + 0.14). Although an allylpentafluorosilicate is probably not the active species in these reactions, it can be imagined that the binding of the aldehyde to the pentacoordinate silicate to a form a hexacoordinate, aldehyde-substituted tetrafluorosilicate may have a similar effect on the redistribution of electron density in the adduct and lead to a similar dual activation of both the aldehyde and allyl fragments of the complex.…”

“…Computational studies on the electronic structure of a series of fluorosilanes and silicates provide a quantitative picture of how fluoride activation can enhance both the nucleophilic and electrophilic character in allyltrifluorosilane (Figure 8). [293] Whereas the degree of induced polarization, evidenced by increased electron density at C g (q(C g )) and decreased electron density at the silicon atom (q(Si)), is small in changing from the tetracoordinate ground state to the…”

Asymmetric Synthesis with Chiral Cyclic Phosphorus Auxiliaries

“…Enhanced s-p conjugation leads to greater nucleophilicity at the g-carbon. 67 A MOPAC calculation for the crotylation reaction found several conformers for the pentacoordinate allylic silicate with low interconversion barriers, which were in good agreement with the stereochemical outcome of the reaction (Scheme 31).…”

Stereochemistry of the Allylation and Crotylation Reactions of α‐Methyl‐β‐hydroxy Aldehydes with Allyl‐ and Crotyltrifluorosilanes. Synthesis of anti,anti‐Dipropionate Stereotriads and Stereodivergent Pathways for the Reactions with 2,3‐anti‐ and 2,3‐syn‐α‐Methyl‐β‐hydroxy Aldehydes.

“…Toluene was distilled from sodium. 1 H NMR spectra were measured at 400 MHz on a Varian VNMR 400 instrument or at 500 MHz on a Varian Inova 500 instrument. Chemical shifts are reported in δ units with coupling constants reported in Hz.…”