Synthesis and Reactivity of Martin's Spirosilane-Derived Chloromethylsilicate

Abstract

Pentacoordinate silicon derivatives with a chloromethyl ligand are versatile compounds that are usually obtained from the corresponding tetravalent trialkoxy- or trihalogeno(chloromethyl)silane. We describe herein the synthesis of a chloromethylsilicate bearing two Martin's ligands readily obtained by addition of in situ generated chloromethyllithium to a spirosilane. The reactivity of this new species was evaluated and it has been established that the chloride is displaced by strong nucleophiles such as alkyllithiums and (hetero)aryllithiums. In Lewis acidic conditions, the pentacoordinate silicon species rearranges through a formal insertion of a methylene into one Si-C bond, to form a new tetravalent spirosilane with a six-membered ring. The same kind of rearrangement can be triggered also by addition of a Lewis base. The mechanism of the rearrangement in both conditions has been studied by means of DFT calculations.

Keywords: DFT calculations; Sila-Matteson rearrangement; pentacoordination; silicon; spirosilane.

Figure 1

Synthesis of pentacoordinate chloromethylsilane derivatives and presentation of the objectives.

Scheme 1

Synthesis of chloromethylsilicate 2.

Figure 2

X-Ray structure of chloromethylsilicate 2 (CCDC number 2124348). Ellipsoids are shown with 50% of probability.

Scheme 2

Nucleophilic substitution of chloromethyl silicate 2 with n-butyllithium.

Scheme 3

Expected products from ring expansion.

Figure 3

Synthesis of rearranged product 5 and the corresponding X-ray structure (CCDC-2129139) Ellipsoids are shown with 50% of probability.

Scheme 4

Gibbs free energy profile for the aryl migration with or without magnesium(II) bromide at the B3LYP-D3/6-311+G(2d,p) level of theory. Energies are in kcal·mol⁻¹.

Figure 4

Representation of some relevant calculated structures.

Scheme 5

Rearrangement of methylsilicate 2 in Lewis basic conditions. X-ray structure of hydroxy derivative 7 (CCDC-2124350) protonated DMAP was omitted for more clarity. Ellipsoids are shown with 50% of probability.

Scheme 6

Assessment of the stability of zwitterion 8.

Scheme 7

Gibbs free energy profile for the aryl migration with DMAP at the B3LYP-D3/6-311+G(2d,p) level of theory with the CPCM model (acetonitrile). Energies are in kcal·mol⁻¹.

Scheme 8

Energies associated to the formation of hexavalent adduct F at the B3LYP-D3/6-311+G(2d,p) level of theory with the CPCM model (acetonitrile). Energies are in kcal·mol⁻¹.