Angle-strained sila-cycloalkynes

Abstract

Second row elements in small- and medium-rings modulate strain. Herein we report the synthesis of two novel oligosilyl-containing cycloalkynes that exhibit angle-strain, as observed by X-ray crystallography. However, the angle-strained sila-cyclooctynes are sluggish participants in cycloadditions with benzyl azide. A distortion-interaction model analysis based on density functional theory calculations was performed.

Figure 1.

(a-b) Displacement ellipsoid plots (50% probability level) of sila-cycloalkynes Si-CHy and Si-Coy at 110 K. H atoms are removed for clarity. (c-d) View of distortion from linearity at alkyne.

Figure 2.

Calculated heats of hydrogenation for COy, Si-CHy, and Si-Coy in kcal/mol. Heat of hydrogenation of alkynes were calculated as partial reduction of cycloalkynes to cis-cycloalkenes. Energies at ω-B97M-V/def2-TZVPP with implicit toluene solvent.

Figure 3.

Cropped ¹H NMR spectra (400 MHz, CD₂Cl₂) comparing the results of the SPAAC reaction of Si-CHy and N₃Bn (top) to a sample of 4 (bottom). Benzylic and allylic protons are indicated.

Figure 4.

Displacement ellipsoid plot (50% probability level) of triazole 4 at 110 K. Disorder and H atoms are removed for clarity a) View highlighting endocyclic bond angles. b) View highlighting ring conformation. Hydrogens omitted and exocyclic phenyl rings shown in wireframe for clarity. Black = carbon, blue = silicon, light blue = nitrogen.

Figure 5.

Optimized geometries, activation Gibbs free energies, and alkyne strain energies of [3+2] cycloaddition transition states of COy (top), Si-CHy (middle), and Si-COy (bottom). All energies are in kcal/mol. Energies at ω-B97M-V/def2-TZVPP with implicit toluene solvent.

Scheme 1.

Synthesis of sila-cycloalkynes Si-CHy and Si-COy via alkylation of dianions D1 and D2.

Scheme 2.

Attempted azide-alkyne cycloaddition reactions of Si-CHy and SiCOy with benzyl azide at room temperature in dichloromethane and at high temperature in toluene.

Chart 1.

Examples of known sila-cycloalkynes and cyclooctyne, ranked according to extent of distortion from linearity at the alkyne. Energies at ω-B97M-V/def2-TZVPP with implicit toluene solvent.