The synthesis and molecular structure of tetra(isopropyl)silane

Tetra(isopropyl)silane has been prepared using literature methods, with the individual steps improved by changes in some of the experimental conditions. The key reagent 2‐lithiopropene, which can now be obtained in good yields from 1‐methacrylic acid via 1,2‐dibromo‐1‐methylpropionic acid and 2‐bromopropene by treatment of the latter with ultrasound‐activated lithium metal, was shown to contain mono‐ and dilithiopropyne. The reaction with chlorotrimethylsilane led to the corresponding silylated derivatives, while with silicon tetrachloride tetra(isopropenyl)silane was obtained, which after purification is easily converted into the title compound by catalytic hydrogenation. — The gas phase molecular structure of [(CH₃)₂CH]₄Si has been determined by electron diffraction. The parameters could be successfully refined for a model of S₄ symmetry. Bond distances SiC, CC, and CH as well as bond angles SiCC and CCH show the steric compression of the four isopropyl substituents. Steric strain is minimized by twists of the methyl groups and the isopropyl groups away from the fully staggered conformations, but also by an increase of two of the CSiC angles as compared to the remaining four, which are decreased relative to the tetrahedral standard. The structure differs strongly (mainly in the twist angles) from that of the isoelectronic tetra(isopropyl)phosphonium cation in [(CH₃)₂CH]₄P⁺[B(C₆H₅)₄]⁻, but is very similar to those of tetra(cyclohexyl)silane and of tri(isopropyl)phosphonium isopropylide, where the pyramidal configuration of the ylidic carbon atoms leads to a pseudo homoleptic array of the substituents at phosphorus.