29Si NMR Shielding Calculations Employing Density Functional Theory, Focussing on Hypervalent Silicon Compounds

Phosphine-Catalyzed Activation of Phenylsilane for Benzaldehyde Reduction

Hydrosilylation reactions are commonly used for the reduction of carbonyl bonds in fine chemistry, catalyzed by transition metal complexes. The current challenge is to expand the scope of metal-free alternative catalysts, including in particular organocatalysts. This work describes the organocatalyzed hydrosilylation of benzaldehyde with a phosphine, introduced at 10 mol%, and phenylsilane at room temperature. The activation of phenylsilane was highly dependent on the physical properties of the solvent such as the polarity, and the highest conversions were obtained in acetonitrile and propylene carbonate with yields of 46 % and 97 %, respectively. The best results of the screening over 13 phosphines and phosphites were obtained with linear trialkylphoshines (PMe₃ , Pⁿ Bu₃ , POct₃ ), indicating the importance of their nucleophilicity, with yields of 88 %, 46 % and 56 %, respectively. With the help of heteronuclear ¹ H-²⁹ Si NMR spectroscopy, the products of the hydrosilylation (PhSiH_3-n (OBn)_n ) were identified, allowing a monitoring of the concentration in the different species, and thereby of their reactivity. The reaction displayed an induction period of ca. 60 min, followed by the sequential hydrosilylations presenting various reaction rates. In agreement with the formation of partial charges in the intermediate state, we propose a mechanism based on a hypervalent silicon center via the Lewis base activation of the silicon Lewis acid.

Comprehensive Benchmark Study on the Calculation of 29Si NMR Chemical Shifts

A comprehensive and diverse benchmark set for the calculation of ²⁹Si NMR chemical shifts is presented. The SiS146 set includes 100 silicon containing compounds with 146 experimentally determined reference ²⁹Si NMR chemical shifts measured in nine different solvents in a range from -400 to +828 ppm. Silicon atoms bound to main group elements as well as transition metals with coordination numbers of 2-6 in various bonding patterns including multiple bonds and coordinative and aromatic bonding are represented. The performance of various common and specialized density functional approximations including (meta-)GGA, hybrid, and double-hybrid functionals in combination with different AO basis sets and for differently optimized geometries is evaluated. The role of scalar-relativistic effects is further investigated by inclusion of the zeroth order regular approximation (ZORA) method into the calculations. GGA density functional approximations (DFAs) are found to outperform hybrid DFAs with B97-D3 performing best with an MAD of 7.2 ppm for the subset including only light atoms (Z < 18), while TPSSh is the best tested hybrid functional with an MAD of 10.3 ppm. For ²⁹Si cores in the vicinity of heavier atoms, the application of ZORA proved indispensable. Inclusion of spin-orbit effects into the ²⁹Si NMR chemical shift calculation decreases the mean absolute deviations by up to 74% compared to calculations applying effective core potentials.

The calculation of 29Si NMR chemical shifts of tetracoordinated silicon compounds in the gas phase and in solution

²⁹Si NMR chemical shifts have been calculated for a variety of silicon-based species relevant in organic synthesis.

Mechanisms of silicon alkoxide hydrolysis-oligomerization reactions: a DFT investigation

Silica aerogels possess a variety of unique and remarkable properties, but the mechanisms of silicon alkoxide, Si(OR)(4), hydrolyses and oligomerization in the initial stage of sol-gel processes are still not well understood. On the basis of density functional theory calculations at the B3LYP/6-31G(d,p)//B3LYP/6-311++G(d,p) basis set level, the hydrolysis and oligomerization reactions of Si(OR)(4) in neutral, acidic, and alkaline solutions were systematically investigated and we found that in acidic solutions the precursor Si(OCH(3))(4) was inclined to hydrolyze rather than to condense and the hydrolysis processes were energetically more favorable than the neutral ones. In alkaline solutions, the hydrolysis products oligomerize through an S(N)1 dimerization mechanism and the condensation rates are fast to form denser colloidal aerogels. Our calculations also testify that the subsequent cyclization reactions are energetically unfavorable.