Hydrosilane σ‐Adduct Intermediates in an Adaptive Zinc‐Catalyzed Cross‐dehydrocoupling of Si−H and O−H Bonds

Abstract Three‐coordinate PhBOXMe2 ZnR (PhBOXMe2 =phenyl‐(4,4‐dimethyl‐oxazolinato; R=Me: 2 a, Et: 2 b) catalyzes the dehydrocoupling of primary or secondary silanes and alcohols to give silyl ethers and hydrogen, with high turnover numbers (TON; up to 107) under solvent‐free conditions. Primary and secondary silanes react with small, medium, and large alcohols to give various degrees of substitution, from mono‐ to tri‐alkoxylation, whereas tri‐substituted silanes do not react with MeOH under these conditions. The effect of coordinative unsaturation on the behavior of the Zn catalyst is revealed through a dramatic variation of both rate law and experimental rate constants, which depend on the concentrations of both the alcohol and hydrosilane reactants. That is, the catalyst adapts its mechanism to access the most facile and efficient conversion. In particular, either alcohol or hydrosilane binds to the open coordination site on the PhBOXMe2 ZnOR catalyst to form a PhBOXMe2 ZnOR(HOR) complex under one set of conditions or an unprecedented σ‐adduct PhBOXMe2 ZnOR(H−SiR′3) under other conditions. Saturation kinetics provide evidence for the latter species, in support of the hypothesis that σ‐bond metathesis reactions involving four‐centered electrocyclic 2σ–2σ transition states are preceded by σ‐adducts.


Catalytic Dehydrocoupling Experiments
A representative procedure for the catalytic dehydrocoupling of silanes and alcohols using Ph BOX Me2 ZnMe in benzene as solvent is given. Methanol (0.100 g, 3.15 mmol, 0.127 mL) was added to a solution of 2a (0.032 g, 0.09 mmol) in benzene-d6 containing tetrakis(trimethylsilyl)silane (0.01 M) as an internal standard. Phenylsilane (0.100 g, 0.9 mmol, 0.110 mL) was added to the reaction mixture, and the homogeneous solution was mixed on a vortex stirrer at room temperature. The progress of the reaction was monitored using 1 H NMR spectroscopy, specifically observing the appearance of new signals assigned to the alkoxy groups of the product and disappearance of SiH signal of the phenylsilane.

Solvent-free conditions:
A mixture of methanol (0.100 g, 3.15 mmol, 0.127 mL) and phenylsilane (0.100 g, 0.9 mmol, 0.110 mL) was added to 2a (0.032 g, 0.09 mmol). The reaction mixture becomes effervescent. The progress of the homogeneous reaction was monitored by removing small aliquots for analysis by 1 H NMR spectroscopy.

Computational Details
The ground state geometry optimization, IR frequency and saddle point calculations were performed using Truhlar's Minnesota 06-2X meta-GGA functional, [9] as implemented in NWChem. [10] The Los Alamos National Laboratory double-ζ valence basis set (LANL2DZ) [11] was used along with effective core potentials (ECPs) for Zn and Si. Grimme's dispersion corrections were empirically added through a long-range contribution (DFT-D3) for all calculations. [12] Hessians were performed on all optimized structures. Starting materials, intermediates, and products contain no imaginary (negative) modes. Transition-state structures

Procedure for Kinetic Experiments
General. Reactions were monitored by 1 HNMR spectroscopy using Bruker Avance-III 600 NMR spectrometer. The temperature of the NMR probe was preset to 333.2 K before each experiment and was calibrated using a thermocouple inserted through a septum into an NMR tube containing benzene. The concentration of Ph BOX Me2 ZnMe pre-catalyst was determined by integration vs. a standard of accurately known concentration of tetrakis(trimethylsilyl)silane Si(SiMe3)4 dissolved in benzene-d6 prior to the addition of substrate. During catalytic conversions, single-scan 1 HNMR spectra were acquired at preset intervals (1 min or 3 min). Peak areas of signals assigned to reactants, products, and the Si(SiMe3)4 standard was used as the basis to calculate concentrations.
Concentrations of PhMeSiH2 or ArylOH vs time were fit via non-linear least squares regression analysis to appropriate second-order, first-order (exponential), or zero-order equations. The hole in the septa was then covered with silicon grease to provide an additional seal. The sample was re-inserted into the pre-heated NMR spectrometer probe to commence the kinetics experiment.

Representative kinetic experiment for the catalytic dehydrocoupling of
A.  in Figure 5 of the main text, describing saturation behavior of ArylOH in Regime 1.