Stabilization of an amidinatosilylene with a bulky tris(trimethylsilyl)silyl substituent was realized with the preparation of PhC(NtBu) 2 Si{Si(SiMe 3 ) 3 } (1) from PhC-(NtBu) 2 SiHCl 2 with K{Si(SiMe 3 ) 3 } in more than 90% yield. The highly deshielded 29 Si NMR resonance (δ = 76.91 ppm) can be attributed to the absence of a π-donating substituent. The molecular structure of 1 shows a trigonal-planar geometry around the Si II center with a Si II −Si IV bond length of 2.4339(13) Å. A series of reactions of 1 with Me 3 NO, S, Se, and Te were performed. While siloxane derivatives (2 and 3) are obtained from reactions with Me 3 NO, silachalcogenones (4− 6) are formed with other chalcogens. The presence of SiE (E = S, Se, and Te) bonds in 4−6 have been confirmed by singlecrystal X-ray studies. Silaoxirane (7) formation was observed when 1 was treated with acetone, demonstrating the importance of the tris(trimethylsilyl)silyl group to kinetically and thermodynamically protect the silaoxirane derivative with less bulky substituents on the C atom.
Simple and readily accessible lithium compounds such as 2,6-di-tert-butyl phenolate lithium (1a), 1,1' dilithioferrocene (1b) and nacnac lithium (1c) are found to be efficient single site catalysts for hydroboration of a range of aldehydes and ketones with HBpin at room temperature. The efficacy of 1a-1c as catalysts is extended to the cyanosilylation of aldehydes and ketones with Me3SiCN.
Ever increasing demand for green and sustainable chemical processes has set up a drive to replace transition metals with earth-abundant, nontoxic, and environmentally benign alternatives. In this regard, the alkaline earth metal complexes have attracted significant attention. Herein, we have used a β-diketiminato ligand with methyl-pyridine side arm to synthesize magnesium (1) and calcium (2) compounds. The constitutions of 1 and 2 have been confirmed by single crystal X-ray studies, which show that the magnesium and calcium atom in 1 and 2 possesses octahedral geometry. Subsequently, we have used them as catalysts (1 mol %) for hydroboration of a wide range of aldehydes using pinacolborane (HBpin) at room temperature. The strategy has further been extended to ketones with 2 mol % catalyst loading. DFT calculations have been performed to understand the mechanism.
The transition metal free catalytic hydroboration of aldehydes and ketones is very limited and has not been reported with a well-defined silicon(iv) compound. Therefore, we chose to evaluate the previously reported silicon(iv) hydride [PhC(NtBu)SiHCl], (1) as a single component catalyst and found that it catalyzes the reductive hydroboration of a range of aldehydes with pinacolborane (HBpin) under ambient conditions. In addition, compound 1 can catalyze imine hydroboration. DFT calculation was carried out to understand the mechanism.
The recently reported hypersilylsilylene PhC-(NtBu) 2 SiSi(SiMe 3 ) 3 (1) reacts with BH 3 , 9-BBN, and PhBCl 2 to yield the respective Lewis acid base adducts 2−4, respectively. Compound 4 undergoes isomerization to form a ring expansion product 5. The same silylene was found to initially form an adduct with HBpin (6) and subsequently isomerized to 7 via the rupture of the B−H bond of HBpin ( 7), where the hydride was bound to the carbon atom of the amidinate ligand and the Bpin unit was attached to the silicon center. Surprisingly, the reaction of 1 with HBcat results in PhC(NtBu) 2 Bcat (8). Subsequently, we have shown that HBcat forms the same product when it reacts with related silylene PhC(NtBu) 2 SiN(SiMe 3 ) 3 (1′). With all of these reactions in hand, we ponder if silylene can activate two small molecules at one time. In this work, we delineate the three-component reactions of silylenes 1 and 1′ with 4-fluorobenzaldehyde and HBpin, which afforded unusual coupling products, 9 and 10, respectively. Note that 9 and 10 were prepared from the cleavage of the B−H and CO bonds by silylene in a single reaction and are the first structurally attested Si−C−O−B coupled products.
This study demonstrates the preparation and structural characterization of a Si(iv) hydride (PhC(NtBu)SiH(CH)Cl) (1) and its use as a catalyst for the cyanosilylation of a variety of aldehydes. Compound 1 represents the first neutral penta-coordinate silicon(iv) species that catalyzes cyanosilylation of aldehydes under mild conditions.
Readily accessible lithium compounds have been employed to catalyze the hydroboration of alkene and alkynes including terpenes using HBpin with anti-Markovnikov selectivity. The mechanism is proposed on the basis of experimental and DFT studies.
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