Coordinatively Saturated Tris(oxazolinyl)borato Zinc Hydride-Catalyzed Cross Dehydrocoupling of Silanes and Alcohols

Abstract

The four-coordinate zinc compound ToMZnH (1, ToM = tris(4,4-dimethyl-2-oxazolinyl)phenylborate) catalyzes selective alcoholysis of substituted hydrosilanes. The catalytic reaction of PhMeSiH2 and aliphatic alcohols favors the monodehydrocoupled product PhMeHSi-OR. With the aryl alcohol 3,5-C6H3Me2OH, the selectivity for mono(aryloxy)hydrosilane PhMeHSiOC6H3Me2 and bis(aryloxy)silane PhMeSi(OC6H3Me2)2 is controlled by relative reagent concentrations. Reactions of secondary organosilanes and diols provide cyclic bis(oxo)silacycloalkanes in high yield. The empirical rate law for the ToMZnH-catalyzed reaction of 3,5-dimethylphenol and PhMeSiH2 is −d[PhMeSiH2]/dt = k′obs [ToMZnH]1 [3, 1,2 Our interest in the catalytic preparation of alkoxysilanes was inspired by our interactions with Prof. Victor Lin, as we sought to synthesize precursors for advanced silica-based functionalized nanomaterials for catalytic applications. Silyl ethers are also important building blocks in organosilicon chemistry and in synthetic organic chemistry as protecting groups.3 The reaction of chlorosilanes and alcohols provides a straightforward route to SiÀO bonds. However, the HCl byproduct of these reactions must be trapped with base, and this method is not useful for syntheses that require neutral conditions. The degree of condensation is difficult to control when more than one SiÀCl group is present in the silicon substrate. Furthermore, chlorosilanes are water sensitive and must be kept rigorously anhydrous prior to use. Catalytic SiÀH bond alcoholysis (eq 1) can avoid these problems, giving H 2 as the byproduct and bypassing acid formation and/or basic conditions. Organosilanes are not hydrolytically sensitive (in the absence of a catalyst), and catalysts can control selectivity. -hydrosilane or silyl hydride transition-metal complexes formed via oxidative addition steps. 7,13,14 In contrast, copperand gold-catalyzed silane alcoholyses are proposed to involve discrete metal hydride and metal-alkoxide intermediates (no MÀSi bond). 18,20 In situ generated zinc(II) catalysts are also proposed to follow this mechanism.23 However, these zinc, copper, and gold catalysts are generated in situ, the catalytic speciation is unknown, and the turnover-limiting step has not been established.Zinc(II) catalysts are particularly interesting, given the low cost, favorable biocompatibility, and high natural abundance of this main group metal. Although zinc hydrides are presumed intermediates in hydrosilane alcoholysis, the catalytic activity of the few isolable, monomeric, terminal zinc hydrides is not developed. 24À28

Molecular Zinc Dihydride Stabilized by N‐Heterocyclic Carbenes

“…No traces of differently substituted products were observed. These preliminary results indicate that compounds 1 have higher catalytic activity than [tris(4,4‐dimethyl‐2‐oxazolinyl)phenylborate]zinc hydride (10 mol %, 45 °C, 10 h) 4a. Compounds 1 show activity comparable with that reported for [(κ 3 ‐Tptm)ZnH] 4b…”

Molekulares Zinkdihydrid: Stabilisierung durch N‐heterocyclische Carbene

“…[2] In kondensierter Phase sind nur Komplexe mit terminaler oder verbrückender Hydridgruppe mit zusätzlichen mono(anionischen) Liganden wie nacnac, [3a-c] Tris (4,4-dimethyl-2-oxazolinyl)phenylborat, [3d] [Tris(2-pyridylthio)methyl], [3e] Tris(3-tert-butylpyrazolyl)hydroborat [3f,g] und weiteren Liganden [3h-j] beschrieben. Als Katalysatoren auf Basis unedler Metalle haben Zinkdihydridkomplexe neuerlich Aufmerksamkeit in der Silankupplung mit protischen Substanzen wie Alkoholen [4] wie auch in der homogenen Hydrosilylierung von Kohlendioxid erlangt.…”

Catalytic Formation of Silicon–Heteroatom (N, P, O, S) Bonds

“…The Sadow group reports the selective alcoholysis of hydrosilanes with tris(4,4‐dimethyl‐2‐oxazolinyl)phenylborate zinc hydride (To M ZnH) . The catalyst itself is also prepared by the reaction of To M ZnOR (from the To M ZnMe precursor) with dihydromethylphenylsilane in benzene/toluene under mild conditions (RT).…”