Alkoxysilanes complexes

The second method of catalyst incorporation in mesoporous materials involves, generally, multi-step syntheses where organic moieties (i.e. amines, phosphines, thiols) are immobilized followed by post modification toward the final product. Alternatively, the catalyst may be synthesized as the corresponding alkoxysilane complex followed by immobilization into the mesostructured materials. For instance, Kiihn and co-workers demonstrated covalent... 

Figure 6.1 Formation of alkoxysilanes complexes with chelating ligands, causing increased charge distribution on formation of a chelate cycle and increase in the coordination number [25].

LaRonde FJ, Brook MA. Stereoselective reduction of ketones by histidine-alkoxysilane complexes the role of imidazole in nucleophilic substitution at silicon. Tetrahedron Lett. 1999 40 3507-3510. 

Alkyl silyl ethers are cleaved by a variety of reagents Whether the silicon-oxygen or the carbon-oxygen bond is cleaved depends on the nature of the reagent used Treatment of alkoxysilanes with electrophilic reagents like antimony tri-fluonde, 40% hydrofluonc acid, or a boron tnfluonde-ether complex results in the cleavage of the silicon-oxygen bond to form mono-, di-, and tnfluorosiloxanes or silanes [19, 20, 21) (equations 18-20)... 

Clarke and Shannon also supported copper bis(oxazoline) complexes onto the surfaces of inorganic mesoporous materials, such as MCM-41 and MCM-48, through the covalent binding of the ligand, modified by alkoxysilane functionalities [59]. The immobilized catalysts allowed the cyclopropanation of styrene with ethyldiazoacetate to be performed as for the corresponding homogeneous case, and were reused once with almost no loss of activity or selectivity. 

Silica-supported metal (e.g., Pd/Si02) catalysts also have surface silanol groups that can react with the alkoxysilane groups of the complexes. These combination catalysts consist of a tethered complex on a supported metal. A Rh complex was tethered to the surface of a Pd/Si02 catalyst, and the tethered catalyst was more active for the hydrogenation of aromatic compounds than the free complex or the supported catalyst separately.33 It is possible that the H2 is activated on the supported metal and the hydrogen atoms migrate to the silica, where they react with the reactant molecules coordinated by the tethered complex. 

An alternative strategy to obtain silica immobilised catalysts, pioneered by Panster [23], is via the polycondensation or co-condensation of ligand functionalised alkoxysilanes. This co-condensation, later also referred to as the sol-gel process [24], appeared to be a very mild technique to immobilise catalysts and is also used for enzyme immobilisation. Several novel functional polymeric materials have been reported that enable transition metal complexation. 3-Chloropropyltrialkoxysilanes were converted into functionalised propyltrialkoxysilanes such as diphenylphosphine propyltrialkoxysilane. These compounds can be used to prepare surface modified inorganic materials. Two different routes towards these functional polymers can be envisioned (Figure 3.4). One can first prepare the metal complex and then proceed with the co-condensation reaction (route I), or one can prepare the metal complex after the... 

The use of tri-tert-butylphosphine has produced still higher selectivities, allowing near total control in the synthesis of (A)-vinylsilanes, including alkoxysilanes and disiloxanes.38,39 In the context of a total synthesis of an HMG-CoA reductase inhibitor, hydrosilylation with a chlorosilane catalyzed by a platinum(O) olefin complex, Pt2 [(CH2=CH)Me2Si]20 3 (also known as Karstadt s catalyst), followed by coupling with a 2,6-disubstituted aryl iodide forged a key intermediate shown in Scheme 6.38... 

Combining two different hydrosilylation catalysts in sequence with chlorodimethylsilane has allowed the construction of complex dye assemblies and conducting polymers. In the example shown (Scheme 8), Karstedt s catalyst was chosen after a brief screen for the hydrosilylation of an aromatic diyne with chlorodimethylsilane. After reduction of the chlorosilane, an equimolar mixture of disilane B and diyne A was treated with a catalytic amount of Wilkinson s catalyst, resulting in the formation of polymer C.42 Hydrosilylation of alkynes has also been studied as a means of synthesizing oligo(phenylenevinylene) units with pendant alkoxysilanes to create curable, hole-transporting films.43,43 ... 

It has been well recognized that the hydrolysis of alkoxysilanes and chlorosilanes is effectively catalyzed when fluoride anions are present due to formation of hypercoordinated silicon intermediates.803 More in-depth studies by Bassindale et al. showed that the reaction of PhSi(OEt)3 with stoichiometric amounts of Bu4NF surprisingly yields an encapsulation complex, namely tetrabutylammonium octaphenyloctasilsesquioxane fluoride 830, in which the fluorine atom is situated inside the cubic siloxane cage (Scheme 114). The Si--F distance of average 2.65 A is shorter than the sum of van der Waals radii (3.57 A), which renders the coordination number of the silicon atoms at [4+1]. 

By far, the most W-Si bonds reported in the period that this review covers involve W(CO)n or (t]S-CsRs)W-containing compounds. A significant development has been that of a recyclable catalyst for the hydrosilylation of ketones the system begins with a polar liquid substrate (ketone or ester) and finishes with a non-polar liquid product (alkoxysilane). The rest state of the catalyst is a mixture of the [BlCgFsTH salts of 36 and 37 the tungsten complex is far more active than its molybdenum analog. 

Scheme 12.17 shows a representative example of the covalent attachment of a molecular catalyst to an inorganic support material via a hnker ( tether ) [165]. Here the sequential approach was exploited to immobihze catalyticaUy valuable rare-earth metal BINOL complexes [166]. An alkoxysilane funchonalized (R)-BINOL was obtained by a four-step reaction sequence involving brominahon at the 6-position, protechon of the phenohc groups by O-methylahon with CH3I,... 

Transition metal complexes have been widely investigated as catalysts for the synthesis of alkoxysilanes via alcoholysis of hydrosilanes. The system provides a convenient method for the protection of hydroxy groups in organic synthesis and the synthesis of silyl ethers. The general reaction is shown in Eq. (59). 

It is important to note that catalysts for alkoxysilane hydrolysis are usually catalysts for condensation. In typical silane surface treatment applications, alkoxysilane reaction products are removed from equilibrium by phase separation and deposition of condensation products. The overall complexity of hydrolysis and condensation has not allowed simultaneous determination of the kinetics of silanol formation and reaction. Equilibrium data for silanol formation and condensation, until now, have not been reported. 

The alcoholysis reaction was found to be first order in both the silane and the catalysts, and of intermediate order in the alcohol, when catalyzed by carboxylic acids. When catalyzed by dichloro-acetic acid, the reaction has a Hammet p value of +0.43. This is consistent with a concerted displacement reaction between the alkoxysilane and the complex involving the alcohol and the carboxylate anion. The intermediate is a negatively charged intermediate, probably a penta-substituted silicon species. 

The finding of a small positive value for p was something of a surprise, bui together with the idea that a catalyst complex must somehow be formed, we can draw some conclusions. The complex could be some form of protonation of the alcohol, but it is difficult to envision that species carrying out a substitution. A more likely possibility is that the mechanism more closely resembles Schowen s work with buffered carboxylic acid solutions. In this scheme, we propose that the carboxylic acid first protonates either the alcohol or the alkoxysilane. The resulting carboxylate then forms a complex with the alcohol as postulated by both Schowen and Boe for the base catalyzed reaction. This is then followed by a concerted displacement reaction as shown below ... 

Corriu and coworkers37 showed that alkoxysilanes R Si(OR )4-n [in particular RSi(OMe)3] give better yields for the DDB cyclization than do chlorosilanes. The reaction of anionic pentacoordinated silicon complexes [RSi(02CgH4-0)2] Na+ with DDB and subsequent LiAlPLi reduction give 1-R-diphenylsiloles (R = Me, Ph)38. Bis(silacyclopentadien-l-yl)alkanes were formed from DDB and a, a>-bis(dihalomethylsilyl)alkanes39. 

Carbon monoxide was taken up, however, in the presence of olefins, and among the complex reaction products unsaturated alkoxysilanes predominated. It seems likely that these resulted from reactions such as the following ... 

In this part, we wish to focus on the study of two types of silanes. Aminoorganosilanes are special members of the alkoxysilanes group. They carry the catalyzing amine function, required for chemical bonding with the silica surface, inside the molecule. This makes them more reactive than other organosilanes and reduces the complexity of the liquid phase reaction system to be studied. Only three components, silica, silane and solvent, are present. Furthermore there is a large interest in the reaction mechanism of silica gel with APTS, since this aminosilane is the most widely used compound of the organosilane family. 

Catalytic alcoholysis of silanes by a variety of transition metal based catalysts is a useful method to form silyl ethers under mild conditions (Scheme 19). The process is atom-economical hydrogen gas is the only byproduct. This mild method has not been fully exploited for the preparation of unsymmetrical bis-alkoxysilanes. A catalytic synthesis using silicon alcoholysis would circumvent the need of bases (and the attendant formation of protic byproducts), and eliminate the need for excess silicon dichlorides in the first silyl ether formation. We sought catalytic methods that would ultimately allow formation of chiral tethers that are asymmetric at the silicon center (Scheme 20). Our method, once developed, should be easily transferable for use with high-value synthetic intermediates in a complex target-oriented synthesis therefore, it will be necessary to evaluate the scope and limitation of our new method. 

Oxidative addition of the silane to the metal is fast and reversible 30 therefore unless the pentacoordinated silane drastically slows down the oxidative addition process, pentacoordination will not alter the rate of the reaction at this stage of the cycle. The increased reactivity of le may be explained by the attack of the alcohol on the pentacoordinated silane that would form after oxidative addition (Figure 9A). The rate of the alcohol addition is increased by the higher reactivity of the pentacoordinated silicon center. This may explain the slower reactivity for those alkoxysilanes that cannot form this intramolecular coordination complex due to the absence of a nearby Lewis basic atom. We had observed during the comparison of aliphatic alcohol to benzyl alcohol that the nucleophilicity of the alcohols has an effect on the rate of the reaction. This is evidence that the alcohol and the silane are involved in the rate-determining step with 10 % Pd/C catalytic system. 

---