Bis-alkoxysilane

Silicon tethers based on bis-alkoxysilanes (silyl ketals) are commonly prepared from the dichlorosilanes by reaction with an alcohol in the presence of base. These conditions are not compatible with some base labile compounds. To make unsymmetrical bis-alkoxysilanes requires a method for breaking the symmetry of the dichlorosilane. Without such a method, one must accept a statistically determined mixture of mono-alkoxy and bis-alkoxy products. This may be acceptable for inexpensive readily available alcohols, but it precludes the use of bis-alkoxysilane tethers for high-value synthetic intermediates. To overcome these limitations to... 

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. 

The best results were obtained for the formation of the bis-alkoxy diisopropylsilane products when 10 % Pd/C was used as the catalyst (Scheme 25). Under these conditions excellent yields were obtained in just 2 h at room temperature. We propose that the use of diisopropylsilane is crucial to the success of the second addition, because the mono-alkoxy product from the reaction of (+)-ethyl lactate with diphenylsilane,20 when subjected to the conditions of the second addition using any of the three catalysts evaluated, gave only low yields of the unsymmetrical bis-alkoxysilane. 

Our optimized method was applied to form several unsymmetrical bis-alkoxysilanes in very good yields over two steps (Table 6). Tertiary alcohols, which can be difficult to protect, readily react under the conditions to give good yields of the unsymmetrical bis-alkoxysilane product.21... 

Once again the mono-alkoxy product le was used as our starting point. This mono-alkoxy product was treated with several alcohols using the Mn(CO)sBr as catalyst in CH2CI2 at room temperature in air. The results are shown in Table 8 and Table 9. The success of this reaction was very encouraging to us because we were able to synthesize unsymmetrical bis-alkoxysilanes containing modifiable functional groups. This will allow us to explore stereospecific intramolecular reactions with chiral silanes. 

A mild method has been developed to synthesize unsymmetrical bis-alkoxysilanes (silyl ketals). This method utilizes three different catalysts to synthesize a variety silyl ketals in a stepwise manner. We were able to achieve our initial goal of finding catalytic systems that are mild, compatible with carbon-carbon multiple bonds, easily accessible and cost effective. Our method can couple tertiary alcohols in moderate to high yields. 

We have synthesized mono-alkoxysilanes 2a-2i and unsymmetrical bis-alkoxysilanes 3a-3i, 5a-5f and 7a-7e in moderate to high yields using a combination of Rh2(OAc)4 followed by 10 % Pd/C or Mn(CO)5Br as catalysts. To our knowledge we are the first to show the compatibility of the manganese catalyst with aryl bromides. We have shown that diisopropylsilane was crucial to this process and that employing (+)-ethyl lactate as the alcohol in the first step allows for a faster and more efficient reaction in the second step. 

Treatment of the allylic alcohols with diphenyldichlorosilane and 2,6-lutidine afforded the bis-alkoxysilanes 195 in excellent yield. These silicon-tethered compounds were treated with Grubbs catalyst to induce RCM reaction, furnishing the seven-membered silacycles 196 in 87-95% yield (Equation 37) <1998JOC6768>. 

The first TST-RCM sequence was reported by Grubbs and Fu for the construction of an achiral 1,4-diol, which circumvented the isolation of the silaketal intermediate in order to simplify purification (Equation 8.1) [11]. Treatment of the bis-alkoxysilane 1... 

Ariza et al. utilized the TST-RCM in conjunction with the Ireland-Claisen rearrangement to facilitate the total synthesis of (—)-phaseolinic acid (Scheme 8.3) [14]. The C2-symmetrical silaketal 6 was prepared in 58% overall yield using an adaptation of the protocol described by Evans and Murthy [13], which employed enantiomerically enriched propargylic alcohols to form the symmetrical bis-alkoxysilane rather than allylic alcohols. Selective reduction of the his-alkyne with Lindlar catalyst, followed by RCM with catalyst [Ru]-I, afforded the silaketal... 

Interchanging the order of the CM and TST-RCM reactions allows for the preferential functionalization of the less hindered and more reactive Type I alkene and thus permits the formation of the seven-membered silaketal without competitive RCM. A critical feature for the success of this strategy is that the formation of the cis-silaketal is preferred over the trans-derivative, which provides optimal rates of reaction [22]. Treatment of a mixture of the unsymmetrical bis-alkoxysilane 58 and butenolide 60 (1 4 ratio) with catalyst [Ru]-III provided the gigantecin skeleton 62 in 63% yield in a single step. Additionally, the formation of the homodimer of butenolide 60 is inconsequential since it can be recycled through... 

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. 

Vinylsilane to copper transmetallation has entered the literature,93 93a,93b and a system suitable for catalytic asymmetric addition of vinylsilanes to aldehydes was developed (Scheme 24).94 A copper(l) fluoride or alkoxide is necessary to initiate transmetallation, and the work employs a copper(ll) fluoride salt as a pre-catalyst, presumably reduced in situ by excess phosphine ligand. The use of a bis-phosphine was found crucial for reactivity of the vinylcopper species, which ordinarily would not be regarded as good nucleophiles for addition to aldehydes. The highly tailored 5,5 -bis(di(3,5-di-tert-butyl-4-methoxyphenyl)phosphino-4,4 -bis(benzodioxolyl) (DTBM-SEGPHOS) (see Scheme 24) was found to provide the best results, and the use of alkoxysilanes is required. Functional group tolerance has not been adequately addressed, but the method does appear encouraging as a way to activate vinylsilanes for use as nucleophiles. 

Telechelic polymers usually bear monofunctional groups at each of their extremities. However, sometimes each end-group is bifunctional, such as in a, co-bis-unsaturated telechelics, or trifunctional as in a, co-bis(trialkoxysilyl) telechelics wherein they participate in crosslinking by the sol-gel reactions (hydrolysis and condensation of alkoxysilane groups). 

From silanes 52 are obtained, in high yield, the corresponding silanols 53, which react further to produce disiloxanes 56 and 58-60. Silanes 54 alkoxysilanes 55 and disilanes 57 give high yields of disiloxanes 56. Ozonolysis of tetraethylsilane yields initially acetaldehyde and trimethylsilyl hydroperoxide 61. The latter is partially converted to bis(triethylsilyl) peroxide 62, which is hydrolyzed to silanol 63 and hydrogen peroxide. The ozonolysis is of first order, both in regard to the silanes, and to ozone. The ozonolysis starts with formation of 64 followed by formation of the trioxide 65, which decomposes to acetaldehyde and hydroperoxide 61 (Scheme 14)79 80. 

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. 

S = solvent. Ligands DIOP = 2,3-0-isopropylidene-2,3-dihydroxy-1,4-bis(diphenylphosphino)butane47 NMDPP = neomenthyldiphenylphosphine MDPP = menthyldiphenylphosphine48. b Enantiomeric excess, determined after conversion of the alkoxysilane to a trisubstituted silane. 

From benzophenone (560) and phenylsilane (559), mono- (561) and bis(diphenylmethyl)phenylsilane (562) can be obtained (equation 282)315. The asymmetric dimethyl(2,2-dimethyl-l-phenylprop-l-yloxy)phenylsilane (563) can be synthesized in the presence of a cationic rhodium complex (equation 283)316. An alkoxy replacement can be used for the synthesis of alkoxysilanes with non-identical alkoxy moieties, e.g. for diethoxymethoxysilane (565) (equation 284)317. 

The known reactions of the hydroxy function in the carbon moieties of phosphonic acids and related compounds are many. When treated with an excess of a silane, RsSiH, in the presence of colloidal nickel at about 110 °C, (hydroxymethyl)phosphonic and bis(hydrox-ymethyl)phosphinic acids each undergo silylation at both alcohol and acid OH sites, although equation 12 is a simplification of the overall chemistry. The other main phosphorus-containing product from such reactions is 198 (when R = Et) the 1,4,2,5-dioxadiphosph(V)orinanes (199) are also isolable if reaction product mixtures are kept at ambient temperature. An excess of an alkoxysilane R3SiOR also fully silylates... 

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