Silanes, unsaturated

In addition to the volatile silanes, silicon also forms non-volatile hydrides with formulae (SiHj) , but little is known about their structure. Silicon, however, does not form unsaturated hydrides corresponding to the simple alkenes. 

Silane coupling agents are generally synthesized through addition of silicon hydrides to unsaturated organic molecules ... 

More recently, the Lewis acid promoted asymmetric 1,4-addition of trimethyl(2-propenyl)silane to chiral a,/ -unsaturated /V-acylamides has been published33. Lewis acid mediated reactions of trimethyl(2-propenyl)silanes with a,/I-unsatu rated AT-acyloxazolidinones or iV-enoylsultams show high chemical yield with good diastereomeric excess. The absolute configuration of the new asymmetric center is controlled by the nature of the Lewis acid used. 

The titanium(IV) chloride catalyzed addition of allylic silanes to (E)-(2-nitroethenyl)benzene affords y,<5-unsaturated nitronates which, on treatment with low valent titanium species [generated in situ from titanium(IV) and zinc], give y,<5-unsaturated nitriles. For example, [(Zs)-2-butenyl]-(dimethyl)phenylsilane underwent reaction with ( )-(2-nitroethenyl)benzene to give 3-methyl-2-phenyl-4-pentenenitrile in 65 % yield as a 3 1 mixture of diastereomers of unassigned configuration22. 

The oxidative addition of silanes (with silicon-hydrogen bonds) to coordinatively unsaturated metal complexes is one of the most elegant methods for the formation of metal-silicon bonds. Under this heading normally reactions are considered which yield stable silyl metal hydrides. However, in some cases the oxidative addition is accompanied by a subsequent reductive elimination of, e.g., hydrogen, and only the products of the elimination step can be isolated. Such reactions are considered in this section as well. 

A novel and versatile method for preparing polymer-supported reactive dienes was recently developed by Smith [26]. PS-DES (polystyrene diethyl-silane) resin 28 treated with trifluoromethanesulfonic acid was converted to a polymer-supported silyl triflate 29 and then functionalized with enolizable a,jS-unsaturated aldehydes and ketones to form silyloxydienes 30 and 31 (Scheme 4.4). These reactive dienes were then trapped with dienophiles and the Diels Alder adducts were electrophilically cleaved with a solution of TFA. 

FIGURE 29.3 General scheme of the reaction of a polysulfidic silane with an unsaturated polymer. 

Superior Environmental Products, Inc. introduced a product based on liquid poly sulfide containing 40% of a surface-modified scrap tire mbber. The product, ER-IOOR, is a coating that can temporarily contain chemical, oil, and gasoline spills. Rodriguez [97] reported that an unsaturated polyester resin containing silane-treated CGR showed better mechanical properties than that containing untreated CGR. 

A regio- and stereospecific INOC reaction of unsymmetrical silaketals 114, synthesized in one pot from unsaturated alcohols, nitro ethanol, and dichloro-silanes, via the nitrile oxide 115 to isoxazolines 116 has been described (Scheme 14) [37a]. The intermolecular version of the cycloaddition, under similar conditions, proceeds with poor regio and stereoselectivity. 

Fluoride-catalyzed condensation of cyclohexanone with tetra(isothiocyanate)silane 558 in the presence of BU4NF or BusSnF and (Me3Si)2S04 559 affords, in 98% yield, the unsaturated isothiocyanate 560 and Si02 [130]. 

Entries 11 to 13 are examples of iminium ion and acyliminium ion reactions. Note that in Entries 11 and 12, vinyl, rather than allylic, silane moieties are involved. Entries 14 and 15 illustrate the synthesis of (3,-y-unsaturated ketones by acylation of allylic silanes. 

Allylic silanes act as nucleophilic species toward a, (3-unsaturated ketones in the presence of Lewis acids such as TiCl4.130... 

Scheme 9.5. Conjugate Addition of AUylic Silanes to a, p-Unsaturated Enones...

Some examples of conjugate addition reactions of allylic silanes are given in Scheme 9.5. Entries 1 to 3 illustrate the synthesis of several (3-allyl ketones. Note that Entry 2 involves the creation of a quaternary carbon. Entry 4 was used in the synthesis of a terpenoid ketone, (+)-nootkatone. Entry 5 illustrates fluoride-mediated addition using tetrabutylammonium fluoride. These conditions were found to be especially effective for unsaturated esters. In Entry 6, the addition is from the convex face of the ring system. Entry 7 illustrates a ring closure by intramolecular conjugate addition. 

Each of the unsaturated cyclic amines shown below has been synthesized by reaction of an amino-substituted allylic silane under iminium ion cyclization conditions (CH2=0, TFA). By retrosynthetic analysis, identify the appropriate precursor for each cyclization. Suggest a method of synthesis of each of the required amines. 

The hydrogenation of alkenes and alkynes in water can also use silanes as hydrogen sources. Tour reported that by using palladium acetate as catalyst, triethoxysilane reduced C-C unsaturated bonds to saturation in a mixture of THF and water.18 The reaction showed excellent chemo- and stereoselectivity. Water was essential to the reaction. In the absence of water, 95% of the starting alkene remained unchanged (Eq. 3.3). 

Hydrosilylation turned out to be a unique method in organosilicone chemistry, but in some cases it suffers from severe side reactions. An explanation is provided by the generally accepted reaction mechanism known as "Chalk-Harrod mechanism" described elsewhere [7]. Included in this series of reaction steps is an insertion of olefmic ligands into a platinum-hydrogen bond. Since the metal may be bonded to either of the unsaturated carbon atoms and the reaction is also an equilibrium, alkenes may result which are in fact isomerized starting material. Isomeric silanes are to be expected as well (Eq. 1), along with 1-hexylsilane, which is by far, the main compound produced. 

Conversion of ketone 80 to the enol silane followed by addition of lithium aluminum hydride to the reaction mixture directly provides the allylic alcohol 81 [70]. Treatment of crude allylic alcohol 81 with tert-butyldimethylsilyl chloride followed by N-b ro m o s u cc i n i m i de furnishes the a-bromoketone 82 in 84 % yield over the two-step sequence from a.p-unsaturated ester 80. Finally, a one-pot Komblum oxidation [71] of a-bromoketone 82 is achieved by way of the nitrate ester to deliver the glyoxal 71. It is worth noting that the sequence to glyoxal 71 requires only a single chromatographic purification at the second to last step (Scheme 5.10). 

Reactions, under irradiation, involving the addition to unsaturated hydrocarbons of nitrogen and phosphorus compounds and of silanes have been summarized by Spinks and Woods (1990). Some cycloaddition reactions have also been reviewed. These will not be detailed here. 

The ring-opening of the cyclopropane nitrosourea 233 with silver trifiate followed by stereospecific [4 + 2] cycloaddition yields 234 [129]. (Scheme 93) Oxovanadium(V) compounds, VO(OR)X2, are revealed to be Lewis acids with one-electron oxidation capability. These properties permit versatile oxidative transformations of carbonyl and organosilicon compounds as exemplified by ring-opening oxygenation of cyclic ketones [130], dehydrogenative aroma-tization of 2-eyclohexen-l-ones [131], allylic oxidation of oc,/ -unsaturated carbonyl compounds [132], decarboxylative oxidation of a-amino acids [133], oxidative desilylation of silyl enol ethers [134], allylic silanes, and benzylic silanes [135]. 

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