Aryl silanes dimethylphenylsilane

Fluoride ion catalyzes the hydrosilylation of both alkyl and aryl aldehydes to silyl ethers that can be easily hydrolyzed to the free alcohols by treatment with 1 M hydrogen chloride in methanol.320 The most effective sources of fluoride are TBAF and tris(diethylamino)sulfonium difluorotrimethylsilicate (TASF). Somewhat less effective are CsF and KF. Solvent effects are marked. The reactions are facilitated in polar, aprotic solvents such as hexamethylphosphortriamide (HMPA) or 1,3-dimethyl-3,4,5,6-tetrahydro-2(l //)-pyrirnidinone (DMPU), go moderately well in dimethylformamide, but do not proceed well in either tetrahydrofuran or dichloromethane. The solvent effects are dramatically illustrated in the reaction of undecanal and dimethylphenylsilane to produce undecyloxyphenyldimethylsi-lane. After one hour at room temperature with TBAF as the source of fluoride and a 10 mol% excess of silane, yields of 91% in HMPA, 89% in DMPU, 56% in dimethylformamide, 9% in tetrahydrofuran, and only 1% in dichloromethane are obtained (Eq. 164).320... 

Shibata and co-workers have reported an effective protocol for the cyclization/hydrosilylation of functionalized eneallenes catalyzed by mononuclear rhodium carbonyl complexes.For example, reaction of tosylamide 13 (X = NTs, R = Me) with triethoxysilane catalyzed by Rh(acac)(GO)2 in toluene at 60 °G gave protected pyrrolidine 14 in 82% yield with >20 1 diastereoselectivity and with exclusive delivery of the silane to the G=G bond of the eneallene (Equation (10)). Whereas trimethoxysilane gave results comparable to those obtained with triethoxysilane, employment of dimethylphenylsilane or a trialkylsilane led to significantly diminished yields of 14. Although effective rhodium-catalyzed cyclization/hydrosilylation was restricted to eneallenes that possessed terminal disubstitution of the allene moiety, the protocol tolerated both alkyl and aryl substitution on the terminal alkyne carbon atom and was applicable to the synthesis of cyclopentanes, pyrrolidines, and tetrahydrofurans (Equation (10)). 

Just as carbon-based alkyl groups can be oxidized, a silicon-based silane unit can be oxidized under certain conditions. Alkylsilanes can be converted to a hydroxy unit, but either an aryl group (R—SiR 2Ar) or another silyl group (RSi—SiR 3) must be attached to silicon. In an early version of this reaction, Fleming used a two-step process to transform the silane unit to an alcohol unit, treatment with mercuric acetate and peroxyacetic acid, followed by reduction with lithium aluminum hydride (see sec. 4.2.A for reductions with LiAlHq). Comins et al. used this procedure to convert dimethylphenylsilane (397) to alcohol 398 in 93% yield for the two steps, which was part of a synthesis of A-acetyl-A-methylphlegmarine. ... 

In addition to the silylation of aryl halides, alkenyl iodides can also be silylated in the presence of a palladium(O) catalyst (eq 22). This silylation proceeds stereoselectively with retention of the carbon-carbon bond stereochemistry, with neither the a-nor (Z)-isomer being produced. The byproduct that arises from this reaction is the saturated /3-aryl triethoxysilane resulting from hydrosilylative reduction of the olefin (97 3 unsaturated saturated). Interesting to note is the increased observance of this saturated byproduct in the presence of other silanes, including dimethoxymethylsilane (93 7), triethylsilane (87 13), dimethylphenylsilane (87 13) and triphenylsilane (90 10), once again demonstrating the specific advantage of triethoxysilane. The alkenylsilanes produced in this reaction are versatile intermediates that have been used effectively in other synthetic transformations. ... 

---