Trimethylsilyl monosubstitution

Monosubstitution of acetylene itself is not easy. Therefore, trimethylsilyl-acetylene (297)[ 202-206] is used as a protected acetylene. The coupling reaction of trimethylsilylacetylene (297) proceeds most efficiently in piperidine as a solvent[207]. After the coupling, the silyl group is removed by treatment with fluoride anion. Hexabromobenzene undergoes complete hexasubstitution with trimethylsilylacetylene to form hexaethynylbenzene (298) after desilylation in total yield of 28% for the six reactions[208,209]. The product was converted into tris(benzocyclobutadieno)benzene (299). Similarly, hexabutadiynylben-zene was prepared[210j. 

Trimethylsilylethynylpyrazole was deprotected by treatment with tetrabutyl-ammonium fluoride (TBAF) to give monosubstituted acetylene in 90% yield. (96ADD193). The same conditions were used to cleave the trimethylsilyl group in l-tetrahydropyranyl-3-carboxyethyl-4-[2-(trimethylsilyl)ethynyl]pyrazole (96INP 9640704). 

Introduction of trimethylsilyl substituents attached directly to the ot-carbon atom of a-(benzotriazol-l-yl)alkyl thioethers provide new opportunities. Thus, treatment of lithiated monosubstituted a-(benzotriazol-l-yl)alkyl thioethers with chlorotrimethylsilane produces a-(trimethylsilyl)alkyl thioethers 837. In reactions with hexamethyl-disilathiane and cobalt dichloride, thioethers 837 are converted to thioacylsilanes 838 that can be trapped in a Diels-Alder reaction with 2,3-dimethylbutadiene to form 2-alkyl-4,5-dimethyl-2-trimethylsilyl-3,6-dihydro-27/-thiopyrans 839 (Scheme 133) <2000JOC9206>. 

The thermolysis of bis[(trimethylsilyl)methyl]sulfoxides (27) at 100 °C in hexamethylphosphoramide (HMPA) results in the elimination of hexamethyldisi-loxane, and thiocarbonyl ylides appear as reactive intermediates (53). This protocol allows the generation of the parent species la, as well as monosubstituted and alkylidene-substituted representatives such as lb and Ic, which are difficult to obtain by other methods (Scheme 5.9). 

The 3//-l,2,4-diazaphospholes formed from the reaction of diazomethane and its monosubstituted derivatives (R CH=N2 R = H, alkyl, aryl, acyl, phosphoryl) could not be isolated due to a rapid 1,5-H shift leading to 27/-l,2,4-diazaphospholes 227. When diazo(trimethylsilyl)methane or [bis(diisopropylamino)phosphino]dia-zomethane was used, the l,5-SiMe3 [or PR2, R = N(/-Pr)2] shift completely dominates over the H shift (289,290). In the case of open-chain or cyclic a-diazoketones, cycloadducts 228 cannot be isolated due to rapid acyl shifts giving 229 and ultimately 230 (289). This transformation offers a versatile method to prepare [h]-fused 1,2,4-diazaphospholes from cyclic a-diazoketones and phos-phaalkynes (289). 

The photolysis of tris(trimethylsilyl)phenylsilane in the presence of a series of alkynes alforded the silacyclopropene through silylene addition to the triple bond. Those obtained from monosubstituted alkynes underwent photochemical isomerization to the disilanyl-alkyne through a 1,2-hydrogen shift (Scheme 48) (80JOM(190)117). Disubstituted alkynes form silirenes that can be isolated by preparative GLC. 

A new microwave-assisted protocol for the generation of diversely substituted 3,4-dihydropyrimidine-5-carboxylic acid esters 40 has been developed by Kappe and co-workers [88, 89] using trimethylsilyl chloride (TMSCl) as a mediator for the Biginelli MCR. This involved the reaction of S-ethyl acetothioacetate or ethyl acetoacetate, an aromatic aldehyde and (monosubstituted) urea or thiourea as building blocks. Also sterically hindered aromatic and heterocyclic aldehydes... 

Epoxide -> aliylic alcohol. Treatment of an oxirane with equimolar amounts of 1 and DBU in an aromatic solvent affords aliylic trimethylsilyl ethers in moderate yield. 2,2-Di-, tri-, and tetrasubstituted oxiranes, as well as oxides of cycloalkenes, react at 23° or below. 2,3-Di- and monosubstituted oxiranes do not react at this temperature these species react with 1 and DBU at 70-80° to give trimethylsilyl enol ethers. The reaction of epoxycyclooctane gives a product of transannular cyclization. In the case of epoxycyclohexane, the intermediate 2 has been isolated. 

Monosubstituted tetrazene (Mc3Si)HN— N=N— NH2 is very thermo-labile (41, 42). In methylene chloride it decomposes above ca. -40 C with the formation of trimethylsilyl azide and bis(trimethylsilyl)amine, as well as ammonium azide. Thus, the (catalyzed ) decomposition of the compound follows Thermolysis Pathway IV. 

Epoxides can react smoothly with allylsilane reagents. For example, 1,3-bis-trimethylsilyl-Tpropene can react with one of a variety of epoxides under Lewis acid catalysis to generate a highly functionalized alkenol (Equation 35). Yields are modest, with best results being obtained using monosubstituted epoxides <1998TL529>. Trimethyl silyl... 

Table 15 are disubstituted acetylenes. There is one exception 3-(trimethylsilyl)-l-octyne, a monosubstituted acetylene, polymerizes with NbCl5. This indicates that even monosubstituted acetylenes can polymerize with Nb and Ta catalysts if the acetylenes satisfy a certain, probably steric, condition. Since no other monosubstituted acetylenes are known to polymerize with Nb or Ta catalysts, only disubstituted acetylenes will be dealt with as monomers in Sects. 3.2 and 3.3. 

In Table 19 are collected typical monomers that polymerize with group 5 and 6 transition metal catalysts to produce high-molecular-weight (Mw > 1 x 10s) polyacetylenes. Among them, tert-butylacetylene and 3-(trimethylsilyl)-l-octyne are monosubstituted acetylenes, while the others are disubstituted ones. It is noteworthy that all of these monomers are considerably sterically crowded. By judicious choice of polymerization conditions, the polymer yield becomes fair to quantitative in every case. The Rtw s of the polymers reach ca. 3 x 10s 2 x 106. 

The third type of monomer among Si-containing monosubstituted acetylenes is o-(trimethylsilyl)phenylacetylene (3). 

Limitations of the reaction due to the substitution pattern of the allylic alcohols were overcome by the use of tetrapropylammonium perruthenate (TRAP) as a catalyst and monosubstituted, disubstituted and trisubstituted allyl alcohols were converted into the corresponding saturated aldehydes and ketones [5]. Intermediacy of the ruthenium alkoxide in this reaction was evidenced from the complete lack of reactivity of the trimethylsilyl ether derived from the allylic alcohol. 

The silanetriols 14-16 react with trimethylsilyl chloride in a 1 1 ratio to yield the products RSi(OH)2(OSiMe3) (50-52) respectively [43]. These compounds show monosubstitution of the trisilanol by a trimethylsilyl group. The reaction of silanetriol 15 with GeMesCl in a 1 2 ratio yields the acyclic siloxane RSi(OH)(OGeMe3)2 (53). The reactions of the silanetriols 14-16 with tri-methyltin chloride in 1 3 ratio give the acyclic stannasiloxanes RSi(OSnMe3)3 (54-56) [44]. 

The photochemical extrusion of nitrogen from silyl-substituted diazoacetates (hv > 300 nm) in the presence of various alkenes leads mainly to the formation of cyclopropanes (Table 4). Reactions of trimethylsilyl- and triisopropylsilyldiazoacetates with monosubstituted alkenes such as hex-1-ene or styrene (Table 4, entries 1-3) show interesting results. The formation of the thermodynamically less favored Z-isomer increases with growing steric demand of the silyl substituent. The cyclopropanation of ( )- and (Z)-but-2-ene (Table 4, entries 5 and 6) reveals that the addition reaction does not proceed completely stereospecifically. Small amounts of the wrong diastereomer can be detected, which is believed to arise from the triplet spin state of the carbene. Insertion into allylic C-H bonds occurs in the case of di- or trisubstituted alkenes (Table 4, entries 4-7). 

Reactions of 3-methylthio-4-trimethylsilyl-1,2-butadiene with electron-poor monosubstituted and disubstituted alkenes were promoted by a catalytic amount of ethylaluminum dichloride, affording the corresponding methylenecyclobutanes with high selectivities and with yields ranging from 37% for methyl crotonate to 97% for methacrylonitrile. ... 

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