Acyclic silanes

FIGURE 20. Bond lengths in silanethiol derivatives (left) and Si—S—X bond angles in acyclic silan-ethiol derivatives (right)... 

Proton, 13C, and 32Si NMR spectra for different 1,1-disubstituted SCBs have been studied (Table 3) <1980JOM( 190)9, 19830MR412>. The influence on the ring carbon chemical shifts of substituents on the silicon atom is larger for SCBs than for silacyclopentanes and acyclic silanes. The S C-2 and C-3 signals of SCBs are within the ranges 9.5-27.7 and 11.7-22.7 ppm, respectively. 

Trifunctional (meth)acrylate fluorine-containing cyclic and acyclic silane compounds have been prepared that form photocurable resin compositions. Coatings from these resins are anti-fouling and resist organic stains from oil mist and fingerprints without detracting from surface mar resistance. 

Degradation of Larger Silicon Catenates. Although it seems clear that polymeric disilane derivatives photodecompose by bond homolysis to produce silyl radicals, model studies on larger silicon catenates indicate that their photochemistry may be more complex (Scheme IV). Cyclic silane derivatives seem to extrude monomeric silylenes upon irradiation to produce smaller cyclic silanes (52). The proposed silylene intermediates have been identified spectroscopically 49, 53), and trapping adducts have been isolated in solution. Exhaustive irradiation ultimately results in acyclic silanes, which... 

Cyclic siloxanes and their aza analogues, azasilacyclopentanes, can be prepared from appropriate unsaturated, acyclic silane or aza analogue (Equation (13)) <90JOC3438,930M2297>. 

Stereochemical Behavior of Exocyclic Leaving Groups. Important changes in stereochemistry, compared to acyclic silanes, were reported with the cyclic systems shown in Table 26. These stereochemical data lead to the following comments ... 

Acyclic silanes Six-membered rings Silacydopentanes Silacyclobutanes... 

Although the allylation reaction is formally analogous to the addition of allylic boranes to carbonyl derivatives, it does not normally occur through a cyclic TS. This is because, in contrast to the boranes, the silicon in allylic silanes has little Lewis acid character and does not coordinate at the carbonyl oxygen. The stereochemistry of addition of allylic silanes to carbonyl compounds is consistent with an acyclic TS. The -stereoisomer of 2-butenyl(trimethyl)silane gives nearly exclusively the product in... 

Allylic trimethoxysilanes are activated by a catalytic combination of CuCl and TBAF.118 The mechanism of this reaction is not entirely clear, but it seems to involve fluoride activation of the silane. These reactions are stereoconvergent for the isomeric 2-butenyl silanes, indicating that reaction occurs through an acyclic TS. 

The 121/Cl3SiH combination selectively cross-couples alkenes with alkynes intermolecularly to give acyclic homoallylic silanes 127 and 128 (Eq. 22) [73]. 

Table 3.27. Symmetry, bond lengths Raa and Rah, bond angle 6 hah and theoretical strain energy (TSE) for small cyclic alkanes (A = C) and silanes (A = Si), with corresponding geometrical variablesT of acyclic (C2V) propane and...

Silyltitanation of 1,3-dienes with Cp2Ti(SiMe2Ph) selectively affords 4-silylated r 3-allyl-titanocenes, which can further react with carbonyl compounds, C02, or a proton source [26]. Hydrotitanation of acyclic and cyclic 1,3-dienes functionalized at C-2 with a silyloxy group has been achieved [27]. The complexes formed undergo highly stereoselective addition with aldehydes to produce, after basic work-up, anti diastereomeric (3-hydroxy enol silanes. These compounds have proved to be versatile building blocks for stereocontrolled polypropionate synthesis. Thus, the combination of allyltitanation and Mukayiama aldol or tandem aldol-Tishchenko reactions provides a short access to five- or six-carbon polypropionate stereosequences (Scheme 13.15) [28],... 

Reactions of chiral silanes with chiral aldehydes exhibit matching and mismatching characteristics (Eqs. 9.56 and 9.57) [48]. The additions proceed through an acyclic transition state, which favors syn adducts. The matched (M)/(R) pairing of Eq. 9.56 proceeds by way of a favorable Felkin-Anh arrangement to afford the syn,syn homopropargylic alcohol product. However, if the silanes possess an a-hydrogen, a vinylic chloride intermediate is formed, as shown in Scheme 9.13. Subsequent treat-... 

The stereoselectivity is higher for the /i-stannanc.114 This stereochemistry is the same as that observed for allylic silanes and can be interpreted in terms of an acyclic transition state. (See page 571). Either an anti or gauche conformation can lead to the preferred syn product. An electronic n interaction between the stannane HOMO and the carbonyl LUMO is thought to favor the gauche conformation.115... 

Reagents A Convenient Route to sec- and tert- 6-Alkylpurines. Tetrahedron Lett. 1996, 37, 1285-1288. (c) Dvorakova, H. Dvorak, D. Holy, A. Synthesis of Acyclic Nucleotide Analogues Derived from 6-(sec- or tert-Alkyl) purines via Coupling of 6-Chloropurine Derivatives with Organocuprates. Collect. Czech. Chem. Commun. 1998, 63, 2065-2074. (d) Hocek, M. Holy, A. Perfluoroalkylation of 6-Iodopurines by Trimethyl(perfluoro-alkyl)silanes. Synthesis of 6-(Perfluoroalkyl)purine Bases, Nucleosides and Acyclic Nucleotide Analogues. Collect. Czech. Chem. Commun. 1999, 64, 229-241. 

Indeed, Adam observed that, whereas acyclic vinyl silanes 132 undergo smooth epoxidation in excellent yield, the cyclic substrates exhibit varying amounts of allylic oxidation as well. The C-H insertion pathway becomes more prominent when the double bond is deactivated by either steric or electronic influences. Thus, the /-butyl derivative 134 gives as much as 20% of the allylic oxidation product (136) [95TL4991]. 

Si) =—10.2 ppm) and l,l-bis(/-butylamino)-l-silacyclobutane (629Si = —14.9ppm) is increased compared to 1,1-dimethyl-l-silacyclobutane, and the value of 17(29Si-ISN) for the former compound is smaller (17.2 Hz) than for acyclic bis(alkylamino)silanes. 

The reduction of a,p-unsaturated ketones is equally fast and remarkably selective for 1,2-addition of the silane. A variety of both cyclic and acyclic a,p-unsaturated ketones were reduced with diphenylsilane under the standard conditions with yields ranging from 66-99% for 1,2-addition. Cyclic enones were reduced with a higher degree of regioselectivity than the acyclic enone, mesityl oxide. 

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