Heteroatom-substituted silane

Except for silanes, other heteroatom substituted systems have been used to stabilize multiply lithiated centres, e.g. (phenylthiomethyl)germanes 173a-c, 174a-c and 175, -stannanes 176a and 176b (Figure 22), -amines and (phenylthiomethyl)phosphanes . ... 

Under optimized conditions, cycloisomerizations of a number of functionalized hept-l-en-6-ynes took place in good-to-excellent yields (Table 9.3). Heteroatom substitution was tolerated both within the tether and on its periphery. Alkynyl silanes and selenides underwent rearrangement to provide cyclized products in moderate yield (entries 6 and 7). One example of seven-membered ring formation was reported (entry 5). Surprisingly, though, substitution was not tolerated on the alkene moiety of the reacting enyne. The authors surmize that steric congestion retards the desired [2 + 2]-cycloaddition reaction to the point that side reactions, such as alkyne dimerization, become dominant. 

The cyclopropanation of vinyl organometallic and heteroatom substituted vinylic compounds has also been reported using zinc carbenoids. Vinylboronates (equation 37) °, -alanes (equation 38) °, -zincs (equation 39) °, -stannanes (equation 40) ° , -phos-phonates (equation 41) °, -germanes (equation 42) °, and silanes (equation 43) °° "° could be readily converted into cyclopropane derivatives. 

Some /J-heteroatom substituted a,/J-unsaturated acyl silanes react with methyl ketone enolates in a stepwise stereoselective cyclopentannelation process, formally a [3 + 2] annelation, which may proceed through aldol reaction followed by Brook rearrangement and cyclization (Scheme 111)223. 

Summary Reacticm of a-heteroatom-substituted oligosilanes with potassium alkoxides is of interest to obtain insight into the mechanism of the Wurtz type polymerization of halosilanes. It also provides access to building blocks with unique reactivity. Reactions with fluoro- and methoxy-substituted silanes exhibited initial formation of silylenoid species which can undergo self-condensation. This property is less pronounced with the alkoxysilanes, which allowed for the isolation and structural characterization of an a-methoxy-silyl potassium compound (3). 

Table 11 Ratio of Diastereoisomers in the Reactions of Heteroatom-substituted Enol Silanes with Chiral...

On the basis of all the experiments with vinyl-substituted silanes and siloxanes with heteroatom-functionalized alkenes, catalyzed by ruthenium complexes, we were able to propose general synthetic routes. The reaction of vinyl-substituted silanes with vinyl-substituted heteroorganic (N,0,S) compounds proceeds effectively and yields, under optimum conditions (usually a five-fold excess of alkene 80-110 °C) and in the presence of ruthenium complexes containing or generating Ru-H and/or Ru-Si bonds, l-silyl-2-N(0 or S)-substituted ethenes with a high preference for the -isomer, according to Eq. 4, where R3 = Mea, Me2Ph, or (OEt)3. 

Part two (section 3) deals with the hydrosilylation of unsaturated carbon-heteroatom bond, mostly 0=0 and 0=N (but also C N, and C=S), as a catalytic method for the reduction of C=0 and C=N bonds—one of the most fundamental transformations in organic chemistry. Catalytic hydrosilylation of prochiral ketones and imines with substituted silanes and siloxanes that can provide (if followed by hydrolysis) convenient access to chiral alcohols and amines, respectively, discussed from the catalytic and synthetic point of view completes this part. 

Asymmetric Hydrosilylation of Unsaturated Carbon-Heteroatom Bonds. Asymmetric, catalytic hydrosilylation of prochiral ketones with substituted silanes or siloxanes gives silyl ethers that can be easily hydrolyzed to chiral alcohols. Similarly, prochiral imines undergo asymmetric hydrosilylation to give A-silylamines and, after subsequent hydrolysis, chiral amines (Scheme 32). 

The use of alkenylsilyl ethers in palladium-catalyzed cross-coupHng reactions has long been known. In fact, only a year after Hiyama s landmark report on the TBAF-promoted coupling reactions of vinyltrimethylsilane, Tamao and Ito capitahzed on the use of alkoxy-substituted silanes as viable components for such reactions (cf Scheme 7.4). The generahty of the silyl ether coupHng added a useful class of organosilicon substrates that are complementary to the fluorosilanes developed by Hiyama et al. These studies showed that other heteroatom-based... 

Chromium(II) chloride also mediates Wittig-type reactions of a-heteroatom-substituted gem-dihalides and aldehydes. In Scheme 5.27, representative examples of the preparation of alkenylboranes [38], -silanes [39], and -stannanes [40] are shown. In each case, high -selectivity was observed. As these compounds are very important substrates for Suzuki, Hiyama, and Stille couplings, their stereoselective formation enhances the value of the chromium(II) chloride mediated reactions. 

The last example conveys a noteworthy message. Aryl-attached silyl groups are latent hydroxyl functions as they may be oxidized like halo-, alkoxy-, or dialkylamino-substituted silanes and unlike heteroatom-free tetraalkylsilanes. As a matter of fact, arylsilanes are prone to any kind of electrophilic attack. Thus, acids split them into the arene and a silanol and aqueous tetrafluoroboric acid into a fluorosilane. Both are oxidizable of course. 

The meta photocycloaddition is frequently observed in cases of alkyl or electron-donor substituted benzene derivatives. Two exo/endo isomeres 2a,b are obtained in high yields in the reaction of anisole 1 with cyclopentene (Sch. 2) [18]. The reaction was also efficiently carried out in an intramolecular way. Two efficient reactions of this type are depicted in Sch. 2. The azatriquinane 3 [19] and the silane derivative 4 [20] were obtained in high yields. The formation of different regioisomers can be controlled by the heteroatoms in the side chain. The longer C-Si bonds particularly favor the formation of compound 4. 

Mechanistic implications of a general cross-metathesis of vinylsilicon with allyl-substituted heteroorganic compounds have been studied in detail for the reaction with allyl alkyl ethers [13]. The detailed NMR study of the stoichiometric reaction of Grubbs catalyst with allyl-n-butyl ether has provided information on individual steps of the catalytic cycle. A general mechanism of the cross-metathesis of vinyltri(alkoxy, siloxy)silanes (as well as octavinylsilsesquioxane) with 3-heteroatom-containing 1-alkenes in the presence of ruthenium carbene is shown in Scheme 5. 

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