Silanes heteroatom-functionalized

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. 

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. 

Because C-H bonds are usually less reactive towards dioxirane oxidation than heteroatoms and C-C multiple bonds, it is instructive to give a few general guidelines on the compatibility of functional groups within the substrate to be submitted to oxidative C-H insertion Substances with low-valent heteroatoms (N, P, S, Se, I, etc.), C-C multiple bonds, and C=X groups (where X is a N or S heteroatom) are normally not suitable for C-H insertions, because these functionalities react preferably. Even heteroarenes are more susceptible to dioxirane oxidation than C-H bonds, whereas electron-rich and polycyclic arenes are only moderately tolerant, but electron-poor arenes usually resist oxidation by dioxiranes. N-oxides and N-oxyl radicals are not compatible because they catalyze the decomposition of the dioxirane. Oxygen insertion into Si-H bonds by dioxirane is more facile than into C-H bonds and, therefore, silanes are not compatible. Substance classes normally resistant towards dioxirane oxidation include the carboxylic acids and their derivatives (anhydrides, esters, amides, and nitriles), sulfonic acids and their de-... 

There are several limitations on these methods with regard to the heteroatom involved. Reaction of the tin heterocycle with NBS results in cleavage of the aromatic-tin bonds to give o,o -dibromobibenzyl (81). Reaction of the tin heterocycle with DDQ resulted only in products of heteroatom elimination (81). Attempts to generate a silicon heterocycle with a functional group on the Si atom have failed to produce the unsaturated system (XVII). Dehydrobromination reactions of the silane afford highly colored reaction products but no isolable identifiable monomer. It is probable that the dehydrobromination reaction occurs not only at the ethylene bridge but also by transannular elimination. 

Some alkenes with heteroatoms can be used in cross-coupling reactions. Vinyl boranes are an important group, as the carbon-boron bond can subsequently be converted into so many other functional groups, such as halides (Scheme 8.97). ° An example of cross-metathesis of a vinyl borane, followed by Suzuki coupling, can be found in Scheme 11.40. An example of a vinyl silane metathesis can be found in Scheme 2.110. 

The reaction may be of some preparative interest for obtaining alkylbenzonitriles and various a-functionalized alkylbenzonitriles starting from polynitriles (see Scheme 4.10 and Scheme 4.11) [56,57]. Donors that can be conveniently used as the precursors of the radicals include Jt donors, such as alkenes [58,59] and alkyl aromatics [60-63], heteroatom-centered donors, such as carboxylic acids [64] and ierf-butyl esters [65], ethers [66], ketals [67] (as well as cyclopropanone sUyl ketals) [68] and amines, organometallic donors such as silanes, silyl ethers, and silyl amines [69-71] as well as germanes, stannanes, and borates [72]. 

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. 

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