Intramolecular cyclizations tris silane

It is difficult to find crosslinking systems that are ideal in that all functional groups are of equal reactivity and intramolecular cyclization is negligible. The crosslinking of vinyl terminated poly(dimethylsiloxane) polymers with tri- and tetrafunctional silanes appears to be an exception. Thus the calculated and experimental pc values were 0.578 and 0.583, respectively, for the tetrafunctional silane and 0.708 and 0.703, respectively, for the trifunctional silane (with r — 0.999) [Valles and Macosko, 1979]. 

Sultams can also be accessed by intramolecular cyclization of compounds containing preformed C-S-N-C-C fragments with a C-C bond formation as demonstrated in a one-pot synthesis of tricyclic sultam 236 <05SL577>. Tetrahydropyridine 235, obtained from 77,7/-bis(allyl)sulfonamide 234 by ring-closure-metathesis (RCM) followed by isomerization, undergoes radical cyclization in the presence of tris(trimethylsilyl)silane (TTMSS) to give tricycle sultam 236. 

Intramolecular Reactions. Tris(trimethylsilyl)silane is an effective mediator of radical cyclizations. In addition to halides and selenides, secondary isocyanides can be used as precursors for intramolecular C-C bond formation, which is impossible using the tin hydride (eq 11). Selective cleavage of the carbon-sulfur bond of a 1,3-dithiolane, 1,3-dithiane, 1,3-oxathiolane, or 1,3-thiazohdine derivative is an efficient process to generate carbon-centered radicals, which can undergo cyclization (eq 12). 

As in other fields of organic chemistry, the use of microreactors represents a new method to accelerate reactions significantly. Within a larger study on tin hydride and tris(trimethylsilyl)silane-mediated reactions, the continuous flow system has also been applied to conduct an intramolecular Meerwein reaction. The cyclization of bromide 6 to indane 7 was completed in less than 1 min (Scheme 3) [39]. 

In a similar case, hydrosilylation of a 1,6-diene or 5-en-l-one with tris(trimethylsilyl)silane gives the cyclized products with a preferred as orientation of the alkyl groups7. The intermediate silylalkyl or siloxyalkyl radical, respectively, is trapped intramolecularly to give substituted cyclopentanes. 

Intermolecular radical additions are facilitated if the alkene has a hydroxyl substituent that can form an ester to boron, so that the actual addition is intramolecular. Accordingly, bis(2-bromoethyl) alkenylboronates readily cyclize under radical conditions [85]. This concept was quickly extended to a-boryl radicals [86]. One of the more favorable examples is the reaction of diisopropyl (bromomethyl)boronate with 4-methylhex-l-en-3-ol and tris(trimethylsilyl)silane in the presence of azobis(isobuty-... 

---