Radical tris silyl

The action of a catalytic amount of triethylborane on tris(trimethylsilyl)silane induces the formation of tris(trimethylsilyl)silyl radicals, which promote the ring-closure of 1,6-heptadiene to a mixture of the cis- and rirms-cyclopcntanc derivatives 115, together with a small amount of the silicon heterocycle 116 (equation 61)68. 

Since this book mainly deals with the literature on silyl radicals after the 1980s, the references quoted for the early work are not always the seminal ones but the available reviews. I hope that early experts in the field will forgive me if they find their pet paper uncited. I have tried to maintain an essential simplicity and readability of the text, and hope that I have succeeded so that the book is easily consulted also by nonexperts. I also hope that this book serves as an important link between the various areas of chemistry. 

Persistent and stable silyl radicals have attracted considerable attention [42]. Bulky aryl or alkyl groups that generally make carbon-centred radicals persistent [43,44] have a much weaker effect on the silyl radicals. The high reactivity of the Ph3Si radical contrary to the stable Ph3C radical is mentioned above. The decay of the trimesitylsilyl radical at 63°C follows a first-order kinetics with a half-life of 20 s [37]. Tri-tert-butylsilyl radical is also not markedly persistent showing the modest tendency of tert-h Ay groups to decrease pyramidalization... 

Table 1.2 EPR data for a variety of tris(trialkylsilyl)silyl radicals...

The rates of bromine atom abstraction by tris(trimethylsilyl)silyl radicals from a range of /Jara-substitutcd benzyl bromides has indicated that the silyl radical is nucleophilic. In addition both the polar and spin-delocalization effects of the substituents play a role in the abstraction reaction with the latter effect greater than for H-atom abstractions.166 The perfluoroalkylation of aromatics and alkenes has been investigated using C4F9I as the source of C,. Measurement of rate constants indicated that perfluoroalkyl radicals were 2-3 orders of magnitude more reactive than the corresponding alkyl radicals. This was attributed primarily to the reaction enthalpy and far less to the electrophilic nature of the radicals.167... 

Tris(trimethylsilyl)silyl radical is relatively stable and can therefore serve as a radical leaving group. This reaction has been extended to the radical-initiated allylation of organic halides202 203. Thus, thermolyses of bromides a to a carbonyl substituent 144 or of simple iodides with allyltris(silyl)silane in the presence of a radical initiator gives the corresponding allylation products (equation 112). 

The radical-initiated allylation of alkyl halides with allyltris(trimethylsilyl)silanes proceeds via an SH2 process mediated by a tris(trimethylsilyl)silyl radical.225 The radical-allylating agents react with alkenes, alkynes, and aldehydes via a radical chain process to give the corresponding allylsilylation products.226... 

Radical reactions have some stereochemical features that can be compared directly with their ionic counterparts, especially when the radical centre is adjacent to an existing stereogenic centre. The tris(trimethylsilyl)silyl radical adds to chiral ketones like 3-phenyl-2-butanone 7.59 to give a radical 7.60 flanked by a stereogenic centre. The hydrogen atom abstraction from a thiol, determines the relative stereochemistry, and the products 7.61 and 7.62 are analogous to those from the hydride reduction of the ketone. They are formed in the same sense, and the stereochemistry is explained by the Felkin-Anh picture 7.60. 

Similar products were produced from copolymers such as poly-(cyclohexylmethylsilane-co-dimethylsilane), and materials characteristic of the copolymer composition were obtained. Three disilanes (i.e., 1,2-dicy-clohexyl-1,2-dimethyl- 1,1,2,2-tetramethyl- and 1-cyclohexyl-1,2,2-tri-methyldisilane), as well as the two substituted silylene adducts, were produced from the predominantly random copolymer. The disilanes, which are taken to be diagnostic of silyl radical abstraction, were assumed to accumulate in the mixture, because they no longer absorbed light significantly at 254 nm. The photoinstability of a model trisilane, 2-n-butyl-l, 1,1,2,3,3,3-hepta-methyltrisilane, was demonstrated under the reaction conditions even though its absorption maximum occurred at 215 nm. 

Major drawbacks of tris(trimethylsilyl)silane relative to tri-n-butyltin hydride, however, are the cost of the reagent, the need to handle the reagent under argon and the propensity of the tris(trimethysilyl)silyl radical to add to multiple bonds. Many other silicon and germanium hydrides, as well as sulfur, selenium, and phosphorous radical transfer reagents have been investigated as tin hydride alternatives and while many have niche applications, none so far have the flexibihty or range of apphcations of tin hydrides. ... 

Asymmetric induction is used in the stereoselective synthesis of silanes via the hvdrosilyla-tion of a,/l-unsaturated esters30. The addition of the tris(trimethylsilyl)silyl radical to the double bond is highly regioselective. yielding an ester-substituted radical that abstracts hydrogen diastereoselectively. 

Silyloxy-substituted alkyl radicals, which are generated via addition of tris(trimethylsilyl)silyl radicals to chiral ketones, abstract hydrogen from thiols with moderate diastereoselectivitics37. The svnjunti ratio is dependent on the steric bulk of the neighboring alkyl substituents. 

Trisylbenzene [tris(trimethylsilyl)methylbenzene] when photolyzed is highly susceptible to homolysis (ESR spectroscopy demonstrated the presence of radicals) the radicals recombined by attack of the silyl radical on the aromatic ring as the major pathway16 (equation 5). 

A further variant on the radical carbonylation/acyl radical cyclization theme involves the silylcarbonylation of 1,5-hexadienes [48]. Here, the sequence is initiated by the addition of a tris(trimethylsilyl)silyl radical to the least substituted terminus of the diene. Carbonylation and acyl radical cyclization then ensues in the normal way. It should be noted that this type of carbonylation cannot be achieved with tin hydride, since the carbonylation rate is not sufficient to capture y -tin-attached alkyl radical, which quickly reverts to tin radical and the 1,5-diene. 

We also attached three steroid substrates to an iodophenyl template in 16 (Scheme 6-7) using a tris-silyl ether link, and saw that the template could direct selective radical-relay chlorination to all three tethered substrate species in the same work we reported a similar finding when a thiophene ring was the triply tethered template [49]. Thus the sulfur atom of thiophene can perform radical relay. In these triple catalytic functionalizations, the more reactive sulfuryl chloride was the preferred reagent. 

Crich and co-worker report an attractive solution to this problem by a combination of efficient iodine abstraction by tributyltin or tris(trimethylsilyl)silyl radicals from an aryl iodide to form an aryl radical and successive intramolecular homolytic attack at sulfur by the aryl radicals to generate acyl radicals (Scheme 8) [60]. 

In photoreactions of di-, tri-, and oligosilanes in the presence of acceptors (A), initiation is by electron transfer from the silanes to A and Si-Si bond cleavage occurs to give silyl radicals and silyl cation components. The silyl radicals display reactivity similar to diat of allyl and aiylmethyl radicals with the radical anions of the electron acceptors. The result is the introduction of silyl groups into the electron-deficient alkenes and arenes. When nucleophiles such as alcohols are... 

Reagent for Polarity Reversal Catalysis in Radical Reactions. Polarity reversal catalysis (PRC) has been established by Roberts in free-radical chemistry as an efficient alternative to the use of stannanes (e.g., tri-/>butylstannane) and their associated toxicity and purification problems. Silyl radicals can be a valid alternative to tin radicals for one of the most common radical reactions, that is, radical dehalogenation, but silanes, contrary to stannanes, cannot sustain an effective radical chain reaction, due to the stronger Si-H bond. 

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