Radical reactions tris silane

Radical reactions with acyl radicals sometimes involve decarbonylation as side-reactions, especially when stabilized secondary or tertiary radicals can be formed. These side-reactions can be suppressed using low-temperature reaction conditions together with different reducing agents such as tris(trimethylsilyl)silane.254... 

Ryu, Sonoda and coworkers reported that tris(trimethylsilyl)silane is a useful mediator for a three-component coupling reaction [45]. Table 4 summarizes examples of radical carbonylations mediated by (TMS)3SiH. The first example shows a three-component coupling reaction in which hexyl iodide, CO, and acrylonitrile combine to form a P-cymo ketone. The CO addition step is in competition with the addition to the alkene and the hydrogen abstraction from radical mediator. Thus, it is anticipated that a set of less efficient hydrogen donors, such as (TMS)3SiH, and the use of a smaller excess amount of an alkene is most favorable. Indeed, the reaction can be carried out at only 20 30 atm of CO pressure, substantially below the 80-90 atm which is used for carbonylative acyl radical reactions which are mediated by tin hydride, and a nearly stoichiometric amount (1.2 equiv) of acrylonitrile is sufficient. Some other examples, which include vinyl radical carbonylation, are also shown in Table 4. 

Alkyl, alkenyl, aryl and acyl radicals can all be used in cyclization reactions. Acyl radicals can be generated by addition of alkyl radicals to carbon monoxide, or more conveniently from acyl selenides, and undergo a variety of radical reactions. A synthesis of the sesquiterpene (—)-kamausallene made use of the radical cyclization from the acyl selenide 69 (4.61). Tris(trimethylsilyl)silane and triethylborane in air were used to promote the reaction, which is highly selective (32 1) in favour of the cis stereoisomer 70, as expected from a chair-tike transition state. Best yields in the cyclization reactions of acyl radicals are found with electron-deficient alkenes, indicating the nucleophilic character of acyl radicals. 

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. 

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). 

These reactions result in iodine atom transfer and introduce a potential functional group into the product. The trialkylborane method of radical generation can also be used in conjunction with either tri-n-butyl stannane or fnT-(trimethylsilyl)silane, in which case the product is formed by hydrogen atom transfer. 

A new effective metal-ffee radical approach by Murphy et al generates the free radical by treatment with tetrathiafulvalene (TTF).1491 As depicted in scheme 16 the aromatic amine 79 is transformed into the diazonium salt 81 which on treatment with TTF leads to the radical 82. The following stereoselective cyclization gives the hexahydrocarba-zole scaffold 80, a substructure of alkaloids like aspi-dospermidin, strychnin and vinblastin. Also the non-toxic tris(trimethylsilyl)silane was employed for domino reactions, eg. for the preparation of the alkaloid aspidospermidin. 

Tris(trimethylsilyl)silane reacts with phosphine sulfides and phosphine selen-ides under free radical conditions to give the corresponding phosphines or, after treatment with BH3-THF, the corresponding phosphine-borane complex in good to excellent yields (Reaction 4.45) [82]. Stereochemical studies on P-chiral phosphine sulphides showed that these reductions proceed with retention of configuration. An example is given in Reaction (4.46). 

Reaction 7.74) [84], That is, (TMS)3Si radical added to the double bond of allyl sulfides, giving rise to a radical intermediate that undergoes (3-scission with the ejection of the thiyl radical. Hydrogen abstraction from the silane completes the cycle of these chain reactions. 2-Functionalized allyl tris(trimethylsilyl)si-lanes (71) have been employed in the radical-based allylation reactions. 

The same research group has further performed radical carbonylation reactions on the same microreactor system [36]. First, alkyl halides were initiated and effectively reacted with pressurized carbon monoxide to form carbonyl compounds. The principle was subsequently successfully extrapolated to the multicomponent coupling reactions. 1-Iodooctane, carbon monoxide and methyl vinyl ketone were reacted in the presence of 2,2 -azobis(2,4-dimethylvaleronitrile) (V-65) as an initiator and tributyltin hydride or tris(trimethylsilyl)silane (TTMSS) as catalyst (Scheme 15). 

The radical source must have some functional group X that can be abstracted by trialkylstannyl radicals. In addition to halides, both thiono esters and selenides are reactive. Allyl tris(trimethylsilyl)silane can also react similarly.232 Scheme 10.11 illustrates allylation by reaction of radical intermediates with allylstannanes. 

Coupling reactions have also been observed in the reduction of vinyl silanes with potassium in DME at low temperatures. Although reduction of trans-1,2-bis(trimethylsilyl)ethylene or l,l,2-tris(trimethylsilyl)-ethylene gives rise to radical anions detectable by ESR, the reduction of l,l-bis(trimethylsilyl)ethylene gives rise to an ESR spectrum attributed to the radical anion of l,3-bis(trimethylsilyl)-... 

Tris(trimethylsilyl)silane is found to be an efficient reducing agent for a variety of functional groups. In particular, the reduction of halides, chalcogen groups, thiono esters and isocyanides are the most common ones. The efficiency of these reactions is also supported by available kinetic data. The rate constants for the reaction of (TMS)3Si radicals with a variety of organic substrates are collected in Table 2. 

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