Triethylsilane, reaction with radicals

Scheme 11 has lead to the development of an alternative method for conducting radical chain reactions with inexpensive reagents such as triethylsilane and alkylthiols. 

For the acetoxy radical, the for decarboxylation is about 6.5 kcal/mol and the rate is about 10 s at 60°C and 10 s at —80°C. Thus, only very rapid reactions can compete with decarboxylation. As would be expected because of the lower stability of aryl radicals, the rates of decarboxylation of aroyloxy radicals are slower. The rate for p-methoxybenzoyloxy radical has been determined to be 3 x 10 s near room temperature. Hydrogen donation by very reactive hydrogen-atom donors such as triethylsilane can compete with decarboxylation at moderate temperatures. 

The reaction of thiyl radicals with silicon hydrides (Reaction 8) is the key step of the so-called polariiy-reversal catalysis in the radical chain reduction. The reaction is strongly endothermic and reversible with alkyl-substituted silanes (Reaction 8). For example, the rate constants fcsH arid fcgiH for the couple triethylsilane/ 1-adamantanethiol are 3.2 x 10 and 5.2xlO M s respectively. 

The low reactivity of alkyl and/or phenyl substituted organosilanes in reduction processes can be ameliorated in the presence of a catalytic amount of alkanethiols. The reaction mechanism is reported in Scheme 5 and shows that alkyl radicals abstract hydrogen from thiols and the resulting thiyl radical abstracts hydrogen from the silane. This procedure, which was coined polarity-reversal catalysis, has been applied to dehalogenation, deoxygenation, and desulfurization reactions.For example, 1-bromoadamantane is quantitatively reduced with 2 equiv of triethylsilane in the presence of a catalytic amount of ferf-dodecanethiol. 

Bennasar et al. reported a new radical-based route for the synthesis of calothrixin B (378) (869). This synthesis starts from the 2,3-disubstituted N-Boc indole 1558 and uses a regioselective intramolecular acylation of a quinoline ring as the key step for the construction of the calothrixin pentacyclic framework. Chemoselective reaction of in s/fM-generated 3-lithio-2-bromoquinoline [from 2-bromoquinoline 1559 with LDA] with the 3-formylindole 1558 followed by triethylsilane reduction of the... 

Reduction of RX to RH.1 In the presence of di-r-butylhyponitrite (initiator) and a thiol,2 triethylsilane reduces alkyl chlorides, bromides, or iodides to alkanes in >91% yield by a chain reaction in which the thiol effects transfer of H from the silane to an alkyl radical. This reduction is generally effected with a R3SnH, which is toxic and more difficult to remove from the products. 

Again, no reaction was observed in the absence of mercaptoethanol. The mechanistic steps for the transformation were elucidated by the Chatgilialoglu et ah and are represented in Scheme 4.13, in analogy with the pathway reported for the radical reduction of aromatic azides with triethylsilane in toluene, with the addition of silyl radicals to the azide function, liberation of nitrogen and formation of silyl-substituted aminyl radical. The thiol is the hydrogen atom donor to this intermediate and it can be regenerated by its interaction with the silane, thus propagating the chain. The hydrolysis of the silylamine occurred... 

Isayama has reported an elegant procedure for the hydration of alkenes with molecular oxygen and triethylsilane catalyzed by a cobalt(II) complex followed by a reductive treatment with Na2S203 [41]. The reaction is efficient with terminal alkenes and a,/9-unsaturated esters. The radical nature of this reaction is ques-... 

The chemistry of )9-(thiocarbonyloxy)alkyl radicals stands in complete contrast to that of the (acyloxy)alkyl radicals, with elimination, while not the rule, being the norm [I]. The difference between the acyloxy and thiocarbonyloxy series is likely a consequence of the much weaker thiocarbonyl bond and the related higher stability of sulfur-centered radicals. The method has been developed in combination with the Barton deoxygenation method (Volume 1, Chapter 1.6) as a means of converting a vicinal diol, via the dixanthate, into an alkene (Scheme 33) [60-62]. Tributyltin hydride has been the reagent of choice for this reaction but it may also be conducted with the triethylsilane/benzoyl peroxide couple [63] and, doubtless, tris(trimethylsilyl)silane. 

The study of the reactivity of trialkylsilyl radicals in solution has been placed on a firmer foundation by the measurement of absolute rate constants for some reactions of triethylsilyl radicals (generated by the reactions of tert-butoxyl radicals with triethylsilane), with some organic halides and benzil/ These data show for example, the greater reactivity of Et3Si in halogen abstraction than that of trialkyltin radicals. Kinetic isotope effects (kn/fco) for the insertion of photochemically generated dimethylsilylene and methylphenylsilylene into Si-H and 0-H single bonds are about 1.3 and 2.1 - 2.3, respectively. The preferred mechanism for insertion of silylenes into O-H bonds is shown in equation (1). Other workers haye shown that dimethylsilylene inserts preferentially into O-H bonds of alcohols compared with S-H bonds in silanes or Si-O bonds in alkoxy-silanes. ... 

Recently, the photochemistry of azide 42 was studied by laser flash photolysis (Aex = 266nm) techniques in Fieon-113 (CF2CICFCI2) at room temperature. The formation of at least two intermediates, viz., triplet nitrene M3 (Aa,ax = 4(X)nm, lifetime 1.5 fis) and ethoxycarbonyl radical 44 (2 = 333 nm, lifetime 0.4/is), was observed (Scheme 11.22). The singlet nitrene M3 was deduced to have a lifetime between 2 and 10 ns in Freon-113 at ambient temperature. The kinetics of reactions of M3 with tetramethyleth-ylene and triethylsilane were also measured. ... 

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