Tributylstannyl radical

The electrolysis of unsaturated a-tributylstannyl ethers 12 yields cyclic compounds by a similar reaction, involving in this case carbon-carbon bond formation. Here, too, the evidence points to a facile ET with formation of an a-stannyl ether radical cation. This in turn cleaves to the tributylstannyl radical and an unsaturated ene-oxonium residue. 

The indolyltributylstannanes, which are more robust than their trimethylstannyl counterparts, are prepared similarly [166, 167]. Labadie and Teng synthesized the IV-Me, N-Boc, and jV-SEM (indol-2-yl)tributylstannanes [167], and Beak prepared the A-Boc trimethyl- and tributyltin derivatives in high yield [166]. Caddick and Joshi found that tributylstannyl radical reacts with 2-tosylindoles to give the corresponding indole tin compounds as illustrated [168]. 

The kinetics of reactions involving the tributylstannyl radical have been refined by laser flash time-resolved UV spectroscopy. The measured extinction coefficient of the BujSn- radical in benzene was 1620 40 M 1cm 1 at 400 nm, the rate constant of the reaction of the /-butoxyl radical with Bu3SnH was (3.5 0.3) x 108M 1 s 1, and the rate constant for the self-reaction of the Bu3Sn radical was (3.6 0.3) x 109 M s1. S29... 

Transformation of a substrate into its ion radical enhances the species reactivity. Sometimes, this can overcome steric encumbrance of the substituent to be removed. Thus, l,4-di-iodo-2,6-dimethybenzene expels only one, sterically congested, iodine from position 1 upon the action of the tributylstannyl radical. Upon the action of the enolate ion of pinacolone (Me3CCOCH2) in the photoinitiated SrnI process, both iodines (from positions 1 and 4) are substituted (Branchi and co-authors 2000). 

The transformation of 6-bromo-l-hexene (38) into methylcyclopentane by the action of tributyltin hydride (Scheme 7) typifies the richness of the C—C bond forming chemistry in question. A knowledge of the critical rate constants (kc, ku and Br in Scheme 7) allow, through control of substrate concentration, necessary selectivity criteria to be met. Specifically the 5-hexenyl radical (39) must undergo intramolecular addition to form the cyclopentylmethyl radical (40), 40 must abstract a hydrogen atom from tributyltin hydride and the tributylstannyl radical must abstract the halogen in 38 to form 39. These processes must proceed faster than any competing side reaction. 

Zard has developed the use of N-amidyl radicals. The precursors of the radical intermediates are 0-benzoyl hydroxyamines such as 37. Addition of a tributylstannyl radical to the carbonyl group of the benzoate moiety is followed by the cleavage of the weak N - O bond. A subsequent 5-exol6-endo tandem cyclization takes place to yield the skeleton of the natural product deoxyserratine (Scheme 12) [49]. Later, the same group disclosed a tin-free source of amidyl radicals that relies on the use of M-(0-ethyl thiocarbonyl-sulfanyl) amides and lauryl peroxide as initiator. Examples of polycyclization were also given [50]. On the occasion of a model study toward the synthesis of kirkine, the use of thiosemicarbazide precursors gave access to the tetracyclic structure of the natural product [51]. 

Tokuyama et al. designed a highly elegant synthesis of indoles by reacting tributylstannyl radicals or hypophosphite salts with imsaturated... 

Homolytic cleavage of the N-X bond usually requires activation by light, heat or peroxides, but can also be accomplished by reaction with redox systems (metal salts) or tributylstannyl radicals, depending on the nature of X. 

An alternate radical-induced addition-fragmentation sequence relies on the 1,4-elimination of a leaving group to drive the reaction. In this sequence, because the fragmentation is accompanied by the expulsion of a tributylstannyl radical, the requirement for the initial tributyltin hydride is only catalytic (Scheme 79). The stereospecificity of the reaction course has been rationalized in terms of a conformational preference for the elimination step, as depicted in Scheme 79 where the rrans-substituted cyclohexanone forms only one double bond geometry in the product. As shown in equations (40) and (41), the cij-substituted cyclohexanones undergo the four- and three-carbon expansions stereospecifically to give the (Z)-alkenes. 

Curran gives the approximate scale of reactivities towards tributylstannyl radicals that is shown in Table 20.3.61. 

The first propagation step concerns the reaction of the substrate R X with the tributylstannyl radical. R-X can be a halide (but not a fluoride), a chalco-genide, an isocyanide, a nitro compound or some more complicated radical precursor group such as xanthates, Barton and/or Kim esters.14 The rate of this step depends on X, the weakness of the R-X bond and the stability of the ensuing radical R (roughly speaking, the more stable the radical the weaker is the R-X bond) some typical rate constants are collected in Table 4.1.15... 

Some efficient oxidative and reductive intramolecular carbon-radical additions can be carried out. /pra-replacement of toluenesulfonyl by tributylstannyl radical occurs readily at C-2 (but not at C-3) as does intramolecular replacement by carbon radicals. " ... 

Recently, Kim has reported a modified Hofmann-Loffler-Freytag reaction under reducing conditions [70], Treatment of alkyl azides with Bu3SnH afforded N-tributylstannyl radicals. These radicals proved to be more reactive than ordinary aminyl radicals towards hydrogen atom abstraction presumably because of their higher nucleophilicity. Two examples are shown in Scheme 14. In the first one (Eq. 

The -fragmentation can be used as one of the cascade steps in processes which do not start from the addition of a sulfur-centered radical. Rearrangements of sulfur compounds, catalyzed by tributylstannyl radical, which lead to 3-vinyldihydrothiophene or dihydrothiopyran derivatives, have been developed [70]. The mechanism is shown in Scheme 15 the cyclic y9-thioalkyl radical 9, produced in two steps, fragments, thus producing the thiyl radical 10, which cyclizes in a 5- or 6-endo fashion onto the stannylated double bond subsequent elimination leads to the final heterocyclic compound. 

The cyclization of (104) to the pyrrolizidine derivative (105) involves addition of a tributylstannyl radical to the thiocarbonyl group and intramolecular addition of the radical formed to the double bond <92JOC6803>. 

Hypervalent Sulphur Compounds and their Selenium and Tellurium Analogues.—The reaction of the trithiapentalene (682 R = Me) with trimethylsilyl or tributylstannyl radicals, which occurs at S-1, has been studied by e.s.r. spec-troscopy. The n.m.r. spectra of trithiapentalene (682 R = H) and the dioxathiapentalene (683), determined for the nematic phase, indicate that both... 

Fukuyama devised a novel tin-mediated indole ring synthesis leading directly to 2-stannylindoles that can capture aryl and alkyl halides in a Pd-catalyzed crosscoupling termination reaction [110-112]. The presumed pathway is illustrated and involves initial tributylstannyl radical addition to the isonitrile 65, cyclization, and final formation of stannylindole 66. 

A tandem radical sequence was observed when the indole amide 120 was treated with tributylstannyl radical [81]. The initially formed aryl radical is proposed to undergo a 1,5-hydrogen atom abstraction to form an a-amido radical en route to the cyano spiroindolines 121a and 121b. Treatment with potassium tert-butoxide and oxygen in THF gave the C-3 spirooxindole in modest yield. 

Trithiapentalenyl radicals can be generated by the reaction of l,6,6aA -trithiapentalenes (189 R = H or Me) with trimethylsilyl or tributylstannyl radicals. The hyperfine e.s.r. splitting of the trithiapentalenyl radicals is significantly lower than that of pentadienyl radicals, indicating considerable delocalization of unpaired spin on to sulphur. ... 

An alkyl radical is produced by the reaction of tributylstannyl radical and iodoalkanes to trigger the reaction. An alkyl radical reacts with stannyl enol ethers via Sh2 mechanism to give an fl -alkyl ketone and regenerate a stannyl radical. In the presence of an electron-deficient alkene like fumarate and maleate, an alkyl radical adds to the alkene to generate a radical whose conformation is governed by a dipole-dipole repulsion between the two ester carbonyl groups. Subsequent reaction with the stannyl enol ether occurs preferentially in the opposite side to R to give an anti (erythro) adduct preferentially (Scheme 3-198). 

Ketyl radicals are also available using tributylstannyl radicals as oxophilic species. O-tributylstannyl ketyls are produced under neutral radical conditions by reaction of the carbonyl group with TBTH and AIBN (142 143). The overall process (142—>146, Scheme 25.68) leads to compound 146, which is formed by a cascade process involving the ring opening of cyclopropylmethyl radical 143, cyclization of the resulting 144, and protonation of enolate 145 after its radical reduction. 

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