Stannyl radicals, generation

If selenide additions are carried out in the presence of tri- -butylstannane, the radical generated by addition can be reduced by hydrogen abstraction. The chain is then continued by selenide abstraction by the stannyl radical. This leads to nonselenated addition and cyclization products. 

Radicals for addition reactions can be generated by halogen atom abstraction by stannyl radicals. The chain mechanism for alkylation of alkyl halides by reaction with a substituted alkene is outlined below. There are three reactions in the propagation cycle of this chain mechanism addition, hydrogen atom abstraction, and halogen atom transfer. 

Entry 21 involves addition to a glyoxylic hydrazone and the cis ring junction is dictated by strain effects. The primary phenylselenyl group is reductively removed under the reaction conditions. Entry 22 involves generation of a stannyloxy radical by addition of the stannyl radical at the carbonyl oxygen. Cyclization then ensues, with the cis-trans ratio being determined by the conformation of the cyclization TS. 

Radical homologation. This tin pinacolate is known to generate trimethyltin radicals at 60° and appears to be superior to tributyltin hydride as a source of stannyl radicals for addition of alkyl halides to O-benzylformaldoxime (equation I).1 Iodides, bromides, and selenides can be used as radical precursors. The same... 

Most of the many applications of the tin hydrides in organic synthesis proceed by radical chain reactions in which one step involves the reaction of a radical X- with the tin hydride to abstract hydrogen and generate a stannyl radical,... 

Stannyl radicals are usually generated by homolytic substitution at hydrogen in a tin hydride, or at tin in a distannane, or, conjugatively, at the y-carbon atom in an allylstannane.453 The initiator is commonly AIBN at ca. 80 °C. In the presence of a trace of air, organoboranes are oxidized by a radical chain mechanism, and triethylborane is now commonly used as an initiator at temperatures down to —78°C,519 and it can be used in aqueous solution.520 9-Borabicyclo[3.3.1]nonane (9-BBN) has similarly been used to initiate the reaction of tin hydrides at 0 and —78°C,521 and diethylzinc works in the same way.522... 

Free-radical cyclizations using ethyl radicals generated by EtsB/air system or stannyl radicals systems provide a range of carbocyclic and heterocyclic hydroxylamines (equation 77). Stereoselectivity in these reactions is variable but can be semiquaUtatively predicted by Beckwith-Houk models . Depending on the substitution pattern of the emerging cyclic system, stereoselectivity can be very high, especially in fused polycyclic systems (equation... 

Selenenyl groups can be abstracted from acyl selenides to generate radicals on reaction with stannyl radicals.201 202 203 Normally, some type of stabilization of the potential reaction site is necessary. Among the types of selenides that are generated by selenenyl abstraction are x-sclcncnyl cyanides and a-selenenyl phosphates. 

Each of the syntheses of seychellene summarized in Scheme 20 illustrates one of the two important methods for generating vinyl radicals. In the more common method, the cyclization of vinyl bromide (34) provides tricycle (35).93 Because of the strength of sjp- bonds to carbon, the only generally useful precursors of vinyl radicals in this standard tin hydride approach are bromides and iodides. Most vinyl radicals invert rapidly, and therefore the stereochemistry of the radical precursor is not important. The second method, illustrated by the conversion of (36) to (37),94 generates vinyl radicals by the addition of the tin radical to an alkyne.95-98 The overall transformation is a hydrostannylation, but a radical cyclization occurs between the addition of the stannyl radical and the hydrogen transfer. Concentration may be important in these reactions because direct hydrostannylation of die alkyne can compete with cyclization. Stork has demonstrated that the reversibility of the stannyl radical addition step confers great power on this method.93 For example, in the conversion of (38) to (39), the stannyl radical probably adds reversibly to all of the multiple bond sites. However, the radicals that are produced by additions to the alkene, or to the internal carbon of the alkyne, have no favorable cyclization pathways. Thus, all the product (39) derives from addition to the terminal alkyne carbon. Even when cyclic products might be derived from addition to the alkene, followed by cyclization to the alkyne, they often are not found because 0-stannyl alkyl radicals revert to alkenes so rapidly that they do not close. 

Free radical additions of phenylthio or stannyl radicals to 2-alkenyl 2-siloxycyclo-propanes afford similar products although a completely different mechanism is operative 84). This direct generation of protected y-oxoesters 144 and 145 is of interest since the silyl enol ether function might be usable for regioselective C-C-bond formation and the allyl stannane moiety in 145 could be activated for subsequent transformations. Yet further examples have to demonstrate utility and scope of this mode of ring opening. 

With a metal hydride as the nucleophile, the organotin hydrides, R SnH4. are formed, which, by addition to an alkene or alkyne (hydrostannation), usually by a radical chain mechanism involving stannyl radicals, RaSn", provide the second way of generating the tin-carbon bond (Scheme 1-2). 

Cyclopentadienyltin compounds undergo the same reaction, but much more readily, to generate the cyclopentadienyl and stannyl radicals.3 These reactions have been used for studying substitutent effects in cyclopentadienyl ([5]annulene) radicals (see Sections 9.3.3 and 20.1.1). 

The cyclopentadienyltin compounds (and cyclopentadienyl derivatives of other metals such as mercury and lead) are very sensitive to photolysis, and on irradiation with UV light in an ESR spectrometer they show the spectrum of the cyclopentadienyl radical the stannyl radical is difficult to observe directly under these conditions, but its presence is established by the reactions which it shows with substrates such as alkyl halides. This provides a useful route to a variety of stannyl radicals (see Section 20.1.1), and a method of generating cyclopentadienyl radicals for study by ESR spectroscopy.125 It would be interesting to extend these ESR studies to the more complex compounds 9-6-9-9, which are known to be photosensitive. 

For studies of R3Sn radicals in fluid solution by ESR spectroscopy, the radicals are usually generated by abstraction of hydrogen from an organotin hydride, and this can conveniently be carried out by photolysing a mixture of di-t-butyl peroxide and the hydride within the cavity. The spectral resolution is often better at low temperatures, and this was ascribed to the slowing of the symmetrical exchange reaction between stannyl radicals and the stannane (Section 15.3.5.1).7 8... 

Radiation has to be continuous to maintain an adequate standing concentration of the stannyl radicals. If the t-butoxyl radicals are generated by laser flash photolysis and the formation and decay of the R3Sn radicals is monitored by optical spectroscopy (e.g. BusSn, vmax 400 nm), the rate constant at 295 K for the self-reaction of the BusSn radicals is found to be ca. 1.4 x 109 M 1 s, and that for the reaction of BulO radicals with the stannane is 1.9 x 108 M 1 s 1... 

Scheme 20-2 Generation of stannyl radicals from stannylcobaloximes.

Both allyl- and cyclopentadienyl-stannanes will undergo SH2 (SH2-y) reactions to generate a stannyl radical.42-43 The intermediate adduct radical is stabilised by the presence of the p-stannyl substituent (see below). These reactions find application in organic synthesis and are discussed in Section 9.1.3.3. 

When oxime benzoates, prepared by benzoylation of dialkyl 1-hydroxyiminophosphonates in the presence of Py, are submitted to the action of tin hydrides under radical conditions, the weakness of the N-0 bond is responsible for the fast formation of the reactive iminyl radical. In the example displayed in Scheme 7.71, the intermediate iminyl radical undergoes cyclization faster than the P-scission to nitrile to give a cyclic aminophosphonale. "- The reaction has also been explored with dialkyl l-benzoyloxyimino-2,2-dimethyl-3,4-pentadienylphosphonate and BujSnH/AIBN in refluxing cyclohexane. In this case, the iminyl radical generated by stannyl radical addition on the benzoyl moiety leads to the sole formation of phosphonylated dihydropyridine in quantitative yield. ... 

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