Tributyltin hydride, free radical addition

Stannylation of lithiated allyl ethers gives (Z)-3-alkoxyallylstannanes (1)115,116, whereas mixtures of (Z)- and ( )-tributyl(3-methoxy-2-propenyl)stannanes (2) were obtained from free-radical addition of tributyltin hydride to l-methoxy-l,2-propadienel16. 

Free radical additions to alkenylboronic esters provided the first access to (a-haloalkyl)boronic esters [8]. Intramolecular cyclizations involving radical additions to alkenylboronic esters have been reported and, also, a (l-iodo-S-hexenyl)boronic ester cyclizes to the cyclopentyl derivative under radical conditions [83]. Reaction of pina-col (l-iodopentyl)boronate with tributyltin hydride and butyl vinyl ether yielded the addition product pinacol (l-butoxy-3-heptyl)boronate (71%), but addition of the bory-lalkyl radical to methyl acrylate was inefficient and yielded mainly the simple reduction product, pinacol pentylboronate [84]. 

A detailed mechanistic analysis of the factors affecting the success of these tin hydride mediated addition reactions has been provided by Giese.3 This analysis, which is especially illustrative of how experimental conditions for free radical reactions are planned, is summarized in Scheme 27. Three intermediate radicals, (8), (9) and (10), are involved. As is characteristic of all radical reactions, these radicals are simultaneously exposed to the same reagent pool and each can potentially undergo an addition reaction or an atom transfer reaction. The required reaction of the tributyltin radical (8) is atom ab-... 

Giese and coworkers determined on the basis of experimental results obtained by Scheffold et al. [268, 299] that addition reactions of alkyl bromides 249 to a,p-unsaturated nitriles or esters 248 catalyzed by cobalamine 247 are free radical reactions (Fig. 61) [300], This conclusion was based on the similar cis/trans-selectivities in addition reactions of the 4-tert-butylcyclohexyl radical to different electron-poor alkenes 248 using 247 as a catalyst on one hand and classical tributyltin hydride conditions on the other. The kinetics of the radical addition was determined. 

Tributyltin hydride adds across C=C 77 bonds by a free-radical mechanism. Addition of BusSnH across alkynes is one of the best ways of making alkenyltin compounds, which are useful reagents in organic synthesis. The mechanism is exactly the... 

The free radical chain reaction between PhCOCHjHgCl and 1-morpholinocyclohex-cne has been reported to involve addition of the acceptor radical PhCOCHj- to the jS-position of the enamine followed by electron transfer to regenerate the attacking radical (Scheme 19). Photostimulated reactions of simple alkylmercury halides failed since an electrophilic radical is required. Photolysis of p-nitrobenzyl chloride in the presence of enamines gave the -/>-nitrobenzyl ketone on hydrolysis . Radical mediated reductive alkylation of acyclic-enamines has also been reported with radical precursors such as PhSCH2CN, PhS02CH2Cl and Me3CS02CH2SePh . Reductive alkylation also occurred with chloromethyl p-tolyl sulphone in the presence of tributyltin hydride and azobis(isobutyronitrile) (AIBN) (Scheme 20). 

The methodology of EtsB-mediated addition of RsSnH to an acetylenic bond has also been successfully applied in the cyclization of enynes . An example is given in equation 35. The use of triphenylgermanium hydride, thiophenol, diphenyl diselenide (equation 36), tris(phenylseleno)borane, Se-phenyl areneselenosulfonates (equation 37) and diphenylphoshine under free-radical conditions induces cyclization of enynes like tributyltin hydride. That is, if the chain-transfer step in Scheme 11 is much faster than the ring expansion, the methylenecyclopentane adduct should be the sole product. 

The NMR spectroscopy of poly(vinyl chloride), which was reduced with tributyltin hydride, showed that the original polymer contained a number of short four-carbon branches [300]. This, however, may not be typical of all poly(vinyl chloride) polymers formed by free-radical polymerization. It conflicts with other evidence from NMR spectroscopy that chloromethyl groups are the principal short chain branches in poly(vinyl chloride) [301, 302]. The pendant chloromethyl groups were found to occur with a frequency of 2-3/1,000 carbons. The formation of these branches, as seen by Bovey and coworkers, depends upon head to head additions of monomers during the polymer formation. Such additions are followed by 1,2 chlorine shifts with subsequent propagations [301, 302]. Evidence from still other studies also shows that some head to head placement occurs in the growth reaction [303]. It was suggested that this may be not only... 

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