Tributyltin hydride—Oxygen

Intermediates such as 224 resulting from the nudeophilic addition of C,H-acidic compounds to allenyl ketones such as 222 do not only yield simple addition products such as 225 by proton transfer (Scheme 7.34) [259]. If the C,H-acidic compound contains at least one carbonyl group, a ring dosure is also possible to give pyran derivatives such as 226. The reaction of a similar allenyl ketone with dimethyl mal-onate, methyl acetoacetate or methyl cyanoacetate leads to a-pyrones by an analogous route however, the yields are low (20-32%) [260], The formation of oxaphos-pholenes 229 from ketones 227 and trivalent phosphorus compounds 228 can similarly be explained by nucleophilic attack at the central carbon atom of the allene followed by a second attack of the oxygen atom of the ketone at the phosphorus atom [261, 262], Treatment of the allenic ester 230 with copper(I) chloride and tributyltin hydride in N-methylpyrrolidone (NMP) affords the cephalosporin derivative 232 [263], The authors postulated a Michael addition of copper(I) hydride to the electron-... 

A number of cyclic and sugar-derived halo acetals 273 were subjected to radical 5-exo cyclizations catalyzed by a cobalt salen catalyst 274 with NaB H4 as the stoichiometric reductant but in the presence of air (entry 11) [321, 322]. Under these conditions, bicyclic oxygenated tetrahydrofurans 275a were obtained in 50-84% yield. Diastereomeric isomers 275b were isolated as the minor components. The yields were similar to those obtained with tributyltin hydride. The oxygen concentration proved to be important, since air gave better yields... 

The reaction of triethylboron with oxygen is a convenient way to generate ethyl radicals at room temperature, or at lower temperatures. These conditions at room temperature in the presence of tributyltin hydride efficiently perform the Barton-McCombie deoxygenation reaction. Recently, we showed that the tin hydride could be replaced by diphenylsilane also at room temperature, for the deoxygenation of secondary alcohols.20 Many different thiocarbonyl derivatives can now be used for the Barton-McCombie reaction. Data were collected for R = Ph, />-F-C6H4, C6F5, C6C13H2 with various substrates (Scheme 2). The yields were excellent for all the secondary... 

Methoxyphenyl tellurium compounds with cyclic ether or lactone functionalities in the imposition to the carbon atom linked to tellurium were detellurated by tributyltin hydride in oxygen-free toluene with elimination of the aryltelluro group2. 

Cyclization of an allyioxy-iodobenzene under conditions which are broadly similar to those described in Section I.l gives a good-to-excellent yield of the corresponding oxygen heterocycle. A reduced benzofuran is formed in a hom-olytic process when the bromoacetal (39.1) is heated with tributyltin hydride and a catalytic amount of AIBN the first-formed product is dehydrogenated at room temperature in the presence of 4-toluenesulphonic acid to give the naturally occurring evodone. 

Radical Initiator. 9-BBN-H effectively initiates the reduction of common radical precursors by tributyltin hydride at low temperatures (eq 56). Importantly, the radical reaction proceeds without the need for oxygen, whereas similar initiation by ttialkylboranes requires at least some oxygen. 

The rate of cyclopropylmethyl-allylmethyl radical interconversion has now been measured by e.s.r. methods k was found to be = 1.3 x 10 s at 25"C ( , = 5.94 kcalmol" log A 12.48). The radical addition of tributyltin hydride to cyclopro-pylketones involves oxygen-tin bonding and cyclopropylcarbinyl radical formation. 

The experimental conditions are not specified precisely with respect to the presence of oxygen. This point is particularly important for a mechanistic interpretation due to the role of oxygen traces in the occurrence of an ET process in related reductions.The procedure was extended to the reduction of halo and alkoxy derivatives of Group IV elements, in apolar solvents instead of ethers. Deuteration can be carried out with lithium aluminum deuteride in perdeutero-cyclohexane. 75 The time shortening brought about by sonication in the reduction of veratraldehyde by tributyltin hydride on silica gel was mentioned but not exploited.Little if any effect was noted in the reduction of various aroyl... 

R R = O) suffers lithium-oxygen co-ordination and monobromocyclo-propane generation (Scheme 65), the configuration being the opposite of that obtained from reduction with tributyltin hydride. In (424), however, there is greater... 

---