Reduction reactions Tributyltin hydride

A very talented collaborator, Dr S. W. McCombie, quickly developed an improved Vilsmeier procedure for the synthesis of the desired thionobenzoate 11 and then showed that 11 was smoothly reduced to the desired deoxy-compound 15 at 80° under reflux in benzene using a suitable initiator. Our first example was with cholesterol, but a further example with the glucose derivative 16 showed the power of the new reaction. Derivative 16 gave the thionobenzoate 17 without difficulty. Reduction with tributyltin hydride under the same conditions as above gave a high yield of the desired deoxy-compound 18. 

The reaction of the iodoalkene (178) with Pd(dppe)2 in HMPA followed by reduction with tributyltin hydride gave a mixture of the 6-exo trans product (179), the 6-exo cis product (180), and the 1-endo product (181) in a ratio of 1 1 2 in combined yield of 76% (Equation (18)), identical to the result of using hexalkylditin in benzene <90TL933>. This has led Curran and Chang to propose that the low valent palladium ene-halogenocyclization should be classed as a radical chain reaction that proceeds by way of an atom transfer mechanism and that palladium(O) may be a useful substitute for ditin in other iodine atom transfer reactions. 

Room temperature imidazolium ionic liquids have been shown to be useful solvents for the Michaelis-Arbuzov rearrangement, sometimes allowing the reaction to be performed at room temperature in a short period of tirne. Halolactones, which are the products of halolactonisation of y,5-unsaturated acids, have been shown to be dehalogenated by trialkyphosphites in the presence of water. This makes a useful alternative to the commonly used reduction by tributyltin hydride. The reactions of dibromoethane and dibromopropane with silyl phosphites have been studied. Mono and diphosphonoalkanes (37) and (38) were prepared and their chemical properties were studied. In the reaction of trifluoromethylated P-alkoxyketones with tris(trimethylsilyl)phosphite, a mixture of the 1,2- and 1,4-adducts (39) and (40) is produced. 

Another type of reactivity was disclosed for difluorochloro group in pyrimidine series. Recently Iaroshenko, Langer and co-workers shown that difluorochloro substituted pyrimidines 1227 can be converted into corresponding difluoromethyl pyrimidines 1228 by radical reduction with tributyltin hydride in the presence of AIBN in moderate yields. In addition, CFaCl-substituted pyrimidines 1227 were transformed to the corresponding l,l-difluorobut-3-enyl pyrimidines 1129 by reaction with aUyltributyltin and AIBN. In this case the yields of transformation are lower in comparison with tributyltin hydride reduction (Schane 269) [412]. 

The hydrogenolyaia of cyclopropane rings (C—C bond cleavage) has been described on p, 105. In syntheses of complex molecules reductive cleavage of alcohols, epoxides, and enol ethers of 5-keto esters are the most important examples, and some selectivity rules will be given. Primary alcohols are converted into tosylates much faster than secondary alcohols. The tosylate group is substituted by hydrogen upon treatment with LiAlH (W. Zorbach, 1961). Epoxides are also easily opened by LiAlH. The hydride ion attacks the less hindered carbon atom of the epoxide (H.B. Henhest, 1956). The reduction of sterically hindered enol ethers of 9-keto esters with lithium in ammonia leads to the a,/S-unsaturated ester and subsequently to the saturated ester in reasonable yields (R.M. Coates, 1970). Tributyltin hydride reduces halides to hydrocarbons stereoselectively in a free-radical chain reaction (L.W. Menapace, 1964) and reacts only slowly with C 0 and C—C double bonds (W.T. Brady, 1970 H.G. Kuivila, 1968). 

Our group has also reported that the alkylation products of 4-cyano-l,3-diox-anes can serve as substrates for radical atom transfer reactions [41]. One such example is shown below (Eq. 17). Slow addition of tributyltin hydride/AIBN to a refluxing solution of cyanohydrin 115 generated the radical nitrile transfer product 116. This method, though somewhat limited in scope, can provide access to syn-l,3-diols which maybe unstable to the vigorous Li/NHg reduction conditions. 

Tributyltin hydride reduction of carbonyl compounds. The reduction of carbonyl compounds with metal hydrides can also proceed via an electron-transfer activation in analogy to the metal hydride insertion into TCNE.188 Such a notion is further supported by the following observations (a) the reaction rates are enhanced by light as well as heat 189 (b) the rate of the reduction depends strongly on the reduction potentials of ketones. For example, trifluoroacetophenone ( re<1 = —1.38 V versus SCE) is quantitatively reduced by Bu3SnH in propionitrile within 5 min, whereas the reduction of cyclohexanone (Erea — 2.4 V versus SCE) to cyclohexanol (under identical... 

Another complication that can arise in these reactions is that a cycloaddition reaction of the radical intermediate formed in the tin hydride reduction, to an adjacent carbon-carbon double bond, can compete with the simple reduction reaction. This occurs when a five- or six-membered ring can be formed in an intramolecular cycloaddition reaction. For example, Beckwith and Lawrence96 found both five- and six-membered rings in the product when l-bromo-2,2,5-trimethylhex-l-ene was treated with tributyltin hydride (Scheme 15). 

The reductive removal of the tellurium moiety from aryldichlorotellurolactones is achieved by treatment with tributyltin hydride (TBTH) in refluxing toluene. High yields are conditional on a previous careful deoxygenation of the reaction mixture. 

Other catalytic reactions involving a transition-metal allenylidene complex, as catalyst precursor or intermediate, include (1) the dehydrogenative dimerization of tributyltin hydride [116], (2) the controlled atom-transfer radical polymerization of vinyl monomers [144], (3) the selective transetherification of linear and cyclic vinyl ethers under non acidic conditions [353], (4) the cycloisomerization of (V2V-dia-llyltosylamide into 3-methyl-4-methylene-(V-tosylpyrrolidine [354, 355], and (5) the reduction of protons from HBF4 into dihydrogen [238]. 

Hoping to improve or to shorten our total synthesis of L-daunosamine, we explored the following reactions with 112 and 113, the adducts of the racemic 7-oxanorbomene derivative 47 + 48 to PhSeCl and PhSCl, respectively. Saponification of 112 and 113, followed by treatment with formalin, afforded ketones 114 and 115, respectively. Treatment of with tributyltin hydride in toluene/benzene (AIBN 1 - 2%, 80 °C) gave the key intermediate ( )-I03 in 69 % yield. Under the same conditions, 115 was reduced to ( )-I03 in 40 - 45 % yield. Raney nickel reduction of 115 afforded ( )-103 in 50 % yield together with 40 % of ( )-7-oxabicyclo[2.2.1]heptan-2-one. However, we found the multistep procedure 32 100 101 102 103 easier to scale up. Several intermediates in our synthesis do not have to be isolated. For instance, transfomation of 32 into 102 can be carried out in the same pot in 94 % yield. 

A high degree of stereoselectivity was achieved in reductive radical cyclizations with Coordination of the oxime function (e.g. 108) with samarium cation seems to play an important role, since the identical reaction with a tributyltin hydride/radical initiator system produces poor stereoselectivity (equation 79). ... 

Crich and Rumthao reported a new synthesis of carbazomycin B using a benzeneselenol-catalyzed, stannane-mediated addition of an aryl radical to the functionalized iodocarbamate 835, followed by cyclization and dehydrogenative aromatization (622). The iodocarbamate 835 required for the key radical reaction was obtained from the nitrophenol 784 (609) (see Scheme 5.85). lodination of 784, followed by acetylation, afforded 3,4-dimethyl-6-iodo-2-methoxy-5-nitrophenyl acetate 834. Reduction of 834 with iron and ferric chloride in acetic acid, followed by reaction with methyl chloroformate, led to the iodocarbamate 835. Reaction of 835 and diphenyl diselenide in refluxing benzene with tributyltin hydride and azobisisobutyronitrile (AIBN) gave the adduct 836 in 40% yield, along with 8% of the recovered substrate and 12% of the deiodinated carbamate 837. Treatment of 836 with phenylselenenyl bromide in dichloromethane afforded the phenylselenenyltetrahydrocarbazole 838. Oxidative... 

Previously, vinyl radicals for cyclization reactions were produced by the reduction of vinyl halides with tributyltin hydride. In the present... 

The 2,5-dibromo-2,5-dideoxy-D-xylono-1,4-lactone (6) (Scheme 2) can also be selectively reduced to give 5-bromo-2,5-dideoxy-D-fhreo-pentonolactone (7) by reaction with either aqueous hydrazine as mentioned above, or by consumption of 1 mole of hydrogen [ 14]. Further reaction with hydrogen and Pd/C leads to 3-hydroxypentanoic acid by an initial reductive elimination [15]. Reduction of the primary bromine in these cases can be obtained by means of tributyltin hydride [25] to give 8. 

The reaction of alkyl dihalogenoacetate magnesium enolates with 2,3-isopropylidene-D-glyceraldehyde affords the expected /3-hydroxy-a-dihalogenoesters . The erythro isomer is obtained with isopropyl dichloroacetate magnesium enolate. This result is in agreement with theoretical models. 2-Deoxy-pentono-1,4-lactones are obtained after removal of the halogen atom by either Raney nickel or tributyltin hydride reduction (equation 89). 

The tributyltin hydride reduction of dihaloaziridines, e.g. (266), represents another example where the ring system has been maintained (79CJC1958). Especially noteworthy is the retained configuration associated with the reaction. This behavior differs from the cyclopropyl analog and was explained on the basis of increased s-character in the exocyclic bond caused by the nitrogen atom. 

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