Borohydride radical

The value of Eox for BH 4 is not known, but the borohydride radical, BH4 has been characterized by e.s.r and UV spectroscopy in oxidations of the anion by hydroxyl or azide radicals under pulse-radiolysis conditions (Symons et al., 1983 Horii and Taniguchi, 1986). Most borohydride reductions seem to be straightforward hydride transfers, but stepwise processes occur in the reductions of aryl bromides or iodides under photochemical or di-t-biftyl peroxide initiation. Radical intermediates are shown by the formation of 3-methyl-2,3-dihydrobenzofuran in the reduction of o-allyloxyiodobenzene (Abeywickrama and Beckwith, 1986). 

Prandi has improved the previous oxygenation conditions by using only a catalytic amount of cobalt in the presence of two equivalents of sodium borohydride. Radical cyclization/oxygenation has been applied to the syntheses of biologically important carbofuranosides from iodohexenyl carbohydrates (Scheme 7) [16],... 

Preparation of the prototype starts with the radical side I liain bromination of dichloroacetophenone to give the bromoketone (23). The carbonyl group is then reduced by means of sodium borohydride displacement of halogen by means of isopropylamine... 

Sulfonamides are very difficult to hydrolyze. However, a photoactivated reductive method for desulfonylation has been developed.240 Sodium borohydride is used in conjunction with 1,2- or 1,4-dimethoxybenzene or 1,5-dimethoxynaphthalene. The photoexcited aromatic serves as an electron donor toward the sulfonyl group, which then fragments to give the deprotected amine. The NaBH4 reduces the radical cation and the sulfonyl radical. 

The reductive replacement of mercury using sodium borohydride is a free radical chain reaction involving a mercuric hydride intermediate.18... 

Radical addition of dibromodifluoromethane to alkenes followed by sodium borohydride reduction is a convenient two-step method for the introduction of the difluoromethyl group.5 Either one or both carbon-bromine bonds in the intermediate dibromides may be reduced, depending on the reaction conditions. In the case of acyclic dibromodifluoromethane-alkene adducts, the reduction occurs regioselectively to yield the relatively inaccessible bromodifluoromethyl-substituted alkanes. The latter are potential building blocks for other fluorinated compounds. For example, they may be dehydrohalogenated to 1,1-difluoroalkenes an example of this methodology is illustrated in this synthesis of (3,3-difluoroallyl)trimethylsilane. 

An unusual case of anion-radical preparation consists of reduction of 4- and 3-nitrocatechol or nitrohydroquinone with sodium borohydride in air and in aqueous solutions. The resulting anion-radicals are stable at pH 9-12 despite the presence of water (Grenier et al. 1995). 

In another search for an alternative to Potier s modified Polonovski reaction of catharanthine A-oxide (45), it has now been found that anhy-drovinblastine (42) can be generated directly, in 77% yield, from a reaction of catharanthine and vindoline in 0.01 N acid, promoted by ionized ferric salts, followed by reduction with sodium borohydride (Scheme 30) (Wl). Remarkably, the cation radical 106 generated by Fe(III), in accord with other simple amine oxidations by Lindsay Smith and Mead (102), resulted in isoquinuclidine fragmentation and coupling to vindoline at 0°C, without the conformational inversion observed in the modified Polonovski reaction at that temperature (see Scheme 15). Other metal oxidants or ligand-bound Fe(lll) did not promote the coupling reaction. It will be of interest to see if the overwhelming competition of C-5-C-6 bond... 

It has been suggested that P BChl (where BChl is one of the two monomeric or "accessory BChls that are not part of P) is a transient state prior to P "I (14,16,19), although the evidence supporting this view has been criticized (23, 24) Recent subpicosecond studies find no evidence for P "BChl (8,9) These new results do not preclude some involvement of a monomeric BChl in the early photochemistry, only that P BChl apparently is not a kinetically resolved transient state Perhaps P itself contains some charge-transfer character between its component BChls, or between P and one or both of the monomeric BChls (8,9,25-27) One of the two monomeric BChls apparently can be removed by treatment of the reaction center with sodium borohydride (28) and subsequent chromatography, with no impairment of the primary electron transfer reactions (29) Thus, at present it appears that P I is the first resolved radical-pair state, and it forms with a time constant of about 4 ps in Rps sphaeroides ... 

The ylide 140 <2000JOC8068> is stable to attempted reduction using sodium borohydride, diisobutylaluminium hydride (DIBAL-H), or lithium aluminium hydride <2000JOC6388>, but eliminates propene on heating to form the annulated parent 141. Oxidation of the latter with lead(iv) oxide and potassium carbonate forms a radical, 142, which is stable to chromatography and can be stored in air for several days. 

The reduction of tosylhydrazones can also be performed with sodium borodeuteride in boiling methanol or dioxane, but the mechanism of this reaction (in boiling dioxane at least) is radically different from that of the lithium aluminum deuteride reductions.82 With sodium borohydride the first step is apparently hydride attack on the carbon atom of the C=N bond which is probably concerted with the elimination of the tosylate anion (110 - 111). Migration of the hydrogen from nitrogen to C-3 in (111) concerted with expulsion of nitrogen, provides the corresponding methylene derivative (100).82... 

J. T. Groves, S. Kittisopikul, Dehalogenation reactions catalyzed by tri-n-butyltin chloride. Competition for carbon radicals by borohydride and tin hydride. Tetrahedron Lett. 49 429I (1977). 

Some strategies used for the preparation of support-bound thiols are listed in Table 8.1. Oxidative thiolation of lithiated polystyrene has been used to prepare polymeric thiophenol (Entry 1, Table 8.1). Polystyrene functionalized with 2-mercaptoethyl groups has been prepared by radical addition of thioacetic acid to cross-linked vinyl-polystyrene followed by hydrolysis of the intermediate thiol ester (Entry 2, Table 8.1). A more controllable introduction of thiol groups, suitable also for the selective transformation of support-bound substrates, is based on nucleophilic substitution with thiourea or potassium thioacetate. The resulting isothiouronium salts and thiol acetates can be saponified, preferably under reductive conditions, to yield thiols (Table 8.1). Thiol acetates have been saponified on insoluble supports with mercaptoethanol [1], propylamine [2], lithium aluminum hydride [3], sodium or lithium borohydride, alcoholates, or hydrochloric acid (Table 8.1). 

On submitting D-isomannide 2,5-dinitrate (15) to flash vacuum thermolysis, 1,4 3,6-dianhydro-D-mannopyranose (145) was obtained as the main product (75 - 80% yield), accompanied by 5 -10% of the monoketone 8a. In contrast, when D-isosorbide 2,5-dinitrate (14) was treated under the same conditions, a 6 3 1 mixture consisting217 of 8a, 145, and its D-gluco epimer 146 resulted (see Scheme 40). These results could be explained by a restricted number of rearrangements of intermediate radicals, with inversion or retention of configuration, respectively. By reduction with sodium borohydride, 8a was stereoselectively converted into D-isomannide (4). 

While reduction is usually best carried out using alkaline sodium borohydride, this reaction proceeds by free radicals and is not stereospecific. By employing Na(Hg) in D2O, reduction with retention is observed. However, the stereospecific reduction of (3-hydroxymercurials in which the mercury moiety is a to a carbonyl group has been reported using alkaline NaBH4-EtOH,316 alkaline H2S317 and HS(CH2>3SH (equation 218).316... 

The demercuration of (3-alkoxymercurials is usually best effected using alkaline sodium borohydride. Few rearrangements during this free radical reduction process have been observed 429 When the mercury moiety is positioned a to a carbonyl group, alkaline H2S430,431 or 1,3-propanedithiol316 provide alternatives that afford complementary stereochemical results (equation 262). The use of sodium-mercury amalgam is also useful for stereospecific reduction.432 433... 

Catalytic procedures (introduced by Kuivila and Menapace92) are easier to conduct and the tin hydride concentration is more easily controlled. A catalytic amount of tributyltin hydride or tributyltin chloride is mixed with the radical precursor, the alkene acceptor and a stoichiometric quantity of a coreductant such as sodium borohydride93 or sodium cyanoborohydride.29 Over the course of the reaction, the borohydride continuously converts the tin halide to tin hydride. The use of the catalytic procedure is probably restricted to halide precursors (tin products derived from other precursors may not be reduced to tin hydrides). This method has several advantages over the standard procedures (i) it is simple to conduct (ii) most functional groups are stable to the coreductants (especially sodium cyanoborohydride) (iii) the tin hydride concentration is known, is stationary (assuming that the tin halide is rapidly reduced to tin hydride), and can be varied by either changing the concentration of the reaction or the quantity of the tin reagent (10% is a typical value, but lower quantities can be used) and finally, (iv) the amount of tin hydride precursor that is added limits the amount of tin by-product that must be removed at the end of the reaction. 

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