Benzyl bromide reduction

Benzyl Bromide [Reductive Bromination of an Acetal].506 To a suspension of tin(II) bromide (5.1 mg, 0.02 mmol) and benzaldehyde dimethyl acetal (54.8 mg, 0.36 mmol) in CH2C12 (2.5 mL) were added successively Et3SiH (65.0 mg, 0.56 mmol) and acetyl bromide (96.8 mg, 0.79 mmol) in CH2C12 (1 mL) at room temperature under an argon atmosphere. The mixture was stirred for 3 hours at room temperature and quenched with a phosphate buffer (pH 7). 

The p-cyanobenzyl ether, prepared from an alcohol and the benzyl bromide in the presence of sodium, hydride (74% yield), can be cleaved by electrolytic reduction (—2.1 V, 71% yield). It is stable to electrolytic removal ( — 1.4 V) of a tritylone ether [i.e., 9-(9-phenyl-10-oxo)anthiyl ether]. ... 

The asymmetric reduction of the benzoxathiin is very appealing because of its simplicity (Scheme 5.3). It was envisioned that intermediate 16 could be prepared from thiol-phenol 7 and bro moke tone 17. Scheme 5.8 summarized the synthesis for 16. The l,3-benzoxathiol-2-one 35 was prepared from 1,4-benzoquinone and thiourea following a literature procedure with minor modifications. Benzylation of 35 with benzyl bromide in the presence of KI gave benzyl ether 36 as a crystalline solid. It was observed that the benzylation gave better results when the reaction was run under anaerobic conditions. Hydrolysis of thiocarbonate 36 gave free thiophenol 7 which was used directly in the next reaction. 

The electrochemistry of cobalt-salen complexes in the presence of alkyl halides has been studied thoroughly.252,263-266 The reaction mechanism is similar to that for the nickel complexes, with the intermediate formation of an alkylcobalt(III) complex. Co -salen reacts with 1,8-diiodo-octane to afford an alkyl-bridged bis[Co" (salen)] complex.267 Electrosynthetic applications of the cobalt-salen catalyst are homo- and heterocoupling reactions with mixtures of alkylchlorides and bromides,268 conversion of benzal chloride to stilbene with the intermediate formation of l,2-dichloro-l,2-diphenylethane,269 reductive coupling of bromoalkanes with an activated alkenes,270 or carboxylation of benzylic and allylic chlorides by C02.271,272 Efficient electroreduc-tive dimerization of benzyl bromide to bibenzyl is catalyzed by the dicobalt complex (15).273 The proposed mechanism involves an intermediate bis[alkylcobalt(III)] complex. 

Williams and Rastetter also accomplished an elegant synthesis of ( )-hyalodendrin (83) in 1980 [39]. Beginning with the sarcosine anhydride-derived enolic aldehyde 78, silyl protection of the enal enabled alkylation of the glycine center with benzyl bromide and thiolation using LDA and monoclinic sulfur a la Schmidt. After protection of the thiol with methylsulfenyl chloride and deprotection of the silyl ether, the enol was sulfenylated with triphenylmethyl chlorodisulfide to afford bis(disulfide) 82 as a 2 1 mixture of diastereomers favoring the anti isomer. Reduction of the disulfides with sodium borohydride and oxidation with KI3 in pyridine afforded ( )-hyalodendrin (83) in 29 % yield (Scheme 9.4). 

Alkyl iodides, benzyl chlorides, benzyl bromides, and adamantyl bromides and iodides undergo reduction with triethylsilane/palladium chloride.195 The reduction of a /3-chloro ether occurs in excellent yield with this system (Eq. 56).195... 

These authors, using a horn system, also noted less striking but still significant switches towards the one-electron products in other sonoelectrochemical reductions [66] including dimethylmaleate at a lead cathode in an aqueous mixed-phosphate buffer, and benzyl bromide at a lead cathode in methanolic tetraethylammonium bromide solution (Tab. 6.16). 

A variety of alkyl halides have been reduced at room temperature, including benzyl halides, primary, secondary and tertiary alkyl halides. The reaction times depend on the halide, and vary between 20min (benzyl bromide, 0.5 mol% 28) up to several days (iodopentane, fluoropentane). The reactivity of alkyl halides decreases in the order R—Br > R—Cl > R—1 when reductions are performed in separate flasks. Several mechanistic details of the reaction have been uncovered by in situ monitoring of the reaction by NMR spectroscopy. The precatalyst 28 appeared to be activated by a rapid reduction of the coordinated acetone to PrO—SiEt3 and concomitant coordination of an alkyl halide (H Scheme 12.11). This complex represents a resting state that is in equilibrium with a o-silane... 

Since ketone R)-16 was prepared in a non-selective way when an achiral imino enolate was alkylated, it was considered whether alkylation of chiral enolates, such as that of oxazoline 18, with benzyl bromide 14, would provide stereoselective access to the corresponding alkylation product 19 with R-configuration at C(8) (Scheme 4). Indeed, alkylation of 18 with 14 gave the biaryl 19 and its diastereoisomer almost quantitatively, in a 14 1 ratio. However, reductive hydrolysis using the sequence 1. MeOTf, 2. NaBH4, and 3. H30", afforded hydroxy aldehyde 20 in 25% yield at best. Furthermore, partial epimerization at C(8) occurred (dr 7.7 1). An alternative route, using chiral hydrazones, was even less successful. 

Benzyl radicals have a reduction potential only slightly more negative than the reduction potential of the corresponding benzyl bromide. At a mercury cathode in... 

Miki and Hachiken reported a total synthesis of murrayaquinone A (107) using 4-benzyl-l-ferf-butyldimethylsiloxy-4fT-furo[3,4-f>]indole (854) as an indolo-2,3-quinodimethane equivalent for the Diels-Alder reaction with methyl acrylate (624). 4-Benzyl-3,4-dihydro-lfT-furo[3,4-f>]indol-l-one (853), the precursor for the 4H-furo[3,4-f>]indole (854), was prepared in five steps and 30% overall yield starting from dimethyl indole-2,3-dicarboxylate (851). Alkaline hydrolysis of 851 followed by N-benzylation of the dicarboxylic acid with benzyl bromide and sodium hydride in DMF, and treatment of the corresponding l-benzylindole-2,3-dicarboxylic acid with trifluoroacetic anhydride (TFAA) gave the anhydride 852. Reduction of 852 with sodium borohydride, followed by lactonization of the intermediate 2-hydroxy-methylindole-3-carboxylic acid with l-methyl-2-chloropyridinium iodide, led to the lactone 853. The lactone 853 was transformed to 4-benzyl-l-ferf-butyldimethylsiloxy-4H-furo[3,4- 7]indole 854 by a base-induced silylation. Without isolation, the... 

Lithium aluminum hydride reduction furnishes racemic 2-alkyl-l-alkanols in good yields13. Thus, from alkylation of 1,3-diacylimidazolidinone 1 with benzyl bromide followed by reduction, a 73% yield of ra< -2-methyl-3-phenyl-l-propanol is obtained. When this sequence is carried out with (R,R)-1 as starting material, (S )-2-methyl-3-phenyl-l-propanol is obtained in 93% ee [a]D -10.2 (c = 1.1, C6H6) 13. 

Reduction of the complex on Raney nickel yielded benzylamine, N-methyl-benzylamine, and N,N-dimethylbenzylamine but no / -phenylbenzylamine, a reduction product resulting under the same reaction conditions from benzyl cyanide. Hydrolysis with dilute sulfuric acid in acetic acid yielded benzylamine only, and oxidation of the complex with potassium permanganate gave 4.2 moles of benzoic acid per mole of complex. The bromide anion can be exchanged metathetically with various other anions such as perchlorate, iodide, and thiocyanate. When heated at 100° C. in vacuum, the complex lost one mole of benzyl bromide and yielded only one dicyanotetrakis(benzylisonitrile)iron(II) complex. 

A mixture of isomers was, however, obtained if the a,a -dibromoxylene was not symmetrical. This difficulty was overcome by using a reductive coupling of a substituted 3,4-dihydroisoquinoline (71) as its salt with an o-methoxycarbonyl-substituted benzyl bromide (73)129 [Eq. (60)]. 

Sodium hydride (25 mg, 0.52 mmol) was added to a cooled (0°C) solution of the crude reduction product in dry THF (10 mL) and the reaction mixture was stirred for 15 min, whereupon benzyl bromide (0.065 mL, 0.55 mmol) was added. The cooling bath was removed, and the mixture was stirred at room temperature. After 2 h the mixture was diluted with ether (50 mL), washed with water (3 x 20 mL), dried (MgS04), and concentrated to dryness. The residue was purified by chromatography on a silica gel column with hexane-ethyl acetate 4 1 to yield 4 (58.5 mg, 50%) as a pale yellow oil fo]D — 15.6° (c 0.6, CHClj). 

The presence of a base is also essential for the efficient reductive dehalogenation of RX by 1-benzyl-1,4-dihydronicotinamide (BNAH) via photoinduced electron transfer [121,122], Since the one-electron oxidation potential of the singlet excited state of BNAH ( BNAH ) is —3.1 V (vs. SCE) [50], which is more negative than the one-electron reduction potential of benzyl bromide (PhCH2Br), photoinduced electron transfer from BNAH to PhCH2Br occurs efficiently with the diffusion-limited rate [122]. This fast process needs no base catalyst to accelerate the electron transfer rate further. However, the photoinduced electron transfer results in... 

---