Reduction borane-pyridine

Aldehydes and ketones have been converted to sulfides by treatment with thiols and pyridine-borane, RCOR -I- R"SH —+ RR CHSR", in a reductive alkylation reaction, analogous to 16-6. 

Most published work has focused on the deposition of Ni, Co, and NiCo alloys from hypophosphite electrolytes [14], and this part of the review will deal primarily with these alloys. Other Co alloys studied include CoZnP [15, 16], the recording characteristics of which were described by Soraya [17] CoSnP [18], which is reported to have enhanced corrosion resistance and the rhenium and manganese alloys used for vertical recording, discussed below. Other reductants, such as hydrazine [19], dimethylamine borane [20-22], pyridine borane [23], and borohydride [24, 25], can be used for the chemical deposition of nickel and cobalt, but to date there has been no significant application of these to the technology of magnetic media. 

Boranes may be prepared by reduction of B—O bonds using LiAlH, e.g., C Hj-B(OCjHj)2 or (CjHjBOj reacts with LiAlH., in (CjHj) containing xs CjHjN at 70°C to form the air-stable pyridine—borane ... 

Zagorevskii et al. have observed the same stereoselective trans reduction with several tetrahydro-T-carbolines by generating the diborane from NaBIlt in situ (equation 79).As expected, treatment of these carbolines with pyridine-borane and acid gives the cw-fused isomers (equation 80).Bosch and coworkers have also utilized an in situ generation of diborane to effect the chemoselective reduction of the indole double bond during the synthesis of a new indolomorphan. 

Secondly, selective chemical modification may be performed because the carbohydrate residues have a distinct low reactivity. This can be done by periodate oxidation which cleaves C-C bonds bearing adjacent hydroxyl groups and converts them to aldehydes [25,26]. The generated dialdehyde can react with a variety of nucleophiles - usually primary amino groups on the surface of carrier materials. The resulting Schiff bases can be further stabilized by sodium boro-hydride, sodium cyanoborohydride or pyridine-borane reduction [27]. 

By the action of NaBH4 in DMF-THF in the presence of pyridine at 0°C [B1]. This reduction generates borane, which coordinates to pyridine, forming a complex that precipitates under these conditions (Figure 3.64). The reduction leaves the halide functional group intact (Figure 3.64). 

The reduction of indoles by pyridine-borane in CF3COOH [K3] or BH3 THF in CF3COOH [MMl] is compatible with amide, nitrile, or ester groups. It is interesting to emphasize that LAH in ether media reduces these groups without affecting the... 

Reduction of ketones in homogeneous medium in the presence of cyclodex-trins gives disappointing results. However, inclusion of pyridine-borane into ciys-talline P-cyclodextrins generates complexes that reduce prochiral ketones or activated C=C bonds with good enantiomeric excesses [578,579],... 

Reduction of heterocycles. Pyridine-borane shows enhanced reducing properties in acetic acid, presumably owing to formation of pyridine-acetyl-borane, Py BH(OAc)2 or Py BHa(OAc) (1). Thus 1 reduces quinoline as shown in equation (I). Some other reductions of heterocyclcs are indole—i-indoline... 

Reductive amination, a suitable method for N-alkylation of amines, can be carried out using methanolic pyridine-borane under conventional conditions at room temperature for 3-16 h (95JOC5995). Under MWI, alkylation of hexahydroazepine 432 with paraformaldehyde and formic acid gave 70% of A -methylhexahydroazepine 433 after 4min of irradiation (Scheme 85) (01JCR(S)292). Similarly, A -alkyl and A -cycloalkylhydroazepines 434 and 435 were obtained in 28-81% yields. 

The steric structure of the matrix-carrier can make an additional contribution to the asymmetric reaction, especially if the carrier contains chiral characteristics. Thus, Smith et al. showed that o// / o-cyclodextrin as the host compound can induce asymmetric reactions in the reduction of acetophenone and Tanaka et al. showed that the pyridine-borane-cyclodex-trin system induces chirality with an ee of 91%. Pd supported on a copolymer of Z>eto-cyclodextrin-epichlorhydrin hydrogenates 2-methylpent-2-enoic acid with an ee of 9.3% whereas Pt supported on Z> to-cyclodextrin polymer produced an optical yield of up to 15.7% (Smith et al. 

Pyridine-substituted nicotines, nornicotines, and anabasines are available from a-cyano-amines by a new synthesis. The syntheses of ( )-[l - N]nornicotine and ( )-[l - N]nicotine have been described, from cyclopropyl 3-pyridyl ketone and [ N]formamide. 4-Methylnicotine has been prepared it shows no nicotine-like pharmacological activity. " An investigation into the stereochemical factors involved in the behaviour of nicotine and related compounds in the Menschutkin reaction has been conducted.A bridged nicotine , 1,2,3,5,6,10b-hexahydropyrido[2,3-g]indolizine (44), has been synthesized by carboxylation of the dilithium derivative of 2-methylnornicotine followed by cyclisation and reduction with borane in tetrahydrofuran. Several 5-halogeno-nicotines have been prepared and their pK values and biological activities measured. ... 

Poly(3-alkylthiophene)s are chemically robust, withstanding strong reductants including boranes [67] and LiAlH4 [72]. The electron-rich backbone is, however, readily functionalized by oxidative methods. Li and co-workers exploited this to replace the 4-proton with Cl, Br or NO2 functionality [73-75]. Reaction at the a-methylene was noted in some instances. Subsequent Pd-catalyzed cross-coupling of the perbrominated polymer could effect >99% derivatization. Oxidation renders the backbone susceptible to nucleophilic attack. Li et al. found that pyridine derivatives efficiently reacted at the 4-position of the radical cation, functionalizing up to 60 % of the putative polaron pentads. Use of l-methyl-4-(4 -pyridyl)pyridinium salts yielded viologen substituents [76]. 

Pyridine/alumina 17, 927 Pyridine borane —, reductions with — 16,113 Pyridine derivatives, 1,2,3,6-fefrahydro- 16, 855 Pyridine hydrochloride as reagent 17, 17, 793, 992 Pyridine ring (s. a. Quinoline ring)... 

The reduction of aldehydes by pyridine-borane is a novel route to either symmetrical ethers or, in the presence of an appropriate alcohol, to unsym-metrical ethers (Scheme 41).Trimethylsilyl trifluoromethanesulphonate is the... 

The insoluble polymeric pyridine-borane system (20) has been reported to reduce aldehydes and ketones in high yields at room temperature in the presence of boron trifluoride-etherate, whereas acid derivatives and a,/3-enones are not reduced. The polymer is used in benzene, and reducing power is better if the polymer beads are swollen in benzene before use. The corresponding polymeric 4-pyridyl-borane (21) is sluggish in reductions but reactivity can be increased using a borane complex of the quaternary polymer (22), which is assumed to... 

Y. Kikugawa. Chemistry of amine-boranes. Part 3. Reduction of tryptophan derivatives with pyridine-borane. J. Ghem. Research (5), 1978, 184. 

Reductions with pyridine borane Prim, alcohols from aldehydes... 

A soln. of p-bromobenzaldehyde and pyridine borane in isopropyl ether refluxed 45 min. p-bromobenzyl alcohol. Y 94%.—The reducing power of pyridine borane is considerably less than that of those hydrides heretofore used for the reduction of carbonyl compounds to the alcohol stage. Acids and acid chlorides gave lower yields and esters could not be reduced. F. e. and reductions s. R. P. Barnes, J. H. Graham, and M. D. Taylor, J. Org. Ghem. 23, 1561 (1958). 

A soln. of pyridine borane in glacial acetic acid added to a soln. of dl-3a-acetoxy-18-nor-5 -androstan-17-one in the same solvent, allowed to stand 3 hrs. at room temp, under Ng, a few drops of 10%-HGl and after 5 min. water added, the crude product treated 0.5 hr. with benzoyl chloride in pyridine at room temp. dl-18-nor-5 -androstan-3a,17j -diol 3-acetate 17-benzoate. Y 78%.— Reduction with NaBH4 in aq. dioxane gave only 57% yield. F. e. s. K. V. Yorka, W. L. Truett, and W. S. Johnson, J. Org. Ghem. 27, 4580 (1962). 

The pyridine-borane reduction of oximines has been applied to the reduction of a-oximino-esters (51) to (52) (Scheme 32). ... 

V. T. Perchyonok and Kellie L. Tuck of Monash University found (Tetrahedron Lett. 2008, 49, 4777) that a concentrated solution of Bu NCl and HjPOj in water effected free radical reductions and cyclizations. Stephane G. Ouellet of Merck Frosst demonstrated (Tetrahedron Lett. 2008, 49, 6707) that an oxazoline such as 3 could be converted to the alcohol 4 by acylation followed by reduction. Elizabeth R. Burkhardt of BASF developed (Tetrahedron I tt. 2008, 49, 5152) a protocol for scalable reductive amination using an easily metered liquid pyridine-borane complex. Mohammad Movassaghi of MIT devised (Angew. Chem. Int Ed. 2008, 47, 8909) a strategy for conversion of an allyUc carbonate 8 by way of the ally lie diazene to the terminal alkene 9. 

The desired pyridylamine was obtained in 69 % overall yield by monomethylation of 2-(aminomethyl)pyridine following a literature procedure (Scheme 4.14). First amine 4.48 was converted into formamide 4.49, through reaction with the in situ prepared mixed anhydride of acetic acid and formic acid. Reduction of 4.49 with borane dimethyl sulfide complex produced diamine 4.50. This compound could be used successfully in the Mannich reaction with 4.39, affording crude 4.51 in 92 % yield (Scheme 4.15). Analogous to 4.44, 4.51 also coordinates to copper(II) in water, as indicated by a shift of the UV-absorption maximum from 296 nm to 308 nm. 

The selective reduction of the D-ring olefin in 106 using a partially poisoned catalyst (Pd/C, 0.25 % pyridine) provided intermediate 107 (83 %), which was epimerized at -78 °C with sodium methoxide (HOAc quench at -78 °C, 89 %) (Scheme 10.9). Deoxygenation by means of tosyl hydrazone 108 and subsequent treatment with catechol borane and tetrabutylammonium acetate gave pentacyclic... 

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