Diborane, reduction

Diborane reduction of pyrrole and indole ketones affords the corresponding alkyl-pyrroles and -indoles <68X1145). 

Bell and Hall have incorporated an organometallic unit into a crown by using the ferrocenyl unit as part of the ring or as a third strand. The unit is incorporated either as the 1,1 -diformylferrocene or the corresponding acid. In the former case, the bis-imine is prepared and reduced to give the saturated crown (see structure 24). In the latter case, the acid is converted into its corresponding chloride and thence into the diamide by reaction with a diamine. Diborane reduction affords the saturated amino-crown. Structure 24 could be prepared by either of these methods but the dialdehyde approach was reported to be poor compared to the amide approach which afforded the product in ca. 60% yield . 

Diborane reduction of an ortho ester that is prepared from a triol with CH3C(OEt)3, PPTS. ... 

The first of the nudeophilic ring-opening reactions of vinylaziridines discussed in this section is diborane reduction, developed by Laurent and coworkers in 1976 (Scheme 2.24). Treatment of N-unsubstituted vinylaziridines 89 with B2H6 gives allyl amines 92 by SN2 reduction via cyclic intermediates 90 [40]. In contrast, treatment with 9-BBN gives 2-(hydroxyethyl)aziridines 93 after oxidative workup (Scheme 2.25) [41]. 

Prepn of the 3,5-dinitro by diborane reduction of H03C.CH2.C8Ha-3I5-(N02)2 is given in... 

Reduction of esters, nitriles, and amides. These groups are rapidly reduced by horanc-dimcthyl sulfide in refluxing THF (b.p. 67°) if the dimethyl sulfide (b.p. 38°) is removed as liberated. Under these conditions, the reagent is comparable to uncomplexed diborane. Reduction of secondary and tertiary amides is best effected in the presence of boron trifluoride etherate otherwise, excess reagent is utilized for formation of complexes with the products. 

Diastereoselective reduction of chiral p-keto sulfoxides.1 The chiral /i-keto sulfoxides (I) are reduced by LiAH4 and several borohydrides mainly to 2. The stereoselectivity is increased as the temperature is lowered. In contrast, DIBAH and diborane reduction results mainly in the alcohol 3. 

A syn displacement of the bromine by benzylamine in the presence of triethylamine led, by a Sn2 reaction, to the a and p amino compounds which were separated into 326 (18%) and 327 (81%) respectively. The dichloroacetamide 328 derived from the latter, when subjected to the action of tri-n-butyltinhydride (2eq) and 2,2 -azobisisobutyronitrile underwent a 5-ero ring closure to furnish via the radical 329, the hydrooxindole 330 (51%) and significant amount of the rearrangement product 331 (30%). The latter is believed to be formed by fragmentation of the cyclohexadienyl radical 332 generated from the cyclohexyl radical 329. On diborane reduction, 330 provided the cis hydroindole 333, which on 0,N-debenzylation afforded ( )-c -fused bicyclic aminoalcohol 334, a compound that had been previously cyclised with formaldehyde to ( )-elwesine (320) by Stevens et al [85]. 

N-Alkyl derivatives were prepared from 7-phenyl-2,3,4,6,7,llh-hexahy-dro-l//-pyrazino[2,l-a]isoquinoline by acylation, followed by diborane reduction (84EUP107825,84JMC995). 

Reduction191 of the imine (367) with diborane followed by a catalytic hydrogenation produced 20a(20S)- and 20/3(20R)-amines, (370) and (371), in the ratio 55 45. Reduction employing (+ )-di-3-pinanylborane gave a similar mixture. However, when chirality was introduced into the imine as in (368) [prepared from (S)-phenylethylamine] diborane reduction yielded uniquely the 20a -amine (370) (funtuphyllamine A). The imine (369), prepared from ( + )-(R)-phenylethylamine, gave (370) (8%) and (371) (92%). 

HOCH2.CH2.C H3(N02)2 mw 212.16, N 13-20%. Prepn of the 3,5-dinitro by diborane reduction of H02C.CH2.C H3-3,5-(N02)2 is given in Ref 3 no props mentioned. Ref 4 mentions prepn by nitration of the parent, but CA does not indicate which isomer. The 2,4-dinitro is mentioned in Refs 2 5 without prepn or props Refs 1) Beil — not found 2) Y. Ogata ... 

Enamines may be reduced by sodium borohydride to give saturated amines, but only if a protonating species is available to convert the enamine initially into the iminium cation (lo). Steroidal g-amino-g,5-dienes are unreactive to sodium boro-hydride alone, but addition of acetic acid leads to rapid reduction via the iminium ion (10) to give gj -amino-A -steroids [224], The possibility that diborane was the reactive species in the NaBHd/HOAc system was excluded by the virtual non-reactivity of the enamine to externally generated diborane. Reduction of the iminium ion derivative (3) of a 6-formyl enol ether has been exploited in a variant of the Vilsmeier synthesis of 6-methyl steroids [22 ], the reduction product was the 6-aminomethyl enol ether (ii) which suffered hydrogenolysis with Raney nickel to give the 6-methylated enol ether (12). 

The configuration of the produced diol is influenced by the relative stabilities of the benzyl- or alkylcarbonium ions formed during the reaction. Similar reactions were investigated earlier. The rate and stereochemistry of the diborane reaction is altered by a small quantity of LiCl. Wide-ranging research has been performed with regard to the mechanism and stereochemistry of the diborane reduction in connection with cyclic and aliphatic a, 3-unsaturated and allylic epoxides, on diterpene models, " and by study of the reduction of epoxy-methylenecyclohexane and 2,3-epoxy-3-methylcyclohexanone. ... 

Diborane reduction of diphenylcyclopropenone followed by alkaline peroxide treatment provides" j5-phenylpropiophenone (320) and diphenylcyclopropene is a likely intermediate (equation 96). 

Opposite regioselectivity to that of regular metal hydride reductions has been observed in the NaBH4 reductions of oxiranes in the presence of triethylamine under photochemical conditions (hydride attacks the more highly substituted carbon) <92CC1133>. Diborane (B2H6) likewise tends to reduce oxiranes at the sterically more hindered oxirane carbon the mechanism and stereochemistry of the diborane reduction in connection with aliphatic oxiranes has been studied <82H(l8)28l>. 

Reduction - An excellent review of diborane reductions has appeared.78... 

A stereospecific synthesis of funtuphyllamine A (7) has been reported (G. Demailly and G. Solladig, Tetrahedron Letters, 1975, 2471) diborane reduction of the chiral iminopregnane (6), followed by hydrogenolysis, yields the alkaloidal base (7) sterospecifically. 

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