DIBAH

A third method of aldehyde synthesis is one that we ll mention here just briefly and then return to in Section 21.6. Certain carboxylic acid derivatives can be partially reduced to yield aldehydes. The partial reduction of an ester by dhsobutylaluminum hydride (DIBAH), for instance, is an important laboratory-scale method of aldehyde synthesis, and mechanistically related processes also occur in biological pathways. The reaction is normally carried out at —78 °C (dry-ice temperature) in toluene solution. 

Problem 21.17 i What product would you expect from the reaction of butyrolactone with LiAIH4 1 With DIBAH ... 

Diazoquinone-novolac resist, 506 Diazotization reaction, 941 DIBAH, see Diisobutylaluminum hydride... 

A TEA/DIBAH mixture can be added to cold (-78°C) monomer until the stable colored complex forms. The purification reaction is then allowed to proceed for 60 minutes at room temperature. This procedure allows for removal of impurities without reduction of the ester. Significantly narrower gel permeation chromatograms (Mw/Mn <1.25) of poly(t-butyl methacrylate) are obtained when the samples are prepared from TEA/DIBAH purified monomer. 

C gives a mixture of dibromoketones 166a and 166b, and when these are treated with zinc-copper in methanol saturated with ammonium chloride at room temperature, the debromination product 167 is obtained in quantitative yield. Reduction of 167 with an excess of DIBAH in THF at — 78°C and then at room temperature leads to a mixture of the 6,7-dehydro-3-hydroxytropanes 168 and 169 (93 7 ratio) (Scheme 10). 

Scheme 10. Noyori synthesis of 6,7-dehydro-3-hydroxytropanes 168 and 169. Reagents i, Fe2(CO)9, C6H6, 50°C ii, 2n-Cu, NH4C1,20°C iii, DIBAH, THF, -78- 20°C.

Hayashi, M., Yoshiga, T., Nakatani, K. et al. (1994) Reduction of or-trialkylsiloxy nitriles with diisobutylalu-minium hydride (DIBAH) a facile preparation of (t-lrialkylsiloxy aldehydes and their derivatives. Tetrahedron, 50, 2821-2830. 

Figure 11. Increase of molecular weight with conversion. Catalyst, Nd(O-n-CkHs)s, DIBAH, EADC Nd, 0.035 X 10 3 m/L solvent, n-hexane DIBAH/Nd, 40 Cl/Nd, 3 monomer concentration, 17.5% wt polymerization temperature,...

RM = LiBH, > LiAlEt/THF > LiAlH4/Et20 > DIBAH/Et20 RM = MgBr > PhLi, PhMgBr, BuMgBr > BuLi... 

Organylzirconocene derivatives do not show any useful reactivity toward a,[5-unsaturated carbonyl compounds, but the reaction can be promoted by the addition of CuOTf [14], The initial version was stoichiometric in Cu [14], but a Cu-catalyzed version was subsequently developed [112,113]. Adaptation of the Ni salt + DIBAH catalysts for cross-coupling [10] to conjugate addition led to the Ni-catalyzed conjugate addition of alkenylzirconocene chlorides [16,17] (Scheme 1.25). 

Stille coupling (12, 56).1 The key step in a synthesis of (E)-neomanoalide (4) involves palladium-catalyzed coupling of an allyl halide with an a-stannylfuran. Thus 1 and 2 couple in the presence of Pd(dba)2 and P(C6H5), to form 3 as a 1 1 mixture of (E)- and (Z)-isomers in 66% yield. Conversion of 3 to 4 involves reduction (DIBAH) and selective singlet oxygen oxidation of a 2-silylfuran to a butenolide. 

Selective reduction of ketones.1 This reagent can be used to effect selective reduction of the more hindered of two ketones by DIBAH or dibromoalane. Thus treatment of a 1 1 mixture of two ketones with 1-2 equiv. of 1 results in preferential complexation of the less hindered ketone with 1 reduction of this mixture of free and complexed ketones results in preferential reduction of the free, originally more hindered, ketone. An electronic effect of substituents on a phenyl group can also play a role in the complexation. This method is not effective for discrimination between aldehydes and ketones, because MAD-complexes are easily reduced by hydrides. MAD can also serve as a protecting group for the more reactive carbonyl group of a diketone. The selectivity can be enhanced by use of a more bulky aluminum reagent such as methylaluminum bis(2-f-butyl-6-( 1,1-diethylpropyl)-4-methylphenoxide). 

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