DIBAL mechanism

Reduction of nitrostyrene with aqueous TiCl3 gives a 3,4-diarypyrrole directly in moderate yield (Eq. 10.46).52 The reaction proceeds via dimerization of anion radicals of nitrostyrene and reduction of the nitro function in the dimer to imines. Reduction of dinitrile with diisobutylalu-minum hydride (DIBAL) gives a-free pyrroles (Eq. 10.47) 53 both reactions may proceed in a similar mechanism. These pyrroles are useful intermediates for functionalized porphyrins. 

The mechanism for both of these reactions is very similar to the mechanism for the reduction of acyl chlorides by LATB—H. The first step is an acid-base reaction between an unshared electron pair on oxygen or nitrogen with the aluminum atom of the DIBAL—H. The second step is the transfer of a hydride ion from the DIBAL—H to the carbon atom of the carbonyl or nitrile group. The last step is the hydrolysis of the aluminum complex to form the aldehyde. 

A similar mechanism is operative in the reduction of carboxylic esters with DIBAL (Figure 17.61). The tetrahedral intermediate A is formed by addition of an A1—H bond of the reducing agent to the ester C=0 bond. This tetrahedral intermediate A does not necessarily decompose immediately to an aldehyde and ROAl(iBu)2. In nonpolar media, A definitely decomposes quite slowly. In fact, at very low temperatures A remains unchanged until it is protonated to a hemiacetal during aqueous workup. The latter eliminates water to give the aldehyde. 

Fig. 17.61. Mechanism of the DIBAL reduction of carboxylic esters to aldehydes and further...

Fig. 17.64. Mechanism of the LiAlH4 reduction (top) and the DIBAL reduction (bottom) of nitriles.

Fig. 14.53. Mechanism of the DIBAL reduction of carboxylic esters to aldehydes and further to alcohols. In nonpolar solvents the reaction stops with the formation of the tetrahedral intermediate A. During aqueous workup, A is converted into the aldehyde via the hemiacetal. In polar solvents, however, the tetrahedral intermediate A quickly decomposes forming the aldehyde via complex B. In the latter situation the aldehyde successfully competes with unreacted ester for the remaining DIBAL. The aldehyde is reduced preferentially, since the aldehyde is the stronger electrophile, and it is converted into the alcohol.

In reactions requiring palladium 0), formation of the active complex may be achieved more conveniently by reduction of a palladium(ll) complex, for example, Pd 0Ac)2- Any phdsphine may then be used in the reaction, without the need to synthesize and isolate the corresponding palladium 0)-phosphine complex. Only 2-3 equivalents of phosphine may be needed, making the palladium(O) complex coordinatively unsaturated and therefore very reactive. The reduction of palladium li)to palladium(o) can be achieved with amines, phosphines, alkenes, and organometailics such as DIBAL-H, butyl lithium, or trialkyl aluminium. The mechanisms are worth giving as they illustrate the basic steps of organometallic chemistry. 

With DIBAL-H, nucleophilic addition of one equivalent of hydride forms an anion (Step [1]), which is protonated with water to generate an imine, as shown in Mechanism 22.13. As described in Section 21.12, imines are hydrolyzed in water to form aldehydes. 

The oxygen atoms of (27) are, however, quite different. One is next to an axial methyl group and is quite open to attack, the other is hindered by an equatorial methyl group. Reactions of compounds of this series with various aluminum hydrides (HAlBr2, HAlCh and DIBAL-H are best) at low temperatures give good yields of the products expected from the retention mechanism (equation 26 Table 1 l).99,ioo jp. version (equation 27) is obtained with triethylsilane and various Lewis acid partners. 

The following abbreviations are used throughout this section Et, ethyl Pr, u-propyl unless otherwise indexed Bu, n-butyl unless otherwise indexed C H2 +1, n-alkyl unless otherwise indexed Bn, PhCH2 Ph, phenyl Ar, aryl Cp, / -cyclopentadienyl, unless otherwise indexed n-, normal iso s-, secondary t-, tertiary c-, cyclo p-, para m-hapto THE, tetrahydrofuran 2,5-DMTHF, 2,5-dimethyltetrahydrofuran TBS, r-BuMe2Si MEM, methoxyethoxymethyl Tf, CF3SO2- acac, acetylacetonate Dibal, diisobutylaluminum hydride. Quite recently, a report on this mechanism of titanium-catalyzed hydromagnesa-tion of acetylenes appeared [120]. 

One of the major advantages of Route A is the possibility of following two independent pathways to obtain either (E)-syn- or (E)-anti-24 in a stereodivergent manner. This would be achieved by selection of reducing agents such as DIBAL for the syn isomers via a nonchelation control mechanism or Zn(BH4)2 for the anti diastereomers by way of bidentate chelation. ... 

Write a mechanism for this reaction. Interestingly, 1 mol of DIBAL-H can reduce up to 3 mol of aldehyde despite possessing only one apparent hydride equivalent. 

Scheme 9.27 Proposed mechanism for the deprotection of benzyl ethers by DIBAL H.

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