LiAlH mechanism

Fig. 2.60 Schematic representation of LiH + AlH —> LiAlH mechanical alloying reaction during ball milhng to produce Hj-transparent LiAlH windows [186]...

In another recent development Kojima et al. [103] mechanically milled LiH and Al without and with the TiClj additive for 24 h in a H gas atmosphere at a pressure of 1 MPa at room temperature. They found that a sm l amount of LiAlH could be directly synthesized by the mechanochemical reaction with concomitant formation of LijAlHg. The latter can be relatively easily formed by mechanochemical synthesis of LiAlH and LiH as originally reported by Zaluski et al. [71] and later by Balema et al. [104],... 

As mentioned before in order to determine whether or not the free A1 formed upon decomposition of LiAlH /LijAlH in the composite could act as a catalyst, we also prepared composites with the content of A1 equivalent to the content of A1 in the Awt%LiAlH. Their DSC desorption peak temperature maxima are also plotted in Fig. 3.31. The composites with the equivalent content of A1 do not seem to follow the ROM behavior. Therefore, one can tentatively conclude that the underlying physical mechanism for the ROM behavior is not related to the catalytic effect of free Al. However, this possibility, however remote, cannot be completely ruled out of hand because the particle size of free Al formed upon decomposition might be much smaller than that obtained by ball milling of Al metal powder added to MgH powder. Nanosized free Al could aquire catalytic behavior. However, at the moment we do not have any evidence for that. 

Figure 3.32 shows XRD patterns of (MgH -i-LiAlH ) composites after DSC testing up to 500°C. The primary phases present are Mg and Al. Peaks of MgO and (LiOH) HjO arise from the exposure of Mg and Li (or possibly even some retained LiH) to the environment during XRD tests. Apparently, XRD phase analysis indicates that a nearly full decomposition of original MgH and LiAlH hydride phases has occurred to the elements during a DSC experiment. In addition, no diffraction peaks of any intermetallic compound are observed in those XRD patterns. That means that no intermetallic compound was formed upon thermal decomposition of composites in DSC. Therefore, the mechanism of destabilization through the formation of an intermediate intermetallic phases proposed by Vajo et al. [196-198] and discussed in the beginning of this section seems to be ruled out of hand. 

The structural stability and bonding mechanisms of LiAlH have been... 

Hydrosilylation of hex-l-ene and ethylene with SiHLt occurs in the presence of LiAIHLt as a catalyst. A reaction mechanism involving SiHLt and the alkyl anion, generated by the initial interaction of the olefin with LiAlH/t, has been proposed.62... 

Carboxylic acids, acid chlorides, acid anhydrides and esters get reduced to primary alcohols when treated with lithium aluminium hydride (LiAlH) (Fig.M). The reaction involves nucleophilic substitution by a hydride ion to give an intermediate aldehyde. This cannot be isolated since the aldehyde immediately undergoes a nucleophilic addition reaction with another hydride ion (Fig.N). The detailed mechanism is as shown in fig.O. 

The structure of the polymer obtained in the polymerization of butadiene and isoprene with heterogeneous Ziegler-Natta catalysts depends on the nature of the monomer, catalyst system, and reaction conditions. Previously reported results are reviewed and a mechanism is proposed for the stereoregulated polymerization of conjugated dienes. The polymerization of cyclopentadiene with LiAlH -TiCl4 or LiAlR4-TiCl4 catalyst system yields a readily oxidized polymer for which a 1,2-structure is proposed. 

The mechanism of this Grignard reaction is similar to that of LiAlH reduction. The first equivalent of Grignard reagent adds to the acid chloride, loss of Cl from the tetrahedral intermediate 3delds a ketone, and a second equivalent of Grignard reagent immediately adds to the ketone to produce an alcohol. 

Write a detailed mechanism for the reduction of acetic acid to ethanol by LiAlH. ... 

Amides differ from carboxylic acids and other acid derivatives in their reaction with LiAlH Instead of forming primary alcohols, amides are reduced to amines (Fig.P). The mechanism (Fig.Q) involves addition of the hydride ion to form an intermediate that is converted to an organoaluminium intermediate. The difference in this mechanism is the intervention of the nitrogen s lone pair of electrons. These are fed into the electrophilic centre to eliminate the oxygen that is then followed by the second hydride addition. 

Lithium aluminum hydride (8) reacts with ketones and aldehydes in the same way as sodium borohydride, except that LiAlH is a more powerful reducing agent. In one experiment, reaction of heptanal (13) with LiAlH4 in diethyl ether, followed by aqueous acid workup, gave 1-heptanol (16) in 86% yield. The mechanism is identical to that of borohydride in that heptanal reacts with the negatively polarized hydrogen of the Al-H unit in 8 via the four-centered transition state 14, This leads to an alkoxyalmninate product, 15, and subsequent treatment with dilute acid... 

Studies on the cleavage of palladium-carbon bonds by LiAlH, provide further evidence that hydride cleavage proceeds with a high degree of stereospecificity. Pseudo-first-order conditions are observed in the reaction of isoprene with [Pdaj - in methanol in the presence of lithium chloride. A possible mechanism... 

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