Lewis acids metal hydrides

Unsymmetrically substituted benzylidene acetals can be cleaved regioselectively in some cases using Lewis acidic metal hydride reducing agents.317 The cleavage results in the formation of a benzyl ether (more hindered position) and an alco-... 

The Meerwein-Ponndorf-Verley reduction of carbonyl compounds and the Oppenauer oxidation of alcohols, together denoted as MPVO reactions, are considered to be highly selective reactions. For instance, C=C double bonds are not attacked. In MPV reductions a secondary alcohol is the reductant whereas in Oppenauer oxidations a ketone is the oxidant. It is generally accepted that MPVO reactions proceed via a complex in which both the carbonyl and the alcohol are coordinated to a Lewis acid metal ion after which a hydride transfer from the alcohol to the carbonyl group occurs (Fig. 1) [1]. Usually, metal ec-alkoxides are used as homogeneous catalysts in reductions and metal t-butoxides in oxidations [1]. 

The reaction of metal alkyl compounds with dihydrogen, leading to hydride species with elimination of alkanes, is commonly observed for Lewis acidic metal centers. This reaction has been used to prepare hydride complexes of the lanthanides, and of titanium(III), see equation 8 [39]. 

The main feature of the Meerwein-Ponndorf-Verley reaction pathway involves the coordination of both reactants to the Lewis-acid metal eentre and hydride transfer from the alcohol to the earbonyl group. Aluminium or titanium alkoxides are usually effective homogeneous catalysts. With tin-Beta catalyst, cyclohexanone reduction with 2-butanol led selectively to cyclohexanol at 100 °C. Ketone conversion was >95%, whereas silicon-Beta, Sn02/Si02 and SnCl4 -5H20 were inaetive under the same experimental conditions. Therefore, the activity is likely due to tetrahedral tin in the zeolite framework, and not to extra-framework tin or to leached tin. ... 

The addition of main group-based organometaUic or hydride reagents to pyridines and similar systems takes place through coordination of the heterocyclic molecule to the Lewis acid metal center followed by formation of dearomatized species, for which there is a small but growing body of structural information. 

For a review of reductions by metal hydride-Lewis acid combinations, see Rerick, M.N. in Augustine Reduction, Marcel Dekker NY, 1968, p. 1. 

Brown, H. C., The Reactions of Alkali Metal Hydrides and Boro-hydrides with Lewis Acids of Boron and Aluminum, Congr. Lect., 17th Int. Congr. Pure Appl. Chem. p. 167. Butterworths, London, 1960. 

The mechanism by which the Group III hydrides effect reduction involves activation of the carbonyl group by coordination with a metal cation and nucleophilic transfer of hydride to the carbonyl group. Hydroxylic solvents also participate in the reaction,59 and as reduction proceeds and hydride is transferred, the Lewis acid character of boron and aluminum becomes a factor. 

MR donor bonds, is o-bond metathesis. Alternatively, as the Lewis-acid strength of M increases, the tendency toward agostic or bridging interactions can finally result in H—H bond scission and formal migration of hydride to the metal atom,... 

Some experimental evidences are in agreement with this proposed mechanism. For example, coordinating solvents like diethyl ether show a deactivating effect certainly due to competition with a Lewis base (149). For the same reason, poor reactivity has been observed for the substrates carrying heteroatoms when an aluminum-based Lewis acid is used. Less efficient hydrovinylation of electron-deficient vinylarenes can be explained by their weaker coordination to the nickel hydride 144, hence metal hydride addition to form key intermediate 146. Isomerization of the final product can be catalyzed by metal hydride through sequential addition/elimination, affording the more stable compound. Finally, chelating phosphines inhibit the hydrovinylation reaction. 

In the direct transfer mechanism, the metal ion coordinates both reactants enabling an intramolecular reaction, and activates them via polarization. Consequently, strong Lewis acids including Alln and the Lnln ions are the most suitable catalysts in this type of reactions. In the hydride mechanism, a hydride is transferred from a donor molecule to the metal of the catalyst, hence forming a metal hydride. Subsequently, the hydride is transferred from the metal to the acceptor molecule. Metals that have a high affinity for hydrides, such as Ru, Rh and Ir, are therefore the catalysts of choice. The Lewis acidity of these metals is too weak to catalyze a direct hydride transfer and, vice versa, the affinity of Alm and Lnm to hydride-ions is too low to catalyze the indirect hydrogen transfer. Two distinct pathways are possible for the hydride mechanism one in which the catalyst takes up two hydrides from the donor molecule and another in which the catalyst facilitates the transfer of a single hydride. 

The mechanism of the Meerwein-Pondorf-Verley reaction is by coordination of a Lewis acid to isopropanol and the substrate ketone, followed by intermolecular hydride transfer, by beta elimination [41]. Initially, the mechanism of catalytic asymmetric transfer hydrogenation was thought to follow a similar course. Indeed, Backvall et al. have proposed this with the Shvo catalyst [42], though Casey et al. found evidence for a non-metal-activation of the carbonyl (i.e., concerted proton and hydride transfer [43]). This follows a similar mechanism to that proposed by Noyori [44] and Andersson [45], for the ruthenium arene-based catalysts. By the use of deuterium-labeling studies, Backvall has shown that different catalysts seem to be involved in different reaction mechanisms [46]. 

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