Direct hydrogenation mechanism

Reduction of acetophenone by PrOH/H has been studied with the ruthenium complexes [Ru(H)(ri2-BH )(CO)L(NHC)], (L = NHC, PPh3, NHC = IMes, IPr, SIPr). The activity of the system is dependent on the nature of the NHC and requires the presence of both PrOH and H, implying that transfer and direct hydrogenation mechanisms may be operating in parallel [15]. 

Figure 5.10. Three step direct hydrogenation mechanism...

The general form of the eq. (4.114) can be used in the case of the direct hydrogenation mechanism shown above giving... 

The value of CM has been determined by a number of groups as 6x 10 5 (Table 6.14.1. " However, the mechanism of transfer has not been firmly established. A mechanism involving direct hydrogen abstraction seems unlikely given the high strength of vinylic and aromatic C-M bonds. The observed value of Cy is only slightly lower than Ctr for ethylbenzene ( 7x 10"5). w... 

On the other hand a direct hydrogen transfer through a Meerwein-Ponndorf mechanism, involving coordination of both the donor alcohol and the ketone to the copper site may also be considered. In this case, by using alcohols other than 2-propanol, we could expect some difference in stereochemistry. This would also imply the possibility of carrying out the enantioselective reduction of a prochiral ketone with a chiral alcohol as donor. 

Since the first use of catalyzed hydrogen transfer, speculations about, and studies on, the mechanism(s) involved have been extensively published. Especially in recent years, several investigations have been conducted to elucidate the reaction pathways, and with better analytical methods and computational chemistry the catalytic cycles of many systems have now been clarified. The mechanism of transfer hydrogenations depends on the metal used and on the substrate. Here, attention is focused on the mechanisms of hydrogen transfer reactions with the most frequently used catalysts. Two main mechanisms can be distinguished (i) a direct transfer mechanism by which a hydride is transferred directly from the donor to the acceptor molecule and (ii) an indirect mechanism by which the hydride is transferred from the donor to the acceptor molecule via a metal hydride intermediate (Scheme 20.3). 

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 MPVO reactions has been investigated and questioned on several occasions, and a variety of direct hydrogen-transfer pathways have been suggested (see Scheme 20.4) [31-35]. Recently, racemization of D-labeled 1-phenylethanol with deuterated samarium(III) isopropoxide (17) proved that the MPVO reaction occurs via a direct hydrogen transfer from the a-position of the isopropoxide to the carbonyl carbon of the substrate (Scheme 20.7) [31]. 

The selectivity of the hydrogen transfer is excellent When employing a catalyst with deuterium at the a-positions of the isopropoxide ligands (17), complete retention of the deuterium was observed. A computational study using the density functional theory comparing the six-membered transition state (as in Scheme 20.3, the direct transfer mechanism) with the hydride mechanism (Scheme 20.3, the hydride mechanism) supported the experimental results obtained [36]. A similar mechanism has been proposed for the MPV alkynylations [37] and cyanations [38]. 

In the charge, or electrolysis mode, the process splits water into hydrogen and oxygen and can produce hydrogen directly without mechanical compression. Water enters the cell and is split at the surface of the membrane to form protons, electrons and gaseous oxygen. 

Buta-1,3-diene (BD) hydrogenation is a widely used model reaction to investigate surface modification by promoters or alloy formation.The reaction is usually described as a consecutive mechanism " without direct hydrogenation of buta-1, 3-diene to butane (BA). ... 

The mechanism for the iridium-catalyzed hydrogen transfer reaction between alcohols and ketones has been investigated, and there are three main reaction pathways that have been proposed (Scheme 4). Pathway (a) involves a direct hydrogen transfer where hydride transfer takes place between the alkoxide and ketone, which is simultaneously coordinated to the iridium center. Computational studies have given support to this mechanism for some iridium catalysts [18]. 

Natarajan A, Wang K, Ramamurthy V, Scheffer JR, Patrick B (2002) Org Lett 4 1443 Chesta CA, Whitten DG (1992) J Am Chem Soc 114 2188. Studies favoring a mechanism involving direct hydrogen atom transfer to form the 1,4-hydroxybiradical intermediate have recently been reported by Wang R, Chen C, Duesler E, Mariano PS (2004) J Am Chem Soc 69 1215... 

What probably happens in the oxychlorination process is that chlorine is formed in situ. The reaction of hydrogen chloride and oxygen to give chlorine and water was discovered by Deacon in 1858. Once the chlorine is formed, it then adds to ethylene as in the direct chlorination mechanism. Cu ... 

There are two possible mechanisms for forming amyl alcohol from valeraldehyde. It may result from a dismutation, yielding valeric acid and amyl alcohol in equivalent amounts, or it may result from direct hydrogenation of the aldehyde. The yields that are obtained are clear... 

Mechanism II. Amines may be able to stabilize free radicals formed during the reaction between the fuel and oxygen. This could be done either by forming an adduct between a fuel radical, R , and the amine, followed by reaction to form stable products (28), or by direct hydrogen abstraction (25, 28, 29) to form a radical, T, from the inhibitor, which does not take any part in the fuel-oxygen chain reaction ... 

The oxidation of hydrazine derivatives with diethyl azodicarboxylate is of particular interest because it involves direct hydrogen abstraction. The oxidation of keto hydrazones with lead tetraacetate leads to azoacetates, presumably by a free radical mechanism. 

This particular reaction mechanism belongs to the family of hydrogen-transfer reactions. This pathway implies the direct transfer mechanism between the donor and the acceptor groups. The MPV mechanism is common for main group elements acting as Lewis acids catalysts such as aluminum isopropoxydes or... 

The chief discrepancy between the experimental results of Chanmugan and Burton and of Bell and Kistiakowsky (see Secs. IV-A and B) is that the latter workers found evidence for the presence of methyl in the reaction of methylene with hydrogen and with methane (or their deuterated isomers), whereas the results of Chanmugan and Burton were more consistent with the direct addition mechanism, at least at low temperatures. 

According to a possible mechanism, transfer hydrogenation requires a catalyst-mediated formation of a donor-acceptor complex, followed by a direct hydrogen transfer. An alternative possibility is a simple consecutive dehydrogenation-hydrogenation process. While the former mechanism on palladium is supported by numerous experimental evidence,78 direct hydrogen transfer on nickel was disproved.79... 

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